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Some design guidelines for improving strength-toughness combinations in medium carbon structural steels are critically reviewed. From this, quaternary alloy development based on Fe/Cr/C steels with Mn Jr Ni additions for improved properties is·described. Transmissjon electron microscopy and X-ray analysis reveal increasing amounts of retained austenite in these alloys with r~n content up to 2 w/o and Ni additions at 5 w/o after quenching from 1100° C. A corresponding improvement in toughness properties is also found. Grain refining results in a further increase in the amount of re--,.tained austenite. In addition, the excellent combinations of strength and toughness in these quaternary alloys are attributed to the production of dislocated lath martensite from a homogeneous austenite phase free from uridi sso 1 ved all oy' carbi des.The question of thermal instability of retained austenite following tempering is considered in detail and it is shown that the decomposition of retained austenite is closely related to the ease of n~cleation and growth of cementite. Thus, graphitizing alloying elements such as Ni are beneficial in postponing tWe decomposition of retained austenite.

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