Efforts to improve Fe-Cr-C steel by modification of heat treatment and alloying additions are described. An important result of this work is in the evaluation of toughness with austenitizing temperature. The resistance to fracture of the conventionally (870/sup 0/C) austenitized structure is greatest ahead of a rounded notch (i.e., in Charpy and K/sub A/ tests), whereas it is least ahead of a sharp crack (i.e., in K/sub Ic/ test). Therefore, the advantages of increased fracture touchness, K/sub Ic/, with increased austenitizing temperature, must be weighed against the deterioration of ductility and Charpy impact energy. However, an optimum austenitizing temperature can be chosen (in the range 1000 to 1100/sup 0/C), where a 30 to 50 percent increase occurs in fracture toughness, K/sub Ic/, with no loss of strength and little reduction in impact energy and ductility. Moreover, low alloy additions of Mn and Ni can further increase the toughness of the alloy without reduction in strength. This was possible because the strengthening in these quaternary alloys was achieved primarily through solid solution hardening and not from substructural twinning in martensite. The toughness properties obtained for Fe-4Cr-2Mn-0.26C steel are superior to conventional 4340 and 300-M steels at the 200 ksi yield strengthmore » level, and yet would be cheaper to produce in large quantities.« less
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