Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of GMAW Surfacing Layers on FV520B Steel After Laser Quenching
Literature Overview
This 2024 paper published in Mechanical Engineering Materials by Deng Dewei and colleagues from Dalian University of Technology and Dalian Marine Valve Co., Ltd., investigates the microstructure, hardness, and corrosion resistance of gas metal arc welding (GMAW) surfacing layers on FV520B steel, with and without subsequent laser quenching treatment. FV520B is a martensitic stainless steel widely used for valve bodies and other components in the marine and chemical industries due to its excellent combination of strength, toughness, and corrosion resistance.
Core Technical Findings
The study examined single-layer single-pass, two-layer multi-pass, and three-layer multi-pass homogeneous surfacing configurations. The surfacing layer microstructure consists of lath martensite, a small amount of delta ferrite, and some carbides. The microhardness of the surfacing layer was 350 HV, representing a 14.7% improvement over the base material.
| Parameter | Single-Layer Single-Pass | Two-Layer Multi-Pass | Three-Layer Multi-Pass | After Laser Quenching |
|---|---|---|---|---|
| Microhardness (HV) | 350 | Up to 380.3 | Up to 373.5 | Up to 390.4 |
| Hardness improvement | 14.7% over base | 8.0% over surfacing | 6.7% over surfacing | 11.5% over unquenched |
| Delta ferrite | Present | Reduced at interfaces | Reduced at interfaces | Minimal |
| Corrosion rate | Lower than base | Stable | Stable | Lower than unquenched |
| Quenched zone depth | N/A | N/A | N/A | 194.3 μm |
| HAZ depth | N/A | N/A | N/A | 186.3 μm |
After laser quenching, the martensite microstructure became finer, and the maximum hardness increased to 390.4 HV. The hardness profile showed an initial increase followed by a decrease with increasing distance from the quenched surface. The corrosion rate of the laser-quenched surfacing layer was lower than that of the unquenched layer.
Process and Metallurgical Analysis
FV520B steel is a precipitation-hardening martensitic stainless steel with a nominal composition of approximately 13% Cr, 5% Ni, and 2% Mo. Its excellent corrosion resistance and mechanical properties make it suitable for valve applications in aggressive environments. The homogeneous surfacing approach uses a matching consumable to maintain the base material's properties while providing a fresh, defect-free surface layer.
The GMAW process was selected for its high deposition efficiency and good weld quality. The single-layer single-pass configuration provides a baseline for comparison, while multi-pass configurations simulate practical repair scenarios where thicker deposits are required. The reduction of delta ferrite at interlayer and interpass boundaries in multi-pass configurations is attributed to the thermal cycling effect, which promotes the dissolution and re-precipitation of ferrite.
The laser quenching treatment introduces rapid heating and self-quenching, producing a fine martensitic microstructure with minimal distortion. The quenched zone depth of 194.3 μm and HAZ depth of 186.3 μm indicate a highly localized treatment, which is advantageous for maintaining the dimensional integrity of precision components such as valve bodies. The finer martensite resulting from laser quenching contributes to the observed hardness increase and improved corrosion resistance.
The corrosion resistance improvement after laser quenching can be attributed to several factors. The finer microstructure reduces the grain boundary area available for preferential corrosion. The reduced delta ferrite content minimizes the risk of intergranular corrosion, as delta ferrite is generally more susceptible to pitting in chloride-containing environments. Additionally, the laser quenching process may promote the formation of a more protective passive film on the surface.
Engineering Practice Integration
This research has direct relevance to the repair and maintenance of valve bodies and other FV520B components in the marine, chemical, and power generation industries. Valve bodies often experience wear and corrosion at the sealing surfaces, and replacing entire valves can be costly and time-consuming. Homogeneous surfacing followed by laser quenching provides an effective repair strategy.
For pipe and fitting manufacturers, the findings can be extended to other martensitic stainless steel applications. The GMAW surfacing process is well-suited for automated production lines, and the laser quenching step can be integrated into the manufacturing workflow. The combination of GMAW surfacing and laser quenching offers a cost-effective alternative to full component replacement.
Key process parameters for industrial implementation include:
- GMAW parameters: wire diameter of 1.2 mm, shielding gas of 98% Ar + 2% O2, current of 180 to 220 A, and voltage of 22 to 26 V.
- Laser quenching parameters: wavelength of 1064 nm, power of 2 to 5 kW, scanning speed of 200 to 500 mm/min, and spot diameter of 2 to 4 mm.
- Post-weld inspection: visual examination, dye penetrant testing, and hardness measurement at multiple depths to verify the quenched zone profile.
Key Questions and Reflections
The study does not address the long-term performance of the laser-quenched surfacing layer under cyclic thermal and mechanical loading. Engineers should consider the potential for microcracking at the quenched zone boundary due to thermal expansion mismatch between the hardened surface and the softer substrate. This is particularly relevant for valve bodies that experience pressure cycling and temperature fluctuations.
Another question is the effect of laser quenching on the weld pool composition. The rapid heating and cooling may cause localized alloy segregation or carbide precipitation, which could affect the corrosion resistance in specific regions. Further research on the chemical homogeneity of the quenched zone would be beneficial.
The corrosion rate measurements in this study are relatively short-term. Long-term immersion tests or accelerated corrosion tests would provide more reliable data on the durability of the laser-quenched surfacing layer in real service conditions.
Study Insights and Implications
This research demonstrates that the combination of GMAW homogeneous surfacing and laser quenching is an effective strategy for enhancing the surface properties of FV520B steel components. The hardness improvement of up to 390.4 HV and the reduced corrosion rate provide a compelling case for this dual-process approach.
The findings also highlight the importance of multi-pass surfacing in practical applications. The reduction of delta ferrite at interlayer boundaries and the increased hardness at these interfaces suggest that multi-pass configurations can produce superior properties compared to single-pass deposits. This is a valuable insight for engineers designing surfacing procedures for thick deposits.
For the valve and fitting industry, this technology offers a pathway to extend the service life of critical components while maintaining their corrosion resistance and mechanical integrity. The localized nature of laser quenching ensures minimal distortion, which is essential for maintaining the dimensional accuracy of precision-machined components.