Ultrahigh-strength steels for aerospace applications

The ultrahigh-strength steels used in aerospace applications are primarily alloys developed 25 or more years ago, which would seem to illustrate the conservatism of alloy producers. However, the now widespread use of AF1410, which was developed in the mid-1970s, suggests that new alloys will be adopted for aerospace applications if their mechanical properties are markedly superior to incumbent alloys. Given the excellent properties of the secondary hardening steels such as AF1410, it would appear that the greatest need is for alloys with yield strengths of 1,725 MPa and higher, which have improved fracture toughness and resistance to stress-corrosion cracking. Control of sulfide-type inclusions, coupled with low sulfur levels, may be a means of achieving quite reasonable toughness at high strength levels. Improving resistance to stress-corrosion cracking appears to be a more difficult problem, but it is hoped that a general solution for the case of intergranular fracture by stress-corrosion cracking will arise from ongoing fundamental studies of the effects of segregating impurities and alloying additions on grain boundary cohesion.