17-4PH鋼における逆変態オーステナイトの生成挙動とその諸特性に及ぼす影響

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A typical martensitic precipitation hardening stainless steel, 17-4PH, contains the reverted austenite from the martensitic matrix at overage. The authors investigated the kinetics of it and the effect of retained austenite on the properties for overaged materials.The results are as follows.(1) The kinetics of the reverted austenite formation is explained by the rate formula of Johnson-Mehl-Avrami, ζ=1-exp(-kt1/2), where ζ: the volume fraction of reverted austenite, t: aging time (sec.)k=1.27×105exp(-Q/RT), Q=33.2kcal/mole.It shows the reverted austenite plates grow in thickness through the diffusion of austenite formers, Ni and Cu, along the martensite lath or prior-austenite grain boundaries. Lamellar morphologies of the retained austenite prove the above kinetics.(2) The austenite reverted at lower aging temperatures enriches Ni and Cu, so that it remains stable at room temperature, while the composition of the austenite at higher temperatures reaches the equilibrium, and transforms to martensite unaged on cooling.(3) The hardness of overaged specimens obeys the rule of mixture, HRC=0.368Ma+0.058γR+0.108αwhere the volume fractions of each phases are defined below.Ma: overaged martensite (%)γR: retained austenite (%)α: unaged martensite less than 25%.(4) The retained austenite depresses the tensile strength, especially the proof stress.0.2%PS=115.5-1.4γR(kgf/mm2)0.02%PS=107.0-1.9γR. (kgf/mm2)(5) The larger amount of γR shifts the transition temperatures to lower and the upper shelf values to higher for Charpy impact tests.(6) The lamellar retained austenite enhances the resistance against stress corrosion cracking in boiling 42% MgCl2 solution.(7) The lower proof stress material with γR posseses the poorer fatigue strength under NaCl aq. drops.
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17-4PH鋼における逆変態オーステナイトの生成挙動とその諸特性に及ぼす影響

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