Valve Trim Hardening: Stellite, Colmonoy, and Hard Chrome Coatings

Valve trim hardening is the application of hard, wear-resistant materials to the seating surfaces, plugs, discs, and other contact areas of valves exposed to erosive flow, abrasive particles, cavitation damage, or high-velocity impingement. Standard stainless steel or carbon steel trim materials erode rapidly in aggressive service conditions, leading to seat leakage, trim damage, and valve failure within months or even weeks of installation. Hard-facing alloys and coatings applied to critical trim surfaces extend valve service life by orders of magnitude in erosive service, providing a cost-effective alternative to complete valve replacement on short intervals.

Stellite Hard-Facing Alloys

Stellite alloys (trademarked family of cobalt-chromium alloys) are the most widely used hard-facing materials for valve trim in demanding service. The cobalt base provides excellent corrosion resistance, while the chromium carbides and tungsten carbides distributed through the alloy matrix provide the hardness and wear resistance. Stellite 6 (approximately Rockwell C 38-42 hardness) is the most common general-purpose grade, offering a good balance of wear resistance, corrosion resistance, and weldability. Stellite 12 and Stellite 21 are alternative grades with different carbide contents that provide higher hardness or better corrosion resistance in specific service conditions. Stellite hard-facing is applied to valve seats and plugs by welding (GTAW, plasma transferred arc, or flame spraying), depositing a 2 to 4 mm thick layer that is then ground to the finished dimension.

• Stellite 6: Rockwell C 38-42, most common general-purpose hard-facing for valve trim

• Stellite 12: higher carbide content, harder than Stellite 6, for more abrasive service

• Colmonoy (nickel-based): alternative to Stellite where cobalt-free material is required

• Tungsten carbide: extremely hard, used for severe abrasion in slurry and erosion service

• Hard chrome: electroplated or HVOF-applied, used for stem surface hardness

Thermal Spray and HVOF Coatings

Thermal spray processes deposit hard coating materials onto valve surfaces at high velocity in a semi-molten state. High-velocity oxy-fuel (HVOF) spraying is the preferred thermal spray process for valve trim because it produces dense, well-bonded coatings with low porosity. HVOF can deposit tungsten carbide-cobalt (WC-Co), tungsten carbide-nickel-chromium (WC-Ni-Cr), chromium carbide, and other hard coating materials at thicknesses of 0.1 to 0.5 mm. These coatings achieve very high hardness (Rockwell C 65-75 for WC-Co) that exceeds what is achievable by welded hard-facing, making them suitable for the most severe abrasive and erosive conditions. The thin coating thickness requires that the substrate be machined undersized before coating and then brought to final dimension after coating by grinding.

Cobalt-Free Alternatives for Nuclear Service

Nuclear power plants restrict the use of cobalt-containing alloys (including Stellite) in reactor coolant systems because cobalt-58 and cobalt-60 activation products contribute significantly to radiation dose rates that complicate maintenance. Cobalt-free alternatives for valve hard-facing in nuclear service include Tribaloy T-400 (cobalt-free but contains cobalt-free molybdenum-based alloy), Norem 02 (iron-based alloy developed specifically as cobalt-free substitute for Stellite 6), and Colmonoy grades (nickel-based). These alternatives provide acceptable wear resistance for nuclear coolant gate and globe valve trim while eliminating cobalt activation products from the primary circuit.