Justin Sims

Fatigue Improvement for Gear Steels in Helicopter Powertrains, Phase 2

/ AEROSPACE HEAT TREAT, AEROSPACE HEAT TREAT TECHNICAL CONTENT, FEATURED NEWS, HARDENING TECHNICAL CONTENT, QUENCHING TECHNICAL CONTENT, TECHNOLOGY

OC"A compressive surface stress can benefit bend fatigue performance by reducing the mean stress experienced during service, effectively offsetting the tensile stress generated by the cyclic loading conditions." In this Technical Tuesday by Justin Sims of DANTE Solutions, learn how a simulation program, funded by the U.S. Army, modeled the method of Intensive Quenching®.

This article covers Phase 2 of the project, a follow up to an article that was previously featured on Heat Treat Today. Check out more original content articles in this digital edition or other editions here.

Justin Sims
Lead Engineer
DANTE Solutions

Helicopter powertrain gearing can be subjected to tremendous loads during service. The high tensile loads experienced in the root of the gear tooth, combined with the cyclic loading conditions inherent in gear operation, can lead to cyclic bend fatigue failures. To improve cyclic bend fatigue performance, low alloy steels are often carburized and quenched. The combination of a high carbon case and low carbon core leads to increased strength and hardness in the carburized case, while maintaining a tough core. In this manner, the case resists wear and can carry a high load without fracture, while the core is able to absorb the energy imparted to it during operation. Besides the increased strength and hardness, the addition of carbon creates a chemical gradient from the surface of the component towards the core. The carbon gradient creates delayed martensite transformations, relative to the low carbon in the core, and is responsible for imparting residual compressive surface stress. A compressive surface stress can benefit bend fatigue performance by reducing the mean stress experienced during service, effectively offsetting the tensile stress generated by the cyclic loading condition

Since the timing of the transformation to martensite is the main driver in the generation of compressive residual surface stresses, it is possible, to some extent, to control the magnitude of the surface stress by changing the quenching process. Historically, transmission gears have been carburized and quenched in oil. However, as more and more attention is paid to improving part performance through processing techniques, other forms of quenching have become available that show promise in increasing surface compressive stresses, and thereby improving bend fatigue performance. Of particular interest, is a quenching method which utilizes high pressure, high velocity water to quench parts.

Source: DANTE Solutions
Table 1. Pyrowear 53 nominal chemistry.

Known as Intensive Quenching®, the method was developed by Dr. Nikolai Kobasko as an alternative means of quenching components to achieve deep residual surface compression and improve bend fatigue performance.1–3

The technology works by inducing a large temperature gradient from the surface to the core of the component. In non-carburized components, the process has been shown to provide an extremely rapid and uniform transformation to martensite in the surface layers, while the core remains austenitic. This creates a hard shell, under extreme compression. As the part continues to cool, the surface is pulled into an even deeper state of compression. As the core transforms, some compression is lost due to the expanding core, but the compression that remains is generally greater than that achieved by oil quenching.4–7

Source: DANTE Solutions
Figure 1. Gear CAD model (left) and actual test gear (right).

To evaluate the possibility of improving bend fatigue of helicopter transmission gears, a program was conceived to compare the bend fatigue performance of carburized gears quenched in oil versus carburized gears quenched using the Intensive Quenching process. Funded by the US Army, the project was comprised of two phases. Phase 1, described in a previous Heat Treat Today article, was a proof-of-concept phase, designed to prove that intensively quenched components could outperform oil quenched components in high cycle bend fatigue testing. Phase 2 then moved to actual transmission gear testing. DANTE heat treatment simulation was used extensively throughout the project to guide processing decisions and understand the mechanisms responsible for improved bend fatigue performance though the creation of residual surface compression. This article will examine Phase 2 of the project.

DANTE Solutions
Table 2. Test gear specifications.

Pyrowear 53 was the material of choice for the project, as it is used extensively in helicopter power transmission gearing. Table 1 lists the nominal alloy chemistry for Pyrowear 53, which is a low-carbon, carburizing grade of steel. Figure 1 shows a CAD model of the test gear (left) and a picture of an actual test gear (right); the actual test gear is copper plated to selectively carburize only the gear teeth. The gears were carburized as one batch, and then hardened and tempered to a tooth surface hardness of 59 HRC and a core hardness of 42 HRC. An oil quenching process was used to harden half of the gears and an Intensive Quenching process was used to harden the other half of the gears. Table 2 lists the dimensional specifications of the gear.

One benefit of using the Intensive Quenching process over a conventional oil quenching process is the development of high residual surface compression. Compressive surface stresses benefit fatigue performance by offsetting any tensile stress generated during loading, effectively reducing, or eliminating, the tensile load experienced by the material. Figure 2 compares the residual stress predicted by DANTE for the test gear subjected to an oil quenching process (top) and an Intensive Quenching process (bottom). It is clear that the Intensive Quenching process induces a greater magnitude of compression in the area of the tooth root, which is the location of most gear bending fatigue failures. The residual stresses present in the tooth flank appear equivalent between the two quenching processes, but the oil quenched component has higher tensile stresses under the carbon case. This could lead to problems should any inclusions or material defects be present in that location.

Source: DANTE Solutions
Figure 2. Residual stress prediction for test gear, comparing oil quench and Intensive Quench.

Figures 3 – 5 compare the residual stress profiles of the two gears at three gear tooth locations: flank, root-fillet, and root, respectively. The residual stress profiles for the two processes at the tooth flank, shown in Figure 3, are equivalent, as inferred from the contour plots shown in Figure 2. Both quenching processes generate a surface compressive stress of 275 MPa on the tooth flank. However, the residual stress profiles in the root area of the gear vary greatly between the two processes. Figure 4 shows the residual stress profile at the root-fillet, which is the location of the highest tensile stress during gear service. At this location, the rapid surface cooling afforded by the Intensive Quenching processes creates a large temperature gradient from the surface to the core, allowing more thermal shrinkage to occur after the surface transforms to martensite. The additional thermal shrinkage, combined with the concave geometry of the gear root area, creates additional compressive stresses in this area.

Source: DANTE Solutions
Figure 3. Residual stress versus depth prediction for test gear at point A, comparing oil quench and Intensive Quench.

Figure 4 shows that the Intensive Quenching process generated a compressive stress of 700 MPa on the surface of the root-fillet, while the oil quenched gear produced a 500 MPa compressive surface stress in this location. The intensively quenched gear also has a deeper layer of high compression, not rising above 600 MPa compression until after 1 mm below the surface. Figure 5 shows a similar trend for the root, but with an even larger difference between the two quenching processes, since the geometry is even more concave at this location. Again, the gear subjected to the Intensive Quenching process has high compression up to 1 mm under the surface and a compressive surface stress magnitude 300 MPa higher than the oil quenched gear at the root location. The modeling results indicate that the intensively quenched gears should outperform the oil quenched gears in bend fatigue given the increased surface compressive stress present.

Figure 4. Residual stress versus depth prediction for test gear at point B, comparing oil quench and Intensive Quench.

Figure 5. Residual stress versus depth prediction for test gear at point C, comparing oil quench and Intensive Quench.

All of the hardened gears were tested at the Gear Research Institute, located at Pennsylvania State University in State College, PA, using a servo-hydraulic testing machine with a specially designed fixture to apply a cyclic bending load to two teeth. A schematic of the fixture is shown in Figure 6. A load ratio of 0.1 was used for all fatigue tests to ensure the gear did not slip during testing by having a constant tensile load applied. The fatigue test was considered successful, defined as a runout, if the gear completed 107 cycles given a certain maximum load. The maximum bending stress, calculated for a stress-free initial condition, was used to compare the two processes.

Figure 6. Schematic of fatigue testing apparatus.

As previously mentioned, the effect of residual compressive stresses during tensile bend fatigue is to offset the tensile stress generated by the load. Figure 7 shows a DANTE model of the test gear subjected to oil quenching showing the residual stress from heat treatment (top) and the stress redistribution during the application of a 900 lb. load (bottom). Figure 8 shows the same conditions for the test gear subjected to the Intensive Quenching process. As can be seen from the two figures, in which the legend ranges are the same, there is substantially more compressive stress remaining in the root-fillet area of the gear subjected to the Intensive Quenching process when the load is applied. This means the effective stress experienced by the intensively quenched gear is less than that of the oil quenched gear, given an identical load.

Source: DANTE Solutions
Figure 7. Stress predictions for the oil quenched gear, showing the residual stress from heat treatment (top) and the stress change when a 900 lb. load is applied (bottom).

Source: DANTE Solutions
Figure 8. Stress predictions for the Intensive Quenched gear, showing the residual stress from heat treatment (top) and the stress change when a 900 lb. load is applied (bottom).

Figure 9 shows the residual stress profile from the surface at the root-fillet for both processes, in the unloaded and loaded conditions. From the plot, a load of 900 lb. generates a tensile stress of approximately 200 MPa, which is offset by the compressive residual stresses. With a 900 lb. load, neither gear sees any tensile stresses during loading, and thus, should runout during fatigue testing.

Source: DANTE Solutions
Figure 9. Comparison of predicted stresses versus depth for the oil quench and Intensive Quench gears in the unloaded (Initial) and loaded (Final) state.

Figure 10 shows the results of the fatigue testing. As expected, the gears subjected to the Intensive Quenching process have an increase in fatigue performance. The endurance limit of the intensively quenched gears is approximately equal to the difference in surface compression, though additional tests should be conducted to confirm this. Regardless, increasing the magnitude of surface compression through a process change can significantly improve fatigue performance of power transmission gearing.

Figure 10. S-N curves for the oil quench and Intensive Quench gears tested.

In conclusion, achieving higher residual surface compressive stresses during hardening of a carburized power transmission gear by way of a process change was shown to improve bend fatigue performance. This was confirmed by the company's simulations, which showed a significant increase in compressive surface and near-surface stresses when the gear was quenched using the Intensive Quenching process, as opposed to an oil quench. The cause of the increased compression was determined from simulations to be due to the combination of martensite formation in the surface layers of the gear and the accompanying thermal shrinkage of the austenitic core, which draws concave geometric features, such as a gear tooth root, into a higher state of compression. The large temperature gradient induced during the Intensive Quenching process is necessary to produce these conditions. Physical fatigue testing confirmed the simulation results, showing a significant improvement in fatigue performance for the gears quenched using the Intensive Quenching process. Accurate process simulation pointed to a heat treatment process change that could be used to achieve increased power density through a transmission as opposed to more expensive and time-consuming design changes.

 

  1. N. I. Kobasko and V. S. Morganyuk, “Numerical Study of Phase Changes, Current and Residual Stresses in Quenching Parts of Complex Configuration,” Proceedings of the 4th International Congress on Heat Treatment of Materials, Berlin, Germany, 1 (1985), 465-486.
  2. N. I. Kobasko, “Intensive Steel Quenching Methods. Theory and Technology of Quenching”, SpringerVerlag, New York, N.Y., 1992, 367-389.
  3.  N. I. Kobasko, “Method of Overcoming Self Deformation and Cracking During Quenching of Metal Parts,” Metallovedenie and Termicheskay Obrabotka Metallov (in Russian), 4 (1975), 12-16.
  4.  M. Hernandez et al., Residual Stress Measurements in Forced Convective Quenched Steel Bars by Means of Neutron Diffraction”, Proceedings of the 2nd International Conference on Quenching and the Control of Distortion, ASM, (1996), 203-214.
  5. M. A. Aronov, N. I. Kobasko, J. A. Powell, J. F. Wallace, and D. Schwam, “Practical Application of the Intensive Quenching Technology for Steel Parts,” Industrial Heating Magazine, April 1999, 59-63.
  6. A. M. Freborg, B. L. Ferguson, M. A. Aronov, N. I. Kobasko, and J. A. Powell, Intensive Quenching Theory and Application for Imparting High Residual Surface Compressive Stresses in Pressure Vessel Components,” Journal of Pressure Vessel Technology, 125 (2003), 188-194.
  7.  B. L. Ferguson, A. M. Freborg, and G. J. Petrus, “Comparison of Quenching Processes for Hardening a Coil Spring,” Advances in Surface Engineering, Metallurgy, Finishing and Wear, SAE (01) 1373, (2002).

About the Author: Justin Sims has been with DANTE Solutions for eight years and is an excellent analyst and expert modeler of steel heat treat processes using the company's software. His project work includes development, execution, and analysis of carburization, nitriding, and quench hardening simulations. For more information, contact Justin at justin.sims@dante-solutions.com.

All images were provided by DANTE Solutions.

Fatigue Improvement for Gear Steels in Helicopter Powertrains, Phase 2 Read More »

Fatigue Improvement for Gear Steels in Helicopter Powertrains

/ AEROSPACE HEAT TREAT, FEATURED NEWS, QUENCHING TECHNICAL CONTENT

OCWhat do helicopter gears and heat treat modeling have to do with improving the bend fatigue performance of low-alloy gear steels? Find the answer in this interesting case study which analyzes the effects on compressive surface stress caused by changing the heat treating process.

This Technical Tuesday is provided by Justin Sims of DANTE Solutions and was featured in the Heat Treat Today's 2021 March Aerospace print edition. Check out more original content articles in this digital edition or other editions here.

Introduction

Justin Sims
Lead Engineer
DANTE Solutions

Helicopter powertrain gearing can be subjected to tremendous loads during service. The high tensile loads experienced in the root of the gear tooth, combined with the cyclic loading conditions inherent in gear operation, can lead to cyclic bend fatigue failures. To improve cyclic bend fatigue performance, low-alloy steels are often carburized and quenched. The combination of a high carbon case and low carbon core leads to increased strength and hardness in the carburized case, while maintaining a tough core. In this manner, the case resists wear and can carry a high load without fracture, while the core is able to absorb the energy imparted to it during operation.

Besides the increased strength and hardness, the addition of carbon creates a chemical gradient from the surface of the component towards the core. The carbon gradient creates delayed martensite transformations relative to the low carbon in the core and is responsible for imparting residual compressive surface stress. A compressive surface stress can benefit bend fatigue performance by reducing the mean stress experienced during service, effectively offsetting the tensile stress generated by the cyclic loading conditions.

Most gear steels contain enough alloying elements to guarantee a transformation to martensite upon quenching to room temperature from the austenite phase field. It is well known that the martensite starting temperature is significantly influenced by the amount of carbon in austenite at the time of transformation, with higher amounts of carbon generally lowering the martensite start temperature. This means the chemical gradient present after carburizing creates a nonuniform phase transformation, with the transformation starting at the base carbon just below the carburized case and progressing inward toward the core.

As the martensite is formed, the atomic rearrangement results in a volume expansion, causing a tensile stress to form on the surface as the core material pushes out on the surface. As the component continues to cool, the martensite start temperature is reached in the carbon rich case, usually well after the core has transformed to martensite or bainite, depending on the cooling rate. The transformation in the case progresses outward, with the surface being the last to transform. This core-to-surface transformation results in a compressive surface stress since the volumetric expansion created by the martensite transformation at the surface is constrained by the core material.

Because the timing of the transformation to martensite is the main driver in the generation of compressive residual surface stresses, it is possible, to some extent, to control the magnitude of the surface stress by changing the quenching process. Historically, transmission gears have been carburized and quenched in oil. However, as more attention is paid to improving part performance through processing techniques, other forms of quenching have become available that show promise in increasing surface compressive stresses, and thereby improving bend fatigue performance. Of particular interest is a quenching method which utilizes high pressure, high velocity water to quench parts.

Figure 1. DANTE residual stress predictions comparing a gear subjected to oil quenching and intensive quenching

Known as Intensive Quenching®, the method was developed by Dr. Nikolai Kobasko as an alternative means of quenching components to achieve deep residual surface compression and improve bend fatigue performance.1-3 The technology works by inducing a large temperature gradient from the surface to the core of the component. In non-carburized components, the process has been shown to provide an extremely rapid and uniform transformation to martensite in the surface layers, while the core remains austenitic. This creates a hard shell under extreme compression. As the part continues to cool, the surface is pulled into an even deeper state of compression. As the core transforms, some compression is lost due to the expanding core, but the compression that remains is generally greater than that achieved by oil quenching. 4 – 7

To evaluate the possibility of improving bend fatigue of helicopter transmission gears, a program was conceived to compare the bend fatigue performance of carburized gears quenched in oil versus carburized gears quenched using the Intensive Quenching process. Funded by the U.S. Army, the project was comprised of two phases. Phase One was a proof-of-concept phase, designed to prove that intensively quenched components could outperform oil quenched components in high cycle bend fatigue testing. Phase Two then moved to actual transmission gear testing. DANTE Solutions Inc. heat treatment modeling was used extensively throughout the project to guide processing decisions and understand the mechanisms responsible for improved bend fatigue performance through the creation of residual surface compression. This article will explore Phase one, with Phase two covered in a follow up article.

Phase One

Before any testing was initiated, the company heat treatment simulation was executed to compare the residual stress induced in a gear tooth root from oil quenching and Intensive Quenching. As can be seen in Figure 1, using Intensive Quenching significantly increased the near surface residual compression. This increase in compression should result in an increase in bend fatigue performance. Satisfied with these preliminary results, a testing regiment was initiated.

Table 1. Pyrowear 53 base chemistry

Figure 2. Coupon dimensions, selectively carburized surface, and finite element model

The steel alloy Pyrowear® 53 was chosen as the candidate material for this project. Table 1 shows the base chemistry of Pyrowear 53. The alloy is used extensively in the aerospace industry as a transmission gear material due to its ability to resist softening at high temperature in the hard carburized case, while maintaining high core impact strength and fracture toughness. A specially designed “V” notch 3-point bend fatigue sample was created by the company in conjunction with input from experts at the Army Gear Research Lab at NASA-Glenn and Bell Helicopter. The design was chosen to mimic behavior of a gear tooth root during loading. Figure 2 shows the dimensions of the coupon, the selectively carburized surface, and the finite element model used to explore the effects of process parameter changes on residual stress.

Figure 3. Schematic of intensive quenching orientation for Phase 1 study

A total of 40 coupons were manufactured and selectively carburized. The coupons were then split into two groups. Both groups were subjected to the same 1674°F (912°C) austenitizing, - 110°F (-79°C) cryogenic treatment, and double temper at 450°F (232°C). However, the two groups differ in the method of quenching, with one group quenched using the standard oil quenching practice for Pyrowear 53 and the second group quenched using the Intensive Quenching method. The two groups were processed separately. The Intensive Quenching unit utilized in this project uses a high velocity water stream to quench one component at a time. Figure 3 shows the coupon orientation within the intensive quenching unit. The blue arrow indicates the direction of water flow over the coupon.

After processing all of the coupons and modeling the two processes using the same heat treatment simulation software, a comparison was made between the two processes and the simulation. Figure 4 shows the hardness profile comparison at the center of the notch. As seen, the hardness profiles are equivalent between the two processes. This is expected as the carbon and other alloy content in the material is identical between the two processes. The simulation also matches the experimental data well. While the hardness profiles are identical between the two processes, the residual stress profiles at the center of the notch are not the same, as shown in Figure 5. The intensively quenched coupon has a surface compressive stress of 800 MPa, more than double the compression induced by oil quenching. However, at 0.4 mm, the profiles converge. This is significant as the surface can now carry a higher load, yet no detrimental effects are seen subsurface. Again, the simulation matches the experimental results well.

Satisfied with the increased surface compressive stress gained through the use of Intensive Quenching, 3-point bend fatigue testing was initiated at Case Western Reserve University. Load control was applied, with a minimum to maximum load ratio of 0.1 used to maintain a state of cyclic tension. This type of loading ensures the sample remains stationary throughout the duration of the test.

Figure 4. Phase 1 hardness profile comparison between oil quench, Intensive Quench, and DANTE simulations of the two processes

Figure 5. Phase 1 residual stress profile at the notch center comparison between oil quench, intensive quench, and DANTE simulations of the two processes

Figure 6 shows the results of the bend fatigue testing. It appears from Figure 6 that the increased residual surface compression of the intensively quenched coupons contributed to an increase in bend fatigue performance when compared to the oil quenched samples. However, some scatter does exist. Several parameters could have influenced these results.

First, during coupon manufacturing, the notch was created in the coupon using a milling operation and then heat treated. After heat treatment, no finishing operation was performed on the notch. Therefore, the possibility of surface defects existed. Any surface defect can create a stress riser, creating a stress condition which exceeds the expected stress given the loading conditions and geometry. However, surface defects would not be consistent coupon to coupon, and therefore have the potential to skew fatigue results.

Figure 6. Phase 1 bending fatigue comparison between oil quench and intensive quench

The second parameter that could have influenced the scatter in the fatigue results is related to the intensive quenching process itself. The process is dependent on a steep temperature gradient to generate the greatest level of compressive stress. This requires high velocity water to impact the component quickly, as any delay or low velocity water impingement can create shallow temperature gradients. Using the DANTE software, it was determined that in order to generate the greatest amount of surface compression, full flow must be achieved in a maximum of one second. This was a significant discovery that may have gone unnoticed if simulation was not used to explore process parameter sensitivities. It was unclear if the equipment operation met this maximum time restraint during processing of all coupons. However, due diligence was given to system operation in future experiments with improved consistency.

Figure 7. Schematic of intensive quenching orientations for Phase 1A study

Another processing parameter that has the potential to influence residual stress generated during an intensive quenching operation is the orientation of certain geometric features relative to the high velocity water flow. Again, the DANTE software was utilized, in lieu of expensive physical testing, to determine the optimum orientation of the fatigue sample in the intensive quenching unit. Figure 7 shows the three orientations evaluated. The orientation in Figure 7(A) has the water impinging on the notch surface and Figure 7(B) has the water impinging on the side of the coupon, with water flowing parallel to the notch. Recall that the original coupon orientation, shown in Figure 3, has the water impinging on the top of the coupon and flowing perpendicular to the notch. The final configuration, shown in Figure 7(C), places two coupons in the chamber side-by-side. This configuration has the potential to create an even steeper thermal gradient within the coupon due to the two coupons sharing thermal energy from being in contact with one another, and thus having a slower cooling rate in the core than a single coupon.

Figure 8 shows the surface residual stress across the width of the notch center, as shown by the red arrow in the Figure 8 inset, for the three orientations predicted by the simulation. Of the three orientations evaluated, orientation (A) resulted in the greatest magnitude of compression, as well as remaining the most consistent across the width of the notch. The residual stress contour plots of the three orientations, shown in Figure 9, confirm the uniformity of the residual stress profile across the width of the notch for orientation (A). The other two orientations show markedly reduced compressive surface stress near the edges of the notch. This type of profile would most likely fail in fatigue at those locations with reduced surface compression. To achieve the most consistent performance results, the most uniform surface condition should be sought.

Figure 8. DANTE residual stress profile predictions across the width of the notch center, as shown schematically in the figure inset, for the Phase 1A study

Figure 9. DANTE residual stress predictions for the Phase 1A study

The residual stress profiles at the center of the notch are shown in Figure 10 for oil quenched coupons, intensively quenched coupons with orientations (A) (“IQ-face”) and orientation (B) (“IQ-side”), and the company simulation results for the three processes. As predicted by simulation, and confirmed by X-ray diffraction measurements, the intensively quenched coupon in orientation (A) results in the highest magnitude of residual surface compressive stress, as well as having the deepest compression. The measurements also revealed that intensively quenching the coupon geometry in orientation (B) results in a slight increase in surface compression, when compared to oil quenching, but the compression is reduced much quicker in the orientation (B) coupon. Based on the simulation results, it was surmised that orientation (A) would outperform orientation (B) in bend fatigue, and oil quench would outperform orientation (B). Due to the poor residual stress distribution predicted for orientation (C), no coupons were processed in this orientation.

Figure 10. Residual stress profile measurements and predictions at the notch center for orientations A and B of the Phase 1A study

Figure 11 shows the bend fatigue results for the oil quenched coupons and the intensively quenched coupons in orientation (A) (“IQ-Face”) and orientation (B) (“IQ-Parallel”). As predicted from information gleaned from the DANTE simulation, orientation (A) outperformed the oil quenched coupons. The orientation (A) coupon recorded an endurance limit of approximately 1800 MPa, while the oil quenched coupons recorded an endurance limit of approximately 1600 MPa. This difference is approximately equal to the difference in near surface compressive stress induced by the two processes. The orientation (B) coupons failed to successfully complete a test at the loads chosen. Convinced that increasing the magnitude of surface compression through a process change could improve fatigue performance in transmission gears, Phase Two was initiated to evaluate the process change on an actual gear component.

Figure 11. Phase 1A bending fatigue comparison between oil quench and Intensive Quench

Conclusion

In conclusion, a project was launched to use heat treatment modeling, in conjunction with physical testing, to determine the effects of a process change designed to induce a greater magnitude of compressive surface stress to improve bend fatigue performance of a low-alloy gear steel. Pyrowear 53 was chosen as the gear steel and Intensive Quenching was chosen as the process change to induce a greater magnitude of residual surface compressive stress. Before any testing was initiated, DANTE modeling was used to show that intensive quenching could indeed produce a greater magnitude of surface compression, possibly improving bend fatigue performance by introducing a compressive mean stress and lowering the actual stress witnessed by the component. This modeling was also used to determine the maximum amount of time which may be used by the intensive quenching equipment to reach a full flow condition and still produce an increase in residual surface compression, as well as evaluate the residual stress profile of several different intensive quenching orientations.
Using this modeling to direct physical testing, hardness, residual stress, and bend fatigue performance were evaluated in coupons quenched in oil and coupons intensively quenched in three different orientations. The fourth orientation was not tested as modeling showed the residual stress profile to be unfavorable. Physical testing confirmed the modeling results: hardness profiles are equivalent between the processes, and residual stress profiles coincide with modeling results. Bend fatigue performance was indeed increased by increasing the magnitude of surface compressive stress. Phase One of the project showed that bend fatigue performance was improved by increasing the magnitude of the part’s surface compressive stress and demonstrated that modeling can be an invaluable tool when evaluating process parameter changes on material performance.

References

1. N. I. Kobasko and V. S. Morganyuk, “Numerical Study of Phase Changes, Current and Residual Stresses in Quenching Parts of Complex Configuration,” Proceedings of the 4th International Congress on Heat Treatment of Materials, Berlin, Germany, 1 (1985), 465-486.

2. N. I. Kobasko, “Intensive Steel Quenching Methods. Theory and Technology of Quenching”, SpringerVerlag, New York, N.Y., 1992, 367-389.

3. N. I. Kobasko, “Method of Overcoming Self Deformation and Cracking During Quenching of Metal Parts,” Metallovedenie and Termicheskay Obrabotka Metallov (in Russian), 4 (1975), 12-16.

4. M. Hernandez et al., Residual Stress Measurements in Forced Convective Quenched Steel Bars by Means of Neutron Diffraction”, Proceedings of the 2nd International Conference on Quenching and the Control of Distortion, ASM, (1996), 203-214.

5. M. A. Aronov, N. I. Kobasko, J. A. Powell, J. F. Wallace, and D. Schwam, “Practical Application of the Intensive Quenching Technology for Steel Parts,” Industrial Heating Magazine, April 1999, 59-63.

6. A. M. Freborg, B. L. Ferguson, M. A. Aronov, N. I. Kobasko, and J. A. Powell, Intensive Quenching Theory and Application for Imparting High Residual Surface Compressive Stresses in Pressure Vessel Components,” Journal of Pressure Vessel Technology, 125 (2003), 188-194.

7. B. L. Ferguson, A. M. Freborg, and G. J. Petrus, “Comparison of Quenching Processes for Hardening a Coil Spring,” Advances in Surface Engineering, Metallurgy, Finishing and Wear, SAE (01) 1373, (2002).

 

About the Author: Justin Sims has been with DANTE Solutions for eight years and is an excellent analyst and expert modeler of steel heat treat processes using the DANTE software. His project work includes development, execution, and analysis of carburization, nitriding, and quench hardening simulations. He has developed the DANTE HELP packages and is the primary trainer and software support person for the DANTE software.

All photos provided by DANTE Solutions.

Fatigue Improvement for Gear Steels in Helicopter Powertrains Read More »

Case Study: The Low-Pressure Carburizing Process Improvement for a Ring Gear

/ CARBURIZING TECHNICAL CONTENT, FEATURED NEWS, MANUFACTURING HEAT TREAT

Justin Sims
Lead Engineer
DANTE Solutions

OC“The original LPC schedule, consisting of six boost-diffuse steps, was producing large amounts of carbides during the process. With large amounts of primary carbides in the case of the heat treated gear, rolling contact fatigue performance was decreased.”

Heat Treat Today‘s Technical Tuesday feature, “Low Pressure Carburizing Process Improvement for a Ring Gear: Controlling Carbide Formation during LPC,” explores a case study, written by Justin Sims, lead engineer at DANTE Solutions, about how software modeling aids heat treaters in improving their low pressure carburizing process. Enjoy today’s Original Content.

Introduction

Low pressure carburizing (LPC) processes are becoming more widespread throughout industry due to the reduced cycle times and the control over the carbon profile through the case. Unlike gas carburizing, which utilizes a constant carbon potential to maintain the available carbon on the part surface at a specific value, LPC utilizes a series of boost and diffuse steps. A boost step involves the temporary addition of a carbon carrying gas to the furnace chamber, usually acetylene, to increase the surface carbon to the saturation limit of austenite. If not properly controlled, the carburized case may have an excessive amount of carbon, which damages the final microstructure. After a requisite amount of boost time, generally half minute to several minutes, the carbon carrying gas is evacuated from the chamber. The concentrated carbon in the shallow surface layer from the boost step is then allowed to diffuse into the part, reducing the surface carbon. These two steps are then repeated until the required case depth and carbon profile are achieved.

For steel alloys that do not contain a significant amount of strong carbide forming elements, the LPC process is relatively easy to control. However, with the advent of high strength steels for the aerospace industry, most of which contain substantial amounts of strong carbide forming elements, such as chromium, molybdenum, and vanadium, the LPC process can be challenging. The primary carbides formed during the LPC process, if not properly dissolved, can damage fatigue performance.

While Fick’s Second Law describes the diffusion of carbon through a low alloy steel with reasonable accuracy, the same is not true of medium and high alloy steels. This is due to the presence of carbides forming and dissolving during the LPC process. During a boost step, the carbides formed increase the total amount of carbon into the surface. During the diffuse step, as the carbon that is in solid solution diffuses into the part, reducing the carbon in austenite, the carbides can dissolve to provide more carbon to the solid-state solution. If the carbides are not allowed to fully dissolve or shrink to a significantly small size before the next boost step begins, they will continue to grow. In order to properly predict the carbon profile of medium and high alloy steels, the carbide formation and dissolution must be considered. The heat treatment simulation software DANTE has implemented this feature.

The following is a case study for redesigning a LPC schedule of a ring gear using DANTE. The original LPC schedule, consisting of six boost-diffuse steps, was producing large amounts of carbides during the process. With large amounts of primary carbides in the case of the heat treated gear, rolling contact fatigue performance was decreased.

Image 1

Geometry and Model

Part: Ring Gear

  • Material: Ferrium C64
  • Outer Diameter: 5.5 inches
  • Inner Diameter: 4.5 inches
  • Height: 0.060 inches
  • Number of Teeth: 40

Model: Single Tooth

  • Cyclic Symmetry: Carbon boundary conditions
    act uniformly on all teeth
  • Number of Elements: 233,850 linear hexagonal
  • Number of Nodes: 245,055
  • Higher mesh density near surface to capture
    steep carbon gradients

 

LPC Experiments vs. Prediction

Image 2

  • Experimental data versus DANTE prediction for 3 LPC runs
  • LPC experiments conducted using a cylinder with a 4-inch OD and a 4-inch height made of Ferrium C64
  • 3 different boost-diffuse schedules executed
    • 6 boost-diffuse steps
    • All 3 schedules used the same first 11 steps
    • Final diffuse time increased for each run, with Run 1 having the shortest and Run 3 having the longest
  • LECO used to measure the carbon profile of the test coupons
  • DANTE model parameters for carbon diffusivity, carbide formation, and carbide dissolution fit from experimental data
    • Simulation matches experimental data reasonably well

 

Baseline (Original Carburizing Process)  Model Results

  • The case depth originally was designed for 0.75 mm (0.030 inch) on the flank of the tooth, with a carbon value of 0.3% resulting in a hardness value of 50 HRC for Ferrium C64 when tempered at 495°C (925°F).
  • The contour plot shows all carbon, the carbon in the austenite matrix and the carbon in primary carbide form, at the end of the process for the baseline model:
    • Areas above 0.011 carbon contain primary carbides
    • Tip contains a high amount of primary carbides
  • Line plot shows the predicted carbon in the austenite matrix (Carbon) and the carbon in primary carbide form (Carbon in Carbides) at the surface of the flank for the baseline model over the total time of the process.
    • Carbides present at a depth of 0.25 mm (0.010 inch)
    • Case depth ~0.35 mm deeper than required

Image 3

Image 4

 

 

 

 

 

 

 

  • Contour plot shows all carbon, the carbon in the austenite matrix and the carbon in primary carbide form, at the end of the 3rd boost and diffuse steps
  • Line plot shows the predicted carbon in the austenite matrix (Carbon) and the carbon in primary carbide form (Carbon in Carbides) at the surface of the flank for the baseline model over the total time of the process
    • Carbides formed during the first boost step continue to grow as the process progresses, indicated by the increasing carbon in carbide
    • Final diffuse not long enough to fully dissolve carbides

Image 5

Image 6

Image 7

Redesigned Carburizing Process Model Results

Image 8

  • To ensure the primary carbides dissolve completely before hardening, a new schedule was developed with the aim of reducing the carbon in primary carbide form:
    1. 3 boost-diffuse steps were removed, and the diffuse times increased substantially.
    2. An increase in diffuse time increased the schedule by approximately one-half hour, which is acceptable given the positive results.
  • The contour plot shows all carbon, the carbon in the austenite matrix, and the carbon in primary carbide form at the end of the process for the redesigned process model.
  • Line plot shows the carbon in the austenite matrix (Carbon) and the carbon in primary carbide form (Carbon in Carbide) from the surface of the flank towards the core for the redesigned model at the end of the process
  • Small carbides (negligible) at a depth of 0.1 mm (0.004 inch)

Image 9

  • Easily removed with finish grinding operation
  • Contour plot shows all carbon, the carbon in the austenite matrix and the carbon in primary carbide form, at the end of the 2nd boost and diffuse steps for the redesigned process
    • Primary carbides are nearly fully dissolved, even in the tip (carbon is higher, but it is not in carbide form), at the end of the diffuse step

Image 10

Image 11

 

 

 

 

 

 

 

  • Line plot shows the predicted carbon in the austenite matrix (Carbon) and the carbon in primary carbide form (Carbon in Carbides) at the surface of the flank for the redesigned model over the total time of the process
    • Carbides are nearly fully dissolved after each diffuse step

 

Image 12

Summary

  1. The heat treatment simulation software DANTE model parameters for carbon diffusivity, carbide formation, and carbide dissociation fit from experimental data.
    • Any steel alloy and LPC equipment can be fit to the DANTE carburizing model.
  2. The software successfully predicted the results of a low-pressure carburizing process that was resulting in poor part performance during rolling contact fatigue:
    • Model showed that large primary carbides exist at a depth of 0.25 mm (0.010 inch).
    • Model showed that the carbides do not have time to dissolve during the boost steps.
  3. The software was used to successfully redesign the boost-diffuse schedule to improve rolling contact fatigue performance:
    • Model showed that small primary carbides (negligible) exist at a depth of 0.1 mm (0.004 inch).
    • Model showed that the carbides nearly fully dissolve during the diffuse steps.
    • Small carbides were removed during the finish grinding operation.
    • Rolling contact fatigue performance improved due to the absence of primary carbides near the surface.
  4. Additionally, the software is not limited to Ferrium C64 with respect to primary carbide formation during LPC:
    • Continually updating the material database with carbide behavior for different alloys
    • Continually validating the model with experiments

 

About the Author: Justin Sims is a lead engineer at DANTE Solutions. For more information, contact Justin at DANTE Solutions

All images were provided by DANTE Solutions.

 

Case Study: The Low-Pressure Carburizing Process Improvement for a Ring Gear Read More »

Process Innovation to Reduce Distortion During Gas Quenching

/ AEROSPACE HEAT TREAT, FEATURED NEWS, QUENCHING TECHNICAL CONTENT

“High-pressure gas quenching (HPGQ) attempts to reduce temperature nonuniformities by reducing the cooling rate; however, this is generally not sufficient to eliminate shape change. Shape change can be predicted by heat treatment simulation software, but it is difficult to reproduce the exact same cooling conditions in the vessel for each batch. Therefore, the distortion of the components will not be consistent from batch to batch.”

Read the case study to see one response to this issue in this original content from Heat Treat Today by Justin Sims, lead engineer at DANTE Solutions.

This article first appeared in the latest edition (March 2020) of Heat Treat Today’s Aerospace Heat Treating magazine.

Distortion is generally described by a size change and a shape change. In heat treatment of steels, size change is unavoidable and is mainly due to the volumetric difference between the starting microstructural phase and the final microstructural phase. Shape change of steel parts from heat treatment is due to nonuniform thermal and nonuniform microstructural strains as a result of nonuniform cooling or heating, alloy segregation, poor support of the component while at high temperature, thermal expansion or contraction restrictions, or residual stresses from prior forming operations. Nonuniform cooling or heating can be as fundamental as the temperature gradient from the part surface to its core, or as complex as the flow of fluid around a component feature. Both can result in nonuniform strains, resulting in a shape change. If the stresses causing these strains exceed the yield strength of the material, then permanent shape change will occur. Size change can be anticipated and is predictable, while shape change, or distortion, is usually unanticipated and more difficult to predict.[1-2] 

Justin Sims,
Lead Engineer,
DANTE Solutions

Most thermal processes try to control these nonuniformities using methods of low complexity such as part orientation and rack design. Quenching systems, for example, are generally designed to remove as much thermal energy from the work pieces as possible and to do this as quickly as possible. High-pressure gas quenching (HPGQ) attempts to reduce temperature nonuniformities by reducing the cooling rate; however, this is generally not sufficient to eliminate shape change. Shape change can be predicted by heat treatment simulation software, but it is difficult to reproduce the exact same cooling conditions in the vessel for each batch. Therefore, the distortion of the components will not be consistent from batch to batch.

In response to this issue, a prototype gas quenching unit capable of controlling the temperature of the quench gas entering the quench chamber was devised. With the DANTE Controlled Gas Quench (DCGQ) unit, it is possible to have control of the thermal and transformation gradients in the component by controlling the temperature of the incoming quench gas, thereby significantly reducing, or eliminating entirely, the shape change caused by quenching. In doing so, the size change can easily be predicted by heat treatment simulation software, and post-hardening finishing operations can be reduced or eliminated. This process is ideal for thin parts or components with significant cross-sectional changes. Atmosphere Engineering (now part of United Process Controls) in Milwaukee, Wisconsin constructed the unit and provided the logic to control it. All experiments with the unit were conducted at Akron Steel Treating Company in Akron, Ohio. The project was funded by the U.S. Army Defense Directorate (ADD).

Figure 1 (left) shows the front of the unit, while Figure 1 (middle) shows the back of the unit. The back of the unit contains the human machine interface (HMI), shown in Figure 1 (right), where process parameters can be modified and DCGQ recipes entered. The prototype unit has a working zone of nine cubic ft. and is capable of quenching loads up to 100 lbs. at one atmosphere of pressure.

Figure 2. Comparison of quench gas temperature entering the
quench chamber versus the recipe setpoint temperature for
two different DCGQ process recipes

The ability of the unit to maintain continuity between the recipe setpoint temperature and the actual temperature entering the quench chamber is absolutely paramount. Figure 2 shows two schedules, one aggressive and one conservative, comparing the recipe setpoint (Chamber Inlet SP) to the actual quench gas temperature (Chamber Inlet PV). Figure 2 also shows that the prototype unit has good control of the quench gas temperature between 752°F (400°C) and room temperature, the martensite transformation range for most high hardenable steel alloys. There is some deviation between the two temperatures below 392°F (200°C) for the aggressive schedule as the setpoint reaches its set temperature, due to the relatively small temperature difference between the quench gas and the shop air. This small temperature difference makes it slightly difficult for the air-to-air heat exchanger used in the design to keep up with the rapid drop in temperature, but overall there is very good control of the quench gas temperature.

Figure 3. Micrograph of DCGQ (left) and HPGQ (right) processed coupons, mag. 1000X
There is a copper layer on the surface of the DCGQ processed coupon.

Microstructural examination was conducted on Ferrium C64 coupons processed using the DCGQ process and coupons processedusing a 2-bar HPGQ. C64 was chosen for this study due to its extremely high hardenability and its high tempering temperature. Figure 3 compares the microstructures of the two processes at a magnification of 1000X, and no significant difference is detected. The DCGQ coupons required two hours to complete the transformation, whereas the HPGQ coupons transformed in a few minutes. There is no indication that the slow rate of transformation damaged the microstructure or mechanical properties in any way. Tensile and Charpy properties were equivalent between the two processes.

Distortion coupons, thick disks with eccentric bores, were designed and manufactured with the goal of evaluating the distortion response when subjected to a DCGQ process, and then compared to coupons subjected to a standard 2-bar HPGQ operation. All coupons were manufactured from the same Ferrium C64 bar stock. All coupons were cryogenically treated and tempered at 595°C for eight hours after quenching.

Figure 4. Nomenclature and locations used for out-of-round measurements on the distortion coupon

Figure 4 shows a distortion coupon with the nomenclature and locations used for measuring the out-of-round distortion of the eccentric bore. Due to the uneven mass distribution, the north-south direction will generally be larger than the east-west direction. Five measurements were then made along the axis of the coupon using a Fowler Bore Gauge.

Table 1. Out-of-round distortion measurements of the distortion coupon for a DCGQ and HPGQ process

Table 1 shows the results from four coupons; two hardened using the DCGQ process and two processed using the standard 2 bar HPGQ for C64. The individual measurements (EW1, NS5, etc.) are relative and are dependent on the reference value used for the bore gauge. The individual measurements give an indication of the variation in distortion in the axial direction. The out-of-round measurements are actual values, as they are the difference between the actual measurements. The DCGQ process gave significantly less distortion than the HPGQ process.

While the values reported show a 50% reduction in out-of-round distortion for the DCGQ process, a larger gain could have been realized if two other conditions were addressed. First, the coupon for DCGQ was placed directly into a 1832°F (1000°C) preheated furnace since the prototype unit does not have austenitizing capabilities. Controlled heating, just like controlled cooling, should be utilized to realize the full potential of this process. Second, the DCGQ schedule was designed for another coupon geometry that was processed together with these distortion coupons. Therefore, the schedule was not optimum for this coupon geometry.

Table 2. DANTE simulation results comparing HPGQ and DCGQ using the experimental conditions and a DCGQ with optimized heating and cooling schedulesMARCH 2020

Table 2 compares the DCGQ simulation results in which the two processes executed on the experimental coupons were compared to an optimized process, including controlled heating and cooling schedules designed for this coupon. The optimized schedule predicts an order of magnitude reduction in out-of-round distortion. Comparison of the measurements from the HPGQ and DCGQ experiments in Table 1 to the model predictions in Table 2 shows that the model predictions agree closely with the experimental results.

Simulating the application of the DCGQ process to a gear geometry, the predicted warpage of a bevel gear was examined. The simulation looked at the differences between an oil quench, 10 bar HPGQ, and a 10 bar DCGQ process. From Figure 5, it is clear that the HPGQ process is predicted to produce the most distortion. Even though the 10 bar gas quench has a slower cooling rate than the oil quench, less distortion is not guaranteed since a slower rate does not guarantee a more uniform phase transformation.[3] In this case, both heating and cooling were controlled for the DCGQ simulation.

Figure 5. Comparison of oil quench, HPGQ, and DCGQ processes for a bevel gear

In summary, a prototype gas quenching unit has been constructed with the ability to accurately control the temperature of the quench gas entering the quench chamber. Experimental results have shown that mechanical properties and microstructure are equivalent between the DCGQ process and a 2-bar HPGQ process for Ferrium C64. Thick disks with eccentric bores were machined and then heat treated using DCGQ and HPGQ. It was shown that the DCGQ process reduced distortion in these disks by 50%. Simulation using DANTE then showed that the distortion could be reduced further if controlled heating and cooling are used. Finally, a comparison was made between an oil quench, HPGQ, and DCGQ processes for a bevel gear. This comparison showed that the HPGQ process was predicted to cause the most distortion. HTT

References

[1] Prabhudev, K.H., Handbook of Heat Treatment of Steels, Tata McGraw-Hill Publishing, 1988, p.111-114

[2] Sinha, Anil Kumar, ASM Handbook, Vol. 4: Heat Treating, ASM International, 1991, p.601-619

[3] Sims, Justin, Li Zhichao (Charlie), Ferguson B. Lynn, Causes of Distortion during High Pressure Gas Quenching Process of Steel Parts, Proceedings of the 30th ASM Heat Treating Society Conference, ASM International, 2019, p.228-236

 

About the Author: As an analyst of steel heat treat processes and an expert modeler of quench hardening processes, Justin Sims was the lead engineer for designing and building the DANTE Controlled Gas Quenching (DCGQ) prototype unit. This system was developed to minimize distortion of quenched parts made of high hardenability steels, while still achieving the required properties and performance.

For more information, contact Justin at DANTE Solutions

 

Process Innovation to Reduce Distortion During Gas Quenching Read More »

This Week in Heat Treat Social Media

/ FEATURED NEWS, MANUFACTURING HEAT TREAT

Welcome to Heat Treat Today's second installment of This Week in Heat Treat Social Media. As you know, there is so much content available on the web that it's next to impossible to sift through all of the articles and posts that flood our inboxes and notifications on a daily basis. So, Heat Treat Today is here to bring you the latest in compelling, inspiring, and entertaining heat treat news from the different social media venues that you've just got to see and read!

If you have content that everyone has to see, please send the link to editor@heattreattoday.com.

1. Plibrico Company Sponsors Project for Shriner's Hospitals for Children

The Plibrico Company recently sponsored a Happy Craft Day for Shriner's Hospitals for Children, during which many locations took part in assembling craft kits for kids needing a smile.

2. Innovations and Services on the Front Line

During this difficult and uncertain time, many companies are offering support to fight the spread of COVID-19, and some have come up with unique innovations.

Stack Metallurgical Group has announced its support for manufacturers in fighting the pandemic:

Similarly, Inductoheat has made a statement in the same vein:

ION HEAT has come out with the first prototype of its mechanic lung ventilator:

And Proceq USA Sales Manager Tom Ott demonstrates how to recharge a Proceq UT8000 flaw detector using a common USB power pack:

3. Good Friday Furnace Repair

Capital Refractories' Research & Development Manager Julie Hardy shared images of a 12 ton holding furnace repair that took place on Good Friday:

4. Reading and Podcast Corner

You may have a bit more time to catch up on the reading and podcast listening you've been yearning to do. May we recommend two brief written items of interest and an informative podcast.

Park Ohio Turns 100

Ipsen USA recommends their paper on vacuum furnace maintenance

And, for your listening pleasure, be sure to download the latest Heat Treat Radio episode entitled, Heat Treat Modeling with Justin Sims.

5. 101 Uses for Heat Treat Today Tape

Roseanne Brunello of Mountain Rep came up with a festive use of Heat Treat Today packing tape:

"Heat Treat Today comes through again..."

6. Launch into Your Socially Distanced Weekend with the Family Lockdown Boogie

No explanations necessary. Happy Friday, everyone!

 

This Week in Heat Treat Social Media Read More »

Heat Treat Radio #28: Heat Treat Modeling With Justin Sims

/ FEATURED NEWS, HEAT TREAT RADIO, MANUFACTURING HEAT TREAT, MANUFACTURING HEAT TREAT NEWS

Welcome to another episode of Heat Treat Radio, a periodic podcast where Heat Treat Radio host, Doug Glenn, discusses cutting-edge topics with industry-leading personalities. Below, you can either listen to the podcast by clicking on the audio play button, or you can read an edited version of the transcript. To see a complete list of other Heat Treat Radio episodes, click here.

Audio: Heat Treat Modeling With Justin Sims

In this conversation, Heat Treat Radio host, Doug Glenn, interviews Justin Sims of DANTE Solutions about heat treat modeling. As the heat treat world moves farther way from mysterious black box processes, find out how the latest advances in heat treat simulation software can help your company model specific processes and materials in advance, leading to less guesswork and more profit.

Click the play button below to listen.

Transcript: Heat Treat Modeling With Justin Sims

The following transcript has been edited for your reading enjoyment.

We're going to talk to Justin Sims, lead engineer at DANTE Solutions, Inc., about heat treat modeling.  It's a pretty interesting topic.  With all the advances and sensors and computing power, the heat treat world is moving further and further away from the mysterious black box processes of yesteryear and is allowing companies to model specific processes and specific materials in advance so that there is less guesswork and more profit.  DANTE provides the means by which companies can accurately predict what is going to happen to their part during the heat treat process.

DG: Justin is not only the lead engineer at DANTE Solutions, he is also the author of an article that just appeared in the March 2020 issue of Heat Treat Today and the title of the article was Process Innovation To Reduce Distortion During Gas Quenching. It was a pretty interesting article, something worth reading if you haven't already.  It has to do with DANTE controlled gas quench.

JS:  I got my bachelors in mechanical engineering degree from Cleveland State.  I graduated back in 2015.  I actually started interning at DANTE in 2014 and went full-time in 2016.  I've been the lead/principal engineer at DANTE with mainly responsibilities of managing projects, training our DANTE users, and offering support to our DANTE users.  I helped develop our patent-pending DANTE controlled gas quenching process, which you had just mentioned, and then also a little bit of IT, marketing, sales, and shipping. Being a smaller company, we can kind of do it all.

 

Fig. 1: Bevel gear axial distortion comparison for an oil quench, high pressure gas quench, and a DANTE Controlled Gas Quench

 

DG: Tell us briefly about DANTE.

JS: DANTE Solutions is an engineering consulting and software company.  We offer consulting services as well as licensing our software.  We mainly focus on the aerospace industry, the auto industry quite a bit as well, and we've been starting to get into the mining and energy sectors also.  As I said, we are a smaller company.  There are six of us right now. Two to three guys mainly focus on the software side, and the rest of us focus on more of the training, the support, and the consulting side of the business.

DG: DANTE is located near Cleveland, OH, and Lynn Ferguson, who has been in the heat treat industry for many, many years, was one of the founders. Let's talk about the genesis of the software. Would you say the software is the core product that DANTE Solutions offers?

JS: Yes, it is. We mainly stay in consulting to stay current and to give those users who don't have the capability to run our software (either they don't have the hardware or they don't have the analysts to be able to do such a thing), so we still offer our consulting services for them. But mainly, software is our main line of business.  DANTE was actually formed back in 1982 as Deformation Control Technology, Inc., and we changed our name in 2014 to actually reflect more of the software side, so that's when we changed to DANTE Solutions, Inc.

The project itself that DANTE came out of actually started in 1994 and 1995.  It was a collaboration between Ford, GM, Eaton Corp. and then four national labs--I believe they were Los Alamos, Sandia, Oak Ridge, and Lawrence Livermore--and then us as Deformation Control Technology.  The whole project came out because those large automakers were claiming millions of dollars of lost scrap from distortion.  It was starting to become a major issue and they wanted a way to be able to model the process and be able to optimize the process a little bit better.  After that project ended, DANTE somehow ended up with the software, which has worked out well, as we've been able to commercialize it and we've been updating all the material models and the material database for the last 20 years.  It's actually come quite a long way.

DG:  How did you segue over from auto industry into aerospace?

JS:  It just happens that the aerospace components cost a whole lot more than the auto industry components.  It was a natural fit once they realized that this software was viable and could do what they needed it to do.  And aerospace seems to be more receptive to modeling because their parts are so expensive.

DG:  Let's try to put a little flesh on the bones here. For a manufacturer who has their own in-house heat treat for aerospace, automotive, energy or whatever, what makes this software attractive?  What makes it viable?  Why would someone want it, and why and how do they use it?

JS:  Let's start with viability.  The first thing is that it is easy to use.  DANTE is a set of material routines that link with Abaqus or Ansys finite element solvers.  These are solvers that engineers and analysts in the industry already know pretty well, so there is not a lot of learning of new software.  DANTE is just a material model, so all you're really responsible for is the material name and what microstructural phases you're starting with.  Then we have the ability to modify a few of our control parameters, activating different models; we've introduced stress relaxation, carbon separation, carbide dissolution, and all these different models that you can activate.  But the biggest thing that trips people up . . . [is] understanding your process.  We like to work with people a lot on trying to help them understand what type of thermal behavior their processes are actually imparting on components.  We've done a lot of work with setting up their essentially quench probes and be able to turn around and be able to take that back to heat transfer coefficients that get put into the model. As far as DANTE is concerned, it is fairly easy to use.

We've also developed what they're calling ACT (Ansys Customization Toolkit).  It is essentially a series of buttons where you would click on these buttons, fill out information, and then essentially run your models. Abaqus, for the new version of DANTE, we've also developed a plug-in that essentially does the same thing.  So DANTE has become very point-and-click.  In this world, I think people like that simplicity.

Fig. 2: Axial distortion of a press quenched bevel gear

The next big one would be the accuracy that everybody is concerned about.  Our accuracy is due to the models that we use and the algorithms that we employ.  There are two types of accuracy.  I've touched on the boundary condition accuracy, and that is how your process behaves thermally.  That accuracy can be tough to get.  It's very doable and we've helped people achieve some really amazing accuracy.  The relationship I like to use here is people know static loading models and a lot of engineers have run static loading models.  The loads that you put on these static models are going to determine what deflections you get.  If your load is not correct, then your deflection will not be correct.  In heat treat modeling, the thermal boundary condition is your load.  The more accurate your heat transfer coefficient can be, the more accurate your results are.  But, with that being said, you can still gain a lot of valuable information from being close enough.  We'll talk a little bit about that with the uses and whatnot.

The first important model type that we use is the mechanical model.  We use a multiphase internal state variable model.  A conventional plasticity model considers stress as a function of strain only, where the internal state variable model actually accounts for the history of deformation by relating the stress to dislocation density.  It actually accounts for the history of deformation, which is very important as the steel goes through all the stress reversals that it does going through the process.  Our mechanical model defines each phase, so austinite, pearlite, ferrite, bainites martensite, tempered martensite, all of them, as a function of carbon, temperature, strain and strain rate.  It also accounts for the trip phenomenon.

For our phase transformation model, we like to use analytical models instead of TTT CCT diagrams, and we do this because you don't get any transformation strain information out of the diagram.  So you have no idea how much it is deforming.  In order to figure that out, we like to use dilatometry tests to fit to our analytical models.  We also account for carbide growth and dissolution during carburizing, which is becoming a major point of interest due to the high alloy content of some of these steels that they're now trying to carburize.

DG:  Let's talk a bit more about where manufacturers, who have their own in-house heat treat, might use DANTE's software tool.

JS:  One of the big things we like to use it for is what we call sensitivity analysis.  This would be, "what happens if my normal process has a little bit of variation?"  Or, "what happens if my process parameters change a little bit?" We've also worked into the model now normal material variation.  So if your alloy content is a little on the high side, how would the material behave?  If it's a little on the low side, how will it behave? [This] is a big deal.  One example would be, "I just designed a new part and I want to make sure that it behaves given the range that I know my process can vary."  All processes will vary.  This is no way to make the process exactly the same every time.  Also, in the sensitivity, you can ask the question, “What process variable is a distortion or stress most sensitive to?"  By finding out what process variables cause the most sensitivity, then those are the process variables you really need to pay attention to during processing, then the other ones you can just make sure they're in range and leave them alone.

Development and design are two of the big ones that we're trying to get out there that this software can be used for.  Everybody knows that it can be used for troubleshooting.  Once something goes wrong, yes, sure the software is great and we help figure out a problem; but why not find the problem before it ever even happens?  We've been trying to get people to use it for development of new carburizing and nitriding schedules as well as new recipe and design, and even novel processes.  You had mentioned our DANTE controlled gas quench.  That actually was conceived through all the modeling that we do and watching the response of the material and saying, “Wait a second.  If we can control the martensite transformation rate, we can really control the distortion, so let's see if we can do this.”  Things like that can come out of the software.  Design as well, of optimizing shapes for quench.  You can even do quench to fit, which is, "I know my part distorts this much, so let me machine it distorted and then it will fall into shape."  Optimizing processes.  All of that can be done through design development, and you can find these problems before they ever happen.

Another really big one that I like, and Lynn, our owner, is really keen on this one, is the understanding of your process.  When you start to set up these models, you have to ask a lot of questions about your process.  What is the HTC of my process, which relates back to agitation in the tanks, part racking, flow directions?  You really need to know times and temperatures of every step in your process.  So not just the heat to quench, but what about all those transfers in between?  All of that needs to be done.  So you end up asking a lot of questions like that.

The other one that I always like to say is that the heat treat software removes the black box.  In the past, you know what goes in and you know what comes out, but what happens in the middle is kind of a mystery.  The software helps you figure out what exactly goes on during your process.  It can be very eye-opening.

Fig. 3: Minimum Principal stress of a carburized and oil quenched spur gear

DG:  I've talked with James Jan and Andrew Martin over at AVL, and we talked about a variety of ways they use some of their software, and they mentioned that they work with you guys as well, and they were talking about not even just like a quench agitation, flow direction, and things of that sort, but part orientation as it goes into a quench.  I assume that would be something also that you guys would be able to help analyze, right?  Which way to even put the part into the quench?

JS:  Sure, sure.  And we've done that.  The one that comes to mind is a long landing gear.  This landing gear was about 3 meters in length, and we looked at even slight angles going into the quench tank can have serious consequences on the distortion.  That is definitely something that we've looked at in the past.

DG:  Just that orientation would help, but maybe eliminate vapor stage, or whatever, I assume?  Or pockets?

JS:  Right. And even beyond that, it sets up thermal gradients in different locations of the part.  So now instead of cooling one section faster, you're cooling it a little slower and that kind of thing.  That also relates back to actual vapor stages and how bubbles get trapped.  But that goes back to defining boundary conditions, which is where software like AVL's FIRE can really be helpful in understanding flow patterns.  There is a beneficial relationship there.

DG:  There are a host of different materials that people are using.  How broad is the database, as far as the different types of materials, that you can analyze and model?

JS:  That is a good question.  We have a lot of low alloy, medium alloy, and carburizing grades of steel, the 1000 series, the 8600 series, 9300 series, those types of materials.  We've also worked with some of the high alloy aerospace grades like C64 and the Pyrowear 53 and that sort of thing.  But right now, it's all steel.  There is a lot of talk about being able to do aluminum.  We get that question a lot.

DG:  I was wondering about that specifically- aluminum and/or of course, when we talk aerospace, we're talking titanium.  So titanium is not on the table at the moment?

JS:  It is, but it isn't.  The interesting thing is that there is a phenomena precipitation hardening that goes on in aluminum and titanium.  But it also goes on in these high alloy steels.  It is a secondary hardening mechanism.  We've been working on that and we feel that once we can handle secondary hardening in steel, then the jump to aluminum and titanium should be pretty straightforward.

DG:  So to recap, for those of us who are not as well-versed in the product as you are, basically you've got a simulation software that takes into account the material that is being used, also the thermal process (the recipe), which would include both a controlled heat up and potentially a controlled quench.  Is that a reasonable way to describe it in a very broad way?

JS:  Yes. And also, even the steps before that, like carburizing.  If the part is carburized, you would carburize it first.  Or nitriding; we've just introduced those models.  You can literally do the entire process.  And it's not just quenching either.  We've done martempering, austempering, normalizing, all of these things.  Most all normal thermal processing, DANTE can handle.

DG:  The last question I want to ask is, Who is the ideal person/company that would really find the product/service that you're providing useful? I know you mentioned aerospace and automotive, but can we be more specific than that? Where are you finding the most success?

Fig. 4: Displacement versus temperature curves showing the shift in martensite start temperature for 3 carbon levels

JS:  That's a tough question. Generally, everybody that has used our software has found real benefit in it.  We've tried to get testimonials from a lot of folks, but this can be difficult because of their companies.  But from Cummins, we've gotten good responses and also from GM we've gotten good responses.  One of them has used it to actually introduce new material and replace legacy material that is now saving them quite a bit of money.  GM has used it to look at process design and optimization.  But I would say mainly the people that are going to benefit the most are the folks that have an analyst to be able to do the simulation almost on a daily basis.  It's one of those things where the more you do, the more you see and the more you understand what is happening.  But really anybody that does heat treatment can benefit from understanding what's going on in their process.

DG:  You mentioned Cummins, and I'm looking at your website, and I just want to read a paragraph:

DANTE heat treat simulation software has been a great boon to Cummins.  Since we've started using their software, we have gone through several projects that have increased our understanding of heat treatment and some of which have saved us production costs.  One example was enabling us to gain the leverage needed to make a material and process change on a legacy product that is now saving us at least 25% on material costs.  The team at DANTE Solutions has always been very accommodating and is very quick to give assistance and feedback whenever troubles arise, even when the troubles are caused by other parts of the simulation and not DANTE itself.  I look forward to working with DANTE team in the coming years as we expand our list of engineers who use this software. -- Brian W. at Cummins

So that leads me to one other question.  When a person interacts with you, are they buying software as a service?  Is it cloud-based or is it something that they purchase a license for one computer, one user?  How does it work?

JS:  There are a couple of different ways.  They can lease it annually or they can essentially buy the software and lease a license annually.  The software can go either on their computer or it can go on a server at their company.  We also have options for corporations where you can essentially get software at different locations.  We have a lot of options and we can work with customers if they [have] unique needs.  That's one of the benefits of being a smaller company, we're pretty flexible like that.

DG:  DANTE's mission statement from their website has a nice ring to it: “DANTE Solutions is determined to promote the use of simulation in the heat treat industry.  From design to troubleshooting, DANTE Solutions believes everyone can benefit from a little simulation in their life.”

If you'd like to get in touch with Justin Sims at DANTE, please email me, Doug Glenn, directly at doug@heattreattoday.com and I'll put you in touch with Justin.

Doug Glenn, Publisher, Heat Treat Today
Doug Glenn, Heat Treat Today publisher and Heat Treat Radio host.

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

Heat Treat Radio #28: Heat Treat Modeling With Justin Sims Read More »

Testing Underway for Innovative Gas Quenching Unit

/ AEROSPACE HEAT TREAT, AEROSPACE HEAT TREAT NEWS, FEATURED NEWS, QUENCHING TECHNICAL CONTENT

 

A Cleveland-based heat treatment software and engineering firm, specializing in metallurgical process engineering and thermal/stress analysis of metal parts, recently announced that mechanical and fatigue testing is underway on an innovative gas quenching unit designed to minimize component distortion during the hardening process.

The DANTE Controlled Gas Quenching (DCGQ) unit is capable of quenching single components following a time-temperature schedule designed for a specific component and steel alloy using the DANTE software.

DANTE Solutions proposed the concept of the process and the DANTE Controlled Gas Quench (DCGQ) unit and collaborated with Milwaukee-based Atmosphere Engineering (now part of United Process Controls), which built the unit, and Akron Steel Treating. The project is funded by the US Army Defense Directorate (ADD), and the aim market is aerospace, where high hardenability steels are used for gears, bearings, and shafts.

Justin Sims, mechanical engineer, DANTE Solutions

According to Justin Sims, a mechanical engineer with DANTE Solutions, the project began with Phase 1, wherein the team had to “make sure that a relatively slow cooling rate through the martensite transformation did not degrade material properties.”

“Phase 1 showed that we had comparable results for hardness, tensile properties, Charpy impact properties, and bending fatigue to the standard quenching practice for Ferrium C64,” said Sims. “We then initiated Phase 2 and had a unit built that was capable of controlling the temperature of the incoming quench gas to within +/- 5°C.” Phase 2 will end December 2018 after two years. The Phase 1 process currently has a patent pending.

Mechanical & Fatigue testing is currently underway at Akron Steel Treating Company where the unit is installed, and samples have been processed to compare the DCGQ process to standard HPGQ of high alloy steels. The current steel under investigation is Ferrium C64. Sims noted that DANTE is overseeing the processing of the test materials, and commercial metallurgical testing companies are performing the tests.

Tensile Testing Metallurgical Laboratory completed the hardness, tensile and Charpy impact testing, and the results are similar for conventionally hardened C64 samples and DCGQ processed samples. IMR Test Labs is conducting the bending fatigue tests. The US Army at Fort Eustis will conduct the rolling contact fatigue tests.

“We have hardness, tensile, and Charpy impact results from the unit we can share with anyone who is interested,” said Sims. “Distortion, bending fatigue, and rolling contact fatigue are currently being evaluated and the results will be available before the end of 2018.”

“We believe that the DANTE Controlled Gas Quench (DCGQ) process, patent pending, has the potential to change the way heat treating is performed on high hardenability steels,” added Sims. “By controlling the temperature of the incoming quench gas, components experience a near uniform transformation to martensite. This near-uniform transformation has the potential to eliminate post-heat treatment correction operations by minimizing part distortion and allowing designers to account for the size change distortion in the initial design of a component. To date, mechanical and dynamic properties for Ferrium C64 processed using the standard hardening process and the DCGQ process has been identical. Bend fatigue and rolling contact fatigue are currently being evaluated.”

 

All images provided by DANTE Solutions.

Testing Underway for Innovative Gas Quenching Unit Read More »