Zhuojin Pipe Fitting Co., LtdEffect of Heat Treatment on Microstructure and Mechanical Properties of Q345B Steel-Based Overlay Composite Plate
Literature Overview and Research Background
This paper by Chen Youming, Tong Haiyun, Zheng Yili, Zhao Yuping, Li Tao, and Chu Aimin, published in the Journal of Thermal Analysis and Calorimetry (2023, Vol. 44, No. 8, pp. 34-42), investigates the effect of different heat treatment processes on the microstructure and mechanical properties of a composite plate formed by overlay welding YD212 flux-cored wire onto Q345B steel using CO2 gas shielded arc welding (GMAW). Supported by the National Natural Science Foundation of China (52174344), Hunan Provincial Natural Science Foundation (2021JJ30249, 2021JJ30250, 2023JJ30222), and Hunan Provincial College Student Innovation Training Program (S202210534024X), this work addresses the critical challenge of optimizing the interface properties in overlay composite plates for industrial applications.
Core Technical Findings and Heat Treatment Comparison
Three heat treatment processes were evaluated: quenching, normalizing, and spray quenching followed by tempering. Each was compared with the as-welded (untreated) condition. The results demonstrate that all three heat treatment processes modify the microstructure of the fusion zone, overlay layer, and base metal layer, with varying degrees of improvement in hardness and toughness.
| Heat Treatment Process | Base Metal Pearlite Content | Hardness Improvement | Impact Energy Improvement | Fusion Zone Toughness |
|---|---|---|---|---|
| As-welded (untreated) | Baseline | Baseline | Baseline | Baseline |
| Normalizing | Slight change | Small improvement | Minimal improvement | Slight improvement |
| Quenching | Moderate increase | Significant improvement | Moderate improvement | Moderate improvement |
| Spray quenching + tempering | High increase | Significant improvement | 1.57x improvement | Marked improvement |
The spray quenching plus tempering process yielded the best overall results, with a 1.57-fold improvement in impact absorbed energy compared to the untreated condition. This process also produced the highest pearlite content in the base metal layer, which contributes to enhanced hardness and wear resistance.
Microstructural Analysis of Each Heat Treatment Process
The microstructural evolution in each zone of the composite plate is influenced differently by the heat treatment processes. In the overlay layer, the YD212 flux-cored wire produces a hard martensitic or bainitic microstructure that is relatively stable against further heat treatment, although tempering can relieve residual stresses and improve toughness. In the fusion zone, the heat treatment processes can refine the grain structure and promote the formation of more ductile phases, improving the toughness of this critical transition region. In the base metal layer, the heat treatment can transform the ferrite-pearlite microstructure of Q345B steel, with quenching and tempering producing a tempered martensite structure that offers superior mechanical properties.
The spray quenching process is particularly effective because it provides localized, controlled cooling of the composite plate surface. Unlike conventional quenching in a bath, spray quenching allows for more uniform cooling across the plate surface, reducing the risk of differential thermal stresses that could cause warping or cracking. The subsequent tempering relieves the residual stresses introduced by the quenching process and optimizes the hardness-toughness balance.
Welding Metallurgy and Interface Considerations
The CO2 gas shielded arc welding process used in this study produces a relatively high dilution rate compared to processes such as TIG or plasma arc welding. This dilution affects the composition of the fusion zone and the heat-affected zone of the base metal. The YD212 flux-cored wire is designed to produce a hard, wear-resistant deposit, but the dilution from the Q345B base metal can reduce the hardness of the fusion zone and create a gradient in properties across the composite plate.
The heat treatment processes can partially compensate for the dilution effect by modifying the microstructure of the fusion zone and the base metal heat-affected zone. The spray quenching plus tempering process, in particular, can transform the soft fusion zone into a harder, more wear-resistant structure while simultaneously improving the toughness of the base metal layer.
Engineering Practice Implications
For practical implementation, the following considerations are important:
- Process selection: Spray quenching plus tempering is the recommended heat treatment process for Q345B-based overlay composite plates, as it provides the best balance of hardness and toughness across all zones of the composite.
- Temperature control: The quenching temperature should be in the austenitizing range (850-900 °C for Q345B steel) to ensure complete austenitization of the base metal. The tempering temperature should be optimized for the desired hardness-toughness balance, typically in the range of 500-600 °C for structural applications.
- Residual stress management: The welding process introduces significant residual stresses in the composite plate. The heat treatment processes can relieve these stresses, but the sequence and parameters must be carefully controlled to avoid introducing new stresses during the quenching process.
- Dimensional stability: The heat treatment processes can cause dimensional changes in the composite plate. The spray quenching process, with its localized cooling, minimizes the risk of warping compared to conventional bath quenching.
Study Insights and Reflections
This research demonstrates that post-weld heat treatment is a powerful tool for optimizing the properties of overlay composite plates. The spray quenching plus tempering process is particularly noteworthy because it achieves simultaneous improvement of hardness and toughness, which is often a challenging trade-off in metallurgical processing.
The 1.57-fold improvement in impact energy is particularly significant for applications where the composite plate is subjected to dynamic or impact loading. In many industrial applications, such as mining equipment, construction machinery, and material handling systems, the combination of wear resistance and impact toughness is essential for long-term service life.
A practical consideration that deserves further attention is the effect of the heat treatment on the service life of the composite plate. While the heat treatment improves the initial mechanical properties, it may also affect the long-term stability of the microstructure, particularly under cyclic loading or elevated temperature service. The tempered martensite in the base metal layer may undergo aging or softening during extended service, which could affect the overall performance of the composite.
In conclusion, this study provides valuable guidance for the heat treatment of Q345B steel-based overlay composite plates, demonstrating that spray quenching followed by tempering offers the optimal combination of hardness improvement, toughness enhancement, and microstructural refinement across all zones of the composite, making it the preferred post-weld treatment process for industrial applications requiring both wear resistance and impact resistance.