Recently, I had a question via email regarding the development of residual stress in steel during quenching, and the stress-reversal that often occurs. In this column, I will discuss the stress reversal, and how residual stresses form during quenching. This is often the source of distortion in heat-treated components.
Introduction
During the heat treatment of steels, the steel is first heated to convert the microstructure uniformly to austenite. There are two different situations that can occur. First is that the part is neutral hardened, and that no carburizing occurs. The other alternative is that the part is carburized, creating a surface layer on the part of increased carbon. Once the part is at temperature, and the part is fully converted to austenite, it is quenched in a gas, oil, polymer, or water.
During quenching, the stress field that is developed is driven by three different strain components. These are:
- Thermal strains from heat transfer and thermal gradients.
- Transformational strains from the conversion of austenite to martensite or another microstructure.
- Mechanical elastic-plastic (including transformation-plastic) strains.
The quench path and the composition of the steel governs the size and direction (compressive or tensile) of the residual stress formed [1] [2]. The physical properties of the material being heat treated, such as the thermal conductivity and specific heat capacity, change as a function of temperature. The temperature field is dependent on the boundary conditions of heat transfer, which also change as a function of time and position. The resulting temperature field creates thermal strain, which is proportional to the thermal expansion coefficient and the temperature gradient. This results in differential contraction between the surface and the core.
There are also the metallurgical phase transformations that can occur. These can be diffusion-related transformations, such as the decomposition of austenite into ferrite, pearlite, and bainite. These can be coupled phase transformations that are dependent on just temperature, such as the transformation of austenite to martensite. These phase transformations have volume changes associated with the phase (Table 1). This transformational strain depends on the local microstructure changes, with the most important being the austenite to martensite transformation. This transformation alone can contribute strains up to 2 percent or more.
Finally, there are the stresses generated by transformational and thermal stresses that occur as the part is cooled. These elastic and plastic stresses and strains are a function of quench path and the thermal field (x, y, and z) within the part. Plastic strain occurs whenever the combined elastic and transformational stresses exceed the local yield strength. Transformational plasticity (TRIP) contributes to the size and timing of residual stress. Even modest residual stress during martensite transformations can produce additional residual stress [5].
Stress-Reversal in Neutral-Hardened and Carburized Steels
The stress evolution of a through-hardened steel cylinder during quenching can be broken down into several stages [5]. During early stages of cooling, the surface rapidly cools through the pearlite/bainite range, and then into the martensite temperature range. The core of the cylinder stays near the austenite range. From thermal expansion or contraction, the surface attempts to contract, but is constrained by the hot, soft core. The surface has high radial and tangential tension. The core remains in compression.
Upon transformation to martensite at the surface, a volumetric expansion occurs. This expansion occurs against the still-hot interior. This expansion partly offsets the previous thermal contraction, reducing surface tension. Depending on the hardenability of the steel and its chemistry driving transformation kinetics, the surface can be driven into a transient compression while the interior becomes tensile. As transformation progresses inward from the surface, the fresh martensite layer at the surface becomes stiff, while the interior is still soft. The hard martensitic shell constrains the expansion of the transforming interior. Stresses in mid‑radius and core may reverse from compression to tension as they attempt to expand but are held in by the martensitic “hoop.”
Finally, as the core cools into the martensite temperature range, the volumetric transformation of the interior occurs, loading the already hardened shell. This forces the surface into tension, while the core is under compression. Carburized steels harden because of the presence of the carbon gradient, higher carbon at the surface, and a low-carbon core. The higher carbon content in the case lowers the martensite start temperature (Ms) relative to the core [6] and increases the magnitude of transformation strain and hardness [1] [2] [5]. Further, because of the lower hardenability of the core, transformation to martensite may not occur, but non-martensitic transformation products, which expand less during transformation. The presence of the carbon gradient creates a mechanical property mismatch, with a high hardness case, and a softer, more ductile core.
During initial cooling, the outer surface of the carburized case cools rapidly and experiences high tensile thermal stresses, such as through‑hardening. However, in this case, it stays austenitic longer, delaying its martensitic transformation. The lower carbon core may reach its martensite start temperature earlier than the high carbon case, beginning martensitic or bainitic transformation while the case is still mostly austenitic. The resulting core expansion against the still‑ductile case can drive the case from tension toward compression, representing an early stress reversal at or near the case–core interface.
As the case finally reaches its martensite temperature, it begins to expand while the core has already transformed and hardened. This results in a compressive surface layer with a core that is in tension. A comparison of the differences between neutral-hardened and carburized gears is shown in Table 2.
Conclusions
The sequence of stress reversals in neutral-hardened steels is the result of the differences in carbon content and thermal gradients. It is driven by both thermal and transformation strains.
Should there be any questions regarding this article, or suggestions for further articles, please contact the writer or editor.
References
- S. Augustine and K. N. Prabhu, “Residual Stress and Distortion during Quench Hardening of Steels: A Review,” Journal of Materials Engineering and Performance, vol. 31, no. 7, pp. 5161-5188, 2022.
- D. S. MacKenzie, “Distortion and Residual Stress development during Quenching,” Gear Solutions, no. April, pp. 26-27, 2025.
- K. E. Thelning, Steel and its Heat Treatment, 2nd ed., London: Butterworths & Co., 1984.
- F. Legat, “Why Does Steel Crack During Quenching?,” Kovine Zlitine Technol., vol. 32, no. 3-4, pp. 273-276, 1998.
- J. Sims, “Stress evolution during quench hardening of Steel,” Thermal Processing, no. July, pp. 20-21, 2022.
- G. Krauss, Steels – Processing, Structure, and Performance, 2nd ed., Metals Park, OH: ASM International, 2015.
- W. P. Oliveira, M. A. Savi and P. M. Pacheco, in 20th International Congress of Mechanical Engineering, Gramado, RS, Brazil, Nov 15-20, 2009.
