Stress Relaxation by Transformation Plasticity under the Martensitic Transformation in Steels

Abstract

Transformation plasticity is considered as a phenomenon limiting the actual stress for any specified strain rate under conditions of phase transformation. A one-dimensional model of a steel bar strained under martensitic transformation is provided. Discussion is limited to a series of accepted assumptions, such as athermal kinetics of martensite transformation, monotonic evolution of temperature and constant transformation plasticity parameter. An equivalent tangent modulus is deduced indicating the decisive parameters influencing the softening modes. The obtained model is utilized together with well-known transformation plasticity laws and martensite transformation kinetics to demonstrate an actual softening of steels under the martensitic transformation.

Influence of Microsegregation on the Onset of the Martensitic Transformation

“…The form of this equation is such that the martensite transformation rate decreases monotonically from the onset of the transformation at M s . 7) Due to the volume change accompanying fcc to bcc or bct crystal structures in steels, it is common practice to determine transformation temperatures, e.g. during continuous cooling, using dilatometry.…”

Transformation plasticity during continuous cooling of 700XL and its effect on residual stress

“…The transformation plasticity parameter of the 700XL ferritic steel is obtained from Equation (16). The data are fitted to obtain the transformation plasticity coefficient K.…”

“…In contrast, Denis 12 and Zhang 13 et al argued that transformation plasticity should be considered when predicting residual stress during cooling. In studying whether transformation plasticity is taken into account for stress prediction, Xu 14 and Yao 15 et al pointed out that the presence of transformation plasticity can regulate the internal stress-strain state of the material, and Ivanov 16 et al studied transformation plasticity in martensite and found that the maximum value of stress relaxation occurs at the early stage of transformation; at the same time, significant relaxation also occurs at the late stage of 60% of the phase transformation. There is no consensus on the effect of transformation plasticity on residual stress during cooling.…”