CHRONOLOGY OF DEVELOPMENTS IN Ni-Mo ALLOYS: THE LAST 70 YEARS

摘要

Commercial quasi-binary Ni-Mo alloys containing molybdenum as the primary alloying element between 26 % and 30%rnhave been widely used in many chemical processes since the late 1920's. The first major alloy of this family, Alloy B (UNS #rnN10001), was developed to be resistant to hydrochloric acid and was used in many applications such as in the distillation,rncondensation, and handling of this and other reducing acids such as sulfuric acid under 60% concentration. Many processesrnexisted where use of catalysts like aluminum chloride led to HCl formation by hydrolysis thus requiting use of Ni-Mo alloys.rnHowever, a major problem when using alloy B in the as-welded condition was the decreased corrosion resistance of the heataffectedrnzone due to inter-granular precipitates consisting chiefly of carbides. This meant that the welded alloy B components hadrnto be fully annealed and rapidly cooled prior to its use in severe corrosive environments. This presented a great handicap for widernusage of this alloy in some very large process vessels as demanded by the chemical process industry of the late 1960's and earlyrn1970's. To solve this problem, alloy B-2 (UNS # N 10665), which was a lower carbon, silicon & iron containing version of thernalloy B, was introduced to the industry in 1975, thus allowing its use in the as welded condition. Even though the problem of HAZrncorrosion / knife line attack was solved, another problem of fabricability and stress corrosion cracking in certain environmentsrnemerged, a phenomenon which was not fully addressed or understood at the time of lowering the iron content in alloy B-2 to 2%rnmax with no minimums This paper addresses the fundamental work on understanding this phenomenon and solving it by precisernchemistry control within the current alloy B-2 ASME/ASTM chemistry specifications without degrading its mechanical orrncorrosion properties. Since then other modifications of Ni-Mo alloys to solve both the fabricability and stress corrosion crackingrnproblems have emerged such as alloy B3 ( N10675 ), alloy B4 ( N10629 ) and alloy B10 ( N10624 ). This paper gives a detailedrndescription of the various alloys in the Ni-Mo family along with some guidelines on fabrication.