Effects of silicon additions and retained austenite on stress corrosion cracking in ultrahigh strength steels

A study has been made of the effects of silicon additions and of retained austenite on the stress-corrosion cracking (SCC) behavior of commercial ultrahigh strength steels (AISI 4340 and 300-M) tested in aqueous solutions. By comparing quenched and tempered structures of 4340 and 300-M i) at equivalent strength and ii) at their respective optimum and commercially-used heat-treated conditions, the beneficial role of silicon addition on SCC re-sistance is seen in decreased Region II growth rates, with no change in K_ISCC. The beneficial role of retained austenite is demonstrated by comparing isothermally transformed 300-M, containing 12 pct austenite, with conventionally quenched and tempered structures of 300-M and 4340, containing less than 2 pct austenite, at identical yield strength levels. Here, the isothermally transformed structure shows an order of magnitude lower Region II SCC growth rates than quenched and tempered 300-M and nearly two orders of magnitude lower Region II growth rates than 4340, K_ISCC values remaining largely unchanged. The results are discussed in terms of hydrogen embrittlement mechanisms for SCC in martensitic high strength steels in the light of the individual roles of hydrogen diffusivity and carbide type.