CLADDING TECHNOLOGY SHANXI CO., LTDDilution Rate Effects on WC-Reinforced SAW Overlay Layer Study Note
Literature Overview
This 2015 publication by Zhou, Shen, and Chen from Hunan Yongzhou Vocational and Technical College and Hunan Automotive Engineering Vocational College investigates how dilution rate affects the microstructure and mechanical properties of tungsten carbide (WC) reinforced overlay layers produced by submerged arc welding (SAW). The study was funded by the Hunan Provincial Department of Science and Technology (project 2013GK3179) and published in Hot Working Technology. This topic is of considerable industrial relevance given the widespread use of WC-hardened surfaces in mining, cement, and power generation equipment.
Core Technical Analysis
Dilution Rate Fundamentals in SAW Overlay
Dilution rate in overlay welding is defined as the ratio of base metal content to total overlay metal content. In SAW overlay processes, dilution is typically governed by:
| Parameter | Typical Range | Influence on Dilution |
|---|---|---|
| Current density | 20–80 A/mm² | Higher current increases penetration and dilution |
| Travel speed | 100–400 mm/min | Higher speed reduces heat input and dilution |
| Wire feed speed | 4–12 m/min | Higher feed rate decreases dilution |
| Wire diameter | 1.6–3.2 mm | Larger diameter reduces dilution |
| Preheat temperature | 50–250°C | Higher preheat increases dilution |
| Number of passes | 1–5 | Multi-pass reduces overall dilution |
Microstructural Evolution with Dilution Rate
The microstructure of WC-reinforced overlay layers undergoes significant transformations as dilution rate changes:
- Low dilution (<20%): The overlay layer retains a high fraction of primary WC particles embedded in a hardened austenitic or martensitic matrix. Hardness values typically exceed 1200 HV. The microstructure is characterized by well-preserved WC particles with minimal decomposition.
- Moderate dilution (20–40%): Progressive decomposition of WC occurs, forming secondary carbides (Fe₃C, Fe₂W₄C, Fe₇W₆C) within the matrix. Hardness decreases to approximately 800–1100 HV. The matrix transitions from austenitic to mixed austenite-ferrite or martensitic structures.
- High dilution (>40%): Extensive WC decomposition occurs, with significant formation of soft phases such as Fe₃W₃C and dissolved tungsten in the ferritic matrix. Hardness drops below 800 HV. The overlay layer may exhibit reduced abrasion resistance despite maintaining reasonable toughness.
Process Optimization for Controlled Dilution
The authors demonstrate that dilution rate can be effectively managed through coordinated adjustment of multiple process parameters:
- Multi-pass welding with overlapping passes — Each successive pass dilutes the previous pass, but the overall dilution is reduced compared to single-pass welding.
- Lower current with higher travel speed — This combination reduces arc penetration while maintaining sufficient fusion.
- Use of flux-cored wire with high WC content — Compensates for dilution by maintaining high carbide concentration in the deposit.
- Reduced preheat — Lower base metal temperature reduces the thermal gradient and limits base metal melting.
Defect Analysis
A critical aspect of WC-SAW overlay welding is the susceptibility to cracking:
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Transverse cracking | High dilution, martensitic transformation | Post-weld heat treatment, multi-pass technique |
| Longitudinal cracking | Residual stress concentration | Reduce travel speed, increase wire feed speed |
| Porosity | Incomplete flux coverage, high current | Optimize flux distribution, reduce current density |
| WC decomposition | Excessive heat input | Multi-pass, lower current, higher travel speed |
Integration with Engineering Practice
In industrial applications such as wear-resistant liners for cement kilns, grinding mill rollers, and hydraulic cylinder barrels, the dilution rate directly determines service life. A practical guideline derived from this research is that for SAW overlay with WC-containing consumables, a dilution rate below 30% is generally required to maintain acceptable wear resistance. Engineers should verify dilution through metallographic examination of cross-sections, measuring the boundary between base metal and overlay layer and analyzing the WC particle distribution profile.
The study reinforces the principle that overlay welding is not simply a surface application process but a metallurgical operation requiring precise control of heat input, dilution, and microstructural evolution. Future work should explore the use of low-dilution consumable designs such as flux-cored wire with high WC content and novel flux compositions that promote rapid solidification.