Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of Wear-Resistant Overlay Layer on Q235 Steel Plate
Literature Overview
This study by Dong Shengrong from Hunan Industrial Technician College was published in Materials Development and Application (Vol. 33, Issue 5, 2018, pp. 54–59). The research investigates the effect of CaB6 (calcium hexaboride) addition to flux-cored wire on the microstructure, phase composition, hardness, and wear resistance of wear-resistant overlay layers deposited on Q235 carbon steel plates. The study identifies an optimal CaB6 addition level of 3.0% that maximizes wear resistance while maintaining adequate hardness and metallurgical quality.
Application Background
Q235 is a widely used carbon structural steel in China, equivalent to S235JR in European standards. It is commonly used for structural applications but lacks the wear resistance required for components subjected to abrasive service. Overlay welding provides an economical method of adding a wear-resistant surface layer to Q235 components, allowing the base material to provide structural strength while the overlay provides surface durability.
The use of CaB6 as an alloying addition to the overlay consumable is notable because calcium hexaboride is a compound that introduces both calcium and boron into the weld metal. Boron is a potent carbide-forming element that can form hard borides and carboborides, while calcium serves as a strong deoxidizer and can modify inclusion morphology. The combined effect of these elements in the overlay weld metal creates a microstructure with enhanced wear resistance.
Experimental Design and Variables
The study systematically varied the CaB6 addition level in the flux-cored wire and characterized the resulting overlay welds:
| Variable | Levels Tested | Measurement |
|---|---|---|
| CaB6 addition | Multiple levels (including 3.0% and 6.0%) | Added to flux-cored wire |
| Base material | Q235 carbon steel | Standard structural steel |
| Consumable type | Flux-cored wire with CaB6 | Welded in overlay configuration |
| Tests | Metallographic examination | Phase identification, grain structure |
| Phase analysis | XRD or optical identification | |
| Hardness testing | Vickers or Rockwell | |
| Wear testing | Relative wear resistance measurement | |
| Interface examination | Fusion zone width, bonding quality |
Microstructural Characterization
The overlay weld metal microstructure consists of several phases:
Matrix phases:
- Austenite: The retained austenite phase provides toughness and contributes to wear resistance through work hardening
- Martensite: The hardened martensitic matrix provides the primary hardness contribution to the overlay weld metal
Hard phases:
- Eutectic carboborides: Formed during solidification as a result of the high boron and carbon content
- Primary carboborides: Formed as primary precipitates during solidification, appearing as blocky white particles
The distribution and morphology of these hard phases are directly influenced by the CaB6 addition level. As the CaB6 addition increases, the number of blocky white carboboride particles in the matrix increases, providing more load-bearing hard particles for abrasion resistance.
Phase Composition and Interface Analysis
The study provides detailed phase analysis of the overlay weld metal at different CaB6 addition levels:
At 6.0% CaB6 addition, the overlay weld metal consists of:
- Cr-Fe solid solution
- (Cr,Fe)7(C,B)3 carboboride
- (Cr,Fe)3(C,B) carboboride
- Austenite
- Martensite
The interface between the overlay weld metal and the Q235 base metal was examined, revealing:
- Fusion zone width: approximately 18 μm
- Interface bonding: good metallurgical bond with no visible defects
- Dilution: controlled at a level that maintains adequate base metal strength
The narrow fusion zone (18 μm) indicates that the overlay process achieved good dilution control, which is critical for maintaining the base metal's structural properties while providing the overlay's wear resistance. The good interface bonding ensures that the overlay layer will not delaminate during service.
Hardness and Wear Resistance Results
The study reports a clear relationship between CaB6 addition level and overlay weld properties:
| Property | Trend with Increasing CaB6 | Optimal CaB6 Level |
|---|---|---|
| Hardness | Gradually increases | Increases continuously |
| Wear resistance | Increases then decreases | 3.0% |
| Blocky carboboride content | Increases | Continues to increase |
The key finding is that while hardness continues to increase with CaB6 addition, wear resistance peaks at 3.0% CaB6 and then decreases at higher addition levels. This non-monotonic behavior of wear resistance is explained by the competing effects of hardness and toughness:
- At low CaB6 levels, the overlay has insufficient hard phase content for optimal wear resistance
- At the optimal CaB6 level (3.0%), the balance between hard phase content and matrix toughness is maximized
- At high CaB6 levels, the excessive hard phase content creates a brittle microstructure that is susceptible to crack initiation and propagation during wear
This finding is consistent with the well-established principle in wear-resistant materials that optimal wear resistance requires a balance between hardness and toughness, not simply maximum hardness. The brittle microstructure achieved at high CaB6 levels may initiate cracks under wear loading, leading to material removal through fracture rather than gradual abrasion.
Engineering Practice Considerations
The study has several practical implications for overlay welding applications:
Consumable design: The optimal CaB6 addition level of 3.0% provides a clear target for consumable development. Engineers designing wear-resistant overlay consumables for Q235 applications should target this addition level to achieve maximum wear resistance.
Process control: The narrow fusion zone and good interface bonding demonstrate that the overlay process can be controlled to achieve good metallurgical quality. Engineers should ensure that welding parameters are optimized to maintain low dilution and good interface bonding.
Performance prediction: The relationship between CaB6 addition, microstructure, and wear resistance provides a basis for predicting overlay performance. Engineers can use this knowledge to select appropriate consumables for specific wear environments.
Study Insights and Practical Implications
This study provides a clear demonstration of the importance of optimizing alloy addition levels in overlay welding consumables. The non-monotonic relationship between CaB6 addition and wear resistance illustrates a fundamental principle: more is not always better in materials design. The optimal addition level represents a balance between competing property requirements, and exceeding this level can degrade performance.
The study's focus on Q235 steel, a common and economical structural steel, makes the findings particularly relevant for practical applications. Many industrial components are fabricated from Q235 or similar carbon steels and require wear-resistant surfaces for specific applications. The overlay welding approach demonstrated in this study provides a cost-effective solution for upgrading these components without requiring expensive base metal replacements.
The detailed microstructural analysis provided in the study is valuable for understanding the wear resistance mechanism. The combination of austenite and martensite in the matrix, with dispersed carboboride particles, creates a microstructure that combines toughness with hardness. The austenite provides work hardening capacity during wear, while the martensite provides baseline hardness, and the carboboride particles provide primary abrasion resistance. This multi-phase microstructure is superior to single-phase hard materials because it can accommodate wear loading without catastrophic failure.
For engineers involved in overlay welding procedure development, this study reinforces the importance of systematic optimization of consumable composition. The CaB6 addition level is not simply a variable to be maximized but must be optimized for the specific application requirements. The study's methodology—varying the addition level, characterizing the microstructure, and measuring the properties—provides a template for optimizing other alloy additions in overlay welding consumables.