On the Stress Relaxation Characteristics of 12Cr-1 Mo-1 W-1/4 V Steel at High Temperature

Abstract

The present work has dealt with the stress relaxation characteristics of 12Cr-1 Mo-1 W-1/4 V steel which is used for high temperature equipments.The effect of initial stress on the stress relaxation properties of this steel was studied at 550°C by means of“Step-Down flow Rate”test1) in which the conventional lever type creep testing machines were used.Stress relaxation tests were also processed at several temperatures with constant stress initially applied, by the use of the specially designed stress relaxation testing machines which automatically kept the total extension of the test specimens constant.The magnitude of stress by which the residual stress of the test specimens was reduced with time manually (in“Step-Down”test) or automatically (in automatic type“Relaxation”testing machine), was 1.27kg/mm2 respectively. The test temperature was maintained constant to ±0.5deg. C at each of the tests and 0.06kg/mm2. continued to the period of about 1500 hours.Microstructure change of this steel after long term test procedure was observed with optical and electron-microscopic method.The results obtained are as follows:(1) The group of the residual stress-time curve obtained from“Step-Down”test changing the initially applied stress at given temperature, showed the tendency that each curve of the group approached one another with time.(2) The curves in log-log plots of stress versus strain rate which were derived from the residual stress-time curves of this steel consisted of two or three straight line portions. The stress dependence of the strain rate was larger in“the first straight portion”which indicated the earlier stage of the tests, than in the“second straight portion”which showed the later stage of the tests.(3) These stress versus strain rate curves plotted in log-log scale with different initial stress were nearly in parallel with one another at both the stages.(4) The stress versus minimum creep rate curve obtained from the creep test of this steel was also shown by two straight lines which had nearly the same slope as in the corresponding straight line portions of the stress-relaxation tests at the same temperature.(5) The group of residual stress-time curves obtained from the stress relaxation tests changing the test temperatures with the same initial stress, showed that the stress relaxation process of this steel was thermally activated, and that the activation energy for stress relaxation calculated from the curves in log-log plots of stress versus strain rate, was about 74000cal/mole in“the first straight portion”(stress range of more than 20kg/mm2) which was much the same as that for the self-diffusion of Alpha-Iron; 73000cal/mole9), and was about 58000cal/mole in“the second straight portion” (stress range of less than 20kg/mm2).(6) Some electron-microscopic structure change was observed in the specimen after these tests such as carbide precipitations along the needle-like portions of the tempered martensite phase and inside the Delta-Ferrite phase, and it was found that these structure change would affect the stress relaxation properties of this steel.