TAF綱の応力緩和特性

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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 stress-relaxation 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 10 mm guage dia. and 100 mm 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 30 kg/mm^2 at the temperature range of 550℃〜650℃ (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 30 kg/mm^2 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/mm^2), and test temperature T (°K), within the temperature range of 800°〜900°K and the stress range of 24〜14 kg/mm^2, takes the form; [numerical formula] where [numerical formula] 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 the log ε^^._p-log σ curves), and σ_c is the critical value of about 21 (kg/mm^2).(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-Q4.6T, 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.
社団法人日本材料学会の論文
1967-03-15

TAF綱の応力緩和特性

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