Zhuojin Pipe Fitting Co., LtdEffect of Welding Process on Compatibility Between 1Cr16Ni4Mo2Cu2W1VN Steel and Co6B Alloy Overlay Layers
Literature Overview
This paper by Wu Weijian, Liu Chengmiao, and Li Quan, published in Metal Heat Treatment (2024, Vol. 49, Issue 7, pp. 326–330), investigates the compatibility between a high-alloy steel substrate (1Cr16Ni4Mo2Cu2W1VN) and Co6B alloy overlay layers under two different welding processes: oxy-acetylene welding and argon arc welding (GTAW). The study provides valuable insights for engineers working on surface hardening and repair applications involving high-alloy steels and cobalt-based alloys.
Background and Technical Context
The 1Cr16Ni4Mo2Cu2W1VN steel is a high-alloy austenitic or austenitic-ferritic steel designed for high-temperature service with enhanced wear resistance and oxidation resistance. The Co6B alloy is a cobalt-chromium-tungsten-boron hardfacing alloy widely used for erosion and wear protection in demanding applications. The compatibility between these two materials is critical for ensuring reliable overlay deposition, as high dilution or inadequate bonding can lead to premature failure.
Substrate and Overlay Material Composition
| Element | 1Cr16Ni4Mo2Cu2W1VN (wt%) | Co6B (typical, wt%) |
|---|---|---|
| C | 0.20–0.35 | 2.5–3.5 |
| Cr | 15.5–16.5 | 25–30 |
| Ni | 3.5–4.5 | 5–8 |
| Mo | 1.8–2.2 | 4–6 |
| Cu | 1.5–2.5 | — |
| W | 0.8–1.2 | 3–5 |
| V | 0.2–0.4 | — |
| N | 0.03–0.06 | — |
| Co | — | Balance |
| B | — | 0.5–1.5 |
Welding Process Comparison
The study compares two fundamentally different welding processes for depositing Co6B overlay layers:
| Parameter | Oxy-Acetylene Welding | Argon Arc Welding (GTAW) |
|---|---|---|
| Heat source | Combustion flame | Electric arc |
| Heat input | Lower, more diffuse | Higher, more concentrated |
| Dilution ratio | Lower (typically 5–15%) | Higher (typically 10–25%) |
| Penetration depth | Shallow | Moderate |
| Bonding quality | Good, with proper technique | Excellent |
| Process speed | Slower | Faster |
| Operator skill requirement | Higher | Moderate |
| Equipment cost | Lower | Higher |
Key Findings on Overlay Layer Characteristics
| Property | Oxy-Acetylene (As-Welded) | GTAW (As-Welded) | Oxy-Acetylene (After Heat Treatment) |
|---|---|---|---|
| Fusion transition zone width | Wider | Narrower | — |
| Hardness (HRC) | Higher by 7.3 HRC than GTAW | Baseline | 45.3 HRC |
| Impact energy (J) | — | — | 84.7 J |
| Microstructure | Coarse, with some unmelted particles | Finer, more uniform | Fine, dense, uniform |
| Defect level | Moderate porosity | Low porosity | Minimal porosity |
Metallurgical Compatibility Analysis
Dilution Effects
The wider fusion transition zone observed in oxy-acetylene welding indicates lower dilution compared to GTAW. This is consistent with the lower heat input and more diffuse heat source of oxy-acetylene welding. Lower dilution means the overlay layer composition remains closer to the original Co6B alloy, resulting in higher hardness in the as-welded condition. However, this also means the bonding interface may be less metallurgically compatible, potentially affecting the long-term durability of the overlay.
Microstructural Evolution
The as-welded microstructures of both processes show characteristic Co-Cr-W-B alloy features, including:
- M7C3 carbides (Co,Cr)7C3 — primary hardening phase contributing to wear resistance.
- M23C6 carbides — secondary carbide phase.
- Matrix phase — Co-rich austenitic or martensitic structure depending on cooling rate.
After solution treatment, deep freezing, and tempering (solid solution + deep cryogenic + tempering), the oxy-acetylene welded overlay exhibits:
- A denser microstructure with reduced porosity.
- Finer and more uniform grain structure.
- Enhanced impact toughness (84.7 J), indicating improved ductility without significant hardness loss.
- Optimized carbide distribution for balanced wear resistance and toughness.
Thermal Cycle Effects on Compatibility
The different thermal cycles imposed by oxy-acetylene and GTAW welding lead to distinct solidification behaviors:
- Oxy-acetylene welding produces a slower cooling rate, allowing more time for elemental diffusion at the interface and potentially promoting the formation of a more compatible transition zone.
- GTAW welding produces a faster cooling rate, which can lead to segregation of alloying elements and the formation of brittle phases at the interface.
Engineering Practice Implications
Process Selection Criteria
Engineers should consider the following factors when selecting between oxy-acetylene and GTAW for Co6B overlay on high-alloy steel substrates:
- Required hardness: If maximum as-welded hardness is critical and post-weld heat treatment is not feasible, oxy-acetylene welding may be preferred due to lower dilution.
- Required toughness: If impact resistance is important, GTAW with post-weld heat treatment may provide a more balanced property set.
- Production volume: GTAW is more amenable to automation and higher production rates.
- Equipment availability: Oxy-acetylene requires less specialized equipment but more operator skill.
Heat Treatment Recommendations
The study demonstrates that the solution treatment + deep cryogenic + tempering sequence significantly improves the properties of the oxy-acetylene welded overlay. This treatment sequence:
- Dissolves excess carbides formed during welding.
- Refines the grain structure through controlled nucleation and growth.
- Eliminates residual stresses and improves dimensional stability.
- Optimizes the distribution and morphology of hardening carbides.
Key Questions and Reflections
The study raises an important question about the long-term service behavior of these overlays under actual operating conditions. While the reported hardness and impact properties are encouraging, engineers should consider:
- Thermal fatigue resistance under cyclic temperature exposure.
- Oxidation resistance at elevated temperatures.
- Erosion-corrosion behavior in aggressive environments.
- Thermal cycling stability of the overlay-substrate interface.
Additionally, the study does not report on the mechanical properties of the heat-affected zone (HAZ) in the substrate. For high-alloy steels, HAZ softening or hardening can significantly affect the overall component performance, and this aspect should be investigated in future work.
Study Insights and Implications
This paper provides a clear demonstration that the welding process choice has a profound impact on the metallurgical compatibility and final properties of Co6B overlay layers on high-alloy steel substrates. The finding that oxy-acetylene welding, when combined with appropriate post-weld heat treatment, can achieve superior comprehensive properties compared to GTAW is particularly noteworthy, as it challenges the conventional assumption that arc welding processes always produce better overlay quality. For engineers in surface engineering and tool repair, this study underscores the importance of process-material-heat treatment optimization as an integrated approach rather than evaluating each element in isolation. The 2024 publication date also indicates the continued relevance of fundamental welding process research in addressing practical industrial challenges.