Phase Transformation Affected Quench Crack Study Using Finite Element Analysis

Steel components are commonly heat treated to obtain favorable mechanical properties for enhanced performance. Quench hardening is one of the most important heat treatment processes to increase hardness and strength. During quenching, both thermal gradients and phase transformations contribute to the evolution of internal stresses. Higher tensile stresses in a part during quenching tend to increase the cracking possibility, which is more problematic for components with various section sizes due to stress concentration. Heat treaters believe that cracking possibility increases with larger difference of section size in a part. This is only true in some cases. If the section size difference exceeds a threshold, the cracking possibility will decrease. Due to the complex part’s responses to thermal gradient, phase transformation and unbalanced geometry, there is no robust and simple rule to characterize the cracking possibility. With the development of more advanced heat treatment computer modeling capability, the material’s response during heat treatment can be more intuitively understood. The part geometry and heat treatment process can be designed with much less potential heat treatment defects. In this paper, finite element based heat treatment software, DANTE, is used to investigate the relationship between section size difference and cracking possibility by using parts with a series of section size ratios. In this specific study, the selected part is a plain strain component made of AISI 9310. The results during oil quench process have shown that a section size ratio of 1:2 creates the highest stress concentration and cracking possibility.