HASTELLOY C-4 alloy has an extremely versatile Ni-Cr-Mo alloy with higher than normal resistance to HAZ sensitization HASTELLOY C-4 alloy (UNS N06455) is considered the most (micro structurally) stable of the commonly used nickel-chromium-molybdenum materials, that are famous for their resistance to numerous aggressive chemicals, particularly hydrochloric acid, sulfuric acid, and chlorides. This stableness ensures that the alloy can be welded without fear of sensitization, i.e. the nucleation and growth of deleterious, second phase precipitates in the grain borders of the weld heat-affected zone (HAZ).

Similar to other nickel alloys, it is ductile, simple to form and weld, and has extraordinary resistance to stress corrosion cracking in chloride-bearing solutions. With its high chromium and molybdenum contents, it has the ability to endure both oxidizing and non-oxidizing acids, and is resistant to pitting and crevice attack in the presence of chlorides and other halides.

Normal chemical process industry (CPI) applications include reactors as well as heat exchangers.

Welding & Fabrication

Elgiloy HASTELLOY C-4 alloy is incredibly amenable to the Gas Metal Arc (GMA/MIG), Gas Tungsten Arc (GTA/TIG), and Shielded Metal Arc (SMA/Stick) welding procedures, matching filler metals (i.e. solid wires and coated electrodes) readily available for these processes.

Wrought products of HASTELLOY C-4 alloy are supplied in the Mill Annealed (MA) condition, except if specified. This solution annealing procedure has been created to maximize the alloy’s corrosion resistance and ductility. Following all hot forming procedures, the material needs to be re-annealed, to regain optimum properties.

The alloy also needs to be re-annealed after any cold forming procedures that result in an outer fiber elongation of 7% or more. The annealing temperature for HASTELLOY C-4 alloy is 1066°C (1950°F), and water quenching is recommended. A hold time at the annealing temperature of 10 to 30 minutes is strongly suggested; subject to the thickness of the structure.

HASTELLOY C-4 alloy might be hot forged, hot rolled, hot formed, hot upset and hot extruded. But nonetheless, it is much more sensitive to strain and strain rates than the austenitic stainless steels, and the hot working temperature range is reasonably narrow. For instance, the suggested start temperature for hot forging is 1177°C (2150°F) and the preferred finish temperature is 954°C (1750°F).

Moderate reductions and consistent re-heating gives the best results in “Fabrication of HASTELLOY Corrosion-Resistant Alloys” This reference in addition has guidelines for cold forming, spinning, drop hammering, punching, and shearing. The alloy is stiffer compared to most austenitic stainless steels, and more energy is needed at the time of cold forming. Additionally, HASTELLOY C-4 alloy work hardens more easily than most austenitic stainless steels, and requires numerous stages of cold work, with intermediate anneals.

Although cold work does not typically have an effect on the resistance of HASTELLOY C-4 alloy to general corrosion and to chloride-induced pitting and crevice attack, it actually can affect resistance to stress corrosion cracking. For the best possible corrosion performance, the re-annealing of cold worked parts (following an outer fiber elongation of 7% or more) is extremely important.