CLADDING TECHNOLOGY SHANXI CO., LTDComparison of Co119 Alloy Coatings Prepared by Oxy-Acetylene Welding and Induction Melting
Literature Overview
This 2021 study published in Materials Protection by Shi Haifang and Wang Honglei from Liaoning Technical University presents a systematic comparison of Co119 (a cobalt-chromium-tungsten-based hardfacing alloy, corresponding to the domestic designation CoCrW-119 or similar) coatings produced by two distinct thermal processes: conventional oxy-acetylene (OAW) welding overlay and induction melting. The paper addresses a practical engineering question that has persisted for decades in hardfacing technology: which of these two relatively low-energy-input, cost-effective methods yields superior microstructural integrity and tribological performance for cobalt-based alloy coatings.
Core Technical Content
Process Characteristics and Thermal Parameters
Oxy-acetylene welding overlay is a mature, portable technique that uses a flame temperature of approximately 3,100°C to melt the base metal and hardfacing material simultaneously. The process is characterized by a relatively slow cooling rate, typically in the range of 5–20°C/s, which promotes grain growth and may lead to coarsening of carbides. Induction melting, on the other hand, employs electromagnetic induction to rapidly heat the coating material, achieving a much higher local temperature gradient and cooling rate, often exceeding 50–200°C/s depending on the specific setup.
| Parameter | Oxy-Acetylene Overlay | Induction Melting |
|---|---|---|
| Heat source temperature | ~3,100°C (flame) | Localized EM heating |
| Typical cooling rate | 5–20°C/s | 50–200°C/s |
| Dilution rate | Higher (15–30%) | Lower (8–15%) |
| Equipment portability | High | Moderate |
| Coating thickness per pass | 1–3 mm | 0.5–2 mm |
| Typical hardness (HV) | 700–900 HV | 900–1,100 HV |
Microstructural Analysis
The study reveals that the oxy-acetylene process produces a relatively coarse microstructure with larger dendritic cells and coarser carbide particles. The lower cooling rate allows for more complete diffusion and carbide precipitation, resulting in carbides that are predominantly of the M7C6 and M23C6 type, which are harder but more brittle. The induction melting process, due to its faster cooling, produces a finer microstructure with more uniformly distributed carbides, including a higher proportion of finer WC and Co3W type carbides. This finer distribution contributes to improved wear resistance despite potentially lower peak hardness values in some test conditions.
Wear Performance Comparison
Wear tests typically conducted under dry sliding conditions against alumina (Al2O3) or steel counterparts reveal that induction melting coatings generally exhibit 15–30% lower specific wear rates compared to oxy-acetylene coatings. The superior performance is attributed to the finer, more uniformly distributed carbide network that provides better resistance to abrasive and adhesive wear mechanisms. However, the oxy-acetylene coating may show advantages in high-temperature wear scenarios where thermal stability of the microstructure becomes critical, as the coarser carbides can withstand thermal cycling without significant degradation.
Key Technical Insights and Engineering Reflections
The fundamental insight from this study is that process selection for hardfacing applications cannot rely solely on cost considerations. While oxy-acetylene welding remains the most economical and portable option, the microstructural consequences of its slow cooling rate limit its applicability in demanding wear environments. The dilution rate is a particularly critical parameter: in oxy-acetylene welding, the large heat-affected zone and prolonged melting of the base metal lead to higher dilution, which dilutes the cobalt and tungsten content in the overlay layer and compromises the intended alloy composition.
From a quality control perspective, the induction melting process offers better process consistency and repeatability, which is essential for production environments where coating performance must be guaranteed batch after batch. The tighter process control achievable with induction equipment aligns well with the quality assurance requirements of modern engineering standards such as NB/T 47014 for welding procedure qualification.
Practical Implications for Cladding Engineering
For engineers selecting between these two methods, the decision matrix should consider: the required coating thickness, the severity of the wear environment, temperature exposure, equipment availability, and cost constraints. In field repair applications where portability is paramount, oxy-acetylene welding remains viable provided that the engineer compensates for the higher dilution by using multiple overlay passes and potentially employing a thermal barrier flux to reduce heat input. In manufacturing environments where consistent high-performance coatings are required, induction melting represents the preferred technology.
The study reinforces an important principle in hardfacing metallurgy: the cooling rate is not merely a process variable but a microstructure-determining factor that directly governs the final performance of the coating. Engineers must always correlate process parameters with the resulting microstructure before making material selection decisions.