Ask The Heat Treat Doctor®: Why Use Partial Pressure in Vacuum Furnaces?

February 6, 2026 / HARDENING TECHNICAL CONTENT, MEDICAL HEAT TREAT TECHNICAL, VACUUM FURNACES TECHNICAL CONTENT

Ask The Heat Treat Doctor® has returned to bring sage advice to Heat Treat Today readers and to answer your questions about heat treating, brazing, sintering, and other types of thermal treatments as well as questions on metallurgy, equipment, and process-related issues. In this installment, Dan Herring explains how partial pressure atmospheres prevent evaporation and achieve bright, oxide-free parts in vacuum furnaces.

This informative piece was first released in Heat Treat Today’s December 2025 Annual Medical & Energy Heat Treat print edition.

Have questions or feedback? We’d love to hear from you — reach out to our editorial team at editor@heattreattoday.com.

Operating in vacuum can often lead to problems related to evaporation, that is, literally “boiling away” elements present in the materials being heat treated. This affects surface integrity, functionality, performance, and in some rare cases altering the chemical composition of the base (or filler) metal.

One way to overcome this problem is to introduce a gas partial pressure higher than that of the material’s vapor pressure. Different gas choices, introduction methods, and controls are available to the heat treater. The natural question is, how and when should they be used? Let’s learn more.

What is Partial Pressure?

In simplest terms, the partial pressure of a gas introduced into a vacuum furnace is the force exerted by that gas (or gases) constrained in the vacuum vessel. If only a single gas is present, the partial pressure of the system is the same as the total pressure. For a multi-gas system, air is a good example to look at. At sea level with atmospheric pressure 760 torr (760 mm Hg) and at an altitude of 3,657 m (12,000 ft), the atmospheric pressure is only 483 Torr (Table A).

Table A. Partial Pressure of Individual Gases Present in Air | Source: Jones 1997

In vacuum systems, when the chamber atmosphere is evacuated to a high enough vacuum level — commonly between 10⁻³ Torr (0.1 micron) and 10⁻⁵ Torr (0.01 microns) — issues of evaporation are likely to occur during heat up and holding at temperature. As such, nitrogen or a truly inert gas is introduced below a predetermined temperature at a controlled rate to a fixed partial pressure range and then controlled within this range. One then isolates the high vacuum portion of the pumping system and employs bypass circuitry using the mechanical pump to introduce a continuous flow of gas equal to the pumping capacity (throughput) at the required operating pressure (Figure 1 below).

Figure 1. Typical partial pressure piping on a vacuum furnace
Key:
A: Incoming gas supply line
B: Backfill line
C: Quench solenoid
D: Partial pressure line
E: Partial pressure solenoid valve
F: Partial pressure (micrometer) needle value
G: Inlet into furnace
Source: Courtesy of Vac-Aero International

Why Do We Need to Use Partial Pressure in a Vacuum Furnace?

There is no hard and fast rule for partial pressure settings used for processing various materials in the heat treat industry. However, from a practical standpoint, there are two process considerations for determining partial pressure. The first is the metal-oxide reduction partial pressure. The partial pressure of oxygen at a given temperature determines the direction of the reaction and consequently whether the part is “bright” or “discolored” (oxidized). These values are typically in the range of 10⁻⁶ Torr to 10⁻² Torr. This is why materials like titanium alloys and superalloys must be processed at extremely low vacuum levels. The second consideration is the vaporization of metal at high temperature and hard vacuum. The metal solid-to-vapor partial pressures require higher pressures to avoid alloy depletion. These higher pressures often produce sufficient dilutions of contaminants to drive the reaction to be reducing.

What is often overlooked or misunderstood is that higher levels of partial pressure “dilute” any oxygen or water vapor partial pressure but still can produce oxide free “bright” parts at higher pressures. This dilution also occurs, for example when a retort is purged with nitrogen or argon to achieve clean parts. The oxygen partial pressure is reduced by dilution rather than by vacuum. In addition, it cannot be overemphasized that oxidation present on parts from exposure to the atmosphere and moisture absorbed by the furnace lining when the door is open are critical in running clean work. Oxidation occurs on heat up, but when the temperature is high enough and conditions are right, we can reverse the oxidation reaction so the parts will clean up. This is why it is harder to bright temper than to bright harden.

In batch vacuum furnaces, combination hardening and tempering cycles are used to take advantage of the furnace configuration in which parts stay in the furnace for the full process. Often, the same parts will discolor if tempered in the same furnace after they have been removed and the furnace exposed to air.

Also, a thorough understanding of the required component properties and material characteristics (e.g. alloy composition, grain size, hardenability response) is needed to design the final vacuum heat treat cycles and select the final partial pressure settings.

Figure 2. Chromium deposits / discoloration in the area of a graphite cooling nozzle | Source: The HERRING Group, Inc

For example, stainless steels, tool steels, and more exotic alloys run in a vacuum furnace will benefit substantially from the use of partial pressure atmospheres. In most heat treat shops, partial pressure cycles begin around 760°C (1400°F) at pressure from 1–1.5 Torr (1000–1500 microns). This is primarily because chromium present in many of these materials and in our baskets/fixtures evaporates noticeably at temperatures and pressures within normal heat treatment ranges. At around 990°C (1800°F), chromium will vaporize rapidly as a function of both vacuum level and time. In general, the practical operating vacuum level for most materials is significantly above their equilibrium vapor pressures. It is also helpful at times to know the temperature at which individual elements exceed a critical (10⁻⁶ g/cm²-s) vaporization rate (Herring 2015).

In practice, heat treaters often observe greenish discoloration (chromium oxide) on the interior of their vacuum furnaces (Figure 2), the result of chromium vapor reacting with air leaking into the hot zone. Otherwise, the evaporation deposit is bright and mirror-like. To avoid these types of deposits contaminating both the furnace and the parts run in it, an operating partial pressure between 1 Torr and 5 Torr (1,000 microns to 5,000 microns) is typical for parts that will boil away their elemental constituents.

Chromium Coloration

Heat treaters should be aware that although the most common color of chromium discoloration is green, the color is dependent on chromium’s oxidation state (Table B). For example, Cr (II) compounds typically appear blue, Cr (III) compounds appear green, and Cr (VI) compounds appear orange or red.

Notes: * Most commonly observed colors
Table B. Oxidation Colors of Chromium and Chromium Compounds

Table B provides a more detailed breakdown of chromium’s oxidation states and associated colors.

Which Partial Pressure Gas(es) Can We Use?

Argon, nitrogen, and hydrogen are the most common partial pressure gases. Often, argon is preferred as it is a truly inert gas and tends to “sweep” the hot zone; that is, being a heavier molecule, it tends to reduce evaporation compared with nitrogen or hydrogen. Specialized applications, such as those in the electronics industry, may use helium or even neon (if an ionizing gas is needed). Gases having a minimum purity of 99.99% and a dew point of -60°C (-76°F) or lower should be specified.

Certain cautions are in order. For example, nitrogen may react with certain stainless steels and titanium bearing alloys resulting in surface nitriding. In the case of hydrogen, the normally near neutral vacuum atmosphere can be sharply shifted to a reducing atmosphere to prevent oxidation of sensitive process work or for furnace/fixture bakeout/cleanup cycles. Embrittlement by hydrogen is a concern for certain materials (e.g., Ti, Ta).

Figure 3. 410 stainless steel housings, hydrogen partial pressure (1,000 microns) at 1010°C (1850°F) | Source: Courtesy of Vac-Aero International, Inc

In Summary

Partial pressure atmospheres are required in many heat treating and brazing operations to achieve desired results. Introduction of the partial pressure gas into the furnace hot zone at one or more locations and controlling the partial pressure injection gas stream as a continuous flow, rather than trying to operate at a specific pressure, are critical considerations. The choice of partial pressure gas is also important both from a cost and quality standpoint.

References

Herring, Daniel H. 2014. Vacuum Heat Treatment. Vol. 1. Troy, MI: BNP Media.

Herring, Daniel H. 2015. Vacuum Heat Treatment. Vol. 2. Troy, MI: BNP Media.

Houghton, R., Jr. n.d. Private correspondence, Spectrum Thermal Processing.

Jones, W. R. 1997. “Partial Pressure Vacuum Processing – Part I and II.” Industrial Heating, September/October.

Jones, William. n.d. Private correspondence, Solar Atmospheres Inc.

Fabian, R., ed. 1993. Vacuum Technology: Practical Heat Treating and Brazing. Materials Park, OH: ASM International.

The Boeing Company. n.d. “Practical Vacuum Systems Design Course.”

About the Author

Dan Herring
“The Heat Treat Doctor”
The HERRING GROUP, Inc.

Dan Herring has been in the industry for over 50 years and has gained vast experience in fields that include materials science, engineering, metallurgy, new product research, and many other areas. He is the author of six books and over 700 technical articles.

For more information: Contact Dan at dherring@heat-treat-doctor.com.

For more information about Dan’s books: see his page at the Heat Treat Store.

Ask The Heat Treat Doctor®: Why Use Partial Pressure in Vacuum Furnaces? Read More »

Nitruración anódica por plasma de aleaciones de titanio

December 16, 2025 / MEDICAL HEAT TREAT TECHNICAL, NITRIDING TECHNICAL CONTENT

En esta entrega de Martes Técnico, el Dr. Edward Rolinski y Dan Herring, conocidos respectivamente como “Doctor Glow” y The Heat Treat Doctor®, exploran cómo el nitrurado por plasma anódico para aleaciones de titanio evita los efectos dañinos del nitrurado catódico convencional mientras mejora la resistencia al desgaste, la resistencia a la corrosión y la confiabilidad de los componentes para aplicaciones aeroespaciales y médicas.

Este artículo informativo se publicó por primera vez en Heat Treat Today’s November 2025 Annual Vacuum Heat Treating print edition.

Para leer el artículo en inglés, haga clic aquí.

La nitruración tradicional por plasma/iónica es una tecnología consolidada. Sin embargo, presenta problemas que pueden solucionarse con el nuevo método de nitruración anódica por plasma. Este artículo presenta la idea de utilizar la nitruración anódica por plasma para titanio y aleaciones de titanio, evitando así los efectos perjudiciales de la nitruración catódica por plasma convencional. Descubra cómo este enfoque podría proporcionar capas más duras y sin defectos que mejoran el desgaste, la resistencia a la corrosión y la fiabilidad general de los componentes para piezas críticas de la industria aeroespacial y médica.

Qué es la nitruración anódica?

La nitruración anódica es un tipo de proceso de nitruración por plasma en el que las piezas tratadas se ubican en un potencial anódico (positivo) en lugar del potencial catódico (negativo) habitual. A diferencia de la nitruración por plasma convencional (descarga catódica), donde el componente se bombardea con iones positivos de alta energía, la nitruración anódica implica el bombardeo de electrones de baja energía sobre la superficie del componente.

La nitruración anódica es particularmente efectiva para materiales con una alta energía libre estándar negativa de formación de nitruros (p. ej., titanio, circonio), ya que ayuda a evitar o reducir el efecto de borde, un problema bien conocido en la nitruración catódica que provoca un bombardeo iónico desigual y endurecimiento en esquinas y bordes.

Antecedentes: Complejidades de la nitruración por plasma

La nitruración por plasma con descarga luminiscente se aplica a una amplia gama de materiales, como fundiciones, aceros al carbono, aceros inoxidables, níquel, aleaciones de titanio y pulvimetalurgia (Roliński, 2014). Los procesos de nitruración por plasma y nitrocarburación permiten la formación de capas superficiales con propiedades tribológicas superiores (Roliński, 2014). Sin embargo, la cobertura de las piezas con la descarga luminosa no siempre es uniforme, especialmente cuando se procesan cargas de geometría compleja (véase la Figura 1).

Figura 1. Pequeñas manchas oscuras (ver el área rodeada por el círculo) indican CDS durante la nitruración por plasma pulsado. Nota: Descarga luminiscente no uniforme en la parte superior de la pieza.
Source: Roliński and Herring

La nitruración por plasma de baja descarga es un tratamiento termoquímico que utiliza partículas de alta energía. Los iones de nitrógeno u otras especies gaseosas se aceleran y ganan energía en el espacio oscuro de Crookes (CDS) alrededor de la pieza, que es el cátodo en una configuración de electrodos de corriente directa. Primero activan la superficie mediante pulverización catódica (sputtering) para eliminar cualquier óxido nativo presente. El tratamiento de pulverización catódica también genera una cantidad sustancial de partículas sólidas, generadas por la propia pieza, incluyendo átomos metálicos que flotan cerca de la superficie (Merlino y Goree, 2004; Roliński, 2005). En el procesamiento del titanio, por ejemplo, esto afecta tanto la adsorción como la difusión en la superficie, creando condiciones que degradan la calidad de la capa (Hubbard, et al., 2010). Se ha descrito un impacto negativo de este plasma “polvoriento” en la uniformidad de la capa nitrurada en piezas de geometría compleja (Ossowski et al., 2016).

Además, es bien sabido que durante la nitruración por plasma se observa el denominado efecto esquina/borde (EE), relacionado con la circulación desigual de estas partículas de polvo alrededor del cátodo (véase la Figura 2). En situaciones extremas, especialmente al tratar piezas de geometría compleja, el EE, causado por una distribución desigual del campo eléctrico en esquinas, cavidades, etc., da lugar a una distribución excesiva y desigual de estos depósitos de plasma (PD). De esta manera, el EE agrava el problema ya existente de la redeposición, lo que provoca la formación de diversos microdefectos y un espesor desigual de la capa nitrurada (Merlino y Goree, 2004; Roliński, 2005, 2024; Ossowski et al., 2016).

Figura 3. Componente de titanio después de nitruración gaseosa en amoníaco. Source: Roliński and Herring

La nitruración por plasma del titanio se realiza habitualmente a 680–1100 °C (1256–2012 °F). Entre los aspectos negativos del uso de la polarización catódica en titanio se incluyen el bombardeo de plasma/iónico, que provoca daños superficiales debido principalmente a micro-arcos y la contaminación de la superficie con los compuestos depositados, así como su distribución desigual debido al EE (Merlino y Goree, 2004; Roliński, 2005, 2014, 2024; Ossowski et al., 2016). Aunque el arco eléctrico se ha eliminado mediante la aplicación de técnicas de plasma pulsado, la pulverización catódica solo se puede controlar de forma limitada, especialmente cuando se nitruran piezas de geometría compleja. Por lo tanto, la nitruración gaseosa en amoníaco se ha utilizado ocasionalmente para endurecer piezas de titanio. Se produce un aspecto dorado resultante en la superficie que indica la presencia del nitruro TiN (véase la Figura 3).

Nitruración anódica de aleaciones de titanio

Se han realizado investigaciones sobre la nitruración anódica de aceros por plasma (Zlatanovic 1986; Michalski 1993; Kenĕz 2018). El amoníaco o las especies de nitrógeno activo generadas en el plasma pueden nitrurar el ánodo al igual que al cátodo. Las especies activas que causan la nitruración son átomos de nitrógeno activo y radicales NH altamente reactivos (NH*) formados en el plasma cercano. Los radicales NH (también conocidos como radicales imidógenos) son especies químicas con un enlace nitrógeno-hidrógeno junto con un electrón desapareado. En el caso del titanio, el hidrógeno debe excluirse en muchas situaciones, ya que reacciona con el titanio para formar hidruros estables que fragilizan el producto (Roliński, 2015).

La entalpía libre estándar de formación de nitruros de titanio tiene un valor negativo excepcionalmente alto, lo que significa que el nitruro de titanio se formará espontáneamente cuando el ánodo de titanio reaccione con nitrógeno excitado cercano (Roliński, 2015). Cambiar de polarización catódica a anódica de los componentes tratados ofrece varias ventajas notables. Una descarga luminosa en nitrógeno puro o argón genera únicamente iones positivos que se aceleran hacia el cátodo/pieza de trabajo. Dado que estas mezclas de gases carecen de iones negativos, solo los electrones de la luminiscencia anódica inciden en el ánodo/pieza de trabajo. Esto produce la activación de la superficie sin los efectos negativos de las colisiones de partículas más pesadas, como N₂+ (es decir, un ion molecular de nitrógeno con carga +1), lo que provoca una pulverización catódica excesiva. Al mismo tiempo, partículas de nitrógeno sin carga, como N₂* y N*, reaccionan con el ánodo por quimisorción en la superficie a una temperatura suficientemente alta, lo que finalmente conduce a la formación de la capa de difusión.

Se cree que el proceso de nitruración anódica puede tener efectos positivos en el tratamiento de piezas de precisión de titanio y otras aleaciones para su uso en las industrias aeroespacial y médica. Este método permitirá el tratamiento a la temperatura más baja posible gracias a la activación de la superficie con los electrones de la polarización anódica. La textura, la apariencia y una superficie sin defectos producirán una pieza superior y mejorarán el rendimiento de muchos de esos componentes. Esto será importante cuando la corrosión o las propiedades ópticas de la superficie sean importantes.

La nitruración anódica del titanio puede lograrse mediante un sistema convencional de nitruración por plasma, siempre que el ánodo central esté diseñado y ubicado adecuadamente. Este ánodo, o parte del mismo, debe estar hecho de titanio para evitar la evaporación y la transferencia de impurezas a las piezas.

Aplicaciones

Figura 4. Representación esquemática del aparato de nitruración por plasma anódico. Observe el color dorado característico del nitruro de titanio (TiN) presente en los accesorios y piezas de titanio, todos identificados como “ánodo”. Source: Roliński and Herring

Las aleaciones de titanio son populares en ortopedia debido a su elasticidad, resistencia y biocompatibilidad similares a las del hueso (Roliński 2015; Froes 2015). Los procesos de ingeniería de superficies, como la nitruración anódica, pueden desempeñar un papel importante a la hora de prolongar el rendimiento de los dispositivos ortopédicos varias veces más allá de su vida útil normal.

Los materiales intermetálicos superelásticos, como el 60NiTi, se utilizan en elementos de rodamientos debido a su resistencia a la corrosión y al impacto (Pohrelyuk et al., 2015; Corte et al., 2015). Suelen ser propensos a la degradación por fatiga de contacto rodante (RCF). Cualquier defecto superficial presente en estos componentes, como la concentración local de impurezas o microfisuras, provocará un fallo prematuro. La nitruración por plasma anódico puede utilizarse para endurecer las superficies de los componentes de rodamientos fabricados con estas aleaciones, formando una capa dura y sin defectos, lo que puede mejorar sus propiedades ante el RCF.

Se espera que las piezas de titanio u otras aleaciones con la superficie sometida a nitruración anódica estén libres de micro-defectos, lo que permite su amplia aplicación en el campo médico, la industria aeroespacial y los dispositivos ópticos y semiconductores.

Referencias

Corte, Ch. Della, M. K. Stanford, and T. R. Jett. 2015. “Rolling Contact Fatigue of Superelastic Intermetallic Materials (SIM) for Use as Resilient Corrosion Resistant Bearings.” Tribology Letters 26: 1–10.

Froes, F. H., ed. 2015. Titanium: Physical Metallurgy, Processing and Applications. Materials Park, OH: ASM International.

Hubbard, P., J. G. Partridge, E. D. Doyle, D. G. McCulloch, M. B. Taylor, and S. J. Dowey. 2010. “Investigation of Mass Transfer within an Industrial Plasma Nitriding System I: The Role of Surface Deposits.” Surface and Coatings Technology 204: 1145–50.

Kenĕz, L., N. Kutasi, E. Filep, L. Jakab-Furkas, and L. Ferencz. 2018. “Anodic Plasma Nitriding in Hollow Cathode (HCAPN).” HTM Journal of Heat Treatment and Materials 73 (2): 96–105.

Merlino, R. L., and J. A. Goree. 2004. “Dusty Plasmas in the Laboratory, Industry, and Space.” Physics Today, July, 32–38.

Michalski, J. 1993. “Ion Nitriding of Armco Iron in Various Glow Discharge Regions.” Surface and Coatings Technology 59 (1–3): 321–24. https://doi.org/10.1016/0257-8972(93)90105-W.

Ossowski, Maciej, Tomasz Borowski, Michal Tarnowski, and Tadeusz Wierzon. 2016. “Cathodic Cage Plasma Nitriding of Ti6Al4V.” Materials Science (Medžiagotyra) 22 (1).

Pohrelyuk, I., V. Fedirko, O. Tkachuk, and R. Poskurnyak. 2015. “Corrosion Resistance of Ti-6Al-4V Alloy with Oxidized Nitride Coatings in Ringer’s Solution.” Inzynieria Powierzchni (Surface Engineering) 1: 38–46.

Roliński, E. 2014. “Plasma Assisted Nitriding and Nitrocarburizing of Steel and Other Ferrous Alloys.” In Thermochemical Surface Engineering of Steels, edited by E. J. Mittemeijer and M. A. J. Somers, 413–57. Woodhead Publishing Series in Metals and Surface Engineering 62. Cambridge, UK; Waltham, MA; and Kidlington, UK: Woodhead Publishing.

Roliński, E. 2015. “Nitriding of Titanium Alloys.” In ASM Handbook, Volume 4E: Heat Treating of Nonferrous Alloys, edited by G. E. Totten and D. S. McKenzie, 604–21. Materials Park, OH: ASM International.

Roliński, Edward. 2024. “Practical Aspects of Sputtering and Its Role in Industrial Plasma Nitriding.” In ASM Handbook Online, Volume 5: Surface Engineering. Materials Park, OH: ASM International. https://doi.org/10.31399/asm.hb.v5.a0007039.

Roliński, E., J. Arner, and G. Sharp. 2005. “Negative Effects of Reactive Sputtering in an Industrial Plasma Nitriding.” Journal of Materials Engineering and Performance 14 (3): 343–50.

Zlatanovic, M., A. Kunosic, and B. Tomčik. 1986. “New Development in Anode Plasma Nitriding.” In Proceedings of the International Conference on Ion Nitriding, Cleveland, OH, September 15–17, edited by T. Spalvins, 47–51. Cleveland, OH: NASA Lewis Research Center.

About The Authors:

El Dr. Edward Rolinski, conocido afectuosamente como “Doctor Glow”, es un distinguido científico sénior que ha liderado la investigación sobre nitruración por plasma/iones desde la década de 1970. Posee títulos avanzados en tecnología de fabricación y metalurgia, incluyendo un doctorado en Ciencias. Se ha centrado en los procesos de nitruración por plasma, especialmente en aleaciones de titanio y pulvimetalurgia. A lo largo de su carrera, el Dr. Rolinski ha sido autor de numerosos capítulos y artículos técnicos influyentes, incluyendo para ASTM International y el Manual ASM, y es un prolífico colaborador en publicaciones del sector. Tras décadas de liderazgo e innovación en ingeniería de superficies y tratamiento térmico, ahora es un consultor en la industria del tratamiento térmico.

Dan Herring, conocido como The Heat Treat Doctor®, lleva más de 50 años en la industria. Dedicó sus primeros 25 años al tratamiento térmico antes de fundar su empresa de consultoría, The HERRING GROUP, en 1995. Su amplia experiencia en el campo abarca la ciencia de los materiales, la ingeniería, la metalurgia, el diseño de equipos, la especialización en procesos y aplicaciones, y la investigación de nuevos productos. Es autor de seis libros y más de 700 artículos técnicos.

Para más información: Contacte con Dan en dherring@heat-treat-doctor.com.

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Anodic Plasma Nitriding of Titanium Alloys

December 16, 2025 / MEDICAL HEAT TREAT TECHNICAL, NITRIDING TECHNICAL CONTENT

In this Technical Tuesday installment, Dr. Edward Rolinski and Dan Herring, respectively known as “Doctor Glow” and The Heat Treat Doctor®, explore how anodic plasma nitriding for titanium alloys avoids the damaging effects of conventional cathodic nitriding while improving wear resistance, corrosion resistance, and component reliability for aerospace and medical applications.

This informative piece was first released in Heat Treat Today’s November 2025 Annual Vacuum Heat Treating print edition.

To read the article in Spanish, click here.

Traditional plasma/ion nitriding is a well-established technology. However, it has issues that can be overcome by the newer anodic plasma nitriding method. This article introduces the idea of using anodic plasma nitriding for titanium and titanium alloys to avoid the damaging effect of conventional (cathodic) plasma nitriding. Read how this approach could provide harder, defect-free layers that improve wear, corrosion resistance, and overall component reliability for aerospace and medical critical parts.

What Is Anodic Nitriding?

Anodic nitriding is a type of plasma nitriding process in which the component parts being treated are placed at an anodic (positive) potential instead of the usual cathodic (negative) potential. Unlike conventional plasma (cathodic glow discharge) nitriding, where the component is bombarded by high-energy positive ions, anodic nitriding involves low-energy electron bombardment of the component’s surface.

Anodic nitriding is particularly effective for materials with very high negative Standard Free Energy of nitride formation (e.g., titanium, zirconium) as it helps avoid or reduce the edge effect, a well-known problem in cathodic nitriding that leads to uneven ion bombardment and hardening on corners and edges.

Background: Plasma Nitriding Complexities

Glow-discharge plasma nitriding is applied to a wide range of materials, including cast irons, carbon steels, stainless steels, nickel, titanium alloys, and powder metal (Roliński 2014). The plasma nitriding and nitrocarburizing processes allow for the formation of surface layers known to have superior tribological properties (Roliński 2014). However, coverage of the parts with the glow discharge is not always uniform, especially when complex geometry loads are processed (see Figure 1).

Figure 1. Small dark spots (see circled area) indicate CDS during pulse plasma nitriding. Note: Non-uniform glow discharge at the top of the part.
Source: Roliński and Herring

Glow-discharge plasma nitriding is a thermochemical treatment involving high-energy particles. Ions of nitrogen or other gas species accelerate and gain energy in the cathodic dark space (CDS) around the workpiece — which is the cathode in a direct current electrode setup. They activate the surface first by sputtering to remove any native oxides present. The sputtering treatment also results in the generation of a substantial quantity of solid particles, generated from the part itself, including metal atoms that levitate near the surface of the part (Merlino and Goree 2004; Roliński 2005). In processing titanium, for example, this affects both adsorption and diffusion at the surface creating conditions that degrade layer quality (Hubbard, et al. 2010). A negative impact of this “dusty” plasma on the uniformity of the nitrided layer in complex-geometry workpieces has been reported (Ossowski, et al. 2016).

In addition, it is well known that there is a so-called corner/edge effect (EE) observed during plasma nitriding related to uneven circulation of these dust particles around the cathode (see Figure 2). In extreme situations, especially when complex geometry parts are treated, the EE caused by a non-uniform distribution of the electric field on corners, cavities, etc., results in excessive and non-uniform distribution of these plasma deposits (PD). In this way, the EE amplifies the already-present problem of redeposition, leading to the formation of various microdefects and uneven nitrided layer thickness (Merlino and Goree 2004; Roliński 2005, 2024; Ossowski, et al. 2016).

Figure 3. Titanium component after gas nitriding in ammonia | Source: Roliński and Herring

Plasma nitriding of titanium is usually performed at 680–1100°C (1256–2012°F). Negative aspects of using cathodic polarization on titanium include plasma/ion bombardment resulting in surface damage due primarily to micro arcing and contamination of the surface with the deposited compounds and their uneven distribution due to EE (Merlino and Goree 2004; Roliński 2005, 2014, 2024; Ossowski, et al. 2016). Although arcing has been eliminated by applying pulse plasma techniques, sputtering can only be controlled in a limited way, especially when complex geometry parts are nitrided. Therefore, gas nitriding in ammonia has been used occasionally for hardening titanium parts. A resulting golden appearance representing the presence of TiN nitride is produced on the surface (see Figure 3).

Anodic Nitriding of Titanium Alloys

Research has been conducted on anodic plasma nitriding of steels (Zlatanovic 1986; Michalski 1993; Kenĕz 2018). Ammonia or active nitrogen species generated in the plasma can nitride the anode just as they do the cathode. The active species causing nitriding are active nitrogen atoms and highly reactive NH radicals (NH*) formed in near plasma. NH radicals (aka imidogen radicals) are chemical species with a nitrogen-hydrogen bond along with an unpaired electron. For titanium, hydrogen must be excluded in many situations because it reacts with titanium to form stable hydrides that embrittle the product (Roliński 2015).

The standard free enthalpy of formation of titanium nitrides has an exceptionally large negative value, which means that titanium nitride will form in a spontaneous way when the titanium anode reacts with excited nitrogen nearby (Roliński 2015). Switching the treated components from cathodic to anodic polarization offers several notable advantages. A glow discharge in pure nitrogen or argon generates only positive ions that are accelerated toward the cathode/workpiece. Because these gas mixtures lack negative ions, only electrons from the anodic glow strike the anode/workpiece. This results in activation of the surface without negative aspects of the collisions of the heavier particles, such as N2+ (i.e., a nitrogen molecular ion with a +1 charge), causing excessive sputtering. At the same time, charge-free particles of nitrogen, such as N2* and N*, react with the anode and are chemisorbed at the surface at sufficiently high temperature, leading eventually to formation of the diffusion layer.

It is believed that the anodic-nitriding process may have positive effects in treating precision parts made of titanium and other alloys for use in both the aerospace and medical industries. This method will allow treatment at the lowest possible temperature due to activation of the surface with the electrons from anodic polarization. The texture, appearance, and defect-free surface will produce a superior part and will enhance the performance of many of those components. This will be important when corrosion or optical properties of the surface play a significant role.

Anodic nitriding of titanium can be accomplished within a conventional plasma nitriding system, provided that the central anode is appropriately designed and positioned. This anode or portion of it must be made of titanium to prevent evaporation and transfer of any impurities to the parts.

Applications

Figure 4. Schematic representation of the anodic plasma nitriding apparatus. Note the gold color characteristic for titanium nitride TiN present on the titanium fixturing and parts, all being identified as “anode.” | Source: Roliński and Herring

Titanium alloys are popular in orthopedics due to their bone-like elasticity, strength, and biocompatibility (Roliński 2015; Froes 2015). Surface engineering processes like anodic nitriding can play a significant role in extending the performance of orthopedic devices several times beyond their normal life expectancy.

Super elastic intermetallic materials, such as 60NiTi, are used in rolling element bearings due to their resistance to corrosion and shock (Pohrelyuk, et al. 2015; Corte, et al. 2015). They are typically prone to rolling contact fatigue (RCF) degradation. Any surface defects present in those components, such as local concentration of impurities or micro-cracks, will result in premature failure. Anodic plasma nitriding can be potentially used to harden the surfaces of bearing components made from these alloys by forming a hard, defect-free layer, which may improve their RCF properties.

It is expected that parts made of titanium or other alloys with the smooth surface subjected to the anodic nitriding will be microdefects-free, enabling their broad applications in medical field, aerospace industry, and optical and semiconductor devices.

References

Froes, F. H., ed. 2015. Titanium: Physical Metallurgy, Processing and Applications. Materials Park, OH: ASM International.

Kenĕz, L., N. Kutasi, E. Filep, L. Jakab-Furkas, and L. Ferencz. 2018. “Anodic Plasma Nitriding in Hollow Cathode (HCAPN).” HTM Journal of Heat Treatment and Materials 73 (2): 96–105.

Merlino, R. L., and J. A. Goree. 2004. “Dusty Plasmas in the Laboratory, Industry, and Space.” Physics Today, July, 32–38.

Michalski, J. 1993. “Ion Nitriding of Armco Iron in Various Glow Discharge Regions.” Surface and Coatings Technology 59 (1–3): 321–24. https://doi.org/10.1016/0257-8972(93)90105-W.

Ossowski, Maciej, Tomasz Borowski, Michal Tarnowski, and Tadeusz Wierzon. 2016. “Cathodic Cage Plasma Nitriding of Ti6Al4V.” Materials Science (Medžiagotyra) 22 (1).

Roliński, E., J. Arner, and G. Sharp. 2005. “Negative Effects of Reactive Sputtering in an Industrial Plasma Nitriding.” Journal of Materials Engineering and Performance 14 (3): 343–50.

About The Authors:

Dr. Edward Rolinski, affectionately known as “Doctor Glow,” is a distinguished senior scientist having spearheaded research on plasma/ion nitriding since the 1970s. He holds advanced degrees in manufacturing technology and metallurgy, including a PhD and Doctor of Science. His focus has been on plasma nitriding processes, especially involving titanium alloys and powder metallurgy. Over his career, Dr. Rolinski authored numerous influential technical chapters and articles, including for ASTM International and the ASM Handbook, and is a prolific contributor to industry publications. After decades of leadership and innovation in surface engineering and heat treating, he is now a consultant in the heat-treating industry.

Dan Herring, who is most well known as The Heat Treat Doctor®, has been in the industry for over 50 years. He spent the first 25 years in heat treating prior to launching his consulting business, The HERRING GROUP, in 1995. His vast experience in the field includes materials science, engineering, metallurgy, equipment design, process and application specialist, and new product research. He is the author of six books and over 700 technical articles.

For more information: Contact Dan at dherring@heat-treat-doctor.com.

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Ask The Heat Treat Doctor®: What Masks the Steel’s Surface in Case Hardening?

December 2, 2025 / HARDENING TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH

This informative piece was first released in Heat Treat Today’s November 2025 Annual Vacuum Heat Treating print edition.

Case depth, case uniformity, and final mechanical (as well as other) properties rely not only on controlling both equipment and process variability during heat treatment, but on having clean, properly prepared part surfaces prior to and during heat treating. Expert Dan Herring encourages to learn more below.

Case hardening is a thermochemical surface treatment process designed to add a particular element or combination of elements to a material such as steel. Familiar examples include carbon (carburizing); carbon and nitrogen (carbonitriding); boron (boriding); nitrogen (nitriding); and nitrogen and carbon (nitrocarburizing — ferritic or austenitic). These processes are typically designed to increase the near surface hardness of steel after quenching.

However, various problems can arise due to either the materials or the manufacturing methods employed prior to or during heat treating that will retard or prevent absorption and/or diffusion of the desired element(s) during heat treating. Some of the metallurgical consequences can include:

Shallow or uneven case depths
Surface oxidation
Intergranular oxidation or decarburization
High levels of retained austenite
Soft spots due to incomplete hardening

Machine-Induced Surface Conditions

Improper machining prior to case hardening can compromise surface integrity. Tooling choices, improperly maintained equipment, inadequate operator training, and even environmental factors can contribute to a variety of issues.

While machining problems occur frequently, they are mostly preventable. Attention to part surface condition, cleanliness, and mechanical integrity is essential before heat treating. Training, standardizing machining protocols, planned preventative maintenance programs, and part inspection prior to heat treating will help avoid these issues. Consult Table A for further details on how the causes and effects of undesirable machine-induced surface conditions can be solved.

Splatter of Stop-off Paints on Unintended Areas

A material that masks the surface of steel and delays or prevents case hardening is called a stop-off or maskant. These materials are applied to specific areas of a steel part to prevent the diffusion of hardening elements (like carbon or nitrogen) into the surface during case hardening processes, such as carburizing, nitriding, or carbonitriding. (See Table B.)

Enriching Gas Additions (Sooting)

During the carburizing or carbonitriding process, it is not uncommon to develop a layer of soot on the surface of the parts, especially if the enriching gas additions begin before the entire load is uniformly up to temperature. In some instances, the amount of soot formation is such that the case depth or uniformity is affected. This is often difficult to diagnose, as the soot layer “washes off” during quenching in a liquid, and the part surfaces come out of the furnace looking reasonably clean.

The use of scrap in steelmaking, especially for low alloy case hardening steels can lead to a relatively high level of impurities and tramp elements. At high temperatures these impurities tend to segregate at grain boundaries and migrate toward the surface. This type of segregation can retard case hardening by impeding element (e.g., carbon) transfer. For example, the effects of tin (Sn) and antimony (Sb) on the kinetics of carburization are particularly problematic (Figure 1).

The effect of tramp elements on retardation of carburization can be expressed in the following order (Andreas, et al. 1996), namely Sb > Sn > P > Cu > Pb. To see the effect of one such element, the carbon transfer coefficient (ß) for typical commercial steels is shown as a function of antimony (Sb) content (Figure 2).

In Summary

These are a few of the many causes delaying or preventing case hardening from being effective. There are many others, including alkaline cleaning compounds (in too high a concentration) and even phosphate and other drawing lubricants used in the manufacture of fasteners. Inspection and cleaning of the part surface prior to case hardening will avoid many of these issues. Reviewing material certification sheets for elements known to interfere with case hardening is also an effective way to anticipate problems with case hardening.

References

Herring, Daniel H. 2014. Atmosphere Heat Treatment, Volume 1. Troy, MI: BNP Media.

Herring, Daniel H. 2015. Atmosphere Heat Treatment, Volume 2. Troy, MI: BNP Media.

Ruck, Andreas, Monceau, Daniel, and Grabke, Hans Jürgen. 1996. “Effects of Tramp Elements Cu, P, Pb, Sb, and Sn on the Kinetics of Carburization of Case Hardened Steels.” Steel Research 67 (6): 242–48.

About the Author

For more information: Contact Dan at dherring@heat-treat-doctor.com.

For more information about Dan’s books: see his page at the Heat Treat Store.

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What Is Hydrogen Embrittlement? Part 1

September 9, 2025 / BRAZING TECHNICAL CONTENT, Manufacturing Heat Treat Technical Content

Ask the Heat Treat Doctor® has returned to bring sage advice to Heat Treat Today readers and to answer your questions about heat treating, brazing, sintering, and other types of thermal treatments as well as questions on metallurgy, equipment, and process-related issues.

This informative piece was first released in Heat Treat Today’s September 2025 People of Heat Treat print edition.

If you’ve ever experience internal cracking, surface blistering, loss of ductility, or high pressure hydrogen attack, today’s Technical Tuesday might contain just the information you need to avoid it. Read below to learn from Dan Herring as he addresses what hydrogen embrittlement is, how to avoid it, and what solutions should not be pursued in order to fix it.

The other night, The Doctor decided to relax and watch a rather whimsical movie, The Great Race (1965), directed by Blake Edwards, who is perhaps better known for directing Breakfast at Tiffany’s and The Pink Panther. It is most memorable not for the actors, nor the plot, but for the infamous pie fight involving over 4,000 pies in a scene that took more than five days to film but lasted only four minutes on the big screen. Not one actor was spared the embarrassment of being hit by (multiple) pies in the face!

So, what does THIS have to do with heat treatment, you ask? Well, try as he may to believe the subject has been explained well in the past, The Doctor has been inundated recently with questions about hydrogen embrittlement (aka hydrogen-assisted cracking). Let’s learn more.

What Is It?

Hydrogen-assisted cracking (HAC) is an embrittlement phenomenon responsible for a surprising number of part cracking issues in heat treatment and is found to be the cause of many delayed field failures, especially if the components undergo secondary operations such as plating (Figure 1).

Figure 1. Tin-plated electrolytic tough pitch (ETP) copper battery lugs embrittled during oxy-acetylene brazing

How Does Hydrogen Get In?

It is generally agreed that hydrogen in atomic form will enter and diffuse through a metal surface at elevated or ambient temperatures. The simple rule to remember about hydrogen is fast in, slow out. Once absorbed, atomic hydrogen often combines to form molecular hydrogen or other hydrogen molecules (e.g., methane). As these are too large to diffuse through the metal, pressure builds at crystallographic defects (e.g., dislocations and vacancies) and/or discontinuities (e.g., voids, laps/seams, inclusion/matrix interfaces) causing minute cracks to form. Whether this absorbed hydrogen causes immediate cracking or not is a complex interaction of material strength, external stresses, and temperature.

Figure 2. Intergranular fracture of a plated component (SEM image)

Most heat treaters associate hydrogen embrittlement with the plating process and the lack of a proper bake-out cycle. However, there are many other sources of hydrogen, including heat treating atmospheres; breakdown of organic lubricants left on parts; the steelmaking process (e.g., electric arc melting of damp scrap); dissociation of high-pressure hydrogen gas; arc welding (with damp electrodes); grinding (in a wet environment); and the end-use environment.

Parts undergoing electrochemical surface treatments, such as etching, pickling, phosphate coating, corrosion removal, paint stripping, and electroplating, are especially susceptible (Figure 2).

What Is The Nature and Effect of Hydrogen Attack?

Although the precise mechanism(s) is the subject of active investigation (Figure 3), the reality is that components fail due to HAC. It is generally believed that all steels above 30 HRC are vulnerable, as are materials such as copper, titanium and titanium alloys, nickel and nickel alloys, and the like. See Table A below for examples of hydrogen damage and ways to avoid it.

Figure 3a and 3b. Hydrogen embrittlement mechanism models

Since a metallurgical interaction occurs between atomic hydrogen and the atomic structure, the ability of the material to elastically deform or stretch under load is inhibited. Therefore, it becomes “brittle” under applied stress or load. As a result, the metal will break or fracture at a much lower load or stress levels than anticipated by designers. Since failures can be of a delayed nature, hydrogen embrittlement is insidious.

Table A. Problems with hydrogen damage and ways to avoid them

In general, as the strength of the steel goes up, so does its susceptibility to hydrogen embrittlement. High strength steel, such as quenched and tempered steels (e.g., 4140, 4340), or precipitation hardened steels are particularly vulnerable. It is often called the Achilles heel of high strength ferrous steels and alloys.

Nonferrous Materials and Hydrogen Embrittlement

Nonferrous materials are also not immune to attack. Tough-pitch coppers and even oxygen-free coppers are subject to a loss of (tensile) ductility when exposed to reducing atmospheres. Bright annealing in hydrogen bearing furnace atmospheres or torch/furnace brazing are typical processes that can induce embrittlement of these materials.

In copper, the process involves diffusion and subsequent reduction of cuprous oxide (Cu₂O) to produce water vapor and pure copper. An embrittled copper often can be identified by a characteristic surface blistering resulting from expansion of water vapor in voids near the surface. Purchasing oxygen-free copper is no guarantee against the occurrence of hydrogen embrittlement, but the degree of embrittlement will depend on the amount of oxygen present. For example, CDA 101 (oxygen free electronic) allows up to 5 ppm oxygen while CDA 102 (OFHC) permits up to 10 ppm. A simple bend test is often used to detect the presence of hydrogen embrittlement. Metallographic techniques can also be used to look at the near surface and for the presence of voids at grain boundaries.

Are Low Hydrogen Concentrations Also Problematic?

Of concern today is embrittlement from very small quantities of hydrogen where traditional loss-of-ductility bend tests cannot detect the condition. This atomic level embrittlement manifests itself at levels as low as 10 ppm of hydrogen — in certain plating applications it has been reported that 1 ppm of hydrogen is problematic! Although difficult to comprehend, numerous documented cases of embrittlement failures with hydrogen levels this low are known.

This type of embrittlement occurs when hydrogen is concentrated or absorbed in certain areas of metallurgical instability. This concentrating action occurs via either residual or applied stress, which tends to “sweep” through the atomic structure, moving the infiltrated hydrogen atoms along with it. These concentrated areas of atomic hydrogen can coalesce into molecular type hydrogen, resulting in the formation of high localized partial pressures of the actual gas.

How Does Hydrogen Get Out?

Hydrogen absorption need not be a permanent condition. If cracking does not occur and the environmental conditions are changed so that no hydrogen is generated on the surface of the metal, the hydrogen can re-diffuse out of the steel, and ductility is restored. Performing an embrittlement relief cycle, or hydrogen bake-out cycle (the term “bake-out” is misleading as the process involves both inward diffusion and outgassing), is a powerful method in eliminating hydrogen before damage can occur. Key variables are temperature, time at temperature, and concentration gradient (atom movement).

Electroplating, for example, provides a source of hydrogen during the cleaning and pickling cycles, but by far the most significant source is cathodic inefficiency. To eliminate concerns, bake-out cycles and recommended temperatures/times are shown in ASTM B850-98 (latest revision) as a function of steel tensile strength (see Table 1 of the specification). However, in this writer’s eyes, a “bake-out” cycle of at least 24 hours at temperature is required for the effective elimination of hydrogen as a concern regardless of the tensile strength of the material. Also, caution should be exhibited to prevent over-tempering or softening of the steel, especially on a carburized, or induction hardened part.

Next time we will talk about quench and temper embrittlement, as well as embrittlement due to overheating during forging, all of which are often mistaken for hydrogen embrittlement.

References

ASTM International. 2022. ASTM B850-98 (Reapproved 2022), Standard Guide for Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement. West Conshohocken, PA: ASTM International. https://www.astm.org.

Herring, D. H. 2004. “A Heat Treater’s Guide to Hydrogen Embrittlement.” Industrial Heating, October.

Herring, D. H. 2006. “The Embrittlement Phenomena in Hardened & Tempered Steels.” Industrial Heating, October.

Herring, D. H. 2014–2015. Atmosphere Heat Treatment, Volumes I & II. Troy, MI: BNP Media.

Krause, George. 2005. Steels: Processing, Structure, and Performance. Materials Park, OH: ASM International.

About the Author

For more information: Contact Dan at dherring@heat-treat-doctor.com.

For more information about Dan’s books: see his page at the Heat Treat Store.

Find Heat Treating Products And Services When You Search On Heat Treat Buyers Guide.Com

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What Is Quench Cracking and How Can It Be Prevented?

August 26, 2025 / Manufacturing Heat Treat Technical Content, QUENCHING TECHNICAL CONTENT

The Heat Treat Doctor® has returned to offer sage advice to Heat Treat Today readers and to answer your questions about heat treating, brazing, sintering, and other types of thermal treatments as well as questions on metallurgy, equipment, and process-related issues.

This informative piece was first released in Heat Treat Today’s August 2025 Automotive Heat Treating print edition.

Quench cracking during heat treatment can turn expensive components into scrap metal in seconds. In today’s Technical Tuesday article, Dan Herring (The Heat Treat Doctor®) explores more about the underlying mechanisms and proper preventative measures to save you time, money, and ensure reliable part performance.

As a young heat treater, I learned first-hand about quench cracking while running various dies for our tool and die shop — and succeeded in cracking all of them! I have never forgotten the foreman’s (rather animated) critique of my heat treating abilities. Quench cracking can be a significant problem for heat treaters, its potential consequences ranging from costly rework to premature failure in the field. Let’s learn more.

We must not only understand the mechanisms involved but also take proactive steps to avoid it. This includes careful consideration of such items as:

Material (e.g., chemistry, hardenability, form, mill processing)
Component part design (e.g., sharp radii, thin and thick sections next to one another)
Manufacturing processing steps (e.g., the effect of stress relief after rough machining)
Part loading (e.g., part orientation in relation to the quench, fixturing, total load weight)
Equipment choice (i.e., limitations and capabilities)
Quench medium (e.g., type, agitation, flow characteristics, temperature, temperature rise)
Process parameters (e.g., ramp rates, atmospheres, vacuum levels)

The Heat Treatment Challenge

Quench cracking primarily occurs during the hardening process, typically when materials are rapidly cooled via quenching. Since the cooling process introduces internal stresses within the material, it can result in crack formation. These stresses are a result of the rapid transformation of the material’s microstructure, most notably when transforming to martensite, a very hard, brittle structure.

Figure 1. Quench crack in a 4140 axle shaft

Mechanisms Involved

Failure mechanisms related to quench cracking include the following seven factors.

Material Imperfections

As material is heated, thermally induced stresses can cause existing surface or subsurface defects, such as inclusions, laps, and seams. These defects act as stress risers to open and propagate into cracks. Once a defect reaches “critical flaw size” — the smallest flaw that can lead to failure under expected operational stress levels — crack propagation will begin and lead to part failure.

Rapid or uneven heating only exacerbates this issue, especially when a material undergoes phase transformations that introduce volumetric changes.

Stress Risers

Sharp corners, steep edges around holes, and even grooves in parts create stress concentration points where quench cracking is most likely to occur. These features also result in localized heating and cooling, causing differential stresses that can initiate cracks.

The sharp edges of a part, for instance, cool much faster than the rest of the material, leading to a high risk of cracking.
Proper design modifications, such as adding radii to sharp corners, can reduce the likelihood of stress concentrations.

Rapid Cooling and Phase Transformation

The transformation from austenite to martensite during quenching is a key contributor to internal stresses. The rate at which the material cools can greatly influence these stresses. If cooling is too rapid or if tempering is delayed, the material can become overly brittle, leading to quench cracking.

Improper Heating and Overheating

Overheating during the austenitizing process can lead to coarse-grained structures that are more prone to quench cracking. Coarse grains increase the depth of hardening but reduce the material’s resistance to cracking. It is critical to avoid temperature overshoot, high ramp rates, and excessively long dwell times when heating.

Inadequate Quenching Methods

The choice of quench medium (brine, water, oil, polymer, high pressure gas, etc.) can also contribute to quench cracking. Overly aggressive quenchants may create excessive thermal stresses.

Improper Fixturing

The way parts are positioned during quenching can create problems. If parts are bunched together in a basket, uneven cooling rates will occur, with parts on the edges cooling faster than those in the center. This can lead to differential stresses and increase the risk of cracking.

Delays Between Quenching and Tempering

Quenching produces high residual stresses in the material, and if parts are not tempered soon after quenching, these stresses can lead to cracking. For materials with high hardenability, such as 4340 steel, immediate tempering (usually within 15 minutes of quenching) is critical to prevent in-service failure.

Understanding Fracture Mechanics

Understanding these mechanisms is critically important. A material’s fracture toughness, which is the ability to resist crack growth, is defined by the stress intensity factor (KIC). This value varies based on the material’s properties and the size and geometry of the crack. The important point to remember is that when the applied stress reaches a critical threshold, cracks begin to propagate (literally at the speed of sound), leading to catastrophic failure.

Digging a bit deeper, there are three primary modes of fracture:

Tensile (Mode I): Fracture caused by tensile stress at the crack tip.
Sliding (Mode II): Fracture caused by shear stress that causes the two sides of the crack to slide.
Tearing (Mode III): Fracture caused by shear forces in a direction perpendicular to the crack plane.

Preventive Measures

Several strategies can be employed to minimize the risk of quench cracking during heat treatment. They broadly fall into the following categories.

Material Selection

Choosing the right material for the job is essential. Many designers select materials with high hardenability, forgetting that they can be prone to cracking. Additionally, one should take special care with materials that have high carbon content or are heavily alloyed.

Design Considerations

Ensure that part designs minimize stress risers. Avoid sharp corners and incorporate radii where necessary. Proper design can reduce the likelihood of cracks forming at critical locations.

Improved Manufacturing Practices

Proper stock removal during machining and addressing surface imperfections before heat treatment can prevent the initiation of cracks. Machining should aim to eliminate any seams or inclusions that might act as nucleation sites for cracks. Stress relief after rough machining is almost always a good idea.

Control of Heat Treatment Parameters

Maintain tight control over the heating and quenching processes to ensure uniformity. Avoid overheating and try to ensure that the part enters the quench medium in the best possible orientation to reduce the likelihood of creating differential cooling rates.

Figure 2. Quench crack due to a combination of rapid heating, overheating and improper polymer quench medium concentration in a motor shaft (50x, as polished)

Quenching Media

Select the appropriate quenching medium based on the material, part geometry, and load. Less aggressive quenchants or minimizing time in the quench should be considered for materials with moderate to high hardenability.

Post-Quench Tempering

Temper parts as soon as practical after quenching to avoid concerns with internal stresses. High-hardness materials should be tempered immediately to prevent quench cracking.

Quench Cracking in Other Materials

Quench cracking is not exclusive to steel. Other materials, such as nickel and cobalt superalloys, can also experience cracking due to similar mechanisms. In these materials, the phenomena are often referred to as “fire cracking,” “strain-age cracking,” or “stress cracking.” As with steel, cracks in these materials are often linked to high residual tensile stresses on the surface and the presence of stress raisers. Strategies, such as shot peening, redesigning part geometries, and improving surface finishes, can help mitigate cracking in superalloys.

Summing Up

Quench cracking represents a significant challenge in heat treatment, but by understanding its underlying mechanisms, heat treaters and engineers can take steps to mitigate the risk. Material selection, part design, proper heat treatment procedures, and timely tempering are all critical factors in preventing quench cracking and ensuring the integrity of components. A proactive approach to addressing flaws and stress concentrators combined with careful attention to detail in every stage of the manufacturing and heat treatment process can greatly reduce the likelihood of failure and contribute to the long-term success of heat treated products.

References

Herring, Daniel H. 2012. “Quench Cracking.” Industrial Heating, April.

Herring, Daniel H. 2015. Atmosphere Heat Treatment, Volume 2. BNP Media.

Johnson, D. D. 2005. “Thermal and Mechanical Behavior of Materials.” University of Illinois.

Klarstrom, Dwaine L. 1996. “Heat Treat Cracking of Superalloys.” Advanced Materials and Processes, April.

Krauss, George. 2005. Steels: Processing, Structure and Performance. ASM International.

About the Author

For more information: Contact Dan at dherring@heat-treat-doctor.com.

For more information about Dan’s books: see his page at the Heat Treat Store.

Find Heat Treating Products And Services When You Search On Heat Treat Buyers Guide.Com

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Ask The Heat Treat Doctor®: What Are the Differences Between Intergranular Oxidation (IGO) and Intergranular Attack (IGA)?

June 24, 2025 / HARDENING TECHNICAL CONTENT, Manufacturing Heat Treat Technical Content

Today’s Technical Tuesday is a pre-release foretaste of the great content you can find in Heat Treat Today’s July 2025 Super Brands print edition.

Heat treaters and metallurgists speak a language unique to our industry and it can be confusing at times; terms like intergranular oxidation (IGO) and intergranular attack (IGA) are good examples, as these terms are often (incorrectly) used interchangeably. While these two phenomena sound similar, they have distinct mechanisms, causes, and impacts on material properties. Expert Dan Herring explores them more below.

What is IGO?

IGO is a process by which oxygen preferentially reacts with the metal surface at the grain boundaries, creating oxides (Figure 1). It is not uncommon, for example, to see IGO present after a hardening process run in an Endothermic or nitrogen/methanol atmosphere. As parts are heated to austenitizing temperature, oxygen present due to minute air leaks or produced during the various chemical reactions with the atmosphere results in IGO. The grain boundaries are highly susceptible to oxidation because these are areas where different crystallographic grains meet and are areas of high energy due to the atomic mismatch and disruption of the regular crystal lattice structure. 

Figure 1. Intergranular oxidation (IGO) along the surface of a heat treated chromium steel component. 1000X – As Polished
Source: The HERRING GROUP, Inc.

What is IGA?

Figure 2. Intergranular attack (IGA) along the surface of a martensitic steel component caused by excessive submersion time in a citric acid solution. 1000X – As Polished
Source: The HERRING GROUP, Inc.

IGA, on the other hand, is a broader term that refers to a corrosion phenomenon (aka chemical attack) that specifically targets the grain boundaries of a material. Unlike IGO, intergranular attack (Figure 2) is not limited to oxidation reactions but encompasses a variety of forms of attack involving such things as the formation of precipitates, the dissolution of material at grain boundaries, or the creation of corrosion cracks. Common forms of IGA include stress corrosion cracking (SCC) or sensitization in stainless steel.

In stainless steels, IGA is often triggered by high-temperature environments, usually in the range of 840º – 1560ºF (450 – 850°C) where carbon reacts with chromium to form chromium carbides at the grain boundaries, thus reducing the material’s resistance to corrosion in localized regions. In other alloys, factors like pH, chloride concentration, and temperature can lead to IGA.

Both IGO and IGA weaken the material’s structural integrity or lead to embrittlement compromising the material’s integrity.

Effect on Material Properties

The main effect of intergranular oxidation is the degradation of the mechanical properties, particularly a reduction in both ductility and toughness. As oxidation progresses along the grain boundaries, the material tends to become brittle, which can lead to premature failure under certain types of stress or thermal cycling. IGO often appears visually as a uniform discoloration or thin oxide layer on the surface. Surface pitting is not typically observed.

By contrast, IGA often appears as visible cracks, pits, or localized regions where the metal has been attacked (along the grain boundaries). This leads to a reduction in mechanical strength and can lead to SCC under certain circumstances. IGA can severely compromise the integrity of the material, particularly in critical applications like pipelines, pressure vessels, and nuclear reactors.

Materials Involved

IGO is most commonly observed in steel, aluminum, titanium, and nickel-based alloys, not only during heat treatment but when exposed to oxidizing environments in high-temperature applications, which also result in degradation and loss of material strength and other properties.

IGA tends to be more prevalent in stainless steels, corrosion-resistant alloys, and aluminum alloys. It is especially noticeable in alloys that are susceptible to sensitization (where chromium carbides precipitate at grain boundaries), leading to localized corrosion and cracks. Alloys that form a passivating oxide layer can be more susceptible to IGA if that layer is disrupted.

Principal Concerns

The main concern with intergranular oxidation is material embrittlement, leading to reduced ductility and potential failure under mechanical stress, especially in high-temperature applications. It can also affect the integrity of critical components, such as those used in aerospace or power generation industries.

By contrast, the primary impact of intergranular attack is loss of material strength, leading to structural failure, often without any clear outward signs (e.g., under chloride-induced SCC). It is more likely to cause immediate failure or a dramatic loss in performance, especially in structures exposed to corrosive environments.

How to Detect IGO and IGA

IGO is typically detected by examining the material’s surface using optical or scanning electron microscopy (SEM). Non-destructive techniques, such as X-ray diffraction (XRD), can also be used.

IGA is usually detected through methods like microstructural examination, electrochemical testing, or failure analysis. Techniques, such as SEM or energy-dispersive X-ray spectroscopy (EDS), can be used to examine the grain boundary regions for signs of corrosion.

How to Avoid IGO and IGA

IGO can be avoided by one or more of the following:

Environmental control: Making sure the heat treat furnace has no leaks, reducing oxygen partial pressure or controlling the furnace atmosphere in high-temperature heat treat operations.
Alloy design: The use of materials with stable oxide-forming elements (e.g., chromium, titanium and aluminum) or alloys with high resistance to oxidation (e.g., nickel-based superalloys).
Temperature control: Maintaining lower process temperatures and shorter times where possible to prevent oxidation at the grain boundaries.
Coatings and surface treatments: Application of protective coatings, such as copper plating, post-heat treatment aluminizing, or chrome plating, to reduce oxygen interaction with the grain boundaries during service.

IGA can be avoided by one or more of the following:

Environmental control: Reducing exposure to aggressive chemicals (e.g., chloride ions) by maintaining proper pH levels or using inhibitors in post-cleaning processes.
Proper alloy selection: Selecting materials resistant to intergranular corrosion (e.g., low carbon “L” grades of stainless steel or alloys with improved grain boundary stability).
Heat treatment: Avoiding sensitization of stainless steel by proper heat treatment methods that prevent the formation of chromium carbides at grain boundaries.
Stress relief: Reducing the likelihood of stress corrosion cracking by managing internal stresses during manufacturing and in-service conditions.

Key Differences

The differences between these phenomena are summarized in Table 1.

Summing Up

While both IGO and IGA involve attack at the grain boundaries, they differ in their mechanisms, causes, and effects. From a heat treater’s perspective, IGO most often results at high temperature in oxygen-bearing furnace atmospheres, while IGA often results from pre- or post-heat treatment processing (cleaning, passivation, plating, etc.). Proper material selection, furnace and environmental control, awareness of what can happen, and inspection for these effects are key to preventing them from occurring.

References

Roberge, Pierre R., Corrosion Engineering: Principles and Practice, Mc-Graw Hill LLC, 2008.

Stene, Einar S., Fundamentals of Corrosion: Mechanisms, Causes, and Monitoring.

Schweitzer, Philip A., Fundamentals of Corrosion: Mechanisms, Causes and Preventative Methods, CRC Press, 2009.

About the Author

For more information: Contact Dan Herring at dherring@heat-treat-doctor.com.

For more information about Dan’s books: see his page at the Heat Treat Store.

Find Heat Treating Products And Services When You Search On Heat Treat Buyers Guide.Com

Ask The Heat Treat Doctor®: What Are the Differences Between Intergranular Oxidation (IGO) and Intergranular Attack (IGA)? Read More »

Boronizing — What Is It and Why Is It Used?

April 14, 2025 / Featured, HARDENING TECHNICAL CONTENT, Manufacturing Heat Treat Technical Content

This informative piece was first released in Heat Treat Today’s April 2025 Induction Heating & Melting print edition.

Of all the case hardening processes, boronizing (a.k.a. boriding) is perhaps the least understood and least appreciated. Let’s learn more.

In this era of using coating technologies (e.g., PVD, CVD, DLC) to produce hard, wear-resistant surface layers on component parts, one often forgets that there is a thermo-chemical treatment that often can outperform many of them.

Boronizing (a.k.a. Boriding)

Table 1. Examples of hardness levels achieved by boronizing*
*The hardness of the boride layer depends on the compound formed. For example, FeB is 1900–2100 HV, Fe2B is 1800–2000 HV, while Ti2B is 3000 HV.

Boronizing is a case hardening process that produces a very high surface hardness in steels and is used for severe wear applications (see Table 1). The layer of borides (FeB and Fe₂B) formed also significantly increases corrosion resistance of the steel.

Boron is added to steels for its unique ability to increase hardenability and lower the coefficient of (sliding) friction. In addition, boron is used to control phase transformation and microstructure since the time-temperature-transformation curve for the material when boron is diffused into the surface is shifted to the right.

The Process

The boronizing process is typically run in a solid (pack), liquid, or gaseous medium. Each of these methods involves the diffusion of boron into the steel’s surface, but they differ in how boron is introduced and the conditions under which they operate.

In the pack boronizing, a powder mixture of boron compounds (typically boron carbide or sodium tetrafluoroborate) is packed around the steel workpieces. This pack is placed in a retort-style furnace where it is heated, typically with an argon cover gas, to temperatures ranging from 1300°F to 1832°F (700°C to 1000°C). The heat causes the boron to diffuse into the steel surface, forming a boride layer (Figure 1).
- A key advantage of this method of boronizing is that it is highly effective for producing uniform boride coatings. It is particularly suitable for large parts or components that may not be suitable for immersion in a liquid or exposure to gaseous boron compounds.
In liquid boronizing, the steel is immersed in a molten bath containing boron-bearing compounds, typically a mixture of sodium tetraborate and other chemicals. The steel absorbs boron from the bath, forming a boride layer. The liquid process tends to be faster than the solid method and can be more economical for certain applications.
- One of the challenges with liquid boronizing is that the process can be difficult to control in terms of coating thickness and uniformity. Therefore, this method is often used for smaller, simpler parts rather than large or complex geometries.
Gaseous boronizing involves exposing the steel to a boron-containing gas, typically diborane (B₂H₆) or boron trifluoride (BF₃), at elevated temperatures. The boron diffuses from the gas onto the surface of the steel, forming the boride layer. Gaseous boronizing allows for better control over the process compared to the other two methods, but it requires specialized equipment to handle the toxic and reactive nature of the boron gases.
- The advantage of gaseous boronizing lies in its ability to produce a uniform and controlled boride layer, especially for complex parts or those with intricate geometries.

When working with any boron-containing compounds, adequate ventilation and other safety precautions (e.g., masks, gloves) are required. If boron tetrafloride is present, extra precautions are necessary since it is a poisonous gas.

Typical processing temperature is in the range of 1300°F–1832°F (700°C–1000°C) with time at temperature from 1 to 12 hours. Typical case depths achieved range from 0.003″–0.015″ (0.076 mm to 0.38 mm) or deeper (Figure 2). Case depths between 0.024″ and 0.030″ require longer cycles up to 48 hours in duration.

Figure 1. Typical microstructure of a boronized component

The mechanical properties of the borided alloys depend strongly on the composition and structure of the boride layers. The most desirable microstructure a er boronizing is a single-phase boride layer consisting of Fe₂B₂. Plain carbon and low alloy steels are good candidates for boronizing, while more highly alloyed steels may produce a dualphase layer (i.e., boron-rich FeB compounds) because the alloying elements interfere with boron diffusion. The boron-rich diffusion zone can be up to seven times deeper than the boride layer thickness into the substrate.

The hardness of the borided layer depends on the composition of the base steel (Table 1). Comparative data on steels that have been borided versus carburized or carbonitrided, nitrided or nitrocarburized are available in the literature (see Campos-Silva and Rodriguez-Castro, “Boriding,” 651–702). The surface hardness achieved through boronizing is among the highest for case hardening processes. The boride layers typically exhibit hardness values in the range of 1000 to 1800 HV. This level of hardness helps prevent surface deformation under load, which is particularly beneficial in applications involving high contact pressures, such as gears, bearings, and automotive components.

Boronizing can also lower the coefficient of friction on the surface of the steel. This is particularly useful in applications where reduced friction is necessary, such as in sliding or rotating parts that operate under high pressures. The reduced friction helps to minimize wear and energy consumption, improving the overall efficiency and longevity of the components.

Unlike other surface-hardening methods that can compromise the core properties of the material, boronizing tends to retain the toughness and ductility of the base steel. This means the steel remains strong and resistant to cracking or breaking while also benefiting from a hard, wear-resistant surface.

By contrast, when boron is used as an alloying element in plain carbon and low alloy steels, it is added to increase the core hardenability and not the case hardenability. In fact, boron can actually decrease the case hardenability in carburized steels. Boron “works” by suppressing the nucleation (but not the growth) of proeutectoid ferrite on austenitic grain boundaries. Boron’s effectiveness increases linearly up to around 0.002% then levels off.

Figure 2. Hardness-depth profiles on different borided steel*
* Notes:
1. The boriding temperature was 1740°F (950°C) with six (6) hours of exposure
2. Hardness conversion: 1 GPa = 102 HV (Vickers hardness)
3. Depth conversion: 10 micrometers = 0.00039 inches

Boronizing Applications

Given the range of benefits that boronizing offers, it has found widespread use across many industries. Some of the most common applications include:

Automotive industry: Gears, camshafts, and valve components are often boronized to enhance wear resistance and extend their service life.
Aerospace: Parts exposed to high temperatures and wear, such as turbine blades, landing gears, and other critical engine components, benefit from the hard, wear-resistant coatings created by boronizing.
Cutting tools and dies: The high surface hardness and resistance to abrasion make boronized tools highly effective for machining and forming hard materials.
Mining and earthmoving equipment: Equipment like drill bits, shovels, and conveyor parts subjected to abrasive conditions can be boronized to improve their performance and reduce downtime.
Oil and gas: Valves, pumps, and other equipment exposed to corrosive fluids in the oil and gas industry benefit from the enhanced corrosion resistance of boronizing.

In Summary

Boronizing is not for everyone, but it is safe to say that it is the “forgotten” case hardening process, one that will find increasing use in the future as demand for better tribological properties increases. It is a highly effective surface treatment process that imparts significant benefits to steel, including enhanced wear and corrosion resistance, increased surface hardness, and improved frictional properties. By carefully selecting the boronizing method and optimizing process parameters, manufacturers can produce components with superior performance in demanding applications. As industries continue to push the boundaries of material performance, boronizing can be an essential technique for producing long-lasting, high-performance steel components.

References

Campos-Silva. I. E., and G. A. Rodriguez-Castro, “Boriding to Improve the mechanical properties and corrosion resistance of steels.” In Thermochemical Surface Engineering of Steels, edited E. J. Mittemeijer and M. A. J. Somers. Woodhead Publishing, 2014.

Herring, Daniel H. Atmosphere Heat Treatment, vol. I. BNP Media, 2014.

Kulka, Michal. “Current Trends in Boriding: Techniques.” Springer Nature, 2019.

Senatorski, Jan, Jan Tacikowski, and Paweł Mączyński. “Tribological Properties and Metallurgical Characteristics of Different Diffusion Layers Formed on Steel.” Inżynieria Powierzchni 24, no. 4 (2019).

About the Author

For more information: Contact Dan at dherring@heat-treat-doctor.com.

For more information about Dan’s books: see his page at the Heat Treat Store.

Find heat treating products and services when you search on Heat Treat Buyers Guide.Com

Boronizing — What Is It and Why Is It Used? Read More »

Why Normalize, and Is a ‘Still Air’ Cool Really Important? Part 2

March 13, 2025 / ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, HARDENING TECHNICAL CONTENT, Manufacturing Heat Treat Technical Content, VACUUM FURNACES TECHNICAL CONTENT

This informative piece was first released in Heat Treat Today’s March 2025 Aerospace Heat Treating print edition.

Last time (Air & Atmosphere Heat Treating, February 2025) we addressed the question of why normalizing is necessary. Here we look at the importance of a “still air” cool on the final result. Let’s learn more.

What Is a “Still Air” Cool?

As we learned last month, the term “cooling in air” is associated with normalizing but poorly defined in the literature or in practice, either in terms of cooling rate or microstructural outcome. This lack of specificity has resulted not only in many different interpretations of what is needed, but in a great deal of variability in the final part microstructure.

By way of example, this writer has on multiple occasions asked what changes are made to car bottom furnace cycles where cars are pulled outside of the plant for “air cooling” (Figure 1). Questions such as, is the furnace opened and the car pulled out in inclement weather? And, is this practice done on a particularly windy day, or in a rain or snowstorm or when the temperature is below zero? An all-too-common response is, “Only if it isn’t raining ‘too hard’ or snowing ‘too much’; then, we wait a while.” No wonder part microstructures are often found to vary from part to part and load to load!

Most heat treaters agree, however, that normalizing is optimized by a cooling in “still air.” This term also hasn’t been clearly defined, but it will be here based on both an extensive survey of the literature and the most common heat treat practices. In Vacuum Heat Treatment, Volume II, I define a still air cool as: “Cooling at a rate of 40°F (22°C) per minute … to 1100°F (593°C) and then at a rate of 15°F–25°F (8°C–14°C) per minute from 1100°F (593°C) to 300°F (150°C). Any cooling rate can be used below 300°F (150°C).”

Typical car bottom normalizing furnace opening to the outside environment

In addition, many consider nitrogen gas quenching in a vacuum furnace at 1–2 bar pressure to be equivalent to a still air cool. But again, so many factors are involved that only properly positioned workload thermocouples can confirm the above cooling rates are being achieved.

Also, many use the term “air cooling” to differentiate the process from “air quenching,” “controlled cooling,” and “fan cooling.”

Recall from the previous installment of this column that any ambiguity with respect to cooling rate ought to be defined in engineering specifications and/or heat treat instructions so that the desired outcome of the process can be firmly established.

From the literature, several important observations will serve as cautionary reminders. In STEELS, George Krauss points out that: “Air cooling associated with normalizing produces a range of cooling rates depending on section size [and to some extent, load mass]. Heavier sections air cool at much lower cooling rates than do light sections because of the added time required for thermal conductivity to lower temperatures of central portions of the workpiece.”

George Totten’s work in Steel Heat Treatment indicates: “Cooling … usually occurs in air, and the actual cooling rate depends on the mass which is cooled.” He goes on to state:

After metalworking, forgings and rolled products are often given an annealing or normalizing heat treatment to reduce hardness so that the steel may be in the best condition for machining. These processes also reduce residual stress in the steel. Annealing and normalizing are terms used interchangeably, but they do have specific meaning. Both terms imply heating the steel above the transformation range. The difference lies in the cooling method. Annealing requires a slow [furnace] cooling rate, whereas normalized parts are cooled faster in still, room-temperature air. Annealing can be a lengthy process but produces relatively consistent results, where normalizing is much faster (and therefore favored from a cost point of view) but can lead to variable results depending on the position of the part in the batch and the variation of the section thickness in the part that is stress-relieved.

In “The Importance of Normalizing,” this writer offers the following caution: “It is important to remember that the mass of the part or the workload can have a significant influence on the cooling rate and thus on the resulting microstructure.”

Finally, Krauss again observes: “The British Steel Corporation atlas for cooling transformation (Ref. 13.7) establishes directly for many steels the effect of section size on microstructures produced by air cooling.” (Note: Interpretation of continuous cooling transformation (CCT) curves will be the subject of a future “Ask The Heat Treat Doctor” column.)

Since hardness is one of the most commonly used criteria to determine if a heat treat process has been successful, it should also be noted that one can usually predict the hardness of a properly normalized part by looking at the J40 value when Jominy data is available.

The Metallus (formerly TimkenSteel) “Practical Data for Metallurgists” provides an example of the type of data available to metallurgists and engineers to help define a required cooling rate for normalizing (Figure 2).

All literature references to normalizing agree (or infer) that the resultant microstructure produced plays a significant role in both the properties developed and their impact on subsequent operations.

Figure 2. Combined hardenability chart for normalized and austenitized SAE 4140 steel showing approximate still air cooling rates and resultant hardness (data based on a thermocouple located in the center of the bar diameter indicated)

Final Thoughts — The State of the Industry

It is all too common within the industry for some companies who wish to have normalizing performed on their products to specify only a hardness range on the engineering drawing or purchase order callout that is given to the heat treater.

Industry normalizing practice here in North America varies considerably from company to company. Normalizing instructions are sometimes, but not often enough, provided on either purchase orders, engineering drawings, or in specifications (industry standards or company-specific documents). These instructions range from, in the case of certain weldments, absolutely nothing (i.e., no hardness, microstructure, or mechanical properties) to referencing industry specifications (e.g., AMS2759/1) or specifying complete metallurgical and mechanical testing including hardness and microstructure.

Most commercial heat treaters often perform normalizing to client or industry specifications provided to them. Others prefer so-called “flow down” instructions in which the process recipe is provided to them. It is a common (and mistaken) belief that this removes the obligation of achieving a given set of mechanical or metallurgical properties even if they are called out by specification, drawing, or purchase order.

Also, the final mechanical properties that result from normalizing are seldom verified by the heat treater. Rather, a hardness value (or range) is reported, but hardness is not a fundamental material property, rather a composite value, one which is influenced by, for example, the yield strength, work hardening, true tensile strength, and modulus of elasticity of the material.

References

ASM International. “ASM Handbook, vol. 4, Heat Treating,” 1991.

ASM International. “ASM Handbook Volume 4A, Steel Heat Treating, Fundamentals and Processes,” 2013.

Chandler, Harry, ed. Heat Treater’s Guide: Practices and Procedures for Irons and Steels. 2nd ed, ASM International, 1995.

Grossman, M. A., and E. C. Bain. Principles of Heat Treatment, 5th ed, ASM International, 1935.

Herring, Daniel H. Atmosphere Heat Treatment, vol. I, BNP Media, 2014.

Herring, Daniel H. Atmosphere Heat Treatment, vol. II, BNP Media, 2015.

Herring, Daniel H. Vacuum Heat Treatment, vol. I, BNP Media, 2012.

Herring, Daniel H. Vacuum Heat Treatment, vol. II, BNP Media, 2016.

Herring, Daniel H. “The Importance of Normalizing,” Industrial Heating April 2008.

Krauss, George. STEELS: Heat Treatment and Processing Principles, ASM International, 1990. 463.

Krauss, George. STEELS: Processing, Structures, and Performance, ASM International, 2005.

Practical Data for Metallurgists, 17th ed. TimkenSteel, 2011

Totten, George E., ed. Steel Heat Treatment Handbook, vol. 2, 2nd ed., CRC Press, 2007.

About the Author

For more information: Contact Dan at dherring@heat-treat-doctor.com.

For more information about Dan’s books: see his page at the Heat Treat Store.

Find Heat Treating Products And Services When You Search On Heat Treat Buyers Guide.Com

Why Normalize, and Is a ‘Still Air’ Cool Really Important? Part 2 Read More »

Why Normalize, and Is a ‘Still Air’ Cool Really Important? Part 1

February 21, 2025 / ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, HARDENING TECHNICAL CONTENT, Manufacturing Heat Treat Technical Content, OP-ED

This informative piece was first released in Heat Treat Today’s February 2025 Air/Atmosphere Furnace Systems print edition.

People often ask two fundamental questions related to normalizing. First, is it necessary? Second, just what and how important is a “still air” cool to the end result? Let’s learn more.

Why Normalize?

Normalizing is typically performed for one or more of the following reasons:

To improve machinability
To improve dimensional stability
To produce a homogeneous microstructure
To reduce banding
To improve ductility
To modify and/or refine the grain structure
To provide a more consistent response when hardening or case hardening

For example, many gear blanks are normalized prior to machining so that during subsequent hardening or case hardening dimensional changes such as growth, shrinkage, or warpage will be better controlled.

Normalizing imparts hardness and strength to both cast iron and steel components. In addition, normalizing helps reduce internal stresses induced by such operations as forging, casting, machining, forming or welding. Normalizing also improves chemical non-homogeneity, improves response to heat treatment (e.g., hardening), and enhances dimensional stability by imparting into the component part a “thermal memory” for subsequent lower temperature processes. Parts that require maximum toughness and those subjected to impact are often normalized. When large cross sections are normalized, they are also tempered to further reduce stress and more closely control mechanical properties.

Large paper roll normalized in a car bottom furnace and cooled (due to its mass) using the assistance of a floor fan.

Soak periods for normalizing are typically one hour per inch of cross-sectional area but not less than two hours at temperature. It is important to remember that the mass of the part or the workload can have a significant influence on the cooling rate and thus on the final microstructure. Thin pieces cool faster and are harder after normalizing than thicker ones. By contrast, after furnace cooling in an annealing process, the hardness of the thin and thicker sections is usually about the same.

Micrograph of medium-carbon AISI/SAE 1040 steel showing ferrite grains (white etching constituent) and pearlite (dark etching constituent). Etched in 4% picral followed by 2% nital. (Bramfitt and Benscoter, 2002, p. 4. Reprinted with permission of ASM International. All rights reserved.)

When people think of normalizing, they often relate it to a microstructure consisting primarily of pearlite and ferrite. However, normalized microstructures can vary and combinations of ferrite, pearlite, bainite, and even martensite for a given alloy grade are not uncommon. The resultant microstructure depends on a multitude of factors including, but not limited to, material composition, part geometry, part section size, part mass, and cooling rate (affected by multiple factors). It is important to remember that the microstructure achieved by any given process sequence may or may not be desirable depending on the design and function of the component part.

The microstructures produced by normalizing can be predicted using appropriate continuous cooling transformation diagrams and this will be the subject of a subsequent “Ask The Heat Treat Doctor” column.

In this writer’s eyes, industry best practice would be to specify the desired microstructure, hardness, and mechanical properties resulting from the normalizing operation. Process parameters can then be established, and testing performed (initially and over time) to confirm/verify results.

In many cases, the failure of the normalizing process to achieve the desired outcome centers around the lack of specificity (e.g., engineering drawing requirements, metallurgical and mechanical property call outs, testing/verification practices, and quality assurance measures). Failure to specify the required microstructure and mechanical properties/characteristics can lead to assumptions on the part of the heat treater, which may or may not influence the end result.

“Normalizing is the heat treatment that is produced by austenitizing and air cooling, to produce uniform, fine ferrite/pearlite microstructures in steel … In light sections, especially in alloy hardenable steels, air cooling may be rapid enough to form bainite or martensite instead of ferrite and pearlite.”

What Is Normalizing?

The normalizing process is often characterized in the following way: “Properly normalized parts follow several simple guidelines, which include heating uniformly to temperature and to a temperature high enough to ensure complete transformation to austenite; soaking at austenitizing temperature long enough to achieve uniform temperature throughout the part mass; and cooling in a uniform manner, typically in still air” (Herring, 2014).

It is also important to remember that normalizing is a long-established heat treatment practice. As far back as 1935, Grossmann and Bain wrote:

Normalizing is the name applied to a heat treatment in which the steel is heated above its critical range (that is, heated to make it wholly austenitic) and is then allowed to cool in air.

Since this is one specific form of heat treatment, it will be realized that the structure and mechanical properties resulting from the normalizing treatment will depend not only on the precise composition of the steel but also on the precise way in which the cooling is carried out.

The term ‘normalizing’ is generally applied to any cooling ‘in air.’ But in reality, this may cover a wide range of cooling conditions, from a single small bar cooled in air (which is fairly rapid cooling) to that of a large number of forgings piled together on a forge shop floor … which is a rather slow cool, approaching an anneal. The resulting properties in the two cases are quite different.

In plain carbon steels and in steel having a small alloy content, the air-cooled (normalized) structure is usually pearlite and ferrite or pearlite alone … More rapid cooling gives fine pearlite, which is harder; slow cooling gives coarse pearlite, which is soft. In some few alloy steels, the normalized structure in part may be bainite.

The hardness of normalized steels will usually range from about 150 to 350 Brinell (10 to 35 Rockwell C), depending on the size of the piece, its composition and hardening characteristics.

Importance of Defining Cooling Rate

In 2005, Krauss underscored the importance of defining cooling rate when he wrote: “Air cooling associated with normalizing produces a range of cooling rates depending on section size [and to some extent, load mass]. Heavier sections [and large loads] air cool at much lower cooling rates than do light sections because of the added time required for thermal conductivity to lower temperatures of central portions of the workpiece.”

Microstructures Created by Normalizing

The microstructural constituents produced by normalizing for a particular steel grade can be ferrite, pearlite, bainite, or martensite. The desired microstructure from normalizing adds an important cautionary note, as addressed by Krauss in STEELS (1990 and 2005), namely: “Normalizing is the heat treatment that is produced by austenitizing and air cooling, to produce uniform, fine ferrite/pearlite microstructures in steel … In light sections, especially in alloy hardenable steels, air cooling may be rapid enough to form bainite or martensite instead of ferrite and pearlite.”

Next time: We define a “still air” cool and look at the state of normalizing in North America.

References

ASM International. “ASM Handbook, vol. 4, Heat Treating,” (1991): 35–41.

ASM International. “ASM Handbook Volume 4A, Steel Heat Treating, Fundamentals and Processes,” (2013): 280–288.

ASM International. “Metals Handbook, 8th ed., vol. 1, Properties and Selection of Metals,” (1961): 26.

ASM International. “Metals Handbook Desk Edition,” (1985): 28-11, 28-12.

Chandler, Harry, ed. Heat Treater’s Guide: Practices and Procedures for Irons and Steels. 2nd ed, ASM International, 1995.

Grossman, M. A., and E. C. Bain. Principles of Heat Treatment, 5th ed, ASM International, 1935, 197–198.

Herring, Daniel H. Atmosphere Heat Treatment, vol. I, BNP Media, 2014.

Herring, Daniel H. Atmosphere Heat Treatment, vol. II, BNP Media, 2015.

Herring, Daniel H. “The Importance of Normalizing,” Industrial Heating April 2008.

Krauss, George. STEELS: Heat Treatment and Processing Principles, ASM International, 1990. 463.

Krauss, George. STEELS: Processing, Structures, and Performance, ASM International, 2005. 253–256, 574.

Lyman, Taylor, ed. Metals Handbook, 1948 ed. ASM International, 1948. 643.

Practical Data for Metallurgists, 17th ed. TimkenSteel.

Totten, George E., ed. Steel Heat Treatment Handbook, vol. 2, 2nd ed., CRC Press, 2007. 612-613.

About the Author

For more information: Contact Dan at dherring@heat-treat-doctor.com.

For more information about Dan’s books: see his page at the Heat Treat Store.

Find Heat Treating Products And Services When You Search On Heat Treat Buyers Guide.Com

Why Normalize, and Is a ‘Still Air’ Cool Really Important? Part 1 Read More »

The Heat Treat Doctor

What is Partial Pressure?

Why Do We Need to Use Partial Pressure in a Vacuum Furnace?

Chromium Coloration

Which Partial Pressure Gas(es) Can We Use?

In Summary

References

About the Author

Qué es la nitruración anódica?

Antecedentes: Complejidades de la nitruración por plasma

Nitruración anódica de aleaciones de titanio

Aplicaciones

Referencias

About The Authors:

What Is Anodic Nitriding?

Background: Plasma Nitriding Complexities

Anodic Nitriding of Titanium Alloys

Applications

References

About The Authors:

Machine-Induced Surface Conditions

Splatter of Stop-off Paints on Unintended Areas

Enriching Gas Additions (Sooting)

Material-Related Issues

In Summary

References

About the Author

What Is It?

How Does Hydrogen Get In?

What Is The Nature and Effect of Hydrogen Attack?

Nonferrous Materials and Hydrogen Embrittlement

Are Low Hydrogen Concentrations Also Problematic?

How Does Hydrogen Get Out?

References

About the Author

Find Heat Treating Products And Services When You Search On Heat Treat Buyers Guide.Com

The Heat Treatment Challenge

Mechanisms Involved

Material Imperfections

Stress Risers

Rapid Cooling and Phase Transformation

Improper Heating and Overheating

Inadequate Quenching Methods

Improper Fixturing

Delays Between Quenching and Tempering

Understanding Fracture Mechanics

Preventive Measures

Material Selection

Design Considerations

Improved Manufacturing Practices

Control of Heat Treatment Parameters

Quenching Media

Post-Quench Tempering

Quench Cracking in Other Materials

Summing Up

References

About the Author

Find Heat Treating Products And Services When You Search On Heat Treat Buyers Guide.Com

What is IGO?

What is IGA?

Effect on Material Properties

Materials Involved

Principal Concerns

How to Detect IGO and IGA

How to Avoid IGO and IGA

Key Differences

Summing Up

References

About the Author

Find Heat Treating Products And Services When You Search On Heat Treat Buyers Guide.Com

Boronizing (a.k.a. Boriding)

The Process

Boronizing Applications

In Summary

References

About the Author

Find heat treating products and services when you search on Heat Treat Buyers Guide.Com

What Is a “Still Air” Cool?

Final Thoughts — The State of the Industry

References