Heat treatment for improvement in lower temperature mechanical properties of 0.40 pct C-Cr-Mo ultrahigh strength steel
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I.
INTRODUCTION
ULTRAHIGH strength low alloy steels, such as AIS14340, offer 0.2 pct proof stress which is in excess of 1400 MPa in addition to modest ductility and notch toughness when conventionally heat treated. They have generally not been used at low temperature, however, because of their poor ductility and notch toughness at these levels. Nevertheless, there has been little study to improve their low temperature mechanical properties. We have been studying t-7 the factors that contribute to the mechanical properties of high strength low alloy steels. We have shown8 that isothermal heat treatment of a commercial Japanese 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel (AISI 4340 type) at 593 K for a short time followed by water quenching, in which a mixed structure of 25 vol pct lower bainite and 75 vol pct martensite results in the improvement of low temperature mechanical properties. Following such a heat treatment, notch toughness improved dramatically, indicating a good combination of strength and ductility at low temperatures. The effectiveness of the new heat treatment in improving mechanical properties is attributed to the facts that lower bainite effectively causes a refinement of the lath width and packet size of the parent martensite which leads to increased strength and that in
YOSHIYUKI TOMITA and KUNIO OKABAYASHI, Professor, are with the Department of Metallurgical Engineering, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 591, Japan. Manuscript submitted November 29, 1982. METALLURGICALTRANSACTIONSA
addition, the lower bainite provides significant increased resistance to brittle fracture at low temperatures. There has recently been an increasing demand for economical alloys with superior mechanical properties, and thus considerable effort has been directed toward improving the mechanical properties of these alloys. It is, therefore, important to study whether the new heat treatment will be effective for improving low temperature mechanical properties of other ultrahigh strength steels which contain combinations of the elements found in the more economical alloys in addition to the ultrahigh strength steel that was reported in the previous paper. In the present work, isothermal heat treatment at 593 K for a short time followed by water quenching (reported in the previous paper) has been applied to a commercial Japanese 0.40 pct C-Cr-Mo ultrahigh strength steel (AISI 4140 type), in which the required mechanical properties and hardenability have been achieved by using chromium and molybdenum as the main alloying elements without adding expensive nickel.
II.
EXPERIMENTAL
The steel used in this investigation was a commercial JIS SCM 440 hot-rolled bar stock corresponding to AISI 4140. The chemical composition is given 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 impact specimens were cut from the plates with VOLUME 14A. NOVEMBER 1983-- 2387
Table I. Chemical Composition (Wt
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