Modified heat treatment for lower temperature improvement of the mechanical properties of two ultrahigh strength low alloy steels |
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Authors: | Yoshiyuki Tomita Kunio Okabayashi |
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Affiliation: | (1) Department of Metallurgical Engineering, College of Engineering, University of Osaka Prefecture, 4-804, Mozu-Umemachi, Sakai, 591 Osaka, Japan |
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Abstract: | In the previous papers, a new heat treatment for improving the lower temperature mechanical propertise of the ultrahigh strength
low alloy steels was suggested by the authors 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). In this paper, two commercial Japanese ultrahigh strength steels, 0.40 pct C-Ni-Cr-Mo (AISI 4340 type) and 0.40
pct C-Cr-Mo (AISI 4140 type), have been studied to determine the effect of the modified heat treatment, coupled above new
heat treatment withγ ⇆ α′ repctitive heat treatment, on the mechanical properties from ambient temperature (287 K) to 123 K. The results obtained for
various test temperatures have been compared with those for the new heat treatment reported previously and the conventional
1133 K direct water quenching treatment. The incorporation of intermediate four cyclicγ ⇆ α′ repctitive heat treatment steps (after the initial austenitization at 1133 K and oil quenching) into the new heat treatment
reported previously, as compared with the conventional 1133 K direct water quenching treatment, significantly improved 0.2
pct proof stress as well as notch toughness of the 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel at similar fracture ductility
levels from 287 to 123 K. Also, this heat treatment, as compared with the conventional 1133 K direct water quenching treatment,
significantly improved both 0.2 pct proof stress and notch toughness of the 0.40 pct C-Cr-Mo ultrahigh strength steel with
increased fracture ductility at 203 K and above. The microstructure consists of mixed areas of ultrafine grained martensite,
within which is the refined blocky, highly dislocated structure, and the second phase lower bainite (about 15 vol pct), which
appears in acicular form and partitions prior austenite grains. This newly developed heat treatment makes it possible to modify
the new heat treatment reported previously so as to raise 0.2 pct proof stress to a higher level and keep notch toughness
at the same level. The improvement in the mechanical properties is discussed in terms of metallographic observations and the
modified law of mixtures and so forth. |
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Keywords: | |
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