712 Fabrication Of Aqueous Corrosionresistant Alloys

The nickel alloys for the modern chemical process and petrochemical industries not only have to resist uniform corrosion, localized corrosion, and stress corrosion cracking as characterized by their alloy content, but they must essentially maintain these same corrosion resistance characteristics during manufacture, fabrication, and assembly of various components used in these industries. The industry has experienced some costly mishaps and economic penalties due to premature failures of the component either during fabrication (cracking) or accelerated corrosion during service. This problem in many cases has been traced back to the lack of fundamental understanding of the physical and mechanical metallurgy of the different alloy systems and practice of technically wrong procedures during welding, hot- and cold-working operations, and stress relieving/solution annealing treatments.

The different nickel base alloys of the Ni-Mo and Ni-Cr-Mo families exhibit different physical metallurgical behavior as relates to various phase precipitations during thermomechanical processing and hence the need to follow somewhat different sets of guidelines and precautions in comparison to mild steel and in some cases even stainless steels during various fabrication procedures.

The do's and don'ts of various fabrication techniques such as hot forming vs. cold forming, the need to use proper weld consumables during welding, whether to anneal or not to anneal after welding, is stress relieve heat treatment applicable, the type and degree of cleanliness required for welding, other weld parameters, the heat treatment of formed components, and the heat treatment parameters, the question of iron contact and contamination during fabrication—these and many other such questions generally come up when working with the above alloy classes. The effect of the fabrication parameters on corrosion resistance and toughness is a frequently raised concern. In certain instances weld overlay is employed to combat corrosion, and this technology generates its own set of concerns and questions. Some of these questions are addressed below.

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