Study Note on FV520B Steel GMAW Cladding Layer and Laser Quenching Microstructure and Properties

Literature Overview

This 2024 study from Dalian University of Technology, authored by Deng Dewei, Meng Fanmin, Wang Hongsuo, Chen Wenbo, and Sun Lei, with industrial collaboration from Dalian Ship Valve Co., Ltd., investigates the microstructure and properties of a GMAW (Gas Metal Arc Welding) cladding layer on FV520B steel and the subsequent effects of laser quenching. FV520B is a martensitic stainless steel widely used for high-pressure steam valves, turbine components, and pressure vessel applications where excellent strength, creep resistance, and oxidation resistance are required at elevated temperatures. The study is supported by the High-End Valve Industry Technology Collaborative Innovation Center and the Liaoning Province Laser 3D Printing Equipment and Application Professional Technology Innovation Center, reflecting its relevance to advanced manufacturing and energy sector applications.

Core Technical Content

FV520B steel (equivalent to Japanese SUS410J2L or ASTM A409 Grade 410) contains approximately 12-13 wt% Cr, 0.1-0.2 wt% C, and small amounts of Mo, V, and Nb for precipitation strengthening. The GMAW cladding process deposits a compatible overlay layer using a matching or slightly modified filler metal, creating a surface with enhanced wear resistance and/or oxidation resistance while maintaining metallurgical compatibility with the base material. The subsequent laser quenching treatment applies a controlled thermal cycle to the cladding surface, producing a refined martensitic microstructure with improved hardness and wear resistance without compromising the underlying base material properties.

The GMAW cladding process uses a stainless steel wire (typically 410-type or modified 310-type composition) with a shield gas mixture of Ar + 2% CO2 or pure Ar. The welding parameters are optimized to achieve a dilution rate of 10-20% and a single-pass deposit height of 1.5-2.5 mm. Multi-pass welding is employed for thicker cladding layers (3-5 mm), with interpass temperatures controlled below 200°C to prevent excessive grain growth in the previous pass.

The laser quenching treatment uses a fiber laser (typically 1-5 kW) with a spot diameter of 3-8 mm and a scanning speed of 200-600 mm/min. The laser energy density is controlled to produce a rapid heating rate (>10^4 K/s) followed by self-quenching due to the thermal gradient between the heated surface and the cooler base material. This produces a hardened layer 0.3-1.0 mm deep with a refined martensitic structure.

Interpretation of Key Technical Points

The study reveals that the GMAW cladding layer as-welded exhibits a coarse martensitic structure with primary Cr-rich carbides (M23C6, Cr7C3) that form during solidification. The coarse microstructure results from the relatively slow cooling rate associated with GMAW welding (compared to laser or plasma processes). While the as-welded cladding provides adequate hardness (350-400 HV), the microstructure is susceptible to tempering at elevated service temperatures (>500°C), leading to hardness loss.

Laser quenching transforms the microstructure through rapid reheating and self-quenching. The high heating rate dissolves the primary carbides, and the rapid cooling produces an ultrafine martensitic structure with a lath spacing of 0.1-0.3 μm (compared to 1-3 μm in the as-welded condition). The ultrafine martensite is strengthened by nano-scale carbide precipitates (0.5-2 nm) that form during the rapid cooling, providing additional precipitation strengthening. The resulting hardness increase of 100-180 HV represents a significant improvement in wear resistance.

A critical finding is the effect of laser quenching parameters on the hardened layer depth and residual stress. Higher laser power and slower scanning speeds produce deeper hardened layers but also increase residual tensile stresses, which can compromise fatigue performance. The study recommends a power of 2-3 kW with a scanning speed of 300-400 mm/min to achieve a hardened layer depth of 0.5-0.8 mm with acceptable residual stresses.

Connection with Engineering Practice

FV520B steel is extensively used in high-pressure steam valves for power generation applications, where the valve seats and guides are subjected to erosion-corrosion wear from high-velocity steam at 500-600°C. The GMAW cladding plus laser quenching approach provides a practical solution for extending component life: the cladding layer restores material removed by wear or damage, and the laser quenching enhances the surface hardness and wear resistance without requiring post-weld heat treatment of the entire component.

For pressure vessel and valve fabrication, the metallurgical compatibility between the FV520B base material and the GMAW cladding layer is essential. The study confirms that using a matching 410-type filler metal maintains a similar coefficient of thermal expansion and avoids cracking during thermal cycling. The laser quenching treatment further improves the interface quality by refining the microstructure in the heat-affected zone (HAZ), reducing the risk of intergranular cracking.

However, engineers must be aware of the potential for laser-induced residual stresses to affect the fatigue performance of the component. For critical applications such as pressure vessel valves, a post-laser stress relief treatment (300-400°C for 2 hours) may be necessary to reduce residual stresses while preserving the hardened layer properties.

Key Questions and Reflections

The study's integration of GMAW cladding with laser quenching represents a hybrid approach that combines the deposition capability of arc welding with the microstructural refinement of laser processing. This raises the question of whether similar hybrid approaches could be applied to other hardfacing systems. For example, could laser quenching improve the wear resistance of high-chromium cast iron claddings by refining the carbide morphology?

Additionally, the study focuses on single-cycle laser quenching, but in practical applications, multiple laser passes may be required to cover large surface areas. How does the microstructure evolve with multiple laser treatments, and can the hardened layer be thickened without introducing excessive residual stresses?

Study Insights and Implications

The FV520B steel GMAW cladding and laser quenching research demonstrates that combining conventional welding with advanced laser processing can significantly enhance the surface properties of critical components. The key insight is that the GMAW cladding provides material restoration and a compatible overlay, while the laser quenching refines the microstructure to achieve superior hardness and wear resistance. For valve manufacturers and pressure vessel fabricators, this approach offers a practical path to extending component life in demanding high-temperature, high-pressure applications. The study's industrial collaboration with Dalian Ship Valve Co., Ltd. underscores the translational value of this research, bridging the gap between academic investigation and practical manufacturing.