Modified heat treatment for lower temperature improvement of the mechanical properties of two ultrahigh strength low all
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I. INTRODUCTION U L T R A H I G H strength low alloy steels, such as AISI 4340 and 4140, offer a 0.2 pct proof stress, which is in excess of 1400 MPa, in addition to modest ductility and notch toughness when conventionally heat-treated. However, they have shown poor ductility and notch toughness when used under low temperature conditions. So far, there has been little effort to develop the mechanical properties at these levels. A new heat treatment for improving the lower temperature mechanical properties of the ultrahigh strength low alloy steels has been suggested by the authors 1'2 which produces a mixed structure of 25 vol pct lower bainite and 75 vol pct martensite through isothermal transformation at 593 K for a short time followed by water quenching (after austenitization at 1133 K). The new heat treatment resulted in lower temperature improvement of the mechanical properties of two commercial ultrahigh strength steels such as AISI 4340 and 4140 types. However, when one requires still higher strength in addition to better toughness throughout lower temperature, neither steel treated by the new heat treatment offers the desired strength because the 0.2 pct proof stress gain obtained by using the new heat treatment was reduced as the test temperature was lowered. This, therefore, requires a modification to raise the 0.2 pct proof stress to a higher level throughout lower temperature. YOSHIYUKI TOMITA and KUNIO OKABAYASHI are with the Department of Metallurgical Engineering, College of Engineering, University of Osaka Prefecture, 4-804, Mozu-Umemachi, Sakai, Osaka 591, Japan. Manuscript submitted October 10, 1983. METALLURGICALTRANSACTIONS A
The realm of a y ~-- a ' repetitive heat treatment appears very attractive for developing an improved 0.2 pct proof stress of hardened steel through grain-refining. Grange 3 has shown that by the y ~--- a ' repetitive heat treatment, ultrafine grains could be produced in the hardened steels such as AISI 8640 and 4340, and the 0.2 pct proof stress of the steels could be improved according to a Hall-Petch type relationship between prior austenite grain size and 0.2 pct proof stress, proctor and Paxton 4 have extended this technique to the ultrahigh strength steel, AISI 4340, and improved the stress-corrosion resistance at increased 0.2 pct proof stress. The authors have shown 1'2 that the y ~- a ' repetitive heat treatment significantly improves 0.2 pct proof stress at lower temperature, while fracture ductility and notch tensile strength remarkably decreases, as far as lower temperature improvement of the mechanical properties of AISI 4340 and 4140 type ultrahigh strength steels is concerned. As the review of the literature shows, it can be expected that the introduction of the y ~ a ' repetitive heat treatment steps into the new heat treatment reported previously, 1'2 and will lead to a significant improvement in 0.2 pct proof stress as well as notch toughness of the two ultrahigh strength steels throughout lower temperatures. In the present work, two commercial Japanese u
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