Zhuojin Pipe Fitting Co., LtdMicrostructure and Hardness of Stainless Steel Surfacing Weld Joints
Literature Overview
This paper by Wang Yan from the College of Mechanical and Power Engineering at China Three Gorges University examines the microstructure and hardness of surfacing weld joints produced using A102 stainless steel electrode (E308 equivalent) on Q235A carbon steel substrate. The study investigates the effect of varying welding process parameters and post-weld heat treatment on the microstructure and hardness distribution across the weld joint. Published in Guangdong Chemical Industry in 2018, this work addresses a practical concern in industrial surfacing applications where stainless steel overlays are applied to carbon steel for corrosion resistance.
Core Technical Findings
The study presents several important observations regarding the behavior of austenitic stainless steel surfacing welds on carbon steel substrates:
- Before heat treatment, the weld zone exhibits austenitic structure with dendritic grain morphology and the highest hardness among all regions.
- Grain size in both the overheated zone and weld zone decreases as welding current is reduced, demonstrating the influence of heat input on microstructural refinement.
- After post-weld heat treatment, the weld zone retains austenitic structure but with more uniform distribution compared to the as-welded condition.
- Hardness in the weld zone remains the highest in both as-welded and heat-treated conditions.
- Post-weld heat treatment significantly increases hardness in both the base metal and the weld heat-affected zone (HAZ).
Microstructural Analysis and Process Parameters
The selection of A102 (E308) electrode for surfacing Q235A steel introduces a significant chemical composition mismatch between the austenitic weld metal and the ferritic-pearlitic base metal. This mismatch drives several metallurgical phenomena that are critical for understanding the joint behavior:
| Region | As-Welded Structure | Heat-Treated Structure | Hardness Trend |
|---|---|---|---|
| Weld zone | Austenite, dendritic | Austenite, uniform | Highest (both conditions) |
| HAZ | Mixed phases, variable grain size | Refined phases | Increased after heat treatment |
| Base metal | Ferrite + pearlite | Modified ferrite + pearlite | Increased after heat treatment |
The dendritic structure observed in the as-welded condition is characteristic of rapid solidification from the molten pool. The high hardness in the weld zone is attributed to solid solution strengthening by Cr and Ni, combined with the fine dendritic cell structure that impedes dislocation motion. The reduction in grain size with decreasing welding current is consistent with lower heat input resulting in higher cooling rates and finer microstructures.
Post-Weld Heat Treatment Effects
The post-weld heat treatment results are particularly instructive for engineering practice. The observation that heat treatment significantly increases hardness in both the base metal and HAZ suggests that the treatment temperature and duration were sufficient to induce tempering or secondary phase precipitation in the carbon steel substrate. This is a critical consideration because excessive heat input during surfacing welding can soften the HAZ of carbon steel substrates, reducing the mechanical properties of the base metal. The heat treatment appears to restore and even enhance these properties.
The uniformity improvement in the weld zone after heat treatment is attributed to the dissolution of segregation products at dendrite boundaries and the homogenization of the microstructure. This is beneficial for corrosion resistance, as segregation of elements such as Cr at grain boundaries can lead to intergranular corrosion susceptibility.
Engineering Practice and Defect Considerations
When performing stainless steel surfacing on carbon steel substrates, engineers should be aware of the following potential issues:
- Dilution from the base metal can reduce the Cr and Ni content in the weld metal, potentially leading to a martensitic transformation in the as-welded condition if dilution is excessive.
- Carbon diffusion from the base metal into the weld metal can promote carbide precipitation at grain boundaries, reducing both hardness and corrosion resistance.
- Residual stresses from differential thermal contraction between the austenitic weld metal and ferritic base metal can lead to cracking.
- The choice of welding current directly affects heat input and, consequently, dilution rate and microstructure.
The study's finding that reduced welding current leads to finer grain structures supports the use of lower heat input parameters when microstructural refinement is desired. However, engineers must balance this against the need for adequate fusion and penetration to ensure sound bonding.
Study Insights and Practical Recommendations
This paper provides valuable guidance for engineers performing stainless steel surfacing on carbon steel substrates. The key takeaway is that post-weld heat treatment is not merely beneficial but essential for optimizing the mechanical properties of the entire weld joint, including the base metal. The study also highlights the importance of welding parameter control in achieving the desired microstructure. For pipeline applications where corrosion-resistant overlays are required, the selection of appropriate electrode type, welding parameters, and post-weld treatment must be carefully considered to ensure both corrosion resistance and mechanical integrity of the surfacing weld joint.