Formation of a precipitate, isomorphous with (Fe, Co)7 (Mo, W)6 and relatively rich in tungsten, has been studied in 38 plates, 1/16 to 15/16 in thick, made from 24 commercial heats of Ni-Cr-Mo-Fe-W alloy. Heating temperature, holding time at temperature, rate of heating and rate of cooling affect precipitation. If cooling to below 1050 F (565 C) takes less than 2 minutes, precipitation is determined solely by the heating cycle. No visible precipitation occurs above 2100 F (1150 C). Precipitated particles may or may not be well separated in the range 1975 F (1080 C) to 2100 F (1150 C), depending on response of a given heat, but form a complete grain-boundary envelope in the range 1600 F (870 C) to 1975 F (1080 C) Numerous particles precipitate within the grains in the range 1500 F (815 C) to 1600 F (870 C). From 1250 F (675 C)to 1500 F (815 C) the grain boundary precipitate is much finer but resolvable, with accompanying optical effects and from 1075 F (580 C) to 1250 F (675 C) the precipitate is unresolvable.
Rapid cooling from above 2235 F (1225 C) to suppress precipitation gives best corrosion resistance, but treatment is impractical for large process vessels because of the required heat removal rate and the accompanying distortion. A dependable microstructure with a thermally-stabilized precipitate can be secured, even in the thicker sections, by heating slowly to 2140 ± 20 F (1170 ± 10 C) and holding 1 hr, furnace cooling to 2025 F (1105 C) and holding a minimum of 2 hr and then cooling to below 1050 F (565 C) in less than 30 minutes. A single-pass weld does not sensitize material so treated, but subsequent passes will. Treatment increases tensile strength markedly but has little effect on yield strength and reduces ductility and notch toughness significantly; these effects are more pronounced in wrought than in weld metal. Although weld metal does not meet all ASME Code requirements, welded pressure vessels given the heat treatment have operated successfully for five years.
