A Study on stress relief heat treatment (Report 1)

Tensile relaxation test of HT 60 and deposited metal

Abstract

In order to study the effect of stress relief heat treatment quantitatively, tensile relaxation test and short-time high temperature tensile test were carried out using HT60 and deposited metal. High temperature tensile test was done to measure the stress-strain curve, Young's modulus and yield strength or proof stress at high temperature. The relaxation conditions were as follows; Initial stress 45 Kg/mm² and 25 Kg/mm², relaxation temperature 500°C-650°C, holding time at relaxation temperature 2 hours.
The relaxation test results were discussed on the basis of high temperature tensile test data and discussion is summarized as follows,
I During heating process, residual stress decreased in accordnace with the temperature-rise due to temperature dependence of Young's modulus and conversion of elastic strain to plastic strain. Residual stress, at the instant when the temperature of specimen reached relaxation temperature, was lower than the stress which was obtained from stress-strain curve at the same strain amplitude as relaxation condition. The difference between experimental and calculated stress increased as relaxation temperature rose and as initial stress increased.
2 In the holding period at relaxation temperature, residual stress decreased owing to creep. The reduction of stress became larger as initial stress was higher. The 2/3th of the total reduction of stress took place in the first period within 1 hour.
3 The reduction of heating speed increased the reduction of stress during heating period and decreased it in the holding period. But after 2 hours, residual stress showed a constant value, independent of heating speed.
4 The stress relaxation of deposited metal was much less than that of base metal and its high temperature strength was higher than that of base metal. But in this case, the structural change during relaxation test must be considered.