On the Stress-Relaxation Properties of TAF Steel

Abstract

Despite the extensive studies on the stress-relaxation properties of heat resisting steels, there are several questions left unanswered, in which, for example, the subjects on the extrapolation of stressrelaxation data and effect of re-loading on the stress relaxation properties are included.
The authors have taken interest in these subjects and have been carrying out the stress-relaxation tests on TAF which is 12% chromium heat resisting steel.
Tests were made with the specimens of a 10mm guage dia. and 100mm gauge length by using the automatic type stress-relaxation testing machines of 4 tons capacity, and were processed by the following methods: (1) Applying the same initial stress of 30kg/mm² at the temperature range of 550°∼650°C (this test procedure was termed as“initial-loading 1st”); (2) The specimens subjected to the foregoing procedure being re-loaded to the same initial stress of 30kg/mm² at the same temperature range (termed as“re-loading”); and (3) On the other hand, to another group of specimens applying the initial strain equal to the residual plastic strain left in the“initial-loading 1st”plus initial strain in the “re-loading”(termed as“initial-loading 2nd”).
The results obtained are as follows;
(1) As far as the present tests are concerned, the predicted value of“re-loading”stress-relaxation properties assessed from the data of“initial-loading 2nd”based on the well-known Strain-Hardening Theory deviates significantly from the actual“re-loading”value.
On the other hand, the predicted value calculated from Time-Hardening Theory by using suitable material constants is in comparatively good agreement with the actual one within the range of observed duration.
(2) The equation which correlate plastic strain rate ε_p calculated from“initial-loading”stress-relaxation data for TAF steel, residual stress (kg/mm²), and test temperature T (°K), within the temperature range of 800°∼900°K and the stress range of 24∼14kg/mm², takes the form;
ε_p≈A exp(ασ)exp(-Q'/RT)
where
Q'={124(kcal/mole)……σ_c>σ
267-7σ(kg/mole)……σ_c<σ
where A and α are material constants which are determined separately according to the two stages definend by stress level (corresponding to the two straight portions in log ε_p-log σ curves), and σ_c is the critical value of about 21(kg/mm²).
(3) All the initial loading stress-relaxation data obtained at various test temperatures on TAF steel have been successfully expressed on a single master relaxation curve within the limit range of stress and temperature by using the parameter P (=log Et-Q4.6 T, where E is Young's modulus depending on temperature, t is time, T is absolute temperature, and Q is material constant). This relationship could be also available successfully to other alloys.