In this column, I will discuss the effects prior microstructure plays in the distortion response during heat treatment.
Introduction
In the typical manufacturing process to manufacture a steel or aluminum part, incoming material is purchased; shaped by forming, forging, or casting; machined; heat treated; and finish processed (final machining, plating, or painting).
There are many sources of distortion that have been identified [1] [2]. It is not the purpose of this article to review all the possible sources and permutations of distortion, but to highlight the effect of prior microstructure.
Prior microstructure
The incoming raw material plays a critical role in the quality of a finished part. The prior microstructure and hardenability of the alloy grade selected, as well as the cleanliness of the incoming material, affect distortion and the mechanical properties (fatigue and impact properties). This is illustrated in Figure 1.
Hippenstiel [3] evaluated the effect of prior grain size on distortion and found that finer grain size contributed to reduced distortion. He also found that by reducing the amount of grain size variation, further reductions in distortion could be achieved.
In another study [4], electro-slag remelted, and vacuum melted, and remelted X38CrMoV5-1 hot-work steels were examined for distortion, residual stresses, microstructure, and properties. The vacuum heat treatment was varied to develop a model of phase volume fraction, residual stress, and hardness. Using a Gleeble, the transformation kinetics and flow curves for the alloys were measured. A hardness after tempering was developed based on the volume fraction and hardness of each phase. An additional difficulty was the calculation after tempering was overcome by the inclusion of a coarse secondary hardening carbide. The validation of the model showed good correspondence with experiment.
An examination of the effect of the microstructure prior to heat treatment was examined by Prinz [5]. In this investigation, the microstructure of bar material was varied by normalizing, annealing, or hardening and tempering. The resultant microstructures were primarily ferrite and pearlite in the normalized “as-delivered” condition, with some bainite and martensite due to the high hardenability. The annealed structure was ferrite and pearlite, while the hardened and tempered structure was tempered martensite. The extent of the distortion was evaluated by measurements before and after heat treatment of machined shafts. The effect of hardenability on the distortion of the heat-treated shafts was also examined.
The results of the study showed a strong influence of the prior microstructure on the resulting distortion (Figure 2). Increasing hardenability reduced distortion. However, the authors noted that it is likely that the reduced grain size from the prior heat treatments likely changed the hardenability, so the effects are compounded with prior heat-treatment effects.
The examination of the prior microstructure on the distortion of press-quenched gears was conducted by Reardon [6]. In this study, 464 mm diameter AISI 8620 H steel automotive transmission gears were processed in two separate furnace loads. For the first set of gears, which were designated Series 1, the normalizing temperature that was used was 926°C. These gears received a single normalizing cycle. The second set of gears, labeled Series 2, was normalized twice in succession. The first normalizing cycle they received was identical to that used for the Series 1 gears. For the second normalizing cycle, the temperature was raised to 954°C. Each group of 24 gears was subsequently processed through a gas carburizing furnace using an endothermic atmosphere at a temperature of 926°C to generate effective case depths in the range of 0.0016 to 0.0022 mm. The gears were all cooled to room temperature after carburizing was completed. They were then reheated to the austenitizing temperature of 854°C and individually press quenched.
The resulting out-of-round distortion was measured (Figure 3). It was found that the double normalized gears exhibited substantially lower distortion, and much reduced distortion scatter than the single normalized gears. The double normalized gears had a mean out-of-roundness of 100µm, with a standard deviation of 25.4 µm. The single normalized gears had a mean out-of-roundness of 330 microns, with a standard deviation of 154µm.
Banding or segregation can produce a wide variation in the local hardness, even within the carburized layer. This segregation can cause non-uniform response to heat treatment and result in uncontrolled distortion (Figure 4 and Figure 5).
At my previous employment, large landing gear 900-pound forgings of 300M (such as SAE 4340 but with increased Si and C), were initially forged, then allowed to air cool. Because of the very high hardenability of 300M, and the size of the forgings, there were extreme variations in grain size throughout the forging due to recrystallization. Further, this alloy is also prone to segregation of chromium, leading to a highly banded structure. During machining, large variations of hardness were observed, and this led to non-uniform surface finish and tool breakage. Further, the machined parts experienced non-uniform response to heat treatment and high residual stresses.
After some experimentation, it was decided to perform a long spheroidization anneal on the large forgings as they were received from the forging house. This long spheroidization anneal (approximately 18 hours at 650°C) resulted in a much more uniform microstructure and grain size. The resulting hardness was significantly lower and provided a much more uniform machining response and surface finish. The larger carbides also resulted in better chip breakage.
During heat treatment, the part exhibited much lower distortion and more uniform hardness due to the reduced segregation of chromium, and the uniform grain size.
Conclusions
These studies are all consistent in that they show that the prior microstructure prior to machining and heat treatment plays a critical role in controlling the distortion of a heat-treated component. It is always recommended that parts be properly normalized, or in extreme cases, spheroidized prior to heat treatment for uniform machining response and improved distortion control.
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References
- D. S. MacKenzie, “Control of Residual Stress and Distortion,” Heat Treating Progress, p. 47, July 2007.
- D. S. MacKenzie, “Metallurgical Aspects of Distortion and Residual Stresses in Heat Treated Parts,” in 23rd IFHTSE Heat Treatment and Surface Engineering Congress, April 18-21, Savannah, GA, 2016.
- F. Hippenstiel, “Metallurgical and production-related protocols to reduce heat-treatment distortion in the manufacture of gear components,” in IDE 2005, Bremen, Germany, 2005.
- W. Schützenhöfer, C. Redl, H. Schweiger, I. Siller, N. Dickinger, and R. Schneider, “Comparison of different remelted hot-work tool steels and their distortion, residual stresses and other properties. Experiments and Simulations,” in IDE 2005, Bremen, Germany, 2005.
- C. Prinz, B. Clausen, F. Hoffman, R. Kohlmann, and H.-W. Zoch, “Metallurgical influence on distortion of the case-hardening steel 20MnCr5,” in IDE 2005, Bremen, Germany, 2005.
- A. Reardon, “Controlling Distortion in Heat Treatment Through Press Quenching,” Thermal Processing, pp. 24-29, April 2015.
