Origins and Resulting Effects of Internal Stresses in Martensite

Abstract

This paper aims to present new observations and relate these to other recent findings concerning the influence of local micro-stresses (Type II residual stresses) on the behaviour of martensitic steels. A major source of these stresses is the shearing that accompanies phase transformation in individual austenite grains and which is blocked by constraint from the surrounding matrix. Residual stresses are the principal reason for early plasticity and gradual yielding during tensile testing of martensite.

Diffraction techniques are commonly used to measure residual stresses, where peak profiles are influenced by the micro-strains and also by dislocations. Most publications have considered only dislocations and ignored the role of micro-strains. We demonstrate experimentally that this assumption is untenable and must lead to incorrect values of dislocation parameters. The unusual behaviour whereby diffraction peaks from martensite become narrower during plastic deformation is explained by the progressive relaxation of the micro-strains.

We hypothesise that freshly formed martensite is always tetragonal but that it decomposes spontaneously to a cubic structure by auto-tempering in most low carbon lath martensites where the Ms temperature is sufficiently high. This transformation is examined in detail in higher carbon steels which reveals another surprising effect, namely that diffraction peaks can become broader during annealing, resulting from a newly recognised source of internal micro-stresses. These arise when the contraction of crystals along the c-axis during tempering is inhibited by restraint from their surroundings, so preventing equilibrium atomic spacings from being achieved.