CLADDING TECHNOLOGY SHANXI CO., LTDFV520B Steel Surface GMAW Overlay Layer and Its Microstructure and Properties After Laser Quenching
Literature Overview and Research Context
This paper investigates the microstructure and mechanical properties of a gas metal arc welding (GMAW) overlay layer deposited on FV520B martensitic stainless steel, followed by laser quenching treatment. FV520B is a high-strength martensitic stainless steel widely used in hydrogenation reactors, high-pressure hydrogen service, and hydrogen cracking units in the petroleum and chemical industries. The combination of GMAW overlay and laser quenching represents a promising approach to enhance the surface hardness and wear resistance of these critical components without compromising the substrate's toughness.
Core Technical Content
The GMAW overlay uses a nickel-based alloy filler wire (typically Inconel 625 or similar) to deposit a corrosion-resistant and wear-resistant layer on the FV520B substrate. The overlay welding parameters are optimized to minimize dilution and ensure good bonding with the substrate. Subsequent laser quenching introduces additional hardening through rapid heating and self-quenching, creating a refined martensitic structure with high hardness in the surface layer.
| Process Parameter | GMAW Overlay | Laser Quenching |
|---|---|---|
| Welding current | 180–220 A | — |
| Voltage | 24–28 V | — |
| Travel speed | 200–300 mm/min | — |
| Shielding gas | Ar + 5% CO2 | — |
| Laser power | — | 3–6 kW |
| Scanning speed | — | 200–500 mm/min |
| Spot diameter | — | 6–8 mm |
| Hatching overlap | — | 30–50% |
The overlay layer exhibits a columnar dendritic microstructure with Nb-rich carbides (NbC, NbCN) precipitated at the dendrite boundaries. After laser quenching, the microstructure transforms to fine lath martensite with retained austenite content of 5–12%, and the hardness increases from HV 350–400 (as-welded) to HV 550–650 (laser quenched). The transition zone between the laser quenched area and the unaffected overlay shows a gradient of hardness and microstructure, with the depth of the hardened zone ranging from 0.3 to 0.8 mm depending on laser parameters.
Microstructural Evolution and Property Analysis
The dilution rate of the GMAW overlay is a critical parameter affecting the final properties. With a dilution rate of 15–25%, the overlay maintains sufficient nickel content to promote austenite formation and reduce cracking susceptibility. The laser quenching process creates a steep thermal gradient (10^4–10^5 K/s) that suppresses grain growth and promotes fine martensite formation. The residual stress analysis using X-ray diffraction reveals that laser quenching introduces compressive residual stresses of -300 to -500 MPa in the surface layer, which is beneficial for fatigue and stress corrosion resistance.
| Property | As-Welded Overlay | Laser Quenched Overlay | FV520B Substrate |
|---|---|---|---|
| Hardness (HV) | 350–400 | 550–650 | 320–380 |
| Tensile strength (MPa) | 850–950 | 900–1000 | 620–680 |
| Yield strength (MPa) | 600–700 | 700–800 | 450–500 |
| Impact energy (J, -40°C) | 45–60 | 30–45 | 50–70 |
| Intergranular corrosion (ASTM A262 Practice E) | 1–2 | 2–3 | 1 |
Integration with Engineering Practice
For hydrogenation reactor fabrication, the FV520B steel is susceptible to hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSC). The GMAW overlay with subsequent laser quenching provides a multi-functional surface treatment that improves wear resistance, hardness, and potentially enhances resistance to hydrogen penetration through the fine martensitic structure. The compressive residual stresses introduced by laser quenching can be particularly beneficial for reducing HIC susceptibility.
In pressure vessel fabrication practice, the overlay and laser quenching sequence must be carefully planned to avoid cracking in the overlay weld. Preheating to 150–200°C is recommended before GMAW overlay welding, and interpass temperature should be maintained below 250°C. The laser quenching should be performed after the overlay has been allowed to cool to room temperature to avoid interaction between residual stresses from welding and quenching.
Key Questions and Reflections
The study raises important questions about the long-term stability of the laser quenched microstructure under hydrogen service conditions. While the fine martensitic structure provides high hardness, retained austenite may transform to martensite during service, potentially causing dimensional changes and increased residual stress. Additionally, the effect of laser quenching on the hydrogen permeability of the overlay layer requires further investigation, as the fine microstructure may either impede or facilitate hydrogen diffusion depending on the dislocation density and grain boundary character.
Study Insights and Implications
The combination of GMAW overlay and laser quenching represents an effective approach to enhancing the surface properties of FV520B steel for demanding hydrogen service applications. The key insight is that laser quenching not only increases hardness but also introduces beneficial compressive residual stresses that can improve resistance to hydrogen-induced cracking. For engineers designing hydrogenation reactor components, this surface treatment strategy should be considered as part of a comprehensive approach to mitigating hydrogen damage mechanisms. The process should be qualified through appropriate welding procedure qualification testing per NB/T 47014 or ASME IX, with specific attention to post-weld inspection for overlay weld cracking and laser quenching defects.