CLADDING TECHNOLOGY SHANXI CO., LTDLaser Surface Treatment Effects on Microstructure and Properties of Martensitic Stainless Steel Cladding Layer
Literature Overview
This 2024 publication in Heat Treatment of Metals (金属热处理) by Deng Dewei, Li Zhenhua, Chen Wenbo, Wang Hongsu, and Sun Lei, affiliated with Dalian University of Technology and Dalian Marine Valve Co., Ltd., addresses a critical gap in post-weld treatment technology for martensitic stainless steel cladding layers. The research is funded by the High-End Valve Industry Technology Collaborative Innovation Center (2022DMV003) and the Liaoning Provincial Laser 3D Printing Equipment and Application Professional Technical Innovation Center (DUT2022031), indicating a strong industry-academia collaboration focused on marine valve applications where corrosion resistance and mechanical integrity are paramount.
Core Technical Content
Martensitic stainless steels, such as 410, 420, and 431 grades, are widely used in marine valves, pump impellers, and chemical processing equipment due to their combination of moderate corrosion resistance and high strength. However, weld overlay cladding layers on these substrates often exhibit heterogeneous microstructures with retained austenite, coarse grains, and residual stresses that compromise performance. The authors investigated laser surface treatment (LST) as a post-weld modification technique to refine the microstructure of the cladding layer without introducing significant thermal distortion.
The key findings relate to the interaction between laser parameters—power density, scanning speed, and number of passes—and the resulting microstructural evolution in the martensitic cladding layer. Laser remelting produces extremely high cooling rates (10^3 to 10^6 K/s), which suppresses grain growth and promotes the formation of fine lenticular martensite with reduced retained austenite content.
Microstructural Evolution
| Parameter | Pre-Treatment | Post Laser Treatment |
|---|---|---|
| Grain Size | 80–120 μm | 20–40 μm |
| Retained Austenite | 15–25 vol% | 5–10 vol% |
| Hardness (HV) | 280–320 | 350–420 |
| Tensile Strength (MPa) | 650–750 | 780–900 |
| Residual Stress | Compressive 150–200 MPa | Compressive 300–450 MPa |
The transformation from coarse acicular martensite to fine lenticular martensite is the primary mechanism for hardness improvement. The rapid solidification and subsequent high cooling rate during laser remelting suppresses the diffusion-controlled growth of carbides and promotes a more uniform distribution of chromium-rich carbides (M23C6 and Cr7C3) within the martensitic matrix.
Engineering Practice Integration
For marine valve applications, the corrosion resistance of the cladding layer is equally critical as mechanical properties. The reduction in retained austenite is particularly significant because austenite is thermodynamically unstable in martensitic stainless steels and can transform during service, causing volume expansion and microcracking. Laser treatment stabilizes the microstructure by reducing the metastable austenite fraction.
Practical Considerations for Implementation
- Laser power: 1.5–3.0 kW for typical cladding thicknesses of 1–3 mm
- Scanning speed: 0.5–2.0 m/min to balance penetration depth and cooling rate
- Beam diameter: 1–3 mm depending on the required treatment width
- Atmosphere protection: Argon shielding at 15–20 L/min to prevent oxidation during remelting
- Multiple passes: 2–3 passes for thicker cladding layers (>2 mm) to ensure uniform treatment
Key Questions and Reflections
A critical question arising from this work is the potential for laser-induced microcracking in high-carbon martensitic cladding layers. While the study demonstrates significant property improvements, the thermal gradients introduced during laser remelting could potentially create tensile stresses at the cladding-substrate interface. In practice, a careful balance must be struck between the beneficial microstructural refinement and the risk of cracking. Pre-heating the component to 100–150°C before laser treatment and applying a controlled cool-down rate afterward can mitigate this risk.
Another important consideration is the dimensional accuracy of precision valve components. Laser treatment introduces localized heating that can cause minor distortion, typically in the range of 0.01–0.05 mm for components of moderate thickness. For high-precision valve seats and guides, this distortion must be accounted for in the machining sequence—laser treatment should ideally be performed before final machining operations.
The study's relevance to bimetal pressure vessel fabrication is indirect but meaningful. Components such as valve internals, control valve plugs, and trim assemblies in high-pressure vessels often employ martensitic stainless steel cladding for erosion-corrosion resistance. The laser treatment technology demonstrated here could extend the service life of such components by stabilizing the cladding microstructure against thermal cycling and mechanical fatigue.
Study Insights and Implications
This research represents a significant advancement in the post-weld treatment of martensitic stainless steel cladding layers. The combination of industry-driven funding and academic rigor produces results that are immediately applicable to marine valve manufacturing. The key insight is that laser surface treatment offers a non-destructive, localized method to enhance both the mechanical and corrosion properties of cladding layers without requiring full-component heat treatment, which would be impractical for complex valve geometries. Future work should focus on long-term service testing under actual marine conditions and on developing standardized procedures for laser treatment of cladding layers in accordance with established welding codes such as ASME IX and NB/T 47014.