754Alloy C4 1970s to Present

In addition to the 10-fold decrease in carbon and silicon of alloy C, alloy C-4 had three other major modifications, that is, omission of tungsten from its basic chemical composition, reduction in iron level, and addition of some titanium. The above changes resulted in significant improvement in the precipitation kinetics of intermetallic phases, when exposed in the sensitizing range of 550-1090°C for extended periods of time, virtually eliminating the intermetallic and grain boundary precipitation of the "mu" phase, which has a (Ni, Fe, Co)3(W, Mo, Cr)2 type structure and various other phases. These phases are detrimental to ductility, toughness, and corrosion resistance.

The general corrosion resistance of alloy C-276 and alloy C-4 was essentially the same in many corrosive environments, except that in strongly reducing media like hydrochloric acid, alloy C-276 was better; but in highly oxidizing media, the opposite was true, that is, alloy C-4 was better. Alloy C-4 offers good corrosion resistance to a wide variety of media including organic acids and acid chloride solutions. Details of alloy C-4 development are documented in the open literature [6, 28, 29]. This alloy has found greater acceptance in European countries in contrast to alloy C-276, which is more widely used and accepted in the United States.

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