ENGINEERING CONCEPTS: Transformation-Induced Residual Stresses
Residual stress is an important consideration in the heat treatment of steel, and the development of a compressive residual stress often is considered a good goal because it is believed to improve the fatigue resistance of the heat-treated part. Factors that must be considered include quench severity, hardenability and the type of steel used. Two mechanisms that induce residual stress are thermal straining and nonuniform microstructures. (For a discussion of thermal straining, see Engineering Concepts, July 2000, IH.) Nonuniform microstructures imply that the quenched surface of the steel is martensitic while the interior has transformed to a ferrite-carbide agglomerate such as bainite or pearlite, or both.
Transformation of austenite upon cooling always produces a volume increase, but the volume increase is dependent on both the transformed microstructure and the percent carbon in the steel. Table I shows an example calculation for a 0.35 wt% carbon steel. Consider a steel component where the outer surface transforms to martensite and the interior transforms to pearlite; there is a volume strain difference of 0.67%.
The magnitude of the residual stress often depends on the microstructural gradient, defined as the transition from a fully martensitic to pearlitic microstructure. A high residual stress develops when the transition from martensite to pearlite is sharp, whereas a gradual change in the microstructure minimizes the residual stress, because each layer in the microstructure is only incrementally different. How quickly the microstructure changes depends on quench severity, hardenability and alloy grade-all of which may be engineered for the required effect. Low thermal gradients in combination with steels that transform to lower bainite rather than pearlite develop lower residual stresses. Residual stresses also may be relaxed by tempering above 400¿F (205¿C).
In contrast, C-Mn-B grade steels (for example 15B35) can be used to good effect in developing high surface residual compressive stresses. Boron suppresses the nucleation of ferrite, and, as a result, delays the transformation to pearlite and upper bainite. However, once the reaction starts, the transformation is quite rapid and this produces a sharp microstructural gradient compared with non-boron containing steels alloyed with Cr and Mo, but having the same hardenability. This difference is shown in Fig. 2. Hardenability is defined as the location on the Jominy bar where the microstructure is 50% martensite. This sharp microstructural transition is not nearly as pronounced in boron steels alloyed with Cr and Mo, because the martensite start temperature is depressed by the higher alloy content and boron does not suppress the lower bainite reaction. When C-Mn-B steels are combined with severe quenching, surface compressive stresses on the order of 200,000 psi (1379 MPa) may be obtained. However, to maintain the maximum residual stress, tempering temperatures must be minimized and 300¿F (150¿C) is recommended.