Laser Surface Treatment Effects on Martensitic Stainless Steel Overlay Layer Microstructure and Properties

Literature Overview

This 2024 paper by Deng Dewei and colleagues from Dalian University of Technology and Dalian Marine Valve Co., Ltd., published in Metal Heat Treatment, investigates the effects of laser surface treatment on martensitic stainless steel overlay layers deposited on FV520B steel substrates. The study employs a two-step approach: first, overlay welding using open-arc oscillating deposition with martensitic flux-cored wire, followed by laser surface treatment using either semiconductor or fiber laser sources. The research addresses a practical engineering challenge—repairing surface damage on FV520B steel marine components while simultaneously improving the corrosion resistance of the repaired area.

Overlay Welding Process and Microstructure

The overlay welding was performed using a martensitic flux-cored wire with open-arc oscillating deposition technique. The resulting overlay layer exhibited the following characteristics:

The martensitic microstructure in the overlay layer provides high hardness and strength, while the δ-ferrite content helps prevent cracking during welding and service. The retained austenite contributes to toughness and reduces residual stress. The carbide phases (primarily Cr23C6 and Mo2C) provide additional hardness and corrosion resistance through chromium depletion in the matrix.

Element Distribution Analysis

EDS mapping revealed the elemental distribution across the overlay-base interface:

• Chromium content increased from approximately 12% in the base material to 18-20% in the overlay layer
• Molybdenum content increased from 1.5% to 3-4% in the overlay
• Carbon content was elevated in the overlay due to the flux-cored wire composition
• The transition zone showed a gradual gradient in elemental composition, indicating good metallurgical bonding without sharp compositional discontinuities

Laser Surface Treatment Analysis

Two laser sources were compared: semiconductor laser and fiber laser. The laser surface treatment created three distinct zones in the overlay layer:

The heat-affected zone hardness reduction is attributed to the tempering effect of the laser, which decomposes the hard martensite into tempered martensite with lower hardness but improved toughness. The remelted zone achieves hardness comparable to the original weld zone due to rapid cooling rates that suppress carbide coarsening and promote fine martensite formation.

Corrosion Resistance Improvement

Immersion testing in 3.5% NaCl solution demonstrated that laser surface treatment improved the corrosion resistance of the overlay layer. The improvement is attributed to:

1. Surface refinement: Laser remelting eliminates surface defects, porosity, and micro-cracks that serve as corrosion initiation sites.
2. Carbide redistribution: Rapid solidification in the remelted zone produces finer, more uniformly distributed carbides that deplete less chromium from the matrix.
3. Surface passivation: The laser-treated surface develops a more stable chromium oxide film due to the homogeneous microstructure.

Process Parameter Optimization

The study investigated the effect of scanning speed on laser treatment depth and hardness:

• Lower scanning speeds increase laser interaction depth due to higher energy input per unit area
• Hardness changes minimally with scanning speed variation, suggesting that the laser treatment primarily affects surface integrity rather than bulk properties
• Semiconductor laser achieves greater interaction depth than fiber laser at equivalent power levels
• Surface hardness after treatment is essentially the same for both laser types

The semiconductor laser's greater penetration depth is attributed to its longer wavelength and different beam characteristics, which interact differently with the metallic surface. For marine valve applications where surface damage depth may vary, the semiconductor laser offers more flexibility in treating deeper defects.

Key Insights and Reflections

This study demonstrates a sophisticated multi-step surface engineering approach that combines welding repair with laser surface treatment to achieve superior performance. The key insight is that overlay welding alone, while providing improved hardness and corrosion resistance, may still contain surface defects that limit its performance in aggressive marine environments. The subsequent laser treatment acts as a surface refinement step that eliminates these defects and optimizes the near-surface microstructure. For engineers in the marine industry dealing with FV520B steel component repair, this approach offers a validated methodology that can be incorporated into maintenance procedures. The comparison between semiconductor and fiber lasers provides practical guidance for equipment selection based on the specific repair requirements—semiconductor laser for deeper treatment and fiber laser for surface-level refinement. The finding that hardness is relatively insensitive to scanning speed simplifies process control, while the corrosion resistance improvement validates the approach for marine service applications.