In the past two decades the 6% molybdenum stainless alloys have gone from new inventions to standard materials of commerce. As these alloys mature, new developments continue to occur. Three recent developments in one of these alloys, UNS N08367, will be illustrated.

First, customer demand for increased corrosion resistant alloys has led to the development of a premium version of N08367 alloy. This enhanced product, designated AL-6XN PLUSÔ alloy, has increased Cr and Mo contents, giving it a higher minimum pitting resistance equivalent number (PREN). Careful control of composition keeps it within the broad range assigned to N08367 alloy so that it can be used in all applications for which N08367 alloy has been approved. The composition and properties of AL-6XN PLUS alloy are described.

Second, the 6% Mo stainless steels have much greater resistance to chloride stress corrosion cracking than is exhibited by standard austenitic stainless steels, but they are not immune. Successful use of these alloys in high temperature chloride environments requires development of data on the application limitations. SCC tests have been performed on N08367 alloy at a variety of temperatures and chloride concentrations, and threshold maps are being generated.

Third, mill-produced alloy forms must typically be fabricated to produce useful articles. Such fabrication can exert a profound influence on the properties of an alloy. For the 6% Mo alloys, the potentially detrimental effect of welding was recognized more than a decade ago, and practical guidelines were developed. Other fabrication processes can also potentially impair performance. In particular, the presence of embedded free iron on the surface of stainless steels has been shown to promote pitting corrosion. More recent work has indicated that the nickel alloys containing high Mo are essentially immune to this effect. For the 6% Mo alloys, which occupy an intermediate position, the situation has been less clear. Data on the influence of iron embedded on the surface of N08367 alloy have been generated and will be presented.