During the day-to-day operation of heat treat departments, many habits are formed and procedures followed that sometimes are done simply because that’s the way they’ve always been done. One of the great benefits of having a community of heat treaters is to challenge those habits and look at new ways of doing things. Heat TreatToday’s101 Heat TreatTips, tips and tricks that come from some of the industry’s foremost experts, were initially published in the FNA 2018 Special Print Edition, as a way to make the benefits of that community available to as many people as possible. This special edition is available in a digital format here.
Today we continue an intermittent series of posts drawn from the 101 tips. The tips for this post can be found in the FNA edition under Hardness Testing, CQI-9 Compliance, and Hardening/Tempering.
Heat TreatTip #22
Properly preparing a hardness sample can save time and money.
Inspection Mistakes That Cost
Rockwell hardness testing requires adherence to strict procedures for accurate results. Try this exercise to prove the importance of proper test procedures.
A certified Rc 54.3 +/- 1 test block was tested three times and the average of the readings was Rc 54 utilizing a flat anvil. Water was put on the anvil under the test block and the next three readings averaged Rc 52.1.
Why is it so important that samples are clean, dry, and properly prepared?
If your process test samples are actually one point above the high spec limit but you are reading two points lower, you will ship hard parts that your customer can reject.
If your process test samples are one point above the low spec limit but you are reading two points lower, you may reprocess parts that are actually within specification.
It is imperative that your personnel are trained in proper sample preparation and hardness testing procedures to maximize your quality results and minimize reprocessing.
Whether you need to meet rigid CQI-9 standards or not, what are the top 3, nay 4 best practices that nearly every in-house heat treat department ought to follow to make sure their pyrometer stuff is together?
Daily furnace atmosphere checks. Use an alternative method to verify your controls and sensors are operating properly and that there are no issue with your furnace or furnace gases.
Daily endothermic generator checks. Using an alternate method to verify your control parameter (dew point typically) or the gas composition is accurate will alleviate furnace control issues caused by bad endothermic gas.
Verify/validate your heat treat process every 2 hours OR make sure process deviations are automatically alarmed. this is a solid practice to ensure your controls and processes are running properly. This practice can help ensure that parts are being heat treated to the proper specification intended.
Conduct periodic system accuracy tests (SATs) per pre-defined timelines in CQI-9. Good pyrometry practices are an essential part of heat treatment. Because of the importance of temperature in heat treatment, ensure timeliness of all pyrometry practices addressing thermocouple usages, system accuracy tests, calibrations, and temperature uniformity surveys.
Control of Back Tempering With Induction Heat Treating
Induction heat treating is a selective hardening process. When hardening an induction path close to an area that had previously hardened, the heat from the hardening the second path tempers back the area that was previously hardened. This is a particularly common issue when tooth by tooth hardening of small gear teeth. Back tempering will reduce the hardness on the adjacent area and this effect may range from a few to over 10 HRC points.
Factors to Minimize Back Tempering
Process Issue
Questions to ask
Correct & repeatable placement of quenches
Can quench position be verified and set up repeatedly in the same position?
Verification of quench flow
Is the quench flowing freely through the quench system? Are the quench holes blocked? Are the flowmeters reading accurately?
Integrity of the quench
Was the percentage polymer measured? Is the quench quality okay? Is the quench contaminated?
Inductor design
Is the inductor designed to minimize heat on the tip? Is the quench effectively cooling the part?
Retained heat
Is a skip tooth hardening pattern being used to minimize residual heat in the induction hardening zone? Is the scan speed appropriate?
Induction Hardening Tips: Equipment Selection for Scan Hardening, Part 2
This is the second installment of a multi-part column on equipment selection for induction heat treatment. Part 1, Dr. Valery Rudnev On . . . Induction Hardening Tips: Equipment Selection for Scan Hardening, covered types of scanners, scan hardening system setup, quenching challenges, maximizing process flexibility, and computer modeling. In this installment, Dr. Valery Rudnev discusses another critical aspect of induction scan hardening: inductor design subtleties and a comparison of different fabrication techniques (brazing vs. CNC
machining vs. 3D printing).
Introduction
Hardening inductors are often considered the weakest link in an induction hardening system because they may carry significant electrical power and operate in harsh environments exposed to high temperatures, water, and other coolants while being subjected to mechanical movement and potential sudden part contact.
Single-turn or multiturn inductors may be used in scan hardening (Figure 1). Copper profiling and the number of turns is determined by the workpiece geometry, required hardness pattern, and the ability to properly load match the coil to the power supply without reaching the operational limits or by other specific process requirements, such as the production rate or the hardness pattern runout/pattern cutoff. [1]
Figure 1: Single-turn or multiturn inductors may be used in scan hardening.
The longer (in case of horizontal arrangement) or the higher (vertical arrangement) the scan coil is, the faster the scan rate can be. This is due to the simple fact that the longer inductor leads to a longer period when the part will be inside the coil; therefore, the scan rate can be greater. However, limitations on the maximum length of the inductor’s heating face may be associated with the maximum permissible runout.
Hardness Pattern Runout Control
Single-turn inductors with narrow heating faces (3mm-6mm wide) are used where a sharp pattern runout is needed. An example of this would be the case where a pattern must end near a snap ring groove. Inductors with wider heating faces or two-turn coils can be used when a faster scan rate is desired and an extended runout is permitted. The main disadvantage to the excessively wide heating face is that it may result in an unspecified shift of coil current density when hardening complex geometric parts due to an electromagnetic proximity effect. [1]
Inductor Fabrication Techniques
In applications where high process repeatability is critical (including automotive, aerospace, defense and other industries), the great majority of scan hardening inductors are CNC machined from a solid copper block, thus making them rigid, durable, and repeatable. CAD/CAM/CNC software programs are created that provide appropriate cutter-to-copper spatial relationships, which produce inductors of the required shape and precision regardless of complexity. Figure 2 shows a variety of finished and semi-finished CNC-machined hardening inductors. [2]
Figure 2: finished and semi-finished CNC-machined hardening inductors
In other cases, copper tubing (square, rectangular, round, or die-formed shaped tubes) may be used for coil fabrication (Figure 3). Copper tubing is typically annealed to improve its ductility, bending properties, and workability. When sharp bends or complex coil shapes are required, inductor segments made from tubing are assembled by brazing. Joints are often overlapped, creating tongue-and-groove joints. Butt-joints should not be used.
Figure 3: Copper tubing (square, rectangular, round, or die-formed shaped tubes) may be used for coil fabrication.
A complex geometry inductor that contains numerous brazed joints, and elbow-type 90° joints in particular, could experience impeded water flow in the cooling coil turns, shortening coil life. Poor quality brazed joints are prime candidates for water leaks affecting not only the coil life expectancy but also a quality of hardened components due to a potential soft spotting in the areas of water leaks. Eliminating braze joints or dramatically reducing their number, particularly in current-carrying areas, is the key to fabricating durable, reliable, and long-last inductors.
Additive manufacturing (AM), or 3D printing, delivers successful fabrication of fixtures, tooling, holders, etc. Recently, some inductors have been fabricated using 3D printing as well. It is important to keep in mind that AM is not a single technology but it comprises a number of processes including direct metal laser sintering, electron beam melting, directed energy deposition, direct and indirect binder jetting, and others.
Depending upon a particular AM technique used in fabricating hardening inductors, it may face major challenges to match properties of pure copper. This includes (1) obtaining sufficiently high thermal conductivity (2) or low electrical resistivity, (3) ensuring high volumetric density, and (4) having minimum amount of residuals, just to name a few. All these factors affect coil life. Therefore, if you compare 3D printed inductors with brazed coils comprising numerous brazed joints, in the majority of cases, the life of 3D printed coils will surpass life of brazed inductors because of elimination of brazed joints in current-carrying regions. In addition, fabrication accuracy and repeatability of AM inductors typically surpasses the accuracy of brazed or bended coils.
The situation is different when comparing life of 3D printed coils vs. CNC machined inductors. Fabrication accuracy of both processes is very similar, however, in high-power density applications even small degradation of above discussed four factors associated with AM might become essential causing greater probability of stress-fatigue and stress-corrosion copper failure of 3D printed coils compared to CNC machined inductors fabricated from pure copper. Another factor to consider is repairability of 3D printed inductors. If you need to do a revision then it would be most likely required you to re-manufacture 3D printed coils. Regardless of a fabrication method and for quality assurance purposes, it is beneficial to apply computerized 3D metrology laser scanner technology (Figure 4) to verify coil dimensional accuracy and alignment precision after inductor fabrication and assembly.
Figure 4: It may be beneficial to apply computerized 3D metrology laser scanner technology to verify accuracy and alignment after inductor fabrication and assembly.
Material Selection
Copper and copper alloys are almost exclusively used to fabricate induction coils due to their reasonable cost, availability, and a unique combination of electrical, thermal, and mechanical properties. Proper selection of copper grade and its purity is crucial to minimize the deleterious effects of factors that contribute to premature coil failure including stress-corrosion and stress-fatigue cracking, galvanic corrosion, copper erosion, pitting, overheating, and work hardening. Cooling water pH also affects copper susceptibility to cracking.
Oxygen-free high-conductivity (OFHC) copper should be specified for most hardening inductors. In addition to superior electrical and thermal properties, OFHC copper dramatically reduces the risk of hydrogen embrittlement and developing localized “hot” and “cold” spots. The higher ductility of OFHC copper is also important because coil turns are subjected to flexing due to electromagnetic forces. The higher cost of OFHC copper is offset by improved life expectancy of hardening inductor.
For scan inductors that are intended to heat fillets, an appropriate copper heating face region must be focused into the fillet area. Coil copper profiling and the use of flux concentrators (flux intensifiers) are beneficial to focus the magnetic field into the fillet. These applications require careful design because the induced current has a tendency to take the shortest path and stay in the shaft area rather than flowing into the fillet [1]. Therefore, all efforts must be made to focus the heat generation into the fillet. Typically, higher frequencies work better for this purpose.
Copper Wall Thickness
It is important to maintain sufficient wall thickness to carry the electrical currents. The wall thickness of an inductor’s heating face should increase as frequency decreases. This fact is directly related to both the current penetration depth in the copper δCu. [1] It is highly desirable for the current-carrying copper wall thickness to be 1.6 times greater than the δCu calculated at maximum working temperature. Increased kilowatt losses in the copper, which are associated with reduced coil electrical efficiency and greater water-cooling requirements, will occur if the wall is thinner than 1.6∙δCu.
The table below shows the variation of δCu vs. frequency at room temperature (20°C/68°F).
In some cases, the copper wall thickness can be noticeably thicker than the recommended value of 1.6∙δCu. This is because it may be mechanically impractical to use a tubing wall thickness of, for example, 0.25 mm (0.01 in.).
I recommend Reference #1 to readers interested in further discussion on design of hardening inductors.
Dr. Valery Rudnev, FASM, is the Director of Science & Technology, Inductoheat Inc., and a co-author of Handbook of Induction Heating (2nd ed.), along with Don Loveless and Raymond L. Cook. The Handbook of Induction Heating, 2nd ed., is published by CRC Press. For more information click here.
Induction Hardening Tips: Equipment Selection for Scan Hardening
Introduction
Induction scan hardening is one of the more popular techniques for strengthening various steels, cast irons, and powder metallurgy components. This scanning method is be used to harden flat surfaces or irregular shapes (e.g., rails, bumpers, bed-ways, support beams, track shoes for earth moving machines, teeth of large gears, etc.); however, it is most frequently used for hardening outside and/or inside surfaces of cylindrically shaped components, such as shafts, pins, raceways, etc. In scan hardening, the inductor or workpiece or both moves linearly relative to each other during the hardening cycle.
Depending on the workflow of parts, the induction system can be built as vertical, horizontal, or even at an angle, though vertical scan hardening is by far the most popular design. As an example, Figure 1 shows three variations of the InductoScan® family of modular vertical scan hardening systems.
Figure 1. Variations of the InductoScan® family of vertical modular scan hardening systems. (Courtesy of Inductoheat, Inc.)
What to Choose: Vertical Scanners vs. Horizontal Scanners
Both vertical and horizontal induction scanning systems are viable means to heat treat components. The decision of whether to use a vertical or horizontal scan hardening system is usually based upon the shape and length of heat treated parts, as well as the available space and a workflow throughout the plant or factory in which the equipment is to be installed. Horizontal hardening is often chosen when long workpieces are to be processed (typically 4ft/1.2m or longer) or when high production rates are needed for processing shorter parts.
Vertical scanners are typically associated with a smaller footprint. In the majority of applications, the cylinder-shaped workpiece (e.g., shafts) is positioned between centers or some other tooling or fixture. The workpiece may rotate inside the inductor to even out the hardening pattern around the circumference, or it may be located preferentially with respect to the inductor and processed without rotation when hardening workpieces of certain shapes. The quench spray typically impinges the part approximately 12mm (½”) to 40mm (1.5”) from the coil heating face and is angled away to prevent the quench from splashing back into the inductor. This dimension can vary with different types of steel, the scan rates, and the design specifics.
Setting Up Scan Hardening Systems
Vertical systems can be set up to process as many as four shafts at a time depending on the size of the shafts being processed and the available power source. Parts are loaded either manually or automatically onto a lower center. A loading assist “vee” block or nest may be used to steady the part as it is being loaded and processed. For larger parts, pneumatic cylinders lift the upper centers to facilitate loading. With vertical scan hardening, it may take an appreciable amount of time to process the workpiece because it must be loaded, scanned along the length up to the position where the heating process commences, fast scanned back down to the load-unload position, and then unloaded.
In contrast, a horizontal system is typically set up as a single continuous scanning line that allows parts to be loaded from a magazine and continuously fed to the exit of the machine. Depending on the specific heating requirements for the end of the component, parts are fed end-to-end through the heating coil and pass on to the next process. The loading system can push parts through the inductor by a pinch drive mechanism, conveyor, mechanical pushers, or other means, such as skewed rollers [1]. On a horizontal system, due to heavy duty roller support underneath, gravity, and any required stabilizing devices on top of the workpiece, the part is maintained in the center of the induction coil and quench ring. There is usually less risk of distortion than that which occurs with a vertical system where the part’s shape can change or warp if the part is not always centered.
However, during the heating process on a horizontal system, it may be more difficult to maintain the exact location of features of the part since it is commonly free rolling on the skewed rollers. For this reason, consideration should be given to a part’s shape, the symmetry of its positioning in respect to the heating coil, and selection of support devices. When horizontal systems are used for heat treating long parts of appreciable weight, it might be challenging to speed up or slow down the progress of the workpiece along the skewed rollers as quickly as might be done in vertical scanners with a servo-driven carriage that captures the part.
The roller system of horizontal scan hardeners can interfere with achieving symmetrical cooling of the workpiece since the location of the rollers and the rotation detection mechanism on shorter parts may be too close to the coil or quench barrel. Additionally, a stabilizing fixture may be required to prevent lighter and smaller workpieces from being moved axially by electromagnetic forces rather than the roller system. As with the vertical system, some type of rotation detection must be employed to ensure that the part is actually rotating as it is passing through the heating coil.
Quenching Challenges
Quenching presents a challenge with horizontal scanning [1]. When scanning vertically, quenching takes place below the inductor, which naturally allows gravity to pull the quench fluid down, therefore, the quench fluid continues to flow on the part long after it has passed the quench chamber, which is beneficial to achieving circumferential uniformity of quenching as well as reaching temperatures suitable for handling. When quenching horizontally, the effect of gravity is different and the way the quenchant falls from the workpiece varies leading to the probability of non-uniform cooling along the circumference of the heat-treated component (e.g., quenchant may run along the top of the part but fall off the bottom).
It is also more critical for horizontal scanners to maintain a sufficient distance between the inductor exit and the quenching device due to the higher probability of the liquid quenchant splashing back into the inductor. This could lead to irregular results caused by different cooling rates affecting the hardness consistency as well as the magnitude and distribution of residual stresses.
All of these factors can be summarized as follows:
The main process differences between vertical or horizontal scan hardening systems lie in the part handling and quenching subtleties.
With some scanners, splash shields, deflectors, and drip trays may be needed to prevent the backsplash of the quench fluids.
Maximizing Process Flexibility of Induction Scanners
It is commonly assumed that all scan hardening systems exhibit high process flexibility with respect to the workpiece length and, to some extent, variations in the diameter of the part. Conventional scan hardening provides the ability to vary the speed and power during the process, which controls the amount of heat applied to different areas of the part. Recently developed Statipower-IFP® inverter technology (Figure 2) extends the capability of conventional induction hardening systems to instantly and independently adjust not only power and scan rate but also frequency (5kHz to 60kHz range) during scan hardening cycle [2].
Figure 2. Statipower-IFP® inverter allows instant and independent adjustment of frequency (5kHz to 60kHz) and power during scan hardening cycle. (Courtesy of Inductoheat Inc.).
In the past, the flexibility of induction scanners was limited to using power supplies with single operational frequency. However, when processing a family of parts or components with numerous geometrical irregularities (including large diameter changes, multiple holes, sharp shoulders, combinations of solid and hollow areas, various required case depths, etc., see Figure 3), the fixed frequency in conventional induction scanners can be inadequate, producing “hot” and “cold” spots, as well as unwanted microstructures (e.g., local grain boundary liquation and grain coarsening).
Figure 3. A family of components exhibiting numerous geometrical irregularities
Single frequency scanners have been used to tweak the process in an attempt to promote or suppress thermal conduction [1,2], resulting in a compromise in achieving the desired metallurgical quality, production rate, and process capability. In the heating stage, compromise affects the ability to provide heat-appropriate austenization, but it also presents challenges in the quenching stage.
Austenization is followed by a quenching stage (spray or immersion). If the available, fixed frequency of a conventionally designed induction scanner is considerably higher than optimal then the depth of heat it generates (current penetration depth) is smaller than needed, which might not be sufficient in establishing necessary austenization. In this case, to reach sufficient austenization, the scan rate and applied power must be reduced to allow thermal conduction to the required subsurface depth. Unfortunately, a noticeable heat surplus might still occur.
An Example of Compromised Results
As an example, Figure 4 shows the computer modeling results of the induction scan hardening of a hollow medium carbon steel shaft that has diameter changes, a chamfer, and a groove. Nominal outside diameter is 0.05m (2”); nominal inside diameter is 0.02m (3/4”). Because the shaft is symmetrical, only the top half was modeled. Temperature variations at four selected areas of the shaft are monitored at different inductor positions. Frequency was constant at 15 kHz.
The scan rate and coil power were varied during hardening as an attempt to accommodate changes in the shape of the shaft.
Reducing scan speed (in some cases substantially) not only adds unnecessary cycle time, but if the scan speed is too slow, certain regions of a heat-treated component may cool below the critical temperature before it enters the quench zone, resulting in an undesirable formation of mixed structures and upper transformation products, as well as reduced or spotty hardness readings.
If the fixed frequency of a conventionally designed scanner is noticeably lower than optimal, it may produce a deeper than required austenized layer, affecting hardness depth, transition zone and creating excessive distortion. In this case, increasing scan rate and power density should minimize, but not eliminate, this outcome. Such a compromise can still affect local spray quenching producing undesirable metallurgical results.
Conclusion
It is important to remember that applied frequency has the greatest impact on depth of induction heat generation. A new generation of Statipower-IFP® inverters (Figure 2) eliminates these drawbacks by optimizing the metallurgical quality of induction scan hardening, expanding process flexibility and maximizing a production rate. This patented technology can be effectively used in both vertical and horizontal induction scanners. Reports [2] show changing both coil power and frequency during scan hardening can reduce peak temperatures on 70oC (125oF) while maintaining the required hardness pattern.
I recommend Reference #1 to readers interested in further discussion on induction scan hardening subtleties.
Doyon, V.Rudnev, C.Russell, J.Maher, Revolution-not evaluation-necessary to advance induction heat treating, Advance Materials & Processes, September 2017, p.72-80.
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Dr. Valery Rudnev, FASM, is the Director of Science & Technology, Inductoheat Inc., and a co-author of Handbook of Induction Heating (2nd ed.), along with Don Loveless and Raymond L. Cook. The Handbook of Induction Heating, 2nd ed., is published by CRC Press. For more information click here.
been done. One of the great benefits of having a community of heat treaters is to challenge those habits and look at new ways of doing things. Heat Treat Today’s 101 Heat Treat Tips, tips and tricks that come from some of the industry’s foremost experts, were initially published in the FNA 2018 Special Print Edition, as a way to make the benefits of that community available to as many people as possible. This special edition is available in a digital format here.
Heat Treat Tip #25
