A dilatometric study on the influence of compressive stresses on the tempering of martensitic AISI 4140 steel - Evidence of transformation induced plasticity during cementite precipitation

Modern surface hardening processes intend among other to induce compressive residual stresses in the near-surface layers of the part. During subsequent tempering, these stresses can affect the phase transformations. Dilatometric tempering experiments starting from martensitic AISI 4140 were performed with a uniaxial compressive stress that simulates the compressive residual stresses mentioned above. An unexpected strain drop, increasing with the applied stress, was measured. It is argued that this effect is due to transformation induced plasticity caused by the Greenwood-Johnson effect. For an applied compressive stress of 358MPa, a strain decrease of 0.6% occurs which means that this has imperatively to be taken into account when tempering a hardened surface with compressive residual stresses.