Ni-Cr-Mo alloy system- Special family of Nickel based alloys

The most versatile alloys in Nickel based alloy matrix is Nickel-Chromium-Molybdenum alloy. The presence of chromium and nickel offers resistance to oxidizing and reducing acids and also act as synergistically to offer great resistance to chloride induced cracking of pitting, crevice and stress corrosion cracking. Other families are nickel-chromium-iron and nickel-iron-chromium that are made to bridge the performance and cost gaps between the Nickel-Chromium alloys and austenitic stainless steels. Their advantages over stainless steels including better resistance to stress corrosion cracking.

Nickel-Chromium-Molybdenum alloys are specifically resistant to insidious and variable forms of corrosion resulted by chlorides, pitting, crevice, and stress corrosion cracking. Chromium causes passivation in oxidizing acids as it occurs in the stainless steels. Molybdenum offers resistance to reducing acids specifically to hydrochloric acid. In heat exchangers, not just steam is essential, in fact cooling medium is also considered. Cooling waters are usually chlorinated to combat biofouling and sea water is used as a coolant at various coastal locations.

As Ni-Cr-Mo alloys prevent hydrochloric acids and related salts, they also prevent the related compounds of bromine and fluorine. Actually Ni-Cr-Mo alloys are one of the metallic materials that withstand hot hydrofluoric acid. They resist sulfuric and phosphoric acids and specific contents of caustic soda and caustic potash.

Hastelloy C

Hastelloy C-276- It was made by introduction of argon-oxygen decarburization during melting

Hastelloy C-2000- Copper was added to alloy C2000 for resistance to sulfuric and hydrofluoric acids.

The effect of tungsten in Ni-Cr-Mo alloys is similar to Molybdenum.

NiCrMo alloys are basically face centered cubic structure. They are similar to austenitic stainless steels. They are not fully stable and are hence sensitive towards the production of second phases when subjected to temperatures in about range 650oC to 1100oC.

The NiCrMo alloys are usually supplied in the solution annealed form, the annealing temperature for many alloys is about 1120oC. They are usually cooled in water from their annealing temperatures to lock in their high temperature fcc microstructures. Although they are cooled in gas if they are annealed in hydrogen.

At low temperature limits, a homogenous, long range ordering reaction is feasible however this is slow and of no worry during welding. Nickel alloy Hastelloy C22 wire was designed particularly to take benefit of the long range ordering reaction in a time length of 48 hours with just a moderate reduction in its corrosion functionality. The development of cast Nickel-Chromium-Molybdenum alloys has followed a similar path with initial efforts followed at enhancing heat stability and where feasible improved corrosion resistance. The nickel-chromium-molybdenum alloys are usually supplied in the solution annealed condition, the annealing temperature for many alloys is around 1120oC. They are usually cooled in water from their annealing temperature to hold their high temperature fcc microstructures. 

Concerns over structural instability are extreme during welding, since weld heat affected regions are subjected to temperatures in the sensitizing limit. The second phases of many concerns are M6C carbides that develop in the limit 650 – 1040oC and micro-phase that occurs in the limit 760oC to 1090oC. These phases develop quickly and differently usually within the alloy grain limits, leaving them to be sensitive to preferential corrosion.