_______ is an alloy of chromium, cobalt, and tungsten used to treat interior components of valves to make them very hard and erosion resistant.

• A. Stainless Steel
• B. Monel
• C. Inconel
• D. Stellite

Answer: (D)

Hard-facing materials for valve interiors are chosen for extreme hardness and erosion resistance, since these surfaces endure constant contact with flowing fluids and any entrained particles. Stellite fits this role perfectly as a cobalt-based alloy that incorporates chromium and tungsten. Those elements form hard carbide phases that lock in place and give very high hardness along with excellent wear and erosion resistance, even at elevated temperatures. This makes it ideal for hard-facing valve seats and stems where abrasion would quickly wear softer metals away.

Stainless steel, while superb for corrosion resistance, is iron-based and not optimized for the extreme wear environment of valve interiors. Monel and Inconel are nickel-based alloys valued for high-temperature strength and corrosion resistance, but they aren’t the go-to choice for hard-facing wear surfaces like interior valve components. The distinctive cobalt-chromium-tungsten composition of Stellite is specifically aimed at delivering the hardness and wear resistance needed in those applications.