The effect of solutes on resistance to fracture of body centred cubic iron single-phase solid-solution alloys has been investigated. TheJ-integral method has been used for the measurement of ductile fracture toughness. TheJ_IC values so determined quantitatively indicate the extent of degradation in fracture toughness due to the addition of hardening solute silicon. Cobalt addition results in alloy softening. The measuredJ_IC values clearly demonstrate the toughening effect of cobalt addition as a solute, which result renders the case of Fe-Co solid-solution alloys interesting.

An ultra-high-strength low-alloy NiSiCrCoMo steel has been developed. The development work is part of a major programme at the Defence Metallurgical Research Laboratory in the field of ultra-high-strength, high-fracture-toughness steels. In this context we undertook investigations to understand the effect of solute additions on the fracture behaviour of Armco iron and Fe-C alloys. We investigated Fe-Ni, Fe-Co, Fe-Si, Fe-Mo, Fe-C-Ni and Fe-C-Co alloys for mechanical behaviour. The report by Garrison (1986) on a Fe-C-Ni-Si-Cr alloy was an important pointer to a low-alloy, ultra-high-strength steel with high fracture toughness. The material we have now arrived at is a Fe-C-Ni-Si-Cr-Co-Mo steel with tensile, impact and fracture toughness properties matching those of maraging steel 250 grade in tonnage scale melts.

The effect of alloying additions viz. cobalt, molybdenum, cerium and a combination of cobalt and molybdenum, on theK_ISCC of NiSiCr steel in 3·5% NaCl aqueous solution was studied. Addition of cobalt to NiSiCr steel resulted in an increase in theK_ISCC whereas molybdenum addition decreased theK_ISCC. Cerium addition did not affect theK_ISCC while the combination of cobalt and molybdenum resulted in an increase in theK_ISCC although not as much as in the case of cobalt addition. The effect of alloying elements onK_ISCC could be attributed to their effect on the critical fracture stress and yield strength.

Ultrahigh strength steels have been used increasingly in recent years for critical aircraft and aerospace structural applications. In such applications, though materials performance is of prime consideration, cost and availability makes the low-alloy steels an attractive option. This paper describes the development of an ultrahigh strength NiSiCrCoMo low-alloy steel, supported by significant findings obtained from the basic studies that were aimed at understanding how solute additions influence fracture resistance of iron, with and without the presence of carbon. The results of the basic studies, in combination with the work of Garrison (1986) on a NiSiCr steel, have profitably been employed in the development of a NiSiCrCoMo low-alloy steel possessing a strength-toughness combination quite comparable to the highly alloyed 250-grade maraging steel. Reproducibility of attractive strength and toughness properties has been established in tonnage scale melts. This steel, in the softened condition, has good formability and machinability. Weld parameters have also been established. The NiSiCrCoMo low-alloy steel thus meets the requirements of performance and cost rendering it an attractive option for advanced structural applications.