HTS Coatings — machining & grinding

Chromium Carbide 75/25

75% CrC, 25% NiCr

  • HVOF

Cr₃C₂-NiCr 75/25 is the coating we reach for when boiler tubes have to survive hot ash and a thermal swing. Seventy-five percent chromium carbide, twenty-five percent nickel-chromium binder — softer than the 80/20 grade, meaningfully tougher, same 870°C service ceiling.

The 80/20 variant is the workhorse for steady-state hot erosion — more carbide, higher hardness, best abrasion-at-temperature in a chromium-carbide cermet. It's the wrong coating the minute the duty shifts to cyclic thermal shock or fluidized-bed impact. Drop the carbide by five points, raise the NiCr by five, and the coating absorbs strain instead of cracking. Nano-modified 25NiCr variants have demonstrated 2.2× cyclic life at 750°C versus conventional chromium carbides.

We apply it by HVOF and HVAF onto waterwalls, superheater panels, furnace rolls, fluidized-bed tubes, and gas-turbine hardware below the 870°C ceiling. Hardness 850-1,050 HV on HVOF, bond strength ~10,000 psi, porosity under one percent. HVAF tightens porosity to 0.3% and keeps the coating ductile above 600°C — the combination that lengthens cyclic life on thermally shocked tubes.

Versus 80/20, you lose ~100-200 HV and gain fracture toughness. Versus WC-CoCr, you keep running where cobalt binders decarburize. Similar cost, same guns, same grinding. The decision is the duty.

Send us the tube, the furnace curve, and the start-stop schedule. We'll tell you whether it's the 75/25 or the 80/20.

Technical data

Hardness
700-900 HV typical APS/plasma; 850-1,050 HV typical HVOF; 900-1,100 HV300 Kermetico HVAF; Knoop 1,080-1,170 HK on HVOF-sprayed A516 boiler steel (milled powder, Ra ~1.3 µm)
Bond strength
~10,000 psi (70 MPa @ 500 µm) HVOF typical; plasma 5,000-8,000 psi depending on parameters
Max service temp
1,380-1,600°F (750-870°C) continuous for oxidation/erosion service; research reports useful performance to 900°C in coal-fired boiler environments
Max service temp
750-870°C continuous; 900°C short-excursion in erosion-oxidation duty
As-sprayed porosity
<1% HVOF typical; 0.34% reported for optimized HVAF nanostructured 25NiCr; 2-3% on standard APS
Typical thickness
6-20 mils (150-500 µm) service coatings; 6-10 mils (150-250 µm) is the common boiler and turbine range; 1 mm (40 mils) is the practical HVOF upper bound
Surface finish (Ra)
80-130 as-sprayed; 20-40 ground; <10 on diamond-lapped sealing faces

Where it earns its keep

  • Same 870°C service envelope as the 80/20 grade — full hot-erosion and hot-oxidation capability where WC-Co and WC-CoCr cannot run
  • Five extra points of NiCr binder buy meaningful fracture toughness and thermal-shock tolerance — the right pick when boiler tubes see load swings, start-stop cycles, or fluidized-bed impact instead of pure steady-state erosion
  • Dense (<1%), high-bond (~10,000 psi) HVOF microstructure with oxidation resistance coming from NiCr and erosion resistance from Cr₃C₂ — the classic proven duty cycle for biomass and coal-fired waterwalls
  • Drops into the same HVOF and HVAF guns as the 80/20 grade — same powder pricing tier, same grinding practice, same inspection; the only engineering decision is hardness-versus-toughness
  • HVAF route pushes porosity under 0.4% and retains ductility above 600°C, which lengthens cyclic life on thermally shocked waterwalls versus gas-fuel HVOF

Where it doesn't

  • Softer than the 80/20 grade by roughly 100-200 HV — if the duty is steady-state abrasion or fine-particle erosion at temperature with minimal thermal cycling, step up to the 80/20 variant for more carbide and better wear numbers
  • Not a corrosion coating below 500°C — WC-10Co-4Cr or Inconel 625 outperform at low temperatures; Cr₃C₂-NiCr earns its keep above ~600°C where the other two cannot
  • HVOF parameters (O₂/H₂ ratio, standoff, powder feed) meaningfully shift carbide retention, oxide stringers, and toughness — both 75/25 and 80/20 need qualified params; the milled-powder route has been shown to raise fatigue life ~12% (380 vs 340 MPa on A516)
  • Similar powder cost to the 80/20 grade — spec the 75/25 when thermal cycling, mild impact, or fluidized-bed service is the real duty, not as a default

Typical applications

  • Biomass, waste-to-energy, and pulverized-coal boiler tubes — waterwall, superheater, and reheater panels that see thermal cycling and ash erosion
  • Fluidized-bed combustor tubes and cyclone internals exposed to bed-material impact at temperature
  • Furnace rolls and hearth components in steel and glass processing
  • Gas-turbine compressor hardware and hot-section erosion shields operating below the coating's 870°C ceiling
  • Fuel-rod mandrels and nuclear fuel-handling components (standard AMPERIT 584 recommendation)
  • Valve stems and hydraulic rods running in oxidizing hot-gas service where WC-Co decarburizes
  • Hot-working rolls, continuous-casting rolls, and guide rolls in steel mills
  • Industrial-fan blades and ID-fan housings handling hot particulate streams

Wear modes addressed

  • Solid-particle erosion at elevated temperature (ash, sand, catalyst carryover)
  • Erosion-corrosion and erosion-oxidation under oxidizing and sulfidizing hot-gas atmospheres
  • Sliding and fretting wear to ~870°C where most cobalt-bonded carbides have already decarburized
  • Thermal-shock and cyclic-thermal fatigue — the 25% NiCr binder absorbs strain better than the 80/20 grade
  • Mild impact and vibratory loading in fluidized-bed and fan service
  • Cavitation erosion (secondary duty in hot-water and steam systems)

Industries

  • Power generation (coal, biomass, waste-to-energy, fluidized-bed boilers)
  • Steel and glass manufacturing (furnace rolls, hearth hardware)
  • Chemical and petrochemical process (hot-gas valves, catalyst-handling)
  • Oil & gas refining (reformer internals, FCCU hardware below decarburization limit)
  • Gas-turbine OEM and MRO (compressor hardware, erosion shields)
  • Nuclear fuel fabrication (mandrels, handling tools)
  • Cement and lime (ID-fan blades, cyclone separators)

Substrates

  • Boiler-grade low-alloy steels (ASTM A213 T22, A213 T91, A516 Gr 70)
  • Carbon steels (1018, 1045, A106)
  • Stainless steels (304, 316, 410, 17-4 PH)
  • Nickel-base superalloys (Inconel 625, 718) for hot-section service
  • Cast irons and ductile iron in fan and roll housings

Sources

Data points on this page draw on the following published references. Nothing here replaces a material-specific review by our process engineers — but it's the working starting point.

Material data on this page is provided as a general reference and can vary by lot, substrate, and application. Contact HTS to confirm the right material and specification for your specific part.

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