Stress Relaxation Behavior During the Reheat Treatment Process in the Heat-Affected Zone of Plain Carbon Steels

Abstract

Influence of the microstructure in the Heat-Affected Zone of plain carbon steels (S50C, S25C, S15C and SUYB steel) on stress relaxation behavior during the reheat treatment process were studied. The HAZ structure of plain carbon steels were made by the synthetic heat treatment. Two kinds of microstructures of the synthetic HAZ were prepared by Air Cooling (A.C) or Water Quenching (W.Q) treatment. Stress Relaxation tests were carrid out by means of three point bending method and constant-strain rate test on heating. The deformability of the synthetic HAZ structures at high temperature during the reheat treatment process were examined by constant-load test on heating. The relation between the precipitated behavior of carbides and the change of carbon concentration in the matrix of the synthetic HAZ were investigated during the reheat treatment process. The effect of this relationship on the stress relaxation behavior was discussed in terms of free from alloying elements of plain carbon steels.
The results are summarized as follows.
(1) Stress relaxation behavior during the reheat treatment process of 4 types of plain carbon steels were completely released. Influence of the microstructure in the synthetic HAZ on the stress relaxation behavior appeared in a manner of the delay of relaxation behavior of Ferritic-Pearlitic structures (A. C specimens) during the reheat treatment process.
(2) The deformability of HAZ structure of carbon steels during the reheat treatment process were clearly low with A.C specimens of S50C steel as compared to another 3 types of plain carbon steels. These fractographs mode was a minor ductile fracture containing intergranular fracture surface.
(3) The difference of stress relaxation behavior could be explained by the change of carbon concentration and carbide precipitated site in the matrix of both synthetic HAZ.