Zhuojin Pipe Fitting Co., LtdEffects of Laser Surface Treatment on Microstructure and Properties of Martensitic Stainless Steel Cladding Layer
Literature Overview
This paper, authored by Deng Dewei and colleagues from Dalian University of Technology and Dalian Marine Valve Co., Ltd., published in Heat Treatment of Metals (2024, Vol. 49, No. 3, pp. 56-63), investigates a two-step surface engineering approach for repairing FV520B steel: first applying a martensitic stainless steel cladding layer by open-arc oscillating cladding, then performing laser surface treatment on the cladding layer to enhance surface properties. Two types of lasers—semiconductor laser and fiber laser—were compared, and the effects of laser parameters on microstructure, hardness, corrosion resistance, and treatment depth were systematically evaluated.
Technical Context and Motivation
FV520B is a martensitic stainless steel widely used in marine and chemical valve applications due to its combination of strength, toughness, and moderate corrosion resistance. Surface damage to valve components—caused by cavitation erosion, galling, wear, or corrosion—requires effective repair that not only restores geometry but also enhances surface properties beyond the original condition. The conventional approach of simple cladding repair may not adequately address corrosion resistance requirements, particularly in aggressive marine environments. The proposed two-step approach of cladding followed by laser surface treatment aims to:
- Restore the damaged surface geometry through cladding.
- Improve surface hardness and microstructure through laser remelting.
- Enhance corrosion resistance through microstructural refinement and alloy redistribution.
Cladding Layer Characteristics
The cladding was performed using open-arc oscillating cladding with martensitic flux-cored wire on the FV520B substrate. The resulting cladding layer exhibited the following characteristics:
| Parameter | Value | Comparison with Substrate |
|---|---|---|
| Microstructure | Martensite + δ-ferrite + retained austenite + carbides | Similar to substrate but refined |
| Hardness (HV0.3) | ~480 HV0.3 | +33.33% over substrate |
| Metallurgical bonding | Good | No defects at interface |
| Element distribution | Gradual transition at interface | No sharp segregation |
The presence of δ-ferrite and retained austenite in the cladding layer is beneficial for crack resistance, while the martensitic matrix provides the primary hardness contribution. The carbide phase (likely Cr-rich carbides) contributes to wear resistance.
Laser Surface Treatment: Zone Analysis
After laser surface treatment, the cladding layer can be divided into three distinct zones from the surface inward:
1. Remelted Zone (Surface)
- Microstructure: Fully remelted and rapidly solidified, resulting in a fine-grained martensitic structure with refined carbide distribution.
- Hardness: Comparable to the original cladding zone (~480 HV0.3).
- Corrosion resistance: Significantly improved due to microstructural refinement and homogeneous alloy distribution.
2. Heat-Affected Zone (HAZ)
- Microstructure: Partially tempered martensite with retained austenite transformation.
- Hardness: Reduced compared to the original cladding layer, due to the tempering effect of the laser thermal cycle.
- Corrosion resistance: Slightly improved compared to untreated cladding.
3. Original Cladding Zone
- Microstructure: Unchanged from the original cladding layer.
- Hardness: ~480 HV0.3.
- Corrosion resistance: Baseline (untreated cladding level).
Laser Type Comparison
| Parameter | Semiconductor Laser | Fiber Laser |
|---|---|---|
| Treatment depth | Greater | Shallower |
| Surface hardness | ~480 HV0.3 | ~480 HV0.3 |
| Corrosion resistance improvement | Significant | Significant |
| Energy density distribution | Broader | More concentrated |
| Suitable for | Deep surface modification | Shallow, precise treatment |
The key finding is that while the semiconductor laser achieves a greater treatment depth, both laser types produce similar surface hardness levels. The treatment depth is primarily governed by the scanning speed: lower scanning speeds increase the treatment depth but do not significantly change the surface hardness.
Corrosion Resistance Evaluation
Immersion testing confirmed that laser surface treatment improved the corrosion resistance of the cladding layer. The mechanism is attributed to:
- Microstructural refinement: Finer grains reduce the number of active corrosion initiation sites.
- Homogeneous alloy distribution: Laser remelting redistributes alloying elements, reducing local compositional variations that promote selective corrosion.
- Reduced residual stress: The rapid heating and cooling cycle of laser treatment partially relieves residual stresses from the cladding process, reducing the driving force for stress corrosion cracking.
- Improved surface passivity: The refined microstructure promotes the formation of a more uniform and protective chromium oxide passive film.
Engineering Practice Integration
For marine valve manufacturers, this two-step approach offers a practical repair and enhancement strategy:
- Cladding repair: Restore damaged surfaces using open-arc oscillating cladding with martensitic flux-cored wire.
- Laser surface treatment: Apply laser remelting to enhance surface properties, particularly corrosion resistance.
- Laser selection: Choose semiconductor laser for deeper treatment requirements or fiber laser for more precise, shallow treatment.
- Parameter optimization: Adjust scanning speed to control treatment depth based on the specific application requirements.
The approach is particularly valuable for critical valve components operating in aggressive marine environments where both mechanical integrity and corrosion resistance are essential for long-term reliability.
Key Questions and Reflections
This study raises an important engineering question: what is the optimal balance between treatment depth and surface hardness? The finding that both laser types achieve similar surface hardness but different treatment depths suggests that the choice should be based on the required depth of property enhancement rather than surface hardness alone. The tempering effect in the HAZ, which reduces hardness, is an inherent consequence of the laser thermal cycle and must be accepted as a trade-off for the improved corrosion resistance in the remelted zone. For engineers designing repair procedures for martensitic stainless steel components, this study provides valuable quantitative data on the effects of laser parameters on microstructure and properties, enabling more informed decision-making in repair procedure development.
The systematic comparison of semiconductor and fiber lasers in the context of cladding surface treatment is particularly valuable, as the laser type selection is often guided by availability rather than technical merit. This study provides the technical basis for selecting the appropriate laser type based on specific application requirements.