Zhuojin Pipe Fitting Co., LtdPlasma Surfacing of Stellite Alloy on 1Cr12Ni2W1Mo1V Stainless Steel Microstructure and Performance
Literature Overview
This study by Zhang Youyi, Sun Xuejie, and Ran Chuanhai from Sichuan Vocational and Technical College of Engineering, published in Materials Protection (2019, Vol. 52, No. 1, pp. 88–91), addresses the surface protection of 1Cr12Ni2W1Mo1V martensitic stainless steel through plasma arc surfacing of Stellite alloy. The work is particularly relevant for engineers dealing with water erosion and cavitation erosion in hydraulic components, pumps, and pipeline systems where 1Cr12Ni2W1Mo1V is commonly specified for its combination of strength, toughness, and moderate corrosion resistance. The authors systematically investigated the microstructure, hardness distribution, particle erosion resistance, and water erosion performance of the Stellite overlay, providing quantitative data that can directly inform engineering design decisions.
Core Technical Findings
Microstructure Analysis
The Stellite overlay exhibits excellent metallurgical bonding with the 1Cr12Ni2W1Mo1V substrate, with no observable interfacial defects such as lack of fusion or microcracks. The microstructure consists of a dendritic Co-Cr solid solution matrix with uniformly distributed black carbide particles. The primary carbide phase is M7C3 (Cr7C3), accompanied by a minor amount of WC (tungsten carbide) particles. This carbide distribution is critical for understanding the wear mechanism.
| Microstructural Feature | Description | Engineering Significance |
|---|---|---|
| Matrix phase | Dendritic Co-Cr solid solution | Provides ductile binding matrix |
| Primary carbide | M7C3 (Cr7C3) | Main wear-resistant phase |
| Secondary carbide | WC particles | Enhanced hardness contribution |
| Interface | Good metallurgical bond | No interfacial cracking |
| Grain morphology | Fine and uniform distribution | Improved mechanical homogeneity |
Hardness Distribution
The average microhardness of the Stellite overlay was measured at 382.38 HV (4.9 N), compared to 195.29 HV for the base material, representing a ratio of approximately 1.96 times. The maximum hardness value reached 421.00 HV. A notable finding is that the second surfacing layer exhibits significantly higher hardness than the first layer.
| Layer | Average Hardness (HV 4.9 N) | Maximum Hardness (HV 4.9 N) | Relative to Substrate |
|---|---|---|---|
| Base material (1Cr12Ni2W1Mo1V) | 195.29 | — | 1.00× |
| Stellite layer 1 | Lower than layer 2 | — | ~1.5× (estimated) |
| Stellite layer 2 | 382.38 | 421.00 | 1.96× |
The hardness increment in the second layer can be attributed to the cumulative dilution effect being reduced and the thermal cycling from the first layer providing a pre-heated, refined substrate for the second pass. This is consistent with the general principle in multi-pass surfacing that subsequent layers benefit from the thermal history of preceding layers, leading to finer microstructures and higher hardness.
Erosion Performance
The Stellite overlay demonstrates superior particle erosion resistance compared to the base material. The water erosion rate of the overlay is substantially lower than that of the 1Cr12Ni2W1Mo1V substrate. This improvement is attributed to the synergistic effect of the hard carbide phases (M7C3 and WC) providing abrasive resistance, while the ductile Co-Cr matrix absorbs impact energy and prevents catastrophic spalling.
Process Parameters and Engineering Considerations
Plasma arc surfacing offers several advantages over traditional arc welding processes for overlay applications. The process provides a concentrated heat source with high energy density, resulting in a deep, narrow weld pool that minimizes dilution with the base material. For Stellite surfacing on stainless steel substrates, the following process considerations are critical:
- Shielding gas: Argon or argon-helium mixtures are typically used; helium addition improves arc stability and penetration
- Plasma current: Typically 100–250 A for surfacing applications, depending on wire feed rate
- Wire feed rate: 1.0–3.0 kg/h, matched to current density
- Travel speed: 100–300 mm/min, affecting dilution and microstructure
- Preheating: Generally not required for 1Cr12Ni2W1Mo1V, but may be beneficial for thick sections to reduce residual stress
The dilution rate is a key parameter affecting overlay performance. Lower dilution preserves the beneficial Co-Cr matrix composition and carbide distribution of the Stellite alloy. The plasma process achieves dilution rates typically in the range of 5–15%, which is significantly lower than conventional SMAW or SAW surfacing (20–40%).
Defect Analysis and Countermeasures
From a quality control perspective, several defects are commonly encountered in Stellite plasma surfacing:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Surface cracking | High Cr content promoting brittle carbide network | Control travel speed; use multi-pass with interpass temperature control |
| Porosity | Inadequate shielding; wire surface contamination | Ensure clean wire; optimize gas flow rate and nozzle position |
| Lack of fusion | Insufficient heat input; poor fit-up | Increase current; ensure proper surface preparation |
| Excessive dilution | High travel speed; low current | Optimize current-to-travel-speed ratio |
| Carbide network | Rapid cooling; high cooling rate | Post-weld heat treatment; multi-pass with interpass heating |
Integration with Engineering Practice
In the context of pipeline and piping systems, 1Cr12Ni2W1Mo1V is widely used in Chinese engineering standards (GB/T and SY/T) for applications requiring combined strength, toughness, and corrosion resistance, such as low-temperature service and mildly corrosive environments. However, in water service with solid particle impingement—such as pump casings, valve bodies, and pipeline sections in hydroelectric or marine applications—this grade may suffer from accelerated erosion.
The Stellite overlay approach provides a cost-effective retrofit solution rather than complete component replacement. For new fabrication, the overlay can be applied selectively to erosion-prone areas such as bend sections, elbow interiors, and reducer transitions. The 1.96× hardness improvement translates directly to extended service life, often by a factor of 3–5 in erosive water service.
From a standards compliance perspective, overlay applications must consider:
- API 5L: Overlay requirements for pipeline components in specific service conditions
- ASME B31.3: Post-weld heat treatment requirements for overlays on carbon steel and low-alloy steel piping
- NB/T: Nuclear-related overlay specifications requiring full radiographic examination
- SY/T 0413: Chinese petroleum industry standard for overlay welding of pipeline components
Key Questions and Reflections
Several questions arise from this study that warrant further investigation. First, the study does not report the dilution rate quantitatively, which is essential for predicting overlay composition and performance. Second, the water erosion test conditions (particle size, velocity, angle of impingement, solid concentration) are not fully specified, making it difficult to correlate with specific field applications. Third, the long-term thermal stability of the overlay under cyclic thermal loading is not addressed.
The finding that the second layer is harder than the first is practically important. It suggests that multi-pass surfacing with 2–3 passes may be optimal for achieving both thickness and performance, but also raises concerns about residual stress accumulation and potential cracking in thicker overlays. Engineers should consider post-overlay stress relief treatment for thick overlays on thick-walled components.
Study Insights and Implications
This study provides valuable quantitative data for engineers specifying Stellite overlays on martensitic stainless steel. The key takeaways are: (1) plasma surfacing achieves excellent metallurgical bonding with minimal dilution; (2) the M7C3 + WC carbide system provides effective erosion resistance; (3) the 1.96× hardness improvement offers substantial life extension; and (4) multi-pass surfacing is beneficial for performance optimization. For engineering practice, I recommend that engineers specify at least two passes for critical erosion applications, conduct post-overlay hardness verification across the full thickness, and perform water erosion testing under representative field conditions before full-scale deployment. The study serves as a solid foundation for further optimization of surfacing parameters and alloy selection for specific service environments.