Super' stainless steels resist hostile environments

Alloy developers have focused much of their attention in recent years on improving the resistance of stainless steels to localized corrosion in aqueous environments. The goal: steels that resist pitting, crevice corrosion, and stress-corrosion cracking as well as more costly nickel-base alloys. Optimizing the corrosion performance of super stainless steels requires paying close attention to a variety of metallurgical, processing, and fabricating variables. Without proper control of these parameters, premature failure could result because of the formation of an unwanted microstructure or precipitate. The best way to ensure control is to develop detailed specifications that serve as a framework for a quality assurance program. These specifications will call out chemical composition, microstructure, and, for the super grades in particular, welding procedures. Performance qualification testing is then used to ensure adherence to specifications. Tests can be based on industry standards. However, these methods often do not have fixed acceptance criteria, and the chemically aggressive tests can provide unduly conservative results in some instances, eliminating potentially acceptable materials from further consideration. To overcome these limitations, it is increasingly recommended that tests in simulated service environments be used to provide a link between the actual level of performance required and the results ofmore » standardized tests.« less