Zhuojin Pipe Fitting Co., LtdEffect of Post-Weld Tempering on Microstructure and Hardness Gradient of ZG15MnMoVCu Hardfacing Fusion Zone
Literature Overview
This study by Xiao Xinhua and Xing Zhigang, published in Hot Working Technology in 2015 (Volume 44, Issue 7, pages 203-205), investigates the influence of post-weld tempering treatments on the microstructure and hardness gradient at the fusion zone of ZG15MnMoVCu substrate hardfacing deposits. The research was supported by the Hubei Provincial Department of Education Science and Technology Research Project (B2014028). The authors employed Cr-based flux-cored wire to deposit a wear-resistant overlay on ZG15MnMoVCu cast steel substrate, followed by air cooling and tempering at various temperatures to evaluate the metallurgical evolution across the fusion line.
The ZG15MnMoVCu cast steel is a medium-carbon low-alloy steel widely used in heavy-duty structural applications such as mining equipment, heavy machinery frames, and hydraulic cylinder components. Its chemical composition typically contains approximately 0.15% C, 1.0% Mn, 0.3% Mo, 0.2% V, and 0.3% Cu. The hardfacing application aims to enhance surface wear resistance without compromising the bulk structural integrity of the substrate, which is a critical engineering challenge in surface engineering practice.
Core Technical Findings
The study systematically compared the as-welded air-cooled condition against tempered conditions at progressively higher temperatures, revealing significant metallurgical transformations in both the substrate heat-affected zone (HAZ) and the overlay deposit.
As-Welded Air-Cooled Condition
Under direct air cooling without post-weld heat treatment, the substrate side adjacent to the fusion line exhibited irregular needle-shaped Widmanstätten ferrite structures. This morphology is characteristic of rapid cooling rates that suppress pearlite nucleation and promote ferrite growth from the prior austenite grain boundaries into the interior of the grain. The overlay deposit displayed coarse needle-shaped martensite, indicating that the high carbon and chromium content of the Cr-based flux-cored wire, combined with the rapid solidification and cooling rates typical of hardfacing operations, produced a fully martensitic microstructure.
The chromium distribution across the fusion line was notably asymmetric, with significantly higher Cr concentrations on the overlay side compared to the substrate side. This element partitioning is a direct consequence of the dilution effect during welding and the preferential segregation of alloying elements toward the deposit during solidification. The resulting microhardness gradient across the fusion zone was steep, creating a potential crack initiation site under service loading due to the mismatch in thermal expansion coefficients and mechanical properties between the hard overlay and the softer substrate.
Effect of Increasing Tempering Temperature
As the tempering temperature was progressively elevated, several beneficial metallurgical changes were observed:
- The needle-shaped Widmanstätten ferrite in the substrate HAZ gradually diminished, being replaced by granular pearlite and ferrite structures that provide improved ductility and toughness.
- The coarse needle-shaped martensite in the overlay deposit transformed into fine lenticular tempered martensite, indicating that carbide precipitation occurred at the tempering temperature, relieving internal stresses and improving fracture resistance.
- The chromium distribution across the fusion line became progressively more uniform, suggesting that diffusion processes at elevated temperatures reduced the compositional asymmetry.
- The microhardness gradient across the fusion zone decreased substantially, indicating improved compatibility between the overlay and substrate.
| Condition | Substrate HAZ Microstructure | Overlay Microstructure | Cr Distribution | Hardness Gradient |
|---|---|---|---|---|
| Air-cooled (as-welded) | Irregular needle Widmanstätten ferrite | Coarse needle martensite | Highly asymmetric | Steep |
| Low-temperature tempering | Reduced Widmanstätten, increasing granular pearlite | Beginning of lenticular tempered martensite | Moderately asymmetric | Moderately reduced |
| Medium-temperature tempering | Significant granular pearlite and ferrite | Fine lenticular tempered martensite | Relatively uniform | Substantially reduced |
| High-temperature tempering | Predominantly granular pearlite and ferrite | Well-developed fine tempered martensite | Near-uniform | Minimal |
Interpretation of Technical Points
The fundamental metallurgical mechanism underlying these observations is the interplay between cooling rate, alloy partitioning, and diffusion-controlled phase transformations. During hardfacing welding, the rapid solidification of the molten pool produces a compositionally heterogeneous deposit with high carbon and chromium concentrations, leading to martensite formation upon cooling. The substrate HAZ experiences a thermal cycle that, depending on the cooling rate, can produce either a tempered martensite structure (if the peak temperature exceeds Ac3 and cooling is moderate) or a Widmanstätten ferrite structure (if cooling is rapid from above Ac3).
The tempering process serves multiple beneficial functions simultaneously: it promotes carbide precipitation and coarsening in the martensitic overlay, which reduces hardness but significantly improves toughness; it allows diffusive redistribution of alloying elements across the fusion line, reducing compositional discontinuities; and it promotes the transformation of brittle Widmanstätten ferrite into more ductile granular pearlite and ferrite in the substrate HAZ.
From a practical engineering perspective, the reduction in hardness gradient is particularly significant. A steep hardness gradient across the fusion zone creates a stress concentration under thermal cycling or mechanical loading, which can initiate interfacial cracking. The progressive uniformization of the hardness profile through tempering directly addresses this vulnerability.
Engineering Practice Implications
For industrial hardfacing operations on medium-carbon low-alloy cast steel substrates, this study provides clear guidance on the necessity and optimization of post-weld tempering. The recommended approach would be to select a tempering temperature that balances the following competing requirements:
- Sufficient reduction in hardness gradient to prevent interfacial cracking.
- Maintenance of adequate overlay hardness for wear resistance.
- Avoidance of over-tempering that would excessively soften the overlay.
- Consideration of the substrate's critical cooling rate and the risk of HAZ softening at higher tempering temperatures.
In practice, for ZG15MnMoVCu hardfacing applications, a tempering temperature in the range of 550-650°C for a duration of 2-4 hours per 25 mm of section thickness would likely provide an optimal balance. This temperature range corresponds to the typical tempering range for medium-carbon steels and should promote the beneficial microstructural changes identified in this study without excessive softening of the overlay.
The findings also have implications for welding procedure qualification. When qualifying hardfacing procedures for critical applications, the post-weld heat treatment should be an integral part of the qualified procedure specification, not an optional add-on. The hardness gradient across the fusion zone should be included as a qualification parameter, with acceptance criteria established based on the maximum permissible gradient for the specific service conditions.
Study Insights and Reflections
This study exemplifies the systematic approach that should be applied to hardfacing process optimization. By isolating the tempering temperature as the single variable and maintaining all other welding parameters constant, the authors were able to establish clear cause-and-effect relationships between heat treatment conditions and metallurgical outcomes. This methodological rigor is essential for developing reliable process specifications.
One area that the study does not address but which deserves further investigation is the effect of tempering on the fatigue resistance and corrosion resistance of the hardfaced joint. In many service environments, the hardfaced component is subjected to cyclic loading and corrosive media, and the tempering treatment may influence these properties in ways that are not captured by microhardness measurements alone. Additionally, the study focuses on a single Cr-based flux-cored wire composition, whereas in practice, multiple wire compositions may be used to achieve different overlay hardness levels, and the interaction between wire composition and tempering response should be systematically investigated.
The practical significance of this work extends beyond the specific alloy and wire combination studied. The fundamental metallurgical principles demonstrated—namely, that post-weld tempering improves microstructural compatibility across the fusion zone and reduces hardness gradients—are universally applicable to all hardfacing operations on low-alloy steel substrates. Engineers involved in hardfacing qualification and process development should incorporate these principles into their approach to procedure design and qualification testing.
In conclusion, this study provides valuable empirical evidence that post-weld tempering is not merely a stress-relief operation but a fundamental process step that transforms the metallurgical compatibility of the hardfaced joint, and its systematic application should be considered standard practice in all hardfacing applications on medium-carbon low-alloy steel substrates.