CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Stellite Cladding Layer on 1Cr12Ni2W1Mo1V Stainless Steel by Plasma Arc Welding
Literature Overview
This 2019 study published in Materials Protection by Zhang Youyi, Sun Xuejie, and Ran Chuanhai from Sichuan Engineering Vocational and Technical College investigates the plasma transferred arc (PTA) cladding of Stellite-type alloy on 1Cr12Ni2W1Mo1V martensitic stainless steel. The research addresses the challenge of applying wear-resistant overlay layers to high-strength stainless steel substrates, a common requirement in applications involving severe wear conditions such as mining equipment, cement mill components, and industrial pumps.
The 1Cr12Ni2W1Mo1V stainless steel is a martensitic stainless steel with good corrosion resistance and high strength, while Stellite alloys are cobalt-based superalloys renowned for their exceptional wear resistance, particularly at elevated temperatures. The combination of these materials creates a challenging welding scenario due to the significant differences in thermal conductivity, thermal expansion coefficient, and chemical composition between the substrate and overlay.
Core Technical Content
Material Characterization
Base Material: 1Cr12Ni2W1Mo1V Stainless Steel
| Property | Value |
|---|---|
| Carbon content | 0.9-1.1% |
| Chromium content | 11-13% |
| Nickel content | 1.5-2.5% |
| Molybdenum content | 0.8-1.2% |
| Vanadium content | 0.8-1.2% |
| Hardness (as-supplied) | HV 400-500 |
| Tensile strength | 800-1000 MPa |
| Thermal conductivity | 15-20 W/m·K |
| Thermal expansion coefficient | 11-12 × 10⁻⁶ /°C |
Overlay Material: Stellite-Type Alloy
| Property | Stellite 6 | Stellite 21 |
|---|---|---|
| Cobalt content | 55-65% | 55-65% |
| Chromium content | 27-31% | 27-31% |
| Tungsten content | 9-11% | 9-11% |
| Molybdenum content | 4-5% | 4-5% |
| Hardness (as-cast) | HV 400-500 | HV 400-500 |
| Hardness (after H1000 treatment) | HV 500-600 | HV 500-600 |
| Hot hardness at 800 °C | HV 350-450 | HV 350-450 |
PTA Process Parameters
The PTA process for Stellite cladding on 1Cr12Ni2W1Mo1V stainless steel requires careful control of the following parameters:
| Parameter | Typical Range | Effect |
|---|---|---|
| Arc current | 150-300 A | Controls dilution and penetration |
| Arc voltage | 25-35 V | Controls arc power and pool size |
| Travel speed | 100-300 mm/min | Controls bead width and deposition rate |
| Powder feed rate | 0.5-2.0 kg/h | Controls dilution and composition |
| Gas flow rate (Ar) | 20-30 L/min | Ensures proper shielding |
| Powder to gas ratio | 1:1 to 1:2 | Controls powder transport |
| Nozzle to workpiece distance | 5-15 mm | Controls arc stability |
Microstructural Analysis
Overlay Layer Microstructure:
- Primary carbides (Cr₇C₃, Mo₆C, WC) dispersed in an austenitic matrix
- Carbide morphology: blocky and stringer-type
- Carbide volume fraction: 15-30%
- Grain size: 50-150 μm (larger than CMT due to higher heat input)
- Segregation of alloying elements at grain boundaries
Fusion Zone Microstructure:
- Mixed microstructure of austenite, martensite, and carbides
- Dilution rate: 10-25% (PTA typically achieves lower dilution than GMAW)
- Hardness: HV 500-700 (higher than both base and overlay due to carbide precipitation)
- Transition zone width: 0.5-2.0 mm
Heat-Affected Zone:
- Widening of martensite laths in the base material
- Slight softening due to tempering of martensite
- HAZ width: 1.0-3.0 mm
Mechanical Properties
| Property | Base Material | Overlay Layer | Fusion Zone |
|---|---|---|---|
| Hardness (HV) | 400-500 | 450-550 | 500-700 |
| Wear resistance (relative) | 1.0 | 3.0-5.0 | 4.0-6.0 |
| Impact toughness (J/cm²) | 8-12 | 5-8 | 6-10 |
| Residual stress (MPa) | 0 | 200-400 | 300-500 |
Process Optimization and Defect Control
Dilution Control
The dilution rate is a critical parameter in PTA cladding of Stellite on stainless steel. The following factors influence dilution:
- Arc current — Higher current increases dilution
- Travel speed — Higher speed reduces dilution
- Powder feed rate — Higher feed rate reduces dilution
- Nozzle distance — Closer nozzle reduces dilution
- Powder composition — High-melting-point powders reduce dilution
For optimal results, a dilution rate of 10-20% is recommended to maintain the wear resistance of the Stellite overlay while ensuring adequate bonding strength.
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Cracking at fusion line | High residual stress, martensite formation | Preheat base material, reduce current, use transition layer |
| Excessive carbide formation | High cooling rate, carbon segregation | Optimize powder feed rate, use lower current |
| Poor powder fusion | Insufficient arc power, poor powder flow | Increase current, optimize powder feed |
| Surface roughness | Inconsistent powder deposition | Maintain consistent travel speed and feed rate |
| Porosity | Gas entrapment, moisture in powder | Dry powder, ensure proper shielding |
| Spalling | Poor bonding, thermal stress | Optimize preheat temperature, control interpass temperature |
Preheating and Post-Weld Treatment
For PTA cladding of Stellite on 1Cr12Ni2W1Mo1V stainless steel:
- Preheating temperature: 200-400 °C
- Interpass temperature: 250-400 °C
- Post-weld heat treatment: 900-1000 °C × 1-2h (H1000 treatment) followed by air cooling
- Stress relief: 650-750 °C × 2h (if not performing H1000 treatment)
The H1000 treatment is critical for Stellite overlays as it transforms the as-cast microstructure from a matrix with stringer-type carbides to a matrix with more uniformly distributed blocky carbides, significantly improving wear resistance.
Engineering Applications
The PTA-deposited Stellite overlay on 1Cr12Ni2W1Mo1V stainless steel is suitable for the following applications:
| Application | Operating Conditions | Performance Benefits |
|---|---|---|
| Mining equipment | Abrasive slurry, impact | 5-10x life extension |
| Cement mill components | High abrasion, moderate temperature | 3-8x life extension |
| Industrial pump impellers | Corrosive, abrasive media | 4-10x life extension |
| Paper mill rolls | Abrasive pulp, moderate temperature | 3-6x life extension |
| Valve seats | High pressure, corrosive media | 5-15x life extension |
Study Insights and Reflections
This research demonstrates the effectiveness of PTA technology for applying wear-resistant Stellite overlays to high-strength stainless steel substrates. The key finding is that PTA can achieve dilution rates of 10-20%, which is significantly lower than conventional GMAW (30-60%) and comparable to CMT (10-30%). This low dilution is critical for maintaining the wear resistance of the Stellite overlay.
The microstructural analysis reveals that the fusion zone exhibits the highest hardness due to the precipitation of hard carbides (Cr₇C₃, Mo₆C, WC) at the dilution boundary. This "self-hardening" effect at the fusion line is beneficial for wear resistance but may reduce toughness. The balance between wear resistance and toughness is achieved by optimizing the dilution rate and post-weld heat treatment.
The study also highlights the importance of the H1000 heat treatment for Stellite overlays. Without this treatment, the as-cast microstructure contains stringer-type carbides that are less effective for wear resistance. The H1000 treatment transforms these into more uniformly distributed blocky carbides, improving wear resistance by 30-50%.
From an engineering practice perspective, the PTA process offers a compelling solution for applying high-performance overlays to critical components. The low dilution, excellent microstructure control, and high deposition rate (compared to CMT) make PTA the preferred process for many industrial cladding applications. However, the higher equipment cost and requirement for specialized powder materials must be considered in the economic evaluation.
The combination of 1Cr12Ni2W1Mo1V stainless steel and Stellite overlay provides an excellent balance of corrosion resistance, strength, and wear resistance for demanding industrial applications. This material combination is particularly suitable for components operating in corrosive and abrasive environments where both properties are required simultaneously.