Nickel Base Castings and Nickel Alloy Castings
Nickel-base castings are used primarily for their superior heat resistant and corrosive resistant qualities.
In North America, the designations for cast stainless steel and nickel based alloys are descriptive of their chemistry and purpose. A designation beginning with the letter “C” indicates that the alloy is used primarily for corrosive service. If the first letter is “H”, the alloy is typically used for high temperature service at or above 1200° F.
Common high-nickel alloys include Monel, Inconel, and Hastelloy.
Monel is the common designation for a group of nickel base castings and nickel alloys, primarily composed of nickel and copper, with small amounts of iron, manganese, carbon, and silicon. Stronger than pure nickel, Monel alloys are resistant to corrosion by many agents, including rapidly flowing seawater. Compared to steel, Monel is very difficult to machine as it work-hardens very quickly. It needs to be turned and worked at slow speeds and low feed rates.
Inconel, sometimes called “Inco,” is a family of austenitic nickel-chromium-based superalloys. Inconel alloys are oxidation and corrosion resistant, and are well suited for service in extreme environments subjected to pressure and heat. When heated, Inconel forms a thick, stable, passivating oxide layer protecting the surface from further attack. Inconel retains strength over a wide temperature range, attractive for high temperature applications where aluminum and steel would succumb to creep as a result of thermally induced crystal vacancies. Inconel’s high temperature strength is developed by solid solution strengthening or precipitation hardening, depending on the alloy.
Inconel alloys are typically used in high temperature applications. Common trade names for Inconel Alloy 625 include: Inconel 625, Chronin 625, Altemp 625, Haynes 625, Nickelvac 625 and Nicrofer 6020.
The primary function of the Hastelloy super alloys is that of effective survival under high-temperature, high-stress service in a moderately to severely corrosive, and/or erosion-prone environment where more common and less expensive iron-based alloys would fail, including the pressure vessels,chemical reactors, distillation equipment, and pipes and valves in chemical industry.
Although a super alloy, Hastelloy does experience degradation due to fabricating and handling. Another extremely versatile of the ternary alloy nickel systems is the nickel-chromium-molybdenum (Ni-Cr-Mo) system, common to Hastelloy C grades. The chromium and molybdenum provide resistance to oxidizing and reducing acids, and they also work synergistically to provide outstanding resistance to the chloride-induced phenomena of pitting, crevice attack, and stress corrosion cracking. The remaining two families, nickel-chromium-iron (Ni-Cr-Fe) and nickel-ironchromium (Ni-Fe-Cr), were designed to bridge the performance and cost gaps between the Ni-Cr alloys and the austenitic stainless steels. Their benefits over the stainless steels include enhanced resistance to stress corrosion cracking.
Monel is a trademark of Special Metals Corporation
Hastelloy is a registered trademark of Haynes International