Intergranular stress corrosion cracking (SCC) of alloy B-2 (UNS N1 0665) can occur in dilute acids at low temperatures if there is short range ordering and NiMo in its metallurgical structure. Certain chemistries of alloy B-2 are particularly susceptible to the rapid kinetics of the Ni4Mo transformation which can occur within minutes of exposure to temperatures of 650-750°C such as during welding. The severity of ordering is dependent on alloy chemistry and thermomechanical processing conditions, and can result in reduced ductility at 700°C, and subsequently to SCC. SCC in the heat-aRected zones of welds of fabricated equipment of alloy B-2 have been found to be associated with the presence of Ni4Mo and a short-range ordered structure. Laboratory tests confirmed that only a partially-ordered structure is needed for alloy B-2 to be highly susceptible to SCC in dilute sulfhric acid. A strong correlation was found between degree of ordering, and both susceptibility to SCC and reduced ductility at 700°C. Development of alloy B-3 (UNS N10675) overcomes this thermal instability of alloy B-2, and provides enhanced resistance to SCC in the as-welded condition. In addition to TEM evidence and ductility measurements at 700°C, a stress-corrosion cracking test in boiling 5 wt.% sulfhric acid has also been found useful in evaluating the susceptibility of Ni-Mo alloys to ordering, and to Ni4Mo formation.
Keywords: Aging, ordering, short-range order, Ni4Mo, nickel-molydenum, stress corrosion cracking, ductility, thermomechanical cracking, annealing, alloy B-2, alloy B-3, UNS N10665, UNS N10675, sulfiric acid.