Low temperature improvement of the mechanical properties of 4340 type ultrahigh strength steel with heat treating techniques using interrupted quenching method

Abstract

In many practical applications the use of ultrahigh strength low alloy steels, such as AISI 4140 and 4340, is limited by their poor ductility and notch toughness in low temperature environments. In recent years, considerable effort has been directed toward improving ambient temperature mechanical properties by various heat treating techniques or compositional modification. ~-~2 Unfortunately, these improvements in mechanical properties have not been reflected in the low temperature mechanical properties of the ultrahigh strength low alloy steels. The basic problem is that the martensitic microstructure tempered to retain a high strength level is inherently susceptible to embrittlement which occurs in low temperature environments. We have given one of the potential solutions to this problem by developing 4140 and 4340 type ultrahigh strength steels having a mixed structure of martensite and lower bainite. 13.14,15 The effectiveness of the mixed structure in improving the mechanical properties is attributed to the fact that lower bainite, which appears in acicular form and partitions prior austenite grains, effectively causes a refinement of the martensitic substructure (i .e. , lath width and packet size) which lead to increased strength and that the lower bainite provides a significant increased resistance to brittle fracture at low temperature environments. As noted, the role of the second phase lower bainite in improving the lower temperature mechanical properties has been found to involve a refinement of the martensitic substructure and increasing resistance to low temperature brittle fracture. Thus, it follows that other ductile second phases should have a similar beneficial effect, provided they appear in acicular form and refine the substructure of the parent martensite, resulting in a higher resistance to cracking in low temperature environments. The obvious alternative second phase to consider is highly tempered martensite. This has the great advantage that the required microstructure can be readily achieved by heat treating techniques using interrupted quenching method. 16'17 Also, there is an added advantage that if the resulting microstructure improves the mechanical properties, the heat treating techniques can be applied to larger sections than the new heat treatment reported previously, ~3'Ja''s because the formation of the microstructure predominantly depends upon temperature.