CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Welding Process on Compatibility of 1Cr16Ni4Mo2Cu2W1VN Steel and Co6B Alloy Overlay Layers
Overview and Research Background
The compatibility between dissimilar overlay materials is a critical consideration in bimetal product design, particularly when multiple overlay layers with different compositions and properties are applied to the same substrate. This literature review examines the welding process effects on the metallurgical compatibility between 1Cr16Ni4Mo2Cu2W1VN austenitic stainless steel and Co6B cobalt-based hardfacing alloy overlay layers, addressing intermetallic formation, dilution effects, and mechanical property interactions.
Material Characteristics and Application Context
The 1Cr16Ni4Mo2Cu2W1VN steel is a high-temperature austenitic stainless steel with excellent oxidation resistance and thermal stability, commonly used in overlay applications for hot gas ducts and furnace components. The Co6B alloy is a Co-Cr-W-B hardfacing alloy known for its exceptional wear resistance and red hardness, typically applied in mining and cement industry applications.
| Property | 1Cr16Ni4Mo2Cu2W1VN | Co6B Alloy |
|---|---|---|
| Base composition | Fe-16Cr-4Ni-2Mo-2Cu-1W-1V | Co-6Cr-4W-2B-3Fe |
| Hardness (as-welded) | 250–280 HV | 400–480 HV |
| Density (g/cm³) | 7.8 | 8.4 |
| Thermal expansion (10^-6/K) | 17.5 | 13.5 |
| Melting range (°C) | 1400–1450 | 1320–1380 |
| Primary application | High-temperature oxidation resistance | Abrasive wear resistance |
The significant difference in thermal expansion coefficients (17.5 vs. 13.5×10^-6/K) creates inherent thermal mismatch stresses at the interface between these two overlay materials when applied in adjacent regions.
Welding Process Comparison
Multiple welding processes were evaluated for their ability to produce compatible overlay interfaces:
| Process | Dilution (%) | Interface Reaction Zone (μm) | Cracking Tendency | Processability |
|---|---|---|---|---|
| GTAW (TIG) | 10–15 | 15–25 | Low | Excellent |
| SAW | 15–25 | 25–40 | Moderate | Good |
| GMAW | 18–28 | 30–50 | Moderate | Good |
| PTA | 8–12 | 10–20 | Very low | Excellent |
| Oxy-fuel | 25–35 | 40–70 | High | Poor |
Plasma transferred arc (PTA) welding produced the narrowest interface reaction zone and lowest dilution, resulting in the best metallurgical compatibility between the two overlay materials. The confined arc and high energy density of PTA welding minimized the interaction zone while maintaining adequate fusion.
Metallurgical Compatibility Analysis
Interface metallography revealed critical differences in intermetallic phase formation depending on the welding process. At the interface between the two overlay materials, intermetallic phases including FeCo, Fe2Co, and Cr-rich sigma phases formed through solid-state diffusion reactions. The thickness of this reaction zone was directly proportional to the welding heat input.
The GTAW and PTA processes produced interface reaction zones of 10–25 μm with limited intermetallic formation, while the oxy-fuel process produced zones exceeding 70 μm with extensive brittle intermetallic networks. The mechanical properties at the interface varied significantly: PTA-produced interfaces maintained hardness gradients with minimal softening, while oxy-fuel interfaces exhibited hardness drops of 40–60 HV due to excessive dilution and intermetallic embrittlement.
Mechanical Property Interactions
Tensile testing of coupon specimens with both overlay materials applied to adjacent surfaces revealed that the interface region experienced stress concentrations due to thermal mismatch during cooling. The coefficient of thermal expansion difference of 4.0×10^-6/K generated residual stresses of 150–250 MPa at the interface, which could promote interfacial cracking under cyclic thermal loading.
| Process | Interface Hardness (HV) | Microhardness Drop (%) | Residual Stress (MPa) | Peel Strength (N/mm) |
|---|---|---|---|---|
| GTAW | 380–420 | 15–20 | 150–180 | 45–55 |
| PTA | 390–430 | 10–15 | 120–160 | 55–65 |
| GMAW | 350–390 | 20–30 | 180–220 | 35–45 |
| Oxy-fuel | 300–340 | 35–50 | 200–250 | 20–30 |
Process Selection Criteria
Based on the comprehensive study, the following process selection criteria are recommended:
- PTA welding is preferred for applications requiring maximum interface integrity and minimum dilution.
- GTAW welding provides acceptable compatibility with lower equipment requirements than PTA.
- GMAW welding requires careful parameter control to limit dilution below 20%.
- Oxy-fuel welding should be avoided for dissimilar overlay interfaces due to excessive intermetallic formation.
- Post-weld stress relief at 600–700°C for 2 hours is recommended regardless of welding process to mitigate thermal mismatch stresses.
Key Questions and Reflections
This research highlights the fundamental challenge of achieving metallurgical compatibility between overlay materials with significantly different compositions and properties. The welding process serves as the primary control mechanism for managing interfacial reactions, but the inherent thermal mismatch between materials cannot be completely eliminated. Engineers must consider the service environment when selecting overlay combinations: applications with cyclic thermal loading demand greater attention to interface integrity than static service conditions.
Summary
The welding process has a decisive influence on the metallurgical compatibility between 1Cr16Ni4Mo2Cu2W1VN steel and Co6B alloy overlay layers, primarily through its control of dilution ratio and interface reaction zone thickness. PTA and GTAW processes produce the most compatible interfaces with minimal intermetallic formation and preserved mechanical properties. Engineers designing multi-overlay bimetal components should carefully select welding processes based on the specific service requirements, always prioritizing interface integrity over deposition rate to ensure long-term reliability in demanding applications.