Improvement in lower temperature mechanical properties of 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel with the second p
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INTRODUCTION
IN recent
years, considerable effort has been directed toward improving ambient temperature mechanical properties of ultrahigh strength low alloy steels such as AISI 4340 and 300-M. 1-10However, in spite of frequent use under low temperature conditions in commercial practice, there has been little study of improvement in lower temperature mechanical properties of these steels. It will, therefore, be important in commercial applications to provide new suggestions for developing better lower temperature mechanical properties of these steels by simple and economical heat treatment. Based on our studies 1H5 of the microstructure and mechanical properties of ultrahigh strength low alloy steels having a mixed structure of martensite and nonmartensitic decomposition products, it was found that when lower bainite with comparatively low strength but high ductility, which appeared in acicular form so as to partition prior austenite grains, was associated with martensite tempered at 473 K, it had a beneficial effect on ambient temperature mechanical properties. Based on these experiments, it appears that such a microstructure also improves lower temperature mechanical properties of ultrahigh strength low alloy steels in quenched and tempered conditions. In the present work, for the purpose of improving lower temperature mechanical properties of ultrahigh strength low alloy steels, the effect of the second phase lower bainite on mechanical properties from ambient temperature (287 K) to 123 K has been studied in a commercial Japanese 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel. Y O S H I Y U K I T O M I T A , A s s i s t a n t P r o f e s s o r , and K U N I O OKABAYASHI, Professor, are both with the Department of Metallurgical Engineering, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 591, Japan. Manuscript submitted April 6, 1982.
METALLURGICAL TRANSACTIONS A
EXPERIMENTAL
The steel used in this investigation was a commercial JIS SNCM 439 hot rolled bar stock corresponding to AIS14340. The chemical compositions are shown along with the Ms point in Table I. The steel was received as 130 mm~b bars and subsequently hot-forged to 13 mm thick plates. Tensile and Charpy V-notched specimens were cut from the plates with their length axis parallel to the rolling direction and were fully annealed. In order to obtain a mixed structure of martensite and lower bainite and a fully bainitic structure, specimens were quenched directly into a lead-tin bath followed by isothermal transformation at 593 K above the Ms point from 140 s to 36 ks after austenitization in argon atmosphere at 1133 K for 3.6 ks. The specimens were subsequently waterquenched to ambient temperature. Isothermal heat treatment was conducted in a lead-tin bath with a thermal capacity sufficient to avoid appreciable temperature change during the quenching operation. The fully martensitic structure was obtained by the heat treatment followed by direct water quenching to ambient temperature after austenitization in argon atmo
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