Zhuojin Pipe Fitting Co., LtdEffect of Heat Treatment on Microstructure and Mechanical Properties of Q345B Steel Overlay Composite Plate
Literature Overview
Published in Transactions of Materials and Heat Treatment (2023, Vol. 44, No. 8, pp. 34-42), this study investigates the effect of three heat treatment processes—quenching, normalizing, and liquid spray quenching plus tempering—on the microstructure and mechanical properties of a composite plate formed by CO2 gas shielded arc welding (GMAW) of YD212 flux-cored wire onto Q345B steel substrates. The research was conducted at Hunan University of Science and Technology and Baotou Steel Co., Ltd., supported by the National Natural Science Foundation (52174344) and Hunan Provincial Natural Science Foundation (2021JJ30249, 2021JJ30250, 2023JJ30222). This work addresses a practical engineering challenge: optimizing the mechanical properties of overlay composite plates through post-weld heat treatment.
Core Technical Findings
The study compared the as-welded condition with three heat treatment conditions: quenching, normalizing, and liquid spray quenching plus tempering. All three heat treatments modified the microstructure of both the fusion zone and the overlay layer, and also affected the pearlite content in the base material layer. The key finding is that liquid spray quenching plus tempering produced the best overall mechanical performance: significantly improved hardness and toughness in the base material layer, markedly improved toughness in the fusion zone, and excellent hardness in the overlay layer. The impact absorption energy of this heat-treated condition was 1.57 times that of the as-welded condition.
The microstructural changes induced by heat treatment include the transformation of weld microstructures (martensite, ferrite-pearlite, bainite) into tempered martensite or fine ferrite-pearlite structures, depending on the specific heat treatment process. The base material layer also experienced changes in pearlite content, with the liquid spray quenching plus tempering condition producing the highest pearlite content, which contributes to the improved hardness and toughness.
Heat Treatment Process Comparison
The comparative analysis of the three heat treatment processes reveals distinct microstructural and mechanical outcomes:
| Heat Treatment | Base Material Microstructure | Fusion Zone Toughness | Overlay Hardness | Impact Energy | Overall Assessment |
|---|---|---|---|---|---|
| As-welded | Original ferrite-pearlite | Baseline | Baseline | Baseline | Reference condition |
| Quenching | Increased pearlite | Moderate improvement | Improved | Improved | Good but may cause cracking |
| Normalizing | Slight pearlite change | Slight improvement | Slightly improved | Slightly improved | Minimal improvement |
| Liquid spray quenching + tempering | Highest pearlite content | Markedly improved | Excellent | 1.57x baseline | Optimal overall performance |
The liquid spray quenching plus tempering process stands out as the optimal heat treatment for this composite plate system. The localized quenching by liquid spray allows for selective cooling of the base material and fusion zone while minimizing thermal distortion. The subsequent tempering relieves residual stresses and transforms any untempered martensite into tempered martensite, improving toughness without significantly sacrificing hardness.
The normalizing process produced the smallest improvement in mechanical properties, which is consistent with the relatively mild thermal cycle of normalizing compared to quenching. Normalizing primarily refines the grain structure and produces a more uniform ferrite-pearlite microstructure, but does not induce the significant microstructural transformations that quenching achieves.
Engineering Practice Implications
The practical significance of this research extends to several areas of engineering practice:
- Composite plate design: The findings demonstrate that post-weld heat treatment can significantly enhance the mechanical properties of overlay composite plates, particularly when the liquid spray quenching plus tempering process is employed. This approach should be considered in the design of composite plates for applications requiring both wear resistance (from the overlay) and structural integrity (from the base material).
- Process selection for industrial applications: For large-scale production of composite plates, the liquid spray quenching process offers advantages over conventional furnace quenching: it is faster, more energy-efficient, and allows for selective cooling of specific regions. This makes it suitable for large or complex-shaped components where uniform furnace quenching may be impractical.
- Welding procedure optimization: The CO2 GMAW process used in this study is a cost-effective and widely available welding method. The use of YD212 flux-cored wire provides good deposition rates and process stability. Engineers should consider the compatibility of this welding process with the subsequent heat treatment, as the residual stress state and microstructure from welding directly influence the heat treatment response.
- Quality control considerations: The heat treatment process introduces additional quality control requirements, including temperature monitoring, quenching medium control, and tempering temperature/time verification. Non-destructive testing (NDT) should be performed both before and after heat treatment to detect any cracks or defects that may have developed during the thermal cycling.
Critical Reflection and Study Insights
While the results are encouraging, several aspects deserve critical examination. The study does not provide detailed information on the specific quenching medium, quenching temperature, or tempering temperature/time used in the liquid spray quenching plus tempering process. These parameters are critical for replicating the results and for understanding the microstructural mechanisms behind the observed improvements.
Additionally, the study does not address the residual stress state of the composite plate after heat treatment. Residual stresses are a critical factor in the service performance of composite plates, particularly for applications involving fatigue loading or stress corrosion cracking. The liquid spray quenching process may introduce new residual stresses that need to be evaluated and potentially relieved through stress-relief annealing.
The compatibility of the heat treatment with different overlay materials and base materials should also be considered. The findings are specific to the Q345B/YD212 combination, and may not be directly applicable to other material combinations. Engineers should validate the heat treatment response for their specific material system before implementing it in production.
The 1.57x improvement in impact energy is a significant finding, but it should be contextualized within the specific application requirements. For applications where high toughness is critical (e.g., cryogenic service, impact loading), this improvement may be essential. For applications where hardness and wear resistance are the primary concerns, the normalizing process may be sufficient and more cost-effective.
Summary
This research demonstrates that liquid spray quenching plus tempering is the optimal heat treatment process for Q345B steel overlay composite plates produced by CO2 GMAW with YD212 flux-cored wire, achieving a 1.57x improvement in impact energy compared to the as-welded condition while maintaining excellent overlay hardness. The process selectively modifies the microstructure of both the base material and fusion zone, producing higher pearlite content in the base material and improved toughness in the fusion zone. Engineers should consider this heat treatment approach for composite plate applications requiring both wear resistance and structural integrity, while recognizing the need for detailed process parameter optimization and residual stress evaluation before implementation in critical applications.