High-Molybdenum Alloys13

High-molybdenum stainless and nickel alloys are welded with an overmatching filler metal. This is necessary to maintain corrosion resistance in the weld metal at least equal to the base metal. The reason is that molybdenum and chromium segregate as the weld metal solidifies from the melt. This leaves local areas with high and low molybdenum content. Pitting corrosion can start in the low-Mo areas, with the pits eventually growing even into metal with high molybdenum content. This occurs in alloys ranging from 316L to C-276, for the most part being more severe at higher alloy contents This matter began to receive attention when the 6% Mo stainless steels came on the market. If any of these 6% Mo grades are welded without filler metal, the result is a weld bead that may be as low as 3% Mo in areas. The end result can be that this weld has only the pitting corrosion resistance of 317L stainless. In the case of tubular products autogenously welded in production, a high-temperature anneal is used to homogenize the metal. In addition, a small amount of nitrogen, 3–5%, is added to the torch gas. Fabrications of thin sheet, which cannot be annealed after welding, should have this nitrogen addition to minimize the loss of corrosion. Even so, because thin-sheet welds solidify more quickly, the segregation is less severe. In normal fabrication of a 6% Mo grade, alloy 625 (ERNiCrMo-3) filler metal is used. The weld metal contains 9% Mo. After welding, segregation causes some areas to have as little as 6% Mo. The result is that the alloy 625 weld bead has approximately the same corrosion resistance as the 6% Mo base metal. Higher alloy weld fillers, such as ERNiCrMo- 10 or ERNiCrMo-14, may also be used, though the benefit may be more theoretical than real. ERNiCrMo-4 is not suggested, as it has 5% less chromium than does AL-6XN, for example. Since the mid-1980s nearly all of the 6% Mo alloy fabrications have been made, and put into service, using a 9% Mo weld filler. ERNiCrMo-3 weld filler is widely available and is appropriate for welding lower alloys such as 317L, 317LMN, and 904L for chloride service. The problem of reduced weld bead corrosion resistance from molybdenum and chromium segregation exists with most of the 13–16% Mo nickel alloys as well. Filler 686 CPT (ERNiCrMo-14) does appear to be markedly less susceptible to this effect than other high-molybdenum alloys. 

STAINLESS STEELS
James Kelly
Rochester, Michigan
Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
Edited by Myer Kutz
2006 by John Wiley & Sons, Inc.