Valve Trim Materials: Stainless Steel, Inconel, and Hardened Alloys

Valve trim refers to the internal components of a valve that contact the process fluid and are subject to wear, erosion, corrosion, and high-velocity impingement: typically the closure element (gate, disc, ball, or plug), the seat rings, and the stem. While the valve body material is selected primarily for pressure containment and compatibility with the overall process environment, trim materials are selected specifically to withstand the localized high-velocity flow conditions, metal-to-metal contact, and aggressive fluid environments that exist at the seating surfaces and in the flow path through the valve opening. Trim material selection directly determines valve service life in demanding applications.

Standard Stainless Steel Trim

Grade 316 stainless steel is the most common standard trim material for process valves in corrosive service. Its combination of good corrosion resistance to a broad range of chemicals, adequate hardness (approximately Rockwell B 85 in the annealed condition), and excellent weldability and machinability makes it the default trim material for most general chemical, petrochemical, and pharmaceutical service conditions. For gate and globe valves requiring metal-to-metal seating, the hardness differential between the seat ring and the disc or gate is important: if both surfaces have the same hardness, galling (adhesive welding and metal transfer at high contact stress) can cause rapid seat damage. Standard practice is to harden one seating surface relative to the other, with common combinations of 316 SS seat against Stellite 6 hard-faced disc.

• 316 stainless: general purpose, good corrosion resistance, standard for most process service

• 316/316 combination: acceptable for non-galling soft-seat service, avoid for metal-seated gate valves

• Stellite 6 hard-face: cobalt-chromium alloy, Rockwell C 38-42, standard seat hard-facing

• 17-4 PH stainless: precipitation hardened to Rockwell C 38-40, high strength trim material

• Inconel 625: nickel-chromium alloy, excellent corrosion and high-temperature oxidation resistance

Inconel and High-Nickel Alloy Trim

Inconel alloys (primarily Inconel 625, Inconel 718, and Inconel X-750) are used for valve trim in high-temperature, high-stress, and highly corrosive service where standard stainless steel trim is inadequate. Inconel 625 provides excellent resistance to pitting, crevice corrosion, and stress corrosion cracking in chloride-containing environments, making it the preferred trim material for offshore and seawater service valves. Inconel 718 in the precipitation-hardened condition achieves high strength and hardness (Rockwell C 40-46) suitable for high-pressure trim components that require both corrosion resistance and mechanical strength. Inconel X-750 and other spring-temper nickel alloys are used for spring components in valve assemblies requiring corrosion resistance combined with high fatigue life.

Trim Selection for Erosive and Corrosive Service

Applications combining both erosive flow (high-velocity particulate-laden fluid) and corrosive chemistry require trim materials that resist both attack mechanisms simultaneously. Erosion-corrosion synergy accelerates damage beyond what either mechanism would cause independently: the corrosion removes the protective passive film that would normally slow erosion, and the erosion continuously exposes fresh metal to corrosive attack. Duplex stainless steel trim provides improved erosion resistance over austenitic stainless due to its higher hardness while maintaining corrosion resistance. For severe erosion-corrosion, chromium carbide or tungsten carbide coatings (applied by HVOF thermal spray) on stainless or alloy steel substrates provide the best combination of hardness and corrosion resistance achievable within practical cost constraints.