MODIFIED HEAT TREATMENT FOR LOWER TEMPERATURE IMPROVEMENT OF THE MECHANICAL PROPERTIES OF TWO ULTRAHIGH STRENGTH LOW ALLOY STEELS.

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 with repetitive 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. The microstructure consists of mixed areas of ultrafine grain martensite, within which is the refined blocky, highly dislocated structure, and the second phase lower bainite which appears in acicular form and partitions prior austenite grains. This newly developed heat treatment makes it possible to modify the new heat treatment preported 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.