CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Heat Treatment on Microstructure and Mechanical Properties of Q345B Steel-Based Cladding Composite Plate
Research Overview and Technical Significance
The fabrication of cladding composite plates using Q345B carbon steel as the base material is a common practice in pressure vessel and structural component manufacturing, where the base material provides strength and the cladding layer provides corrosion or wear resistance. The research by Chen, Tong, Zheng, Zhao, Li, and Chu from Hunan University of Science and Technology, published in the Journal of Heat Treatment of Materials in 2023, systematically investigates how post-weld heat treatment conditions influence the microstructure and mechanical properties of Q345B steel-based cladding composite plates. This work was supported by the National Natural Science Foundation of China (52174344) and multiple Hunan Provincial Natural Science Foundation grants.
Microstructural Evolution Under Different Heat Treatment Conditions
Q345B steel has a base microstructure consisting of ferrite and pearlite with a carbon equivalent of approximately 0.40%. The welding process introduces a heat-affected zone (HAZ) with varying degrees of grain growth and phase transformation depending on the peak temperature and cooling rate. Post-weld heat treatment can refine the HAZ microstructure and homogenize the weld metal, directly impacting the mechanical properties of the composite plate.
| Heat Treatment Condition | Temperature (°C) | Duration (h) | HAZ Microstructure | HAZ Hardness (HV) | Tensile Strength (MPa) | Impact Energy at 20 °C (J) |
|---|---|---|---|---|---|---|
| As-welded | — | — | Fine grain martensite + bainite | 320–380 | 580–620 | 25–35 |
| Normalizing | 860–880 | 1–2 | Fine pearlite + ferrite | 200–240 | 520–560 | 55–70 |
| Annealing | 720–740 | 2–4 | Coarse pearlite + ferrite | 170–200 | 480–520 | 60–75 |
| Stress relief | 580–620 | 2–4 | Tempered martensite + bainite | 260–300 | 540–580 | 35–45 |
Mechanical Property Analysis
The normalizing treatment at 860 °C to 880 °C produces the optimal balance of strength and toughness for Q345B-based composite plates. The austenitization at this temperature dissolves the hard phases formed during welding and promotes a uniform fine-grained ferrite-pearlite structure upon air cooling. The resulting microstructure achieves a tensile strength of 520 to 560 MPa while maintaining impact energy above 55 J at 20 °C, meeting or exceeding the Q345B base material specifications per GB/T 1591.
The stress relief treatment at 580 °C to 620 °C, while effective at reducing residual stresses, does not fully restore the toughness of the as-welded condition. The tempered martensite and bainite phases retain hardness values of 260 to 300 HV, which may be acceptable for static pressure applications but could be problematic for components subjected to impact or cyclic loading.
Bond Strength and Interface Integrity
The heat treatment also affects the metallurgical bond between the base material and the cladding layer. In the as-welded condition, the bond interface may contain microcracks and unmelted particles due to incomplete fusion. Normalizing treatment promotes diffusion bonding at the interface, increasing the shear bond strength from approximately 180 MPa in the as-welded condition to 220 to 260 MPa after normalizing. This improvement is attributed to the dissolution of brittle intermetallic phases and the homogenization of the transition zone.
Engineering Practice Recommendations
For pressure vessel applications governed by GB/T 150 and NB/T 47002, the heat treatment condition must be selected based on the design temperature, pressure, and loading conditions. Normalizing is recommended for vessels operating at ambient to moderate temperatures where toughness is critical. Stress relief is appropriate for vessels operating at elevated temperatures where creep resistance and residual stress control are primary concerns. The heat treatment procedure should be qualified per NB/T 47014 with full mechanical testing including tensile, hardness, and impact tests across the weld, HAZ, and base metal regions.
A systematic FMEA approach should be applied to identify potential failure modes associated with heat treatment: under-tempering leading to excessive hardness and cracking, over-tempering leading to strength loss, and grain coarsening leading to reduced toughness. Each failure mode should be assigned a risk priority number and addressed through procedural controls and in-process monitoring.
This research provides a comprehensive understanding of how heat treatment can be leveraged to optimize the performance of Q345B-based cladding composite plates, reinforcing the principle that post-weld heat treatment is not merely a residual stress relief step but a critical process variable that determines the final service capability of the composite component.