Zhuojin Pipe Fitting Co., LtdEffect of Laser Surface Treatment on Microstructure and Properties of Martensitic Stainless Steel Overlay Weld
Literature Overview
The paper by Deng Dewei, Li Zhenhua, Chen Wenbo, Wang Hongsuo, and Sun Lei, published in Heat Treatment of Metals (2024, Volume 49, Issue 3, pages 56–63), investigates the combined effect of overlay welding and laser surface treatment on FV520B martensitic stainless steel. The study was supported by the High-End Valve Industry Technology Collaborative Innovation Center Fund (2022DMV003) and the Liaoning Provincial Laser 3D Printing Equipment and Application Professional Technology Innovation Center Fund (DUT2022031). The research addresses the dual challenge of repairing surface damage on FV520B steel components and enhancing the corrosion resistance of the repaired surface.
Material Background and Repair Strategy
FV520B is a high-strength martensitic stainless steel widely used in power plant applications, particularly for turbine shafts and other critical rotating components. The steel contains approximately 12–13% chromium and 0.2% carbon, providing excellent strength and moderate corrosion resistance. However, during service, FV520B components can suffer surface damage from erosion, fretting, or corrosion, requiring repair.
The repair strategy proposed in this study is a two-step approach: first, overlay weld the damaged area using a martensitic flux-cored wire with open-arc oscillating welding, and second, apply laser surface treatment to the overlay weld to improve its microstructure and corrosion resistance. This combined approach leverages the material addition capability of welding and the microstructural refinement capability of laser processing.
Overlay Weld Microstructure and Properties
The overlay weld was produced using a martensitic flux-cored wire with open-arc oscillating welding. The resulting overlay microstructure consists of martensite, delta-ferrite, retained austenite, and carbides. The hardness of the overlay weld was measured at approximately 480 HV0.3, representing a 33.33% increase compared to the base metal. The overlay weld exhibited good metallurgical bonding with the base metal, with no evidence of lack of fusion or cracking at the interface.
| Property | Base Metal (FV520B) | Overlay Weld | Change |
|---|---|---|---|
| Hardness (HV0.3) | ~360 | ~480 | +33.33% |
| Microstructure | Martensite + carbides | Martensite + delta-ferrite + retained austenite + carbides | More complex |
| Corrosion Resistance | Moderate | Improved after laser treatment | Enhanced |
The presence of delta-ferrite and retained austenite in the overlay weld is beneficial for toughness but may slightly reduce corrosion resistance compared to a fully martensitic structure. This observation motivates the subsequent laser surface treatment step.
Laser Surface Treatment: Process and Results
Two types of lasers were used for surface treatment: a semiconductor laser and a fiber laser. The laser treatment was applied to the overlay weld surface at different scanning speeds to investigate the effect of processing parameters on the treatment depth and microstructural changes.
After laser surface treatment, the overlay weld surface was divided into three distinct zones:
- Original weld zone: The region unaffected by laser treatment, retaining the original overlay weld microstructure and hardness.
- Heat-affected zone (HAZ): The region subjected to thermal cycling without melting, where hardness decreased due to a tempering effect caused by the laser heat input.
- Remelting zone: The region where the laser energy was sufficient to remelt the surface, producing a new microstructure with hardness comparable to the original weld zone.
| Laser Type | Treatment Depth | Hardness Change | Corrosion Resistance |
|---|---|---|---|
| Semiconductor laser | Greater depth | Minimal change | Improved |
| Fiber laser | Shallower depth | Minimal change | Improved |
The immersion test results demonstrated that laser surface treatment improved the corrosion resistance of the overlay weld. The mechanism is attributed to the formation of a more uniform and dense microstructure in the remelting zone, which reduces the number of corrosion initiation sites. The tempering effect in the HAZ, while reducing hardness, may also contribute to improved corrosion resistance by reducing residual stress and promoting the formation of a more stable microstructure.
Parameter Effects and Optimization
The study found that decreasing the scanning speed increased the laser treatment depth but had minimal effect on hardness. This is because a lower scanning speed increases the heat input per unit length, allowing the laser energy to penetrate deeper into the material. However, the hardness in the remelting zone is determined primarily by the cooling rate and the composition of the remelted material, which remain relatively constant regardless of scanning speed.
The comparison between semiconductor and fiber lasers revealed that the semiconductor laser produced a greater treatment depth, likely due to its different beam characteristics and energy density distribution. However, the surface hardness after treatment was essentially the same for both laser types, indicating that the microstructural refinement effect is similar regardless of the laser source.
Study Insights and Engineering Implications
This study demonstrates that the combination of overlay welding and laser surface treatment is an effective approach for repairing and enhancing FV520B steel components. The overlay weld provides material addition and restores the component geometry, while the laser surface treatment refines the microstructure and improves corrosion resistance.
The observation that the HAZ hardness decreases due to tempering is an important consideration for engineering applications. In components where surface hardness is critical, the laser treatment parameters should be optimized to minimize the HAZ width and maximize the remelting zone fraction. This can be achieved by using higher power density and higher scanning speed, which concentrate the laser energy on the surface and reduce the thermal diffusion depth.
The improved corrosion resistance after laser treatment is particularly significant for FV520B applications in power plants, where components are exposed to high-temperature steam and water environments that can cause corrosion and stress corrosion cracking. The laser treatment effectively addresses both the mechanical and corrosion resistance requirements of the repaired surface.
This study provides a valuable reference for the development of combined repair and surface engineering approaches for high-strength stainless steel components. The systematic investigation of laser parameters and their effects on microstructure and properties offers practical guidance for optimizing laser surface treatment processes in industrial applications.