Hardfacing Alloys: Stellite, Chromium Carbide, and Tungsten Carbide Overlay
Hardfacing — the deposition of a wear-resistant alloy onto a substrate by welding or thermal spray — is one of the most cost-effective strategies in materials engineering for extending component service life against abrasion, erosion, cavitation, and metal-to-metal wear. This article examines the three principal hardfacing alloy families (cobalt-base Stellites, iron-base chromium carbide overlays, and tungsten carbide composites), the deposition processes that control deposit quality and dilution, the metallurgical relationships between microstructure and wear performance, and the selection logic that matching alloy to wear mechanism demands.
Key Takeaways
- Hardfacing alloy selection is driven by wear mechanism first: abrasion demands high carbide hardness (CCO, WC), adhesive and cavitation wear demands toughness and low galling tendency (Stellite), and erosion demands a balance of hardness and impact resistance.
- Stellite grades (Co-Cr-W-C) resist wear at elevated temperatures (>500 °C) and in corrosive environments where iron-base overlays oxidise or corrode; Stellite 6 (38–43 HRC) is the global benchmark valve seat alloy.
- Chromium carbide overlay (CCO) plate achieves 55–65 HRC by depositing a hypereutectic Fe-Cr-C microstructure; transverse stress-relief cracks are a normal, acceptable feature.
- Dilution is the critical process variable in hardfacing: PTA achieves 5–15% dilution versus 20–40% for GMAW/SMAW; second-layer deposits are essentially unaffected by substrate composition.
- WC composite overlays embed 2000–2500 HV tungsten carbide particles in a Co or Ni matrix for extreme abrasion; partial WC dissolution during deposition creates a hardened transition zone.
- Post-weld heat treatment is generally avoided for hardfacing deposits because it can dissolve fine carbides; controlled preheat (based on substrate CE) and slow post-weld cooling are the primary thermal management tools.
Hardfacing Alloy Selector & Dilution Calculator
Estimate deposit hardness after dilution and receive an alloy family recommendation based on your wear conditions.
Please check your inputs — hardness 25–75 HRC, temperature 20–900 °C, substrate HRC 0–60.
Wear Mechanisms and Alloy Selection Logic
The first step in hardfacing alloy selection is identifying the dominant wear mechanism. Applying the wrong alloy family — even a more expensive one — will consistently underperform the correct cheaper option. The four primary wear modes relevant to hardfacing are abrasion, erosion, adhesion, and cavitation; industrial applications frequently involve combinations.
Hard particles cut, plough, or scratch the surface. Severity depends on particle hardness vs deposit hardness ratio. Demands: maximum surface hardness; CCO (55–65 HRC) or WC overlays. Sub-modes: low-stress (sliding) vs high-stress (crushing/gouging) — high-stress requires toughness alongside hardness.
Particles impinge at velocity. At low angle (<30°) resembles abrasion — use hard deposits. At high angle (70–90°) resembles impact — use tough, moderate-hardness alloys (Stellite). In slurry pumps both regimes co-exist depending on geometry.
Metal-to-metal contact causes plastic deformation and junction welding. Critical for valve seats, pump shafts, and die surfaces. Demands: low coefficient of friction, work-hardening matrix, corrosion resistance. Stellite 6 and 21 are the benchmark galling-resistant alloys.
Cobalt-Base Alloys: Stellite Family
The Stellite family of alloys — originally developed by Elwood Haynes in the early 1900s — are cobalt-chromium-tungsten-carbon alloys whose wear resistance arises from a combination of hard carbide phases, a work-hardening FCC cobalt matrix, and a tenacious Cr2O3 surface oxide that resists corrosion and adhesive transfer in corrosive environments. Unlike iron-base hardfacing alloys, Stellites retain their hardness at elevated temperatures (700–900 °C) and resist oxidation, making them the only commercially viable hardfacing choice for steam valve seats, gas turbine blade tip repair, and continuous casting rolls operating above 500 °C.
Stellite Alloy Compositions and Phases
The principal phase in high-carbon Stellite grades (Stellite 1, 6, 12) is M7C3 — a complex (Cr,Co,W)7C3 carbide that forms as hyper-eutectic primary dendrites and interdendrite eutectic product during solidification. In low-carbon grades (Stellite 21, 190), M23C6 is the dominant carbide, present in lower volume fractions, which reduces hardness but greatly increases ductility and impact toughness.
| Grade | Co (bal.) | Cr (wt%) | W (wt%) | Mo (wt%) | C (wt%) | Hardness (HRC) | Primary Phase | Key Application |
|---|---|---|---|---|---|---|---|---|
| Stellite 1 | bal. | 30 | 12.5 | — | 2.4 | 51–58 | M7C3 | Severe abrasion, cutting blades |
| Stellite 6 | bal. | 28 | 4.5 | — | 1.1 | 38–43 | M7C3 | Valve seats, pump impellers, bearing surfaces |
| Stellite 12 | bal. | 29 | 8.3 | — | 1.4 | 45–50 | M7C3 | Abrasion + corrosion, intermediate grade |
| Stellite 21 | bal. | 27 | — | 5.5 | 0.25 | 28–35 | M23C6 | Impact, cavitation, biomedical applications |
| Stellite 190 | bal. | 26 | 14.0 | — | 3.3 | 60–65 | M7C3 | Maximum abrasion, not weldable directly |
| Stellite 694 | bal. | 28 | 19.0 | — | 1.5 | 45–52 | M7C3 | High-temperature turbine repair |
Solidification Microstructure of Stellite 6
During PTA or GTAW deposition of Stellite 6, the weld pool solidifies with a dendritic Co-Cr-W solid solution primary phase, with M7C3 carbides segregating to interdendritic regions as the eutectic product. The carbide volume fraction in Stellite 6 is approximately 12–15 vol%, while in Stellite 1 it reaches 30–35 vol%. Rapid cooling (as occurs in GTAW) refines the carbide spacing and increases hardness slightly; slow cooling (as in furnace brazing) coarsens carbides and reduces hardness. This is directly analogous to solidification rate effects on carbide morphology in iron-carbon systems.
Hot Hardness Retention
The critical advantage of Stellite over iron-base hardfacing alloys is hot hardness. Stellite 6 retains approximately 28 HRC at 600 °C and 18 HRC at 800 °C. High-chromium iron (CCO) drops to approximately 15 HRC at 600 °C as the martensite tempers and M7C3 coarsens. For valve seat applications in power stations where seat temperatures can exceed 550 °C, this renders iron-base overlays inadequate and makes Stellite the default specification (ASTM A484, ASME PCC-1 supplementary requirements for power valves).
Iron-Base Chromium Carbide Overlays
Chromium carbide overlay (CCO) is the dominant hardfacing product by volume for bulk material handling in mining, cement, steel, and aggregate processing industries. Its principal advantage over Stellite is economics: iron-base consumables cost 5–15% of equivalent cobalt-base alloys per kilogram. CCO is deposited by FCAW-O (open-arc flux-cored wire), SAW, or PTA and reaches 55–65 HRC through a hypereutectic Fe-Cr-C microstructure in which the primary Cr7C3 carbides are harder than the abrasive particles typically encountered in bulk handling (silica sand: ~800 HV, Cr7C3: 1400–1800 HV).
Fe-Cr-C Alloy Design
The wear-optimal composition lies in the hypereutectic region of the Fe-Cr-C ternary phase diagram at approximately 4–6 wt%C and 25–35 wt%Cr. In this regime, primary Cr7C3 dendrites solidify first, growing to 50–200 μm in length, followed by the ledeburite eutectic (Cr7C3 + austenite). The austenite matrix transforms to martensite on cooling, contributing additional hardness. The total carbide volume fraction is 50–70 vol%, which is why CCO is so effective against low-stress abrasion but relatively brittle and unsuitable for impact.
Carbon equivalent for CCO composition:
Cr7C3 stability requires:
Cr/C ratio (wt/wt) ≈ 4.0 to 7.5
For Fe-30Cr-5C (representative CCO composition):
Cr/C = 30/5 = 6.0 → predominantly Cr7C3
Below Cr/C ≈ 4.0 → Cr3C2 + Cr7C3 mixture
Above Cr/C ≈ 8.0 → M23C6 becomes dominant (lower hardness)
Primary carbide hardness: Cr7C3 → 1400–1800 HV
Cr3C2 → 2100–2300 HV (but more brittle)
M23C6 → 1000–1200 HVTransverse Stress-Relief Cracking
All commercial CCO plates develop a network of transverse cracks perpendicular to the rolling direction at 25–50 mm spacing during cooling from the deposition temperature. These cracks are a thermodynamic inevitability: the hard, brittle deposit (<1% elongation) cannot accommodate the 200–300 MPa tensile residual stress that builds on cooling. Transverse cracks are a design-accepted feature of CCO plate, provided:
- Cracks do not penetrate the buffer layer into the base plate.
- No longitudinal cracks or spallation are present.
- Crack edges are tight (no gap >0.5 mm for slurry applications).
A 3–6 mm austenitic stainless steel (309L or 307) buffer layer between the base plate and CCO deposit prevents crack propagation and allows the brittle overlay to flex slightly relative to the tough substrate.
CCO Plate Standards and Designations
Commercial CCO plates are typically specified as Cr content + C content + minimum hardness, e.g. a “25+5 / 62 HRC” plate (25 wt%Cr, 5 wt%C, 62 HRC minimum). ASTM G65 (dry rubber wheel abrasion) is the standard comparative wear test, with CCO showing mass loss of 0.02–0.08 g versus 0.3–1.2 g for mild steel in the same test. Overlay thickness ranges from 3 mm to 25 mm; total plate thickness includes base plate (typically 6–25 mm mild steel).
Tungsten Carbide Composite Overlays
Tungsten carbide composite hardfacing embeds WC particles — either cast WC (spherical, containing W2C dendrites) or macro-crystalline WC (blocky, single-phase hexagonal) — in a metallic matrix of Co, Ni, or NiCr applied by PTA, PTAW, or thermal spray. WC particles bring extreme hardness (2000–2500 HV) for maximum resistance to coarse abrasive particles, while the metal matrix provides cohesion and toughness. Applications include oilfield drill bit inserts, centrifugal pump wear plates in dredging, coal pulveriser grinding rings, and sugar cane chopper blades.
WC Particle Dissolution During Deposition
The critical challenge in WC composite hardfacing is controlling WC dissolution into the melt pool. Above approximately 1300 °C WC dissolves rapidly, releasing W and C into the matrix. The dissolved W forms W-containing carbides (W2C, (W,Co)6C) at the WC surface during cooling, creating a brittle dissolution halo that reduces particle-matrix adhesion and can cause delamination. Control strategies include:
- Minimising heat input and arc energy per unit length (critical for PTA current and travel speed).
- Using WC powders with a protective NiCr or CoCr coating to inhibit surface dissolution.
- Limiting powder preheating time in the plasma jet.
- Selecting cast WC (higher melting point due to W2C dendrites) over pure WC for high-dilution processes.
WC-Based Hardfacing Matrix Alloys
| Matrix Alloy | Typical WC Loading (wt%) | Matrix Hardness (HRC) | Key Feature | Primary Application |
|---|---|---|---|---|
| Co-Cr (Stellite-type) | 35–50 | 55–65 | Best toughness-hardness balance | Drill bits, pump impellers |
| NiCrBSi (self-fluxing) | 40–60 | 58–68 | Low deposition temp; HVOF compatible | Valve plugs, agitator blades |
| Fe-Cr-C matrix | 30–45 | 55–62 | Lowest cost; adequate toughness | Mining wear plates, bucket lips |
| Ni-base (Inconel type) | 20–40 | 45–55 | Corrosion resistance in acids/brine | Sour-service pump components |
Deposition Processes and Dilution Control
Dilution is the single most important process variable in hardfacing because it directly determines the chemical composition — and thus hardness and wear resistance — of the first deposited layer. Dilution is defined as:
Dilution (%) = (A_base / (A_base + A_deposit)) × 100
where:
A_base = cross-sectional area of base metal melted into the pool
A_deposit = cross-sectional area of filler metal added
Deposit hardness (approximate, first layer):
HRC_deposit = HRC_filler − (HRC_filler − HRC_substrate) × (D/100)
Example — Stellite 6 (42 HRC) on mild steel (15 HRC), 30% dilution (GMAW):
HRC_deposit = 42 − (42 − 15) × 0.30 = 42 − 8.1 = 33.9 HRC
Same deposit, 10% dilution (PTA):
HRC_deposit = 42 − (42 − 15) × 0.10 = 42 − 2.7 = 39.3 HRC| Process | Typical Dilution (%) | Deposition Rate (kg/h) | Heat Input (kJ/mm) | Alloy Forms | Strengths / Limitations |
|---|---|---|---|---|---|
| PTA (Plasma Transferred Arc) | 5–15 | 0.5–5 | 0.3–1.5 | Powder | Lowest dilution; excellent quality; expensive equipment; slow for large areas |
| GTAW (TIG) | 10–20 | 0.5–1.5 | 0.5–2.0 | Rod, wire | Precise heat control; low dilution; very slow; best for small complex geometry |
| GMAW (MIG) | 20–35 | 2–8 | 0.5–1.5 | Wire | High productivity; moderate dilution; good for large flat surfaces |
| SMAW (Stick) | 25–40 | 1–3 | 1.0–3.0 | Electrode | Portable; versatile; high dilution; Stellite rods available (E-CoCr-A, -B, -C) |
| FCAW-O (Open arc) | 15–30 | 4–12 | 0.8–2.5 | Flux-cored wire | Dominant CCO process; high deposition rate; flat/horizontal positions only |
| SAW (Submerged arc) | 35–50 | 5–20 | 2.0–5.0 | Wire + flux | Highest deposition rate; highest dilution (2 layers essential); flat only |
| HVOF thermal spray | 0 (no fusion) | 3–8 | — | Powder | No dilution, no HAZ; bond strength limited; not suitable for impact |
Multi-Layer Strategies
The standard engineering practice for high-performance hardfacing is a two-layer system: a buffer layer (first pass) absorbs dilution and provides a transition zone, while the second layer achieves near-nominal alloy composition and properties. For Stellite on carbon steel, the first layer at 30% dilution (GMAW) might give 33 HRC versus the nominal 42 HRC; the second layer, diluted primarily by first-layer Stellite, will approach 40–42 HRC. For critical valve seats, a third layer may be specified to ensure a minimum 2 mm of full-composition deposit after final machining.
Hardfacing on Specific Substrate Materials
Carbon and Low-Alloy Steels
The most common substrate class. Carbon equivalent (CE) governs preheat requirement. Steels with CE > 0.45 require preheat of 100–200 °C to prevent hydrogen-induced cold cracking in the HAZ, as discussed in the hydrogen-induced cracking article. The heat-affected zone microstructure beneath a hardfacing deposit is typically a coarse-grained HAZ containing martensite, which can crack without adequate preheat and interpass temperature control.
Carbon equivalent (Ito-Bessyo PCM for crack susceptibility):
PCM = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B
Preheat guidelines for hardfacing on carbon steels:
PCM < 0.20 → No preheat required (T_min = 20 °C)
PCM 0.20–0.25 → Preheat 50–100 °C
PCM 0.25–0.30 → Preheat 100–150 °C
PCM > 0.30 → Preheat 150–250 °C minimumAustenitic Manganese Steel (Hadfield Steel)
13% Mn austenitic steels are used in crusher jaws, rail crossings, and dipper teeth — they work-harden in service from ~200 HV to 500+ HV. Hardfacing with austenitic Mn-based filler (AWS EFeMn-B) or chromium carbide is common for rebuilding worn surfaces. The critical constraint is heat input: above approximately 250–350 °C interpass temperature, Mn steel embrittles by carbide precipitation at austenite grain boundaries. Multi-pass hardfacing must use strict interpass temperature control with forced air or water cooling between passes.
High-Chromium Cast Iron Substrates
Hardfacing onto existing CCO plate for repair uses matching-composition FCAW-O wire or, for improved properties, WC-Ni PTA. Preheat of 150–200 °C is required. The existing CCO must be mechanically prepared (grinding or gouging) to remove the work-hardened surface layer before redeposition. Interpass temperature control is especially critical here due to the very low ductility of the substrate.
Quality Assurance and Testing
Post-deposition testing for hardfacing deposits follows a well-established sequence: visual inspection (ISO 17637 for surface integrity, crack pattern acceptability), hardness survey (Rockwell HRC or Brinell per ISO 6506, ASTM E18/E10), bead geometry measurement, and bond test (inclined plane shear test per AWS C2.25 or peel test). For critical applications (valve seats in power stations, oilfield components), the ASTM G65 dry rubber wheel test and ASTM G76 erosion test provide comparative wear data. Metallographic cross-sections etched in Murakami’s reagent (K3[Fe(CN)6] + KOH) reveal carbide distribution and dilution zone depth in Stellite and CCO deposits.
Understanding the coarse-grained and fine-grained HAZ zones formed beneath hardfacing deposits — critical for preheat selection and crack prevention.
The mechanism behind cold cracking in the HAZ of hardfaced carbon steel components — prevention through preheat, low-hydrogen consumables, and PWHT.
HRC, HV, and Brinell measurement of hardfacing deposits — load selection, surface preparation requirements, and cross-section microhardness traverses.
The hard, brittle phase that forms in the substrate HAZ when hardfacing high-CE steels without adequate preheat — and why it cracks under residual stress.
Alloy Family Comparison Summary
| Property / Criterion | Stellite (Co-Cr-W) | Chromium Carbide (Fe-Cr-C) | WC Composite | Iron-Base Martensitic |
|---|---|---|---|---|
| Hardness (HRC) | 28–58 | 55–65 | 55–68 | 45–62 |
| Low-stress abrasion | Good | Excellent | Excellent | Good |
| High-stress abrasion / gouging | Fair | Good | Very Good | Very Good |
| Metal-to-metal galling | Excellent | Poor | Fair | Poor |
| Cavitation erosion | Very Good | Poor | Fair | Poor |
| Elevated temp. performance (>500 °C) | Excellent | Poor | Good | Poor |
| Corrosion resistance | Very Good | Moderate | Fair–Good (Ni matrix) | Poor–Moderate |
| Relative consumable cost | Very High | Low | High | Low–Medium |
| Machinability after deposit | Grind only | Grind only | Grind only | Grind only |
| Preferred deposition process | PTA, GTAW | FCAW-O, SAW | PTA, PTAW | FCAW-O, GMAW |
Frequently Asked Questions
What is hardfacing and how does it differ from cladding?
What is dilution and why does it matter in hardfacing?
What are the Stellite alloy grades and how do they differ?
How does chromium carbide overlay plate resist abrasion?
What is PTA hardfacing and what are its advantages?
What is the difference between abrasive, erosive, and adhesive wear for alloy selection?
Why do chromium carbide hardfacing deposits crack, and is this acceptable?
What preheat and interpass temperatures are required for hardfacing steels?
How are WC hardfacing deposits different from sintered WC-Co cutting inserts?
What are iron-base hardfacing alloys and when are they preferred over cobalt-base?
Recommended Books and References
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