An existing steel was modified to a higher strength level to meet the requirements of airframe structures. Factorial-regression analysis of data determined previously helped to determine the interaction effects of the selected alloying elements on mechanical properties. The range of alloying elements required to optimize toughness at the 1586–1724MNm−2 ultimate strength level was subsequently determined by experiment. The developed alloy 0·16C–2Cr–1Mo–10Ni–14Co (AF 1410), attained a fracture toughness KIC = 156MNm−3/2 at this strength level. These properties result from the precipitation of M2C-type alloy carbides in a highly dislocated, lath martensite matrix during aging.
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