Rolled Alloys

Sigma Phase Metallurgy 101

/ ALLOY FABRICATIONS TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH

In this Technical Tuesday installment, Nick Hicks, metallurgical services manager at Rolled Alloys, emphasizes the importance of mastering the basics of sigma phase metallography in stainless steels. Understanding these fundamentals helps you know when to consult a metallurgist and guarantee top performance of heat treated parts.

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

Heat treaters are always seeking new methods of heat treat and new alloys to improve performance at a lower cost. In the world of stainless steel, there are well-known choices like 310 and newer options like RA 253 MA®. These alloys have exceptional qualities, especially RA 253 MA, which has creep strength up to 2000°F and oxidation resistance up to 2000°F. However, heat treaters should be aware of a potential issue when using such alloys: the formation of sigma phase over time.

In some cases, premature wear in nickel alloys was attributed to sigma phase embrittlement, but it’s important to note that sigma phase does not actually precipitate in nickel alloys. Instead, the actual microstructure may exhibit grain boundary oxidation or carbides. This article seeks to provide a clearer understanding of sigma phase metallography and its impact on stainless steels.

Definition of Sigma Phase

Sigma phase is an intermetallic compound made up of chromium and iron. It is hard, brittle, and non-magnetic. At room temperature, the presence of sigma phase can make the material so brittle that a sudden, hard impact can shatter a piece of metal that contains it, similar to a piece of glass. Pure sigma phase forms when the chromium content is between 42% and 50%, and it is one of the equilibrium phases in the iron-chromium phase diagram as seen in Figure 1.

The peak temperature for sigma phase formation in a 46% Cr alloy is 1510°F. A literature review reveals that different sources cite varying temperature ranges for sigma phase formation. This variation is due to each alloy having its own unique sigma formation range. According to one expert (Kelly 2005), sigma phase can form in the temperature range of 1100°F–1600°F (590°C–870°C).

Metallurgy of Sigma Phase

Many engineers require assistance in distinguishing between sigma phase and the formation of grain boundary oxides and carbides. Otherwise, they might reach incorrect conclusions. Sigma phase is a precipitation product that can manifest in both individual grains and along grain boundaries. Examples of sigma phase formation can be observed in Figures 2–4.

When observing nickel and certain nickel alloys like RA330®, confusion can arise due to the presence of grain boundary oxidation or carbide formation. These occurrences are often mistaken for sigma phase formation by engineers, but it’s important to note that a nominal nickel content of at least 35% is sufficient to prevent sigma phase formation.

Figure 5 depicts RA330 after a 3,000-hour duration at 1900°F. Despite 1900°F being significantly higher than the sigma formation range, some engineers determined that the grain boundary oxides were sigma phase. When there is any uncertainty, it is advisable to consult with a metallurgist who is knowledgeable about the metallography of these alloys.

Physical Properties of Material with and without Sigma Phase

Table A displays the results of impact testing for six different alloys aged at three different temperatures for varying durations. All the alloys experienced some degree of deterioration over time, with certain alloys showing significant losses and reduced ductility. Further analysis revealed that each alloy has its own specific temperature at which sigma phase formation occurs most rapidly. In fact, the formation of sigma phase is dependent on the time at temperature, which makes a C-type curve.

Figure 6 depicts the time temperature transformation curves for sigma phase formation for a few different stainless steels. Any point past a specific alloy’s curve results in the formation of sigma phase.

The results of elevated temperature impact testing for six alloys are presented in Table B. Many of the values in the table indicate that these alloys generally show either no loss of ductility or significantly less loss of ductility when the testing is carried out at elevated temperatures. In most cases, the materials still exhibit sufficient ductility to be safely used at these temperatures.

When these alloys have formed sigma phase and then cooled to room temperature, it’s important to prevent any kind of impact. At operating or heat treating temperatures, these alloys generally maintain enough ductility to be safely used.

Table D displays the results of Charpy testing conducted on RA330 after aging. Although there is a slight decrease in ductility, the material still exhibits sufficient energy absorption to be considered quite ductile and safe for use at room temperature.

Conclusion

Sigma phase precipitation is a phenomenon that occurs in stainless steels and alloys containing less than 35% nominal nickel content. This does not occur in nickel alloys with 35% nickel or more. Sigma phase can make materials very brittle at room temperature. However, at elevated temperatures within typical heat treating ranges, most materials retain sufficient toughness to be used without any concern. It’s important to note that even at high temperatures, toughness is lost. More caution should be exercised in choosing alloys for vibrating systems, as constant vibration can cause premature failure if sigma phase has formed.

Engineers may mistakenly identify grain boundary oxidation or carbides as sigma phase formation in alloys that do not actually form sigma phase. To ensure accurate conclusions, it is important to have interpretations verified by experienced metallurgists who are familiar with the metallography of stainless steels and nickel alloys.

An understanding of the basics of sigma phase metallurgy in stainless steels will help the heat treater, manufacturer, and end user avoid failures associated with sigma phase embrittlement.

References

Andersson, Thomas, and Thomas Odelstam. 1984. Sandvik 253MA (UNS S30815) – The Problem Solver for High Temperature Applications. Sandviken, Sweden: R&D Centre AB Sandvik Steel Bulletin, October.

ASM International. 1986. Binary Phase Diagrams. Metals Park, OH: ASM International.

Crucible Inc., Materials Research Center. 1972. Private Communications, January 10 and June 22.

Herring, Daniel H. 2012. “Sigma Phase Embrittlement.” Industrial Heating. Troy, MI: BNP Media, March.

Kelly, James. 2005. Heat Resistant Alloys. Rolled Alloys. https://www.scribd.com/document/90619472/HeatResistantAlloys-RolledAlloys.

Lien, George E. 1968. Behavior of Superheater Alloys in High Temperature, High Pressure Steam. New York, NY: The American Society of Mechanical Engineers.

Rolled Alloys. n.d. Internal Reports. Temperance, MI.

Rolled Alloys. n.d. Rolled Alloys Bulletin 1353: RA 353 MA® Alloy. Temperance, MI.

About The Author:

Nick Hicks
Metallurgical Services Manager
Rolled Alloys

Nick Hicks is the metallurgical services manager at Rolled Alloys. He holds a bachelor’s degree in mechanical engineering from the University of Toledo and a master’s degree in materials science from Worcester Polytechnic Institute. Nick represents Rolled Alloys at organizations such as the Materials Technology Institute (MTI) and the American Society for Testing and Materials (ASTM). He is also a former Emerging Professional on the ASM Heat Treat Board. Nick specializes in stainless steel and nickel alloy metallurgy for high-temperature and corrosion-resistant applications.

For more information: Contact Nick at nhicks@rolledalloys.com.

This article was initially published in Industrial Heating. All content here presented is original from the author.

Sigma Phase Metallurgy 101 Read More »

A Microalloyed Solution for High-Temp Applications

/ ALLOY FABRICATIONS TECHNICAL CONTENT, Manufacturing Heat Treat Technical Content

Alloy R&D has resulted in a material that combines the affordability of 310 stainless steel with the high temperature properties of more expensive higher nickel alloys, like alloy 600. Be it for your muffle belt conveyor or heat treating trays, this Technical Tuesday installment by Hugh Thompson, applications engineer of Rolled Alloys, will explore the strengths of this alloy variety to determine its best application.

This informative piece was first released in Heat Treat Today’s July 2025 Super Brands print edition.

Increasing nickel prices initiated the development of RA 253 MA®, a versatile alloy used in various thermal applications for equipment construction. With low chromium (Cr) and nickel (Ni) levels, this alloy provides a cost-effective alternative to other pricier nickel-based materials. With microalloying control, it is priced alongside 310 stainless steel while offering high strength properties similar to the more costly 600-series alloys. 

Chemically similar to 309 stainless steel, the alloy offers significantly higher creep resistance and rupture strength than 310. Its benefits include:

  • Oxidation resistance up to 2000°F  
    (1090°C)
  • Significant hot tensile strength  
    comparable to that of the 600-series alloys
  • Noteworthy creep and rupture properties 

This lean austenitic stainless steel uses cerium and silicon to create a very adhesive oxide, resulting in excellent oxidation resistance. The combination of nitrogen and carbon provides creep-rupture strength double that of 310 and 309 stainless steel at 1600°F (870°C). 

Chemistry

RA 253 MA has a specified chemistry, as indicated in Table A.  

Table A. RA 253 MA chemistry

High Temperature Properties 

Figure 1 shows the hot tensile strengths of different materials. RA 253 MA can be seen to have higher hot tensile properties than alloy 600, 310 stainless, and RA330® but lower than RA 602 CA®. It’s worth noting that while its hot tensile strength is reported up to 2200°F (1200°C), practical use is limited to 2000°F (1090°C) in oxidizing environments due to a loss of oxidation resistance at this temperature. 

Figure 1. Hot tensile strengths

Figure 2 displays the allowable design stresses for pressure vessel plates according to Section II-D of the ASME 2023 (2024 revision) code. One can see that the allowable stresses for RA 253 MA are higher than those for 310 stainless and RA330 but not as high as alloy 601. ASME allows design stresses for this alloy up to 1650°F (900°C). However, RA 253 MA is utilized at higher temperatures for various applications because this temperature limit is only for pressure vessels. 

Figure 2. Allowable design stresses

Figure 3 displays the actual 10,000-hour rupture strengths of different high temperature alloys. The data reveal that RA 253 MA exhibits high creep and rupture stress values comparable to alloy 601 and RA 602 CA, and it surpasses RA330; this would also surpass alloy 600.  

Figure 3. 10,000-hour rupture strengths

In Figure 4, data are presented for the minimum creep rate of 0.0001% per hour. Creep refers to the rate at which metal stretches, and it is usually measured in percentage per hour. There is a phase where the creep rate remains relatively constant, known as the secondary creep rate. This rate is a key factor in designing for high temperatures. It’s important to consider that metal will creep even under light loads, as the effects of creep can be observed in material with no load other than its own weight. Therefore, in practical applications, a creep criterion is utilized for design purposes. 

Figure 4. Minimum creep rate of 0.0001% per hour

The furnace industry has traditionally used a design criterion based on the stress required for a minimum creep rate of 1% in 10,000 hours or 0.0001% per hour. The design stress is typically set at a fraction of this value. For one of its criteria, ASME uses 100% of the extrapolated stress for 1% in 100,000 hours (or 0.00001% per hour). It is not recommended to extrapolate stress rupture and creep data to 100,000 hours above 1800°F (980°C). Th is comparison is provided for general guidance only. 

Rupture strength is reported as a stress and number of hours. It is the stress required at a specific temperature to break a specimen within a given time. In the furnace industry, a standard criterion for setting design stresses is to use a fraction of the stress that would result in rupture at 10,000 hours. ASME uses the lower of 67% of the extrapolated 100,000 rupture stress or 100% of the extrapolated 1% in 100,000 hours minimum creep rate. 

Strengths and Limitations 

When compared to alloys like 309 and 310, RA 253 MA has demonstrated equal or superior oxidation resistance. At 2000°F (1090°C), it displays outstanding oxidation resistance, on par with the limit for 310 stainless steel and surpassing 309. It is important to note that although short furnace excursions up to 2100°F (1150°C) can be tolerated, consistent oxidizing temperatures above 2000°F (1090°C) can quickly degrade the material. Therefore, it is best to avoid excursions above the suggested temperature limits for any alloy. 

This material has also proven to perform well in mildly carburizing environments, despite its lower alloy content. Even small amounts of oxygen in the gas, like carbon dioxide or steam, can create a thin and tough oxide layer on RA 253 MA, offering excellent protection against carbon and nitrogen pickup. However, it’s not recommended to use it in carburizing environments. Due to its lower nickel content, it is less resistant to carburization compared to higher nickel alloys such as RA330. 

Table B. Ductility based on room-temperature tensile tests

In a simulation where coupons were exposed to fifteen weeks of simulated bake cycles between 1700°F–1950°F (930°C–1065°C) in “green mix” used for producing carbon electrodes, room-temperature tensile tests revealed the ductility as shown in Table B. 

For RA 253 MA, the sigma phase formation process is much slower compared to 310S and 310, as shown in the TTT diagram in Figure 5 and the micrographs in Figure 6. At temperature, it is very unlikely material containing sigma phase will behave adversely. When the material is cooled to room temperature, it becomes very brittle, making it less resistant to thermal cycling. The material may crack if highly constrained and unable to expand freely during subsequent ramp-up. 

Figure 5. TTT curve for sigma phase formation
Figure 6. RA 253 MA grain structures with and without sigma phase

Corrosion Resistance in Salt Bath Applications 

As shown in Table C, RA 253 MA may be comparable to alloy 600 when exposed to sodium and potassium salts for heat treating high speed steel.  

Table C. Intergranular attack based on exposure to sodium and potassium salts

In this trial, plate samples were exposed to 210–252 cycles in preheat salts at 1300°F–1500°F (700°C–820°C), high heat salt at 2200°F (1200°C), and then quenched in 1100°F (590°C) salt. Table C shows that RA 253 MA has the potential to perform well in a salt bath environment due to its high silicon and chromium levels. While alloy selection is essential, regular maintenance and cleaning of the salt bath and surrounding areas are the most crucial factors. 

In salt bath heat treating, the service life of the pot is primarily determined by maintenance not the alloy. Pots must be desludged regularly, and all old, spilled salt must be removed from the furnace refractory when changing pots 

Corrosion Resistance 

Table D. Sulfidation attack after exposure to an atmosphere containing 13.6% SO2 at 1850°F (1010°C) for 1,860 hours

This alloy performs well, even in hot environments with sulfur in the presence of oxygen. However, it is not resistant to environments with reducing sulfur. Even in the presence of oxygen, the partial pressure of oxygen can be very low while stainless steel is in use. This low pressure can lead to a local sulfidation attack, even in what is considered an oxidizing atmosphere. 

Table D displays the depth of intergranular oxidation and sulfidation in test samples exposed to an atmosphere containing 13.6% SO2 at 1850°F (1010°C) for 1,860 hours. 

Microstructure 

Table E. Charpy v-notch impact results as annealed and after exposure (ft-lb)

The microstructure of RA 253 MA in the annealed and long-term exposure states is shown in Figure 6. In addition, Table E provides the Charpy impact values for the annealed state and at temperatures of 1292°F, 1472°F, and 1652°F (700°C, 800°C, and 900°C) over a long period of exposure.  

Based on the microstructure and Charpy impact data, it is clear that sigma phase precipitation is almost non-existent at 1650°F. Moreover, the TTT diagram in Figure 5 indicates that RA 253 MA requires significantly more time to initiate sigma precipitation compared to 310 and 310S stainless steel. 

Applications for Use 

Given the above capabilities, RA 253 MA can be and has been successfully utilized in a variety of applications. From bell annealing furnace covers, muffle belt conveyors, car exhaust manifolds and exhaust gas flexible tubes to hot air ducts, cooling tower tubes in sulfite process pulp mills, and heat treatment trays for neutral hardening, its abilities can cover a widescope of applications throughout in-house heat treat operations.  

References 

Andersson, T. and T. Odelstam. “Sandvik 253MA (UNS S30815) — The Problem Solver for High Temperature Applications.” A Sandvik Publication, October 1984. 

Kelly, J. Rolled Alloys. Rolled Alloys Bulletin 100. Revised September 2001. 

Kelly, J. Rolled Alloys. Rolled Alloys Bulletin 401, Heat Resistant Alloys©. Revised June 2006. 

Manwell, C. Rolled Alloys. Rolled Alloys Internal Report, Summary of Cyclic Oxidation Testing at 2000°F, August 2005. 

Proprietary Report on the MA Heat Resistant Material Series.  

Saum, W. Rolled Alloys. Rolled Alloys Internal Report, Summary of Oxidation Testing at 2000°F, August 2002. 

About The Author:

Hugh Thompson
Applications Engineer
Rolled Alloys

Hugh Thompson is a metallurgical engineer at Rolled Alloys, leveraging his expertise from The University of Toledo College of Engineering to drive innovation in specialty alloy solutions. Based in Toledo, he combines deep technical knowledge with industry leadership. 

For more information: Contact Hugh Thompson at Hthompson@rolledalloys.com. 

The content of this article was initially published by Industrial Heating. All content here presented is original from the author. 

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

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Considerations To Choose Optimum Fixtures

/ FIXTURE RACKING SYSTEMS TECHNICAL CONTENT, Manufacturing Heat Treat Technical Content

Options abound when it comes to selecting the preferred type of fixture. In this Technical Tuesday installment, Garrett Gueldenzoph, applications engineer at Rolled Alloys, examines various advantages of wrought versus cast alloys in heat treat operations.

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

There are various types of heat treating fixtures, such as trays, racks, boxes, and other part holders available in the market. These fixtures are generally made of castings, wrought fabrications, or hybrids.

For heat treaters, it can be challenging to determine which fixture best suits the job. The decision usually involves a combination of cost and design factors. However, many heat treaters tend to only consider the initial cost and overlook the importance of life cycle costs. It is crucial to consider the cost per pound of heat treated product, which is often overlooked but should be an important consideration.

Cast materials and wrought materials each have their own advantages. The pros and cons of each are summarized in Table 1. Cast materials offer a low cost per unit, the ability to incorporate beneficial elements like Cr and C, higher creep strength, and the ability to be cast into complex shapes that are ready to use.

Wrought alloys can be used in thinner sections, are repairable/weldable, resist thermal fatigue better, and have a better surface finish. Using thinner sections can result in a lower-weight fixture and fewer BTUs to heat the fixture.

Table 1. General comparison of cast vs. wrought materials

Baskets: Wrought and Cast

Baskets are one of the most common heat treating fixtures. A typical basket is shown in Figure 1. This simple basket, made entirely from a wrought round bar, is commonly called a bar basket or rod frame basket. This type of basket is either used as is or lined with wire mesh to hold small parts such as hardware in heat treating facilities. Wire mesh liners are inserted on all five sides to prevent these parts from falling into the furnace. Fully cast baskets or wrought-cast hybrid baskets are also used, but they tend to be heavier due to the larger amount of material they require. These types of baskets are used to support heavier loads than the wrought wire bar basket can handle.

A wrought basket has a lower carbon content and a defined grain structure, making it more resistant to sudden changes in temperature compared to cast baskets or hybrids. This allows it to endure multiple quenching and heating cycles. In contrast, cast baskets may develop cracks from frequent temperature changes. The wrought basket remains resilient to thermal shock until a case is accumulated during case hardening operations.

Cast baskets have a higher carbon content and better resistance to deformation under heavy loads. However, they are more susceptible to cracking than wrought baskets. When choosing between the two, the expected service life and cost per pound for heat treatment are the main economic factors to consider.

Figure 1. Bar basket/rod frame basket

Trays

Trays are commonly used to support heavier parts. There are three main types of trays; two are traditional designs and one is a newer design (see Figure 2). The first traditional tray consists of a serpentine grid made of snakelike bent pieces bordered by consecutive lengths. The pieces are held together by a threaded round bar with nuts welded to each end. A gap is left at one end between the last straight section and the end nut, allowing for free expansion and contraction of the individual pieces. While the serpentine grid can be made from a relatively thin sheet (11 gauge), higher strength can be achieved by increasing the top-to-bottom grid thickness. The second traditional tray is cast with straight legs connecting to round tubes.

The final tray design features a honeycomb pattern by Duraloy, with relatively thick legs. As a result, this heavy duty grid can support heavier weights compared to the traditional cast grid. These grids are becoming more common in heat treat shops due to their ability to handle significant weight. All three tray designs are depicted in Figure 2.

Figure 2. Tray designs for heat treat fixtures

Design

When designing baskets and trays, it is important to decide how thick the supports should be. Thicker supports can hold more weight, but the furnace capacity should also be taken into account to maximize efficiency.

Optimization

Using a tray with thick support members may not always be the best solution, as the furnace has a weight capacity limit. If the furnace can be run at total capacity, the strength of the fixture is well spent. It is best to use a fixture with the highest utilization, which means having the best possible ratio of part weight to total weight. A fixture that is too small will not allow the furnace to be filled to near capacity, while a fixture that is too heavy will limit the number of parts that can be processed.

Damage

Forklifts are a common cause of basket or fixture failure, especially during case hardening operations. The properties of the fixture material must be considered to prevent failure. For example, cast trays are strong but brittle, while wrought material has good impact resistance.

Custom

The final type of fixture is custom designed. One standard fixture is called a daisy wheel because of its grid-like shape. The decision to use a particular fixture depends on its ability to support parts and its expected lifespan. Cast fixtures tend to split in the joint areas, whereas welded wrought fixtures have more ductility and will not break as quickly in the welds. Stiffeners should be avoided unless some means of movement is provided, as they can cause the material to bend, buckle or crack.

Figure 3. Custom fixture

Materials

In the heat treating industry, fixtures and baskets are often made from a versatile alloy called RA330®. This alloy is resistant to oxidation up to 2100°F (1150°C) and has usable creep strength up to 1800°F (980°C). Most steel heat treatment is done below 1750°F (950°C), and many operations are done below 1600°F (870°C). Sigma phase forms in some fixture materials below 1600°F, which makes them brittle at room temperature and prone to failure eve with slight impacts such as forklift hits. But RA330, with 35% nominal nickel, is immune to sigma phase formation, as are nickel alloys with higher nickel content.

RA330 also has good resistance to surface hardening operations like carburizing and nitriding, but carbon and nitrogen can penetrate the protective oxide and diffuse into the base metal over time. Generally, RA330 fixtures last approximately one year in carburizing atmospheres and should last longer in nitriding environments. They may warp from continued use but are resistant to thermal fatigue.

There are other options for wrought materials, but they are often more expensive than RA330. For instance, RA 253 MA® is an alternative with good creep strength and lower cost than RA330. However, due to its lower nickel content, it is subject to sigma phase embrittlement and does not offer much resistance to carburization or nitriding.

If the fixture is used only for neutral hardening in an inert atmosphere or vacuum, then RA 253 MA may be a cost-effective option. On the other hand, RA 602 CA® has performed exceptionally well as a fixturing material for the highest temperature vacuum heat treating operations, up to temperatures just below 2300°F (1260°C). This alloy has one of the highest creep strengths among all potential wrought products.

Despite the other options, RA330 is still the most economical alloy for heat treating fixtures. However, a higher strength alloy may be considered when final heat treat part dimensions are critical and straightness specifications are tight. Other alloys could be considered, but these fixtures would be restricted to that one application.

References

Glasser, Marc. “RA330: Versatile Nickel Based Alloy for Heat Treating.” Industrial Heating, Sept. 2016.

Rolled Alloys. “Cast vs. Wrought.” https://www.rolledalloys.com/resources/cast-vs-wrought/.

Rolled Alloys. “RA 602 CA® Chosen for Heat Treat Baskets for Extreme High Temperature Vacuum Heat Treating.” https://www.rolledalloys.com/wp-content/uploads/2022/07/RA-602-CA-Chosen-for-Heat-Treat-Baskets_nickel-rolled-alloys-metal-supplier.pdf.

About the Author:

Garrett Gueldenzoph
Applications Engineer
Rolled Alloys

Garrett Gueldenzoph specializes in stainless steel and nickel alloy welding at Rolled Alloys. He holds a bachelor’s degree in Mechanical Engineering from the University of Toledo and is actively involved in several respected technical organizations, including the American Welding Society (AWS), the American Society for Metals (ASM), and the American Society for Testing and Materials (ASTM). Garrett has a strong passion for aerospace and space-related applications, and he plays a key role in enhancing the company’s technical expertise in this market.

For more information: Contact Garrett at ggueldenzoph@rolledalloys.com.

This article was initially published in Industrial Heating. All content here presented is original from the author.

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

Considerations To Choose Optimum Fixtures Read More »

Tempering or Annealing, Which Heat Treatment Works for You?

/ ANNEALING TECHNICAL CONTENT, FEATURED NEWS, MANUFACTURING HEAT TREAT, TEMPERING TECHNICAL CONTENT

OCWhat process holds a soft spot in your heart? Tempering or annealing? For Valentine's Day, turn up the heat -- errr heat treatments -- with this look at the differences in tempering and annealing! Heat Treat Today has resources for you to spark some thought and learning on these processes.

Sentiments and strong feelings can certainly be heightened this Valentine's Day. While tempering and annealing may not lend themselves easily to the holiday, we hope you enjoy a bit of a nod to the day in our headings below. Make use of the Reader Feedback button, too, and keep us in the loop with questions and comments on what heat treatment you love.

Problem with Annealing? Get to the Heart of the Issue

An automotive parts manufacturer was running into problems with cracking parts. The variable valve timing plates were returning from heat treatment with this problem. To determine why those parts were cracking after the annealing process, an investigation was launched by metallurgists at Paulo.

The presence of nitrogen combining with the aluminum already present in the particular steel being used was forming aluminum nitrides. What could be done? Read more in the case study article below to find out a workable solution that allowed the annealing to create a crack-free product.

"Part Failure Investigation & Resolution, a Case Study"

Induction, Rapid Air, Oven and Furnace Tempering: Which One do You Love?

Contact us with your Reader Feedback!

This article gives some perspectives, from experts in the field, on what kinds of tempering are available and for what the processes are used.

Hear from Bill Stuehr of Induction Tooling, Mike Zaharof of Inductoheat, and Mike Grande of Wisconsin Oven with some basics and background information on tempering. Those reasons alone make this resource helpful with information like this: "tempering at higher temperatures results in lower hardness and increased ductility," says Mike Grande, vice president of sales at Wisconsin Oven. "Tempering at lower temperatures provides a harder steel that is less ductile."

More specific in-depth study is presented as well. The Larson-Miller equation is considered, and the importance of temperature uniformity is emphasized. Read more of the perspectives: "Tempering: 4 Perspectives — Which makes sense for you?"

Cast or Wrought Radiant Tubes in Annealing Furnaces - is Cheaper Really What to Fall For?

Marc Glasser, director of Metallurgical Services at Rolled Alloys, takes a look at radiant tubes. He particularly discusses the cast tubes and wrought tubes. For use in continuous annealing furnaces, there are several factors contributing to choice of radiant tube type.

Marc says, "Justification for the higher cost wrought alloy needs to take into consideration initial fabricated tube cost, actual tube life, AND the lost production of each anticipated downtime cycle as these downtime costs are often much more than material costs." He probes into areas that may not be considered when thinking of all the costs involved. Read more of his article "Radiant Tubes: Exploring Your Options."

Tempering Furnaces: Improvements are Thrilling

The expert behind this piece shows the importance of tempering, particularly in automotive fastener production. Tim Donofrio, vice president of sales at CAN-ENG Furnaces International Limited examines what's working in the tempering furnaces. The products are meeting and exceeding expectations.

Highly efficient, continuous soft handling mesh belt heat treatment systems are getting the job done. Read more about the advances in tempering furnaces by clicking here: "Mesh Belt Heat Treatment System Advancements for Automotive Fastener Production."

Additional Resource To Catch Your Eye

To wrap up this Technical Tuesday post on tempering and annealing, head over to this additional resource to round out the scope of each process. "What is the Difference: Tempering VS. Annealing" gives a summary perspective on the heat treatments discussed above.

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

 

Tempering or Annealing, Which Heat Treatment Works for You? Read More »

Radiant Tubes: Exploring Your Options

/ ANNEALING TECHNICAL CONTENT, CARBURIZING TECHNICAL CONTENT, FEATURED NEWS, MANUFACTURING HEAT TREAT

OC There are many radiant tube options on the market, so which one is best for your furnace and your budget? In this column that compares radiant tubes in carburizing and continuous annealing furnaces, discover how two major types of radiant tubes stack up.

Marc Glasser, director of Metallurgical Services at Rolled Alloys, investigates more deeply the two choices. This Technical Tuesday discussion on radiant tubes options will be published in Heat Treat Today's February 2023 Air & Atmosphere Heat Treating Systems digital edition.

Introduction

Marc Glasser
Director of Metallurgical Services
Rolled Alloys
Source: Rolled Alloys

Radiant tubes are used in many types of heat treating furnaces from carburizing furnaces to continuous annealing of steel strip. Generally, a heat treater has three options for radiant tubes: cast tubes, wrought tubes, and ceramic silicon carbide tubes. Silicon carbide tubes are rarely used by heat treaters, so this article will not delve too deeply into this option. Suffice it to say, ceramic materials can often handle much higher temperatures at the expense of ductility; ceramics are more brittle than metals, making them prone to failure from the small impacts, so metal cages are sometimes fabricated to protect them. Most of the tubes being used today are cast radiant tubes. With new casting technology — primarily centrifugal casting — thinner tubes are being cast at a lower cost, which then results in a shorter life.

The primary factors for choosing radiant tube material are tube temperature and carbon potential of the furnace atmosphere. Cost-benefit analysis should also be considered. There are multiple applications for radiant tubes, including carburizing furnaces, continuous annealing furnaces for steel sheet galvanizing, steel reheat furnaces, and aluminum heat treating. This article will explore two of the aforementioned radiant tube options, specifically for carburizing and continuous annealing furnaces.

Radiant Tubes for Carburizing Furnaces

Gas carburization is traditionally performed between 1650°F and 1700°F at a carbon potential of 0.8% approximating the eutectoid composition. In today’s competitive environment, more heat treaters are increasing temperatures to 1750°F and pushing carbon potentials as high as 1.6% to get faster diffusion of carbon while spending less time at temperature. INCONEL® HX (66% Ni, 17% Cr) has been a common cast alloy seen in carburizing furnaces. This alloy is regularly selected for its resistance to oxidation and carburization up to 2100°F. Super 22H is more heavily alloyed than HX and is seeing more use as carbon potentials increase but at a premium price. With advances in centrifugal castings, cast tube wall thicknesses have decreased from 3/8-inch to 1/4-inch. Some heat treaters have shared that this decrease in wall thickness has also led to shorter tube life.

Fabricated and welded radiant tubes in alloys 601 and RA 602 CA® have been tested in industry. When tested, these wrought alloys were fabricated to have a wall thickness of 1/8-inch. At the extremes, tubes fabricated from 601 only lasted 50% as long as cast HX. Historically, HX tubes have been approximately 33% higher in cost than that of 601 and utilize heavier 3/8-inch walls. A little-known fact is that by switching to a thinner wall cast tube, the life drops by 50%. By switching to 1/8-inch wall thickness, RA 602 CA tube life has been extended to eight years or more, while running at 1750°F and up to 1.6% carbon potential, at just a 33% premium over cast HX. Life cycle data are presented in Figure 1.

Figure 1. These life cycle comparisons were done in carburizing furnaces only. In non-carburizing furnaces, justification of alloy selection is dependent on actual operating conditions and each individual operator’s own experience.
Source: Rolled Alloys

Radiant Tubes for Continuous Annealing Furnaces

In the area of continuous annealing, the cast alloy of choice is HP/HT (35% Ni, 17% Cr, 1.7% Si, 0.5% C). Here again, this casting has been compared to 601 and RA 602 CA, with the same results. The total life data from these trials are also incorporated into Figure 1. During the collection of this data, there has been no effort to measure the actual tube temperature, so the effect of tube temperature is not clearly defined. In these continuous annealing furnaces, it has been reported that the tubes at the entry end are subject to more heat absorption as burners are firing more due to the continuous introduction of cold material; in trials, the operators have not kept adequate documentation of specific tubes, making justification more diffcult.

Justification for the higher cost wrought alloy needs to take into consideration initial fabricated tube cost, actual tube life, AND the lost production of each anticipated downtime cycle as these downtime costs are often much more than material costs. Only individual fabricators can determine these costs.

The Economics

Table 1
Source: Rolled Alloys

Table 1 above shows the economics of metal alloy choice. To properly interpret, understand that the costs are not actual, but rather relative to 601, so a round number of 1000 was used. With a 30% greater cost of cast tubes, that translates to a relative cost of $1300. The annual cost is the amortized cost over the life of the tube. The total eight-year cost is the relative cost times the number of tubes that would have to be purchased to obtain the life cycle of one tube of the longest-lasting material over its full life cycle.

Missing in this analysis is the additional cost of downtime and lost production. For the replacement of radiant tubes in a carburizing furnace, this typically entails a full week to turn a furnace off, allow it to cool, replace the tubes, and then heat it up again. Many heat treaters do not consider this, and therefore it is a hidden cost. Even without the downtime being considered, by examining the total cost of materials (including replacements) compared to the longest-lasting tube, it turns out that the most expensive tube is the cheapest tube. The obstacle to overcome is whether the heat treater is willing to wait eight years to realize these cost savings.

There can be additional factors to consider. With improvements in the efficiency of casting, the actual costs of the thinner wall casting may be somewhat less, but to match the overall cost of the longest-life material, it would have to be less than half the expected cost. As better, more expensive cast alloys become accepted and actual life data becomes available, these more costly alloys can be added to this table for comparative analysis, too.

This same method of analysis can be applied to radiant tubes for continuous annealing furnaces, but more details will need to be added including furnace position. Different alloy candidates will have to be put to the test in actual operations, carefully document what alloy is in what position or location, and when it gets changed out. This becomes quite cumbersome when annealing furnaces (depending on design and manufacture) can have over 200 radiant tubes.

Conclusion

Currently, cast alloy tubes dominate the market. The concept of total life cycle cost has been introduced as a means of more accurately justifying one’s choice of radiant tube. This comes into play more as processes are pushed beyond traditional process conditions. Cost-benefit analysis must be balanced over acceptable amortization time, of course. However, performing the full analysis as well as the costs saved from downtime may lead some heat treaters to some alternate materials.

About the author: Marc Glasser is the director of Metallurgical Services at Rolled Alloys and is an expert in process metallurgy, heat treatment, materials of construction, and materials science and testing. Marc received his bachelor’s degree in materials engineering from Rensselaer Polytechnic Institute and a master’s degree in material science from Polytechnic University, now known as the NYU School of Engineering. Contact Marc at mglasser@rolledalloys.com

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Heat Treat Radio #60: High-Temperature Material Selection with Marc Glasser, Rolled Alloys

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Heat Treat Today publisher Doug Glenn and Marc Glasser of Rolled Alloys on why choosing the cheapest material is not always the best way to go. Listen to some of the practical tips Mr. Glasser gives for choosing the right alloy for your application.

Below, you can either listen to the podcast by clicking on the audio play button, or you can read an edited transcript.

 

The following transcript has been edited for your reading enjoyment.

Doug Glenn (DG):  We're going to talk today about something that Marc and I had talked about that kind of caught my attention that I thought might be of interest to our listeners, and that's this whole idea that sometimes buying the cheapest material isn't always the best option.  So, that's the topic, but, before we do that, Marc, I want you to tell our listeners and/or viewers a little bit about yourself, your background, and what you're currently doing.

Marc Glasser (MG):  I have been a metallurgist or material scientist for forty years.  Next month will be exactly forty years since I graduated from Rensselaer Polytechnic Institute with a bachelor's degree in materials engineering.  After ten years of working, I went, simultaneously, to a job and to night school for five years and I obtained my Master of Science in material science from, then, Polytechnic University which is now known as the NYU School of Engineering.  I've been working in all areas of metallurgy and material science.  I've worked in rolling, I've worked in forging, I've worked in powder metallurgy, and I've worked in heat treating laboratories.  I'm currently working in metallurgy of heat resistant materials and applications of these alloys in industry.

School of Engineering at NYU

DG:  Let's jump in then, Marc.  I want to talk to you a little bit about this contention that you and I talked about that sometimes, but not all the time, expensive is better and buying the cheapest isn't always the best.  In a nutshell, what are you trying to say on that?

MG:  I'll take it even one step further:  Expensive is cheaper.   Let me expand on that.  You have a part and it's a certain price and you know you have a life of two years. . . so that's cost X.  You have alloy #2 that's going to cost 60% more.  It's going to have a life of eight years.  Again, you're going to pay 60% more for this part than you would for the first part of the less expensive alloy.  But, over the operating life of that less expensive alloy, you're going to have to replace it three times.  You're going to use four separate components.  So, 60% of the cost times four, you're spending 240% more than you would spend on one component that's a little more.

It's cheaper up front, but over the entire life cycle of the part, buying four more parts of the cheaper one is a lot more expensive.

DG:  Let's talk about some of those hidden factors that come into play when you're analyzing the true cost of selecting those materials.  Do you have a couple of examples?

MG:  Absolutely.  The most stark example, that we made our first case history on, is radiant tubes.  For years, the alloy of choice on radiant tubes was a wrought 601 thin wall and you get about two years on it in a typical furnace.  Then the casting industry came in and, because of limitations of the machinery, they had to go with a heavier wall that was three times as thick and that cost 30% more, but it got four years of life.  Now, there's newer technology and they can cast it a lot thinner, but thinner doesn't last as long.  So, for the wrought tube, you're talking about 1/8 of an inch wall thickness.  With cast, for the four-year version, is about 3/8 of an inch and if you go down to 1/4 inch or less, you get maybe two or two-and-a-half years and if you go to the more expense wrought alloy, (again, you're talking about 1/8 inch wall), it's 60% more than the original one, 30% more than the cast, and you get eight years out of it.

Now, again, these numbers are based just on the cost of the material.  But, you've got to dig a little deeper because you're not capturing the true savings of using the more expensive material because, think of this:  If you've been in a heat treating shop and you know your carburizing furnaces, you have to turn it off, cool it down, let it air out because you've got a carbonaceous gas in there and any residual carbon monoxide, if you go in there, you're going to asphyxiate.

The bottom line is, the turnaround can take up to a week.  Each time you have to go down for a week, what everybody doesn't even think about is how much revenue in sales and/or in profits are you losing from that week down?  And, if you're going from cast to the better wrought alloy, you're talking about one week.  If you're still going with the original less alloyed, thinner wrought tube, that's three times.  Those savings can be much larger, depending on the facility, than just the material cost; it's just a few thousand dollars.  I don't know how to evaluate how many tens of thousands or hundreds of thousands of dollars of lost production would be, but each shop has to consider that.  They know the numbers; those are proprietary numbers that need to be considered.

With muffles, it's the same kind of analysis because you have the same alloys except muffles are not typically cast.  But, let me give you an example.  A lot of muffles operate at 2125 and, again, you use a 601 muffle.  They're going to stay perfectly straight and flat at that temp for about six months.  At that point, the typical shop will start seeing a little bit of roof sag and it will sag more and more and more.  But there's plenty of room, so you can get a lot of sag before it starts interfering with the parts being conveyed.  So, my general rule from the shops that I've seen, is that it can sag for about three times as long as it stays straight before the sagging is too great and has to be removed.  Typically, it's about two years.  With the better alloy, again, the case that I've seen was two years without any sagging and that was a higher temperature.

What we've done is we've actually gone to good customers who understand the concept and we work with them on developing case history.  They log in when they put it in and the log in when they take it out.  They have good records, number one.

Now, I'm talking predicted metal temperature based off the process temperature which could be more or less because it's estimated.  But I know that the one that we looked at was at least 2200 on the metal temperature.  And this was one of the really crazy ones because it was replacing a cast material of much higher quality cast material and the cast material was dead straight for a year-and-a-half, it would start to just creep a little, but if you're familiar with casting, there's not a lot of ductility in casting when it starts creeping maybe 3 or 4%, you don't have to worry about more creep; it ruptures!  Then, the gas starts escaping and that's no good so they take it down.  In this case, when you switch from the cast, the best wrought material was actually cheaper and it lasted longer and the particular customer would just change them every two years because they were still in cost savings mode.  Based on my experience, I've predicted that they should be able to get at least six years on it.  But, they're not willing to take that chance.

DG:  The examples that you gave were the radiant tube and the muffles.  I assume the same thing would be true, though, in retorts, for baskets or even fixturing systems, and things like that.

MG:  Absolutely.  I bring those two up because I have more good case histories.

DG:  I assume the same would be somewhat true for fans, and things of that sort, if necessary, although you wouldn't be worrying so, so much about sagging and stuff like that.  But anything, basically, I assume, metal.

MG:  That's correct.

DG:  How about measuring the life cycle of materials components?  Any tips or tricks you've got for people on how exactly to do that and to get an accurate estimate?

MG:  What we've done is we've actually gone to good customers who understand the concept and we work with them on developing case history.  They log in when they put it in and the log in when they take it out.  They have good records, number one.  We've worked with others who've wanted it to work but they didn't do so good of a job tracking it.  In one case, it was a much larger furnace where they had many radiant tubes and they were just working with a few of them.  Personnel changed – one person didn't let the next person know about the trial and the identity got lost.  So, we spent a lot of time for nothing.  But, what we learned on that one is something real simple:  You take a welder and you weld the alloy name somewhere on the tube and that's not going to wear away.  Assuming you choose the right consumable, that weld is not going to go away.

DG:  You already gave a couple of examples, but let me ask you this:  How about a few concrete examples of where a more expensive material produced an overall more cost effective part?  You already kind of gave us those back with the radiant tube, but are there any others that you've got along that line?

MG:  The radiant tube is a great example.  Muffles and retorts.  We've been trying to work with some people on larger heat treating trays, but, again, there the task people have done a pretty good job, so we're trying to find a few people willing to go out on a limb and try something better.

Here, the concept is the idea of something lighter so that we don't look as much about the cost of the component.  If you go with a lighter fixture, your furnace has a weight capacity and if you cut your weight 20-30%, you can put more parts on it and have more of your furnace BTUs going to heat treat parts instead of fixturing.  When you're putting BTUs into parts, you're talking more profit per part.

DG:  Right.  You're not spending as much time, basically, using a basket as a heat sink, or something like that.

MG:  Exactly.  And, that's a concept that I introduced at one of the conferences about a year and a half ago.  These things take time to percolate before they're accepted by people.

DG:  Speaking of acceptance, let me ask you this question:  Are these concepts that we've been talking about, the idea that sometimes less expensive is not better, is it widely accepted, do you think?  I mean, do you think people understand it, generally speaking?

MG:  Some people do.  Not as much as I'd like to see!  The other obstacle you're looking at is when you're looking at four years versus eight years and you look at some of the larger companies, you may have personnel turnover and one person doesn't want his 'replacement' to get all the credit.  These are things that were learned the hard way.  You have to get the right people to try it.  A family-owned business is a perfect place.

I can give you another real good example on heat treating baskets where it made a difference.  I'm going to give the name because I have done papers with him at a conference on this subject so I don't think it's taboo.  I work with Solar Atmospheres on a basket for an extremely high temperature heat treating process that was slightly under 2300 degrees Fahrenheit.  (We can say that because it's in the case history.)  The first baskets that he used were your traditional Inconel 600 601 and they were supporting heavy parts.  After five cycles, they had to cut all the sides off, hand straighten them (each of the sides) and  weld it back together.  That's timely.  So, he went to another alloy, a better alloy, a competitor's alloy (HR120), and got ten cycles on it.  He was very happy.  Then, one of the people at their headquarters heard me give a talk on this new alloy that we had, our 602CA, which we trademark as RA602CA, and he got excited.  He started asking me questions after the presentation and we eventually got kicked out of the room because it went well beyond the break; so we continued out in the hall as we walked to our company's booth and we talked.  It took about ten to twelve months before they were ready to try it.  We worked with their fabricator to get the material.  They were up to forty-five cycles before they straightened it and there's a catch, though, to that.  At forty-five cycles, they probably could've continued, but during the pandemic in 2020, when things were slow, they made a smart business decision that this would be a great time to do the straightening.  I can't fault them, but it would have been nice to know just how many more.  But, at forty-five versus ten, it is probably a similar cost at the time of manufacture.  That's a no-brainer.

DG:  So, we've covered some of the basics.  We understand that it's not necessarily widely accepted so people should pay attention to some of these things that you've said.  Are there any other economic factors that you think people aren't necessarily taking into consideration when they're doing material selection, besides the things we've talked about.  Initial cost, life cycle, cost of replacement, and those types of things.  Is there anything else that they ought to be thinking about?

As I mentioned in one of the cases, when there is significant down time to replace a part, you've got to consider how much money you're not bringing in because you're down for a week, or however long it is.  This is often overlooked, as well.

MG:  As I mentioned in one of the cases, when there is significant down time to replace a part, you've got to consider how much money you're not bringing in because you're down for a week, or however long it is.  This is often overlooked, as well.

DG:  To me, that's cost of replacement, because that's not just a hard replacement cost, but the downtime replacement, right?

MG:  It's a little less obvious, though.

DG:  Those are all good thoughts, Marc.  When people go to do material selection, keep some of these things in mind.  It's not just a matter of what the buyer, the purchaser guy, sees coming across his desk and comparing those two costs, let's talk about the material properties and longevity of the product and things of that sort.

I know that you, being with Rolled Alloys, you guys help customers, I imagine, pretty much continually on things like this.  If people want to get in touch with you or Rolled Alloys, how is it best to do that?

MG:  There are a couple of ways. The first way is my email: mglasser@rolledalloys.com.  You can always ask me a question.  On our website, there is a link to ask a metallurgist a question.  I believe, you can also go www.metallurgical-help@rolledalloys.com and that will bring you to one of the metallurgists in my department and somebody will get an answer to you .

DG:  Thank you very much, Marc.  I appreciate your expertise.  We'll hope it's helpful to the heat treat world.

MG:  Doug, I thank you for having me as your guest and I look forward to more conversations with you.

Doug Glenn <br> Publisher <br> Heat Treat Today

Doug Glenn
Publisher
Heat Treat Today

 

 

 

 

 

 

 

 

 

 

 

 

 

To find other Heat Treat Radio episodes, go to www.heattreattoday.com/radio and look in the list of Heat Treat Radio episodes listed.

 

 

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Marc Glasser on The Tools and Trade Secrets of Heat Resistant Alloy Welding

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Heat resistant alloys used for heat treating fixtures, muffles, retorts, radiant tubes, and other parts are typically stainless steel or nickel based austenitic alloys. Welding of these alloys requires practices that are often exactly the opposite of the practices required for carbon and alloys steels since austenitic stainless steels do not undergo phase transformations. Metallurgists are often asked many questions on the proper welding methods. Carbon and alloy steel welding requires practices and procedures that will minimize or prevent the chances of cracking due to potential martensite formation during weld solidification. Austenitic stainless steels do not undergo any phase transformation. They require rapid cooling to prevent solidification cracks due to hot cracking. Thus different procedures are required.

In this Heat Treat Today Technical Tuesday feature, Marc Glasser, Director of Metallurgical Services for Rolled Alloys, provides some basic information on the metallurgy as well as good welding practices to follow.

Reprinted with permission from Heat Treat 2019: Proceedings of the 30th Heat Treating Society Conference and Exposition, October 15-17, 2019, Detroit, Michigan, USA. ASM International, 2019.

CHEMISTRY CONSIDERATIONS
Most heat resistant alloys used in the heat treating industry for components are austenitic. They can be austenitic stainless steels, or austenitic nickel alloys. The key word is austenitic. One of the virtues of austenitic materials is that they are not subject of phase changes from cooling to heating or heating to cooling. This is markedly different from alloy and carbon steels, which undergo a phase transformation from austenite to ferrite and cementite. The cooling must be slow enough to prevent martensite formation, so preheating and postheating are performed to either prevent this phase transformation or to temper any formed martensite.

Austenitic alloys do not undergo phase transformations to martensite, and as a result slow cooling the material is the worst operation that an austenitic alloy can be subject to. In austenitic alloys, the main concern is the tendency for welds to hot tear upon solidification[1]. In stainless steels with up to approximately 15% nickel, the solution is simple. The composition is adjusted to form small amounts of ferrite during solidification[2]. Prediction of the ferrite number FN, which represents an estimate of the amount of ferrite in the weld after solidification, is predicted by using Schaeffler diagrams. The ferrite nullifies the effect of certain trace elements that cause hot cracking [1]. One of these trace elements, phosphorous cannot be refined out of the material. Since these materials are all melted from scrap metal, the amount of phosphorous found in the heat will mirror the amount in the scrap. Sulfur, silicion, and boron also contribute to hot shortness, but these elements can be refined to very low levels in the steelmaking process.

For higher nickel bearing grades, with more than 20% nickel, the chemistry precludes the possibility of ferrite formation. Therefore, other means must be employed to prevent hot tearing during solidification. In this case, the residual trace elements, particularly P must be kept low, as they lead to hot shortness [2, 3]. Certain alloy additions including manganese (Mn), niobium (Nb), molybdenum (Mo), and carbon (C) all reduce the propensity of austenitic nickel alloys and high nickel stainless steels to crack [4]. 310 stainless steel stans in a unique position having neither ferrite formers nor weldability-enhancing alloy additions. In this alloy, control of chemistry and residuals is of utmost importance.

The other key to successful welding of nickel alloys is to minimize the time spent in the high temperature range where they are susceptible to hot tearing [4].

GOOD WELDING PRACTICES
Good welding practices for nickel alloys are centered on the need to remove heat as quickly as possible in order to minimize the time spent in the hot tearing range. The first consideration is to keep the heat input as low as possible to still get a full penetration weld. The actual input in kJ is dependent on the alloy being welded.

Heat input (HI) is defined as: HI (KJ/in) = Voltage x Amperage x 6/(Speed (inch/min) x 100)

Welds should NOT be preheated and interpass temperatures should be 200°F maximum. The cooler the interpass temperature is, the less likely hot tearing is [5]. A reliable, easy test for a welder is the spit test. Spit on the weld, and if it boils it is still to hot, and further waiting is in order.

One of the most important considerations in welding nickel alloys is to weld in a straight line along the length of the weld and do not weave. Welders tend to weave from side to side especially when welding nickel alloys which are more viscous that carbon steels and this weaving makes the metal flow better. While this technique works well for carbon steel where a higher heat input and slower cooling are necessary, it is exactly the wrong procedure for nickel alloys. Weaving tends to flatten out a weld. This in turn reduces the crown height and strength.

Furthermore, weaving tends to increase the heat going into the weld and slow down the weld speed. The key is to get a nicely shaped, convex weld bead, as illustrated in Figure 1. A concave bead configuration tends to crack along the centerline [5].

Figure 1: Convex vs. Concave Weld

Full penetration welds are important. Beveling one or more of the pieces to be joined may be required to get a full penetration weld. Incomplete penetration leaves a void between the two workpieces. Such a channel can entrap surface treating gases leading to brittle pieces surrounding the weld. Furthermore, the gap can act as a propagation site for cracks which form from thermal cycling from heat treating. This is shown in Figure 2 below.

Figure 2: The effect of non fully penetrated welds

Some suggested joint designs include square butt joint, single V joint, double V joint, single U joint, double U joint, J groove joint, and T Joint. These are shown in Figures 3 to 9 below, along with design criteria. These suggestion grooves are from ASME code[6], but are good guidelines to follow even if code stamps are not required.

Figure 3: Square butt joint. Maximum t = 1/8 ” Gap A = 1/16″ Minimum, 3/32″ Maximum

Figure 4: Single V Joint. Maximum t = 1/2″ Gap A = 1/16″ Minimum, 1/8″ Maximum Land B = 1/16 to 3/32″ Angle C = 60 – 75 degrees

Figure 5: Double V Joint. Gap A = 1/16″ Minimum, 1/8″ Maximum Land B = 1/16 to 3/32″ t = 1/2″ or greater Angle C = 60-75 degrees

Figure 6: Single U Joint. Gap A = 1/16″ Minimum, 1/8″ Maximum Land B – 1/16 to 3/32″ Radius R – 3/8″ Minimum For single groove welds on heavy plate thicker than 3/4 inch. Reduces the amount of time and filler metal required to complete the weld.

Figure 7: Double U Joint. Gap A = 1/16 to 1/8″ Land B = 1/16 to 3/32″ Radius R = 3/8″ Minimum Minimum t = 3/4″

Figure 8: J Groove Joint. Gap A = 1/16 to 1/8″ Land B = 1/16 to 3/32″ Radius R – 3/8″ Minimum For single groove welds on plates thicker than 3/4 inch. Reduces the amount of time and filler metal required to complete the weld.

Figure 9: T Joint.
t = greater than 1/4″
For joints requiring maximum penetration. Full penetration welds give maximum strength and avoid potential crevices.

Regardless of which joint is selected, the purpose is to obtain a full penetration weld with no voids or channels, as shown in Figure 10 below.

Figure 10: Example of Full Penetration Weld

Both the starting and finishing ends of the weld beads can be crack initiation sites. The best practice for starting is to make the start of the weld bead as heavy as the rest of the weld bead [4]. A light or thin start up can cause cracking. This is shown in Figure 11. Furthermore, in nickel alloys, the end of the bead can sometimes yield a star shaped crack. This can be eliminated by backstepping the weld for ½ to 1 inch as shown in Figure 12 [3].

Figure 11: Start welds as heavy as the rest of weld beads

Figure 12: Backstep the weld ends to prevent cracking

Cleanliness is extremely important for welding stainless and nickel alloys. Some general rules include [5]:

  • Remove all shop dirt, oil, grease, cutting fluids, lubricants, etc. from welding surface and on the area 2 inches wide on each side of the weld joint with suitable cleaning agent.
  • Eliminate all sources of low melting metal contaminants from paints, markers, dies, back up bars, etc. Chromium plate copper back up bars can form a barrier between copper and the weld surface. Copper can cause HAZ cracking in nickel alloys. These low melting contaminants cause cracking and failures in nickel alloy and stainless steel welds. Avoid using lead or copper hammers in fabrication shops.
  • Grind clean the surfaces and the HAZ areas. Chromium scales melt at higher temperatures than the base metals and will not be reduced by filler metals.
  • When welding to nickel alloy or stainless to plain carbon steel, the plain carbon steel must be ground on both sides too.

SHIELDING GASES
Bare wire welding requires a shielding gas to protect the weld from oxidation, loss of some elements to slag or oxide formation, and contamination.

Most stainless steel and nickel alloys require 100% argon for shielding for the GTAW or TIG process.

GMAW or MIG welding has two distinct modes of metal transfer. Spray arc processing transfers metal between wire tip and workpiece as droplets. Short circuit processing transfers the metal in sheets or globules. The most common shielding gas for spray arc GMAW welding is 100% argon. 10-20% helium can be added along with small amounts of carbon dioxide (1% max) to improve bead contour and reduce arc wander [1]. Short circuit GMAW welding uses blends of inert gases usually either 75% argon – 25% helium or 90% helium – 7.5% argon – 2.5% carbon dioxide.

In order to prevent hot cracking with the GMAW process, 602CA® requires a unique blend of 90% argon – 5% helium – 5% nitrogen and a trace (0.05%) carbon dioxide. This blend was trademarked as Linde CRONIGON® Ni30. It is not readily available but there are other close alternate quad gas blends that are commercially available. For GTAW welding, argon with 2.5% nitrogen is used to prevent cracking in 602CA. The nitrogen is the key to preventing cracking in 602CA regardless of method.

RESTRAINT AND DISTORTION CONTROL
Weld metal shrinks as it freezes. To accommodate the dimensional changes associated with freezing, either the base metal or the weld must move to prevent cracking or tearing. In complex assemblies with multiple welds, each weld, when solidified functions as a stiffener, further restricting movement of subsequent welds. In such cases, the most difficult or crack susceptible weld in the assembly should be made first and the easiest and strongest welds should be made last [5]. An example is shown in Figure 13 below.

Figure 13: Welding with multiple welds. In this example, the edge weld on the left would be the first weld made. The fillet weld in the middle should be the second made, and the butt weld on the right would be the last one made

When multiple tack welds must be made, they should be sequenced along the length of the plate [5]. Tack welding from one end to the other that is made in order will result in plate edges closing up as shown in Figure 14.

Figure 14: Tack welding in order along plate edge (left) can close up and distort the joint. Sequencing the tack welds (right) can greatly reduce distortion

Finally, multipass welds should be sequenced around the center of gravity of the joint as shown in Figure 15 below.

Figure 15: Proper sequencing of multipass welds

REFERENCES
[1] Schaefer, Anton L, Constitution Diagram for Stainless Steel Weld Metal. Metal Progress. ASM, Metals Park, OH. P 680-683. November 1949.
[2] Ogawa T. & Tsunutomi, E. Hot Cracking Susceptibility of Austenitic Stainless Steel. Welding Journal, Welding Research Supplement. P 825-935. March, 1982
[3] Li, L & Messler, R. W. The Effects of Phosphorous and Sulfur on Susceptibility to Weld Hot Cracking in Austenitic Stainless Steels. Welding Journal. Dec. 1999, Vol 78, No. 12.
[4] Kelly J. Heat Resistant Alloys. Art Bookbindery. Winnepeg, Manitoba, Canada. 2013
[5] Kelly J. RA330, Heat Resistant Alloy Fabrication. Rolled Alloys. Temperance, MI. May, 1999
[6] ASME Boiler and Pressure Vessel Code. American Society of Mechanical Engineers. New York, NY. 2013.

 

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