CLADDING TECHNOLOGY SHANXI CO., LTDLaser Alloying of Edging Die Overlay Layer Surface
Literature Overview and Technical Background
This paper, published in Applied Laser in 2010, investigates the microstructural evolution and application performance of laser alloying applied to the surface of an edging die overlay layer. The research was conducted by Zheng Xiaoqing, Yang Yizhong, Ma Mingxing, and colleagues from Tsinghua University, Inner Mongolia First Machinery Group, and Baotou Jindu Technology Co., Ltd. The work addresses a specific industrial challenge: improving the surface properties of edging die overlay layers through laser surface alloying to enhance wear resistance and extend service life.
Edging dies are critical components in steel strip processing, where they trim the edges of continuously rolled steel strips. The overlay layer on the edging die must withstand repeated impact loading, abrasive wear from the steel strip, and thermal cycling from the hot steel. The conventional overlay layer, typically a hardfacing alloy, provides good initial wear resistance but may suffer from premature degradation due to surface oxidation, spalling, or gradual wear.
Core Technical Analysis
Laser Alloying Process Parameters
Laser alloying involves the selective melting and rapid solidification of the overlay layer surface with a laser beam, often with the addition of alloying elements to modify the surface composition. The key process parameters and their effects are summarized below:
| Parameter | Typical Range | Effect on Microstructure |
|---|---|---|
| Laser power | 1–5 kW | Controls melt pool depth and width |
| Scanning speed | 100–1000 mm/min | Affects cooling rate and grain morphology |
| Spot diameter | 0.5–3 mm | Determines heat input density |
| Powder feed rate | 0.5–5 g/min | Controls alloying element content |
| Shielding gas | Argon or nitrogen | Prevents oxidation during melting |
The optimal parameter combination typically achieves a melt pool depth of 0.5–2.0 mm, which is sufficient to modify the surface microstructure without excessive penetration into the base overlay layer. The cooling rate achieved during laser alloying is typically 10^3–10^5 K/s, which is orders of magnitude higher than conventional welding, resulting in extremely fine microstructures.
Microstructural Refinement
The laser alloying process produces several beneficial microstructural changes in the overlay layer surface:
- Grain refinement: The rapid solidification rate produces grain sizes of 5–50 μm, compared to 100–500 μm in the as-welded overlay layer. This refinement significantly increases hardness through the Hall-Petch mechanism.
- Carbide refinement: Coarse carbides in the as-welded overlay layer are dissolved during laser remelting and reprecipitate as fine carbides (1–10 μm) during rapid solidification. The refined carbides provide superior wear resistance through a combination of dispersion strengthening and increased number of hard particles.
- Solid solution strengthening: The addition of alloying elements (such as Cr, Mo, W, or B) during laser alloying creates a supersaturated solid solution that further increases hardness through solid solution strengthening.
- Nanocrystalline regions: In some cases, the extremely rapid cooling rate can produce nanocrystalline regions (grain size < 100 nm) at the surface, which provide exceptional hardness and wear resistance.
Hardness and Wear Resistance Improvement
The laser alloying process typically produces the following improvements in surface properties:
- Hardness increase: 30–60% increase in surface hardness, from 400–500 HV in the as-welded overlay layer to 550–800 HV in the laser alloyed surface.
- Wear resistance improvement: 2–5 times improvement in wear resistance, as measured by pin-on-disk or block-on-ring wear tests.
- Surface integrity: The laser alloyed surface exhibits improved resistance to oxidation, spalling, and delamination due to the refined microstructure and reduced residual stress.
Engineering Practice and Application
Edging Die Service Conditions
Edging dies operate under severe conditions that demand high surface hardness and toughness:
- Impact loading: The steel strip contacts the edging die at high speed, producing impact loads of 50–200 MPa.
- Abrasive wear: The steel strip surface contains oxide scale and other hard particles that cause abrasive wear of the overlay layer.
- Thermal cycling: The hot steel strip (temperature 800–1200 °C) causes thermal cycling of the edging die surface, leading to thermal fatigue cracking.
- Chemical interaction: At elevated temperatures, chemical interaction between the steel strip and the overlay layer can occur, leading to diffusion wear.
Quality Control and Inspection
The laser alloying process requires careful quality control to ensure consistent results:
- Surface roughness: The laser alloyed surface should have a roughness Ra of 0.2–1.0 μm to minimize friction with the steel strip.
- Crack inspection: Optical microscopy or scanning electron microscopy (SEM) should be used to detect any cracks in the laser alloyed layer.
- Hardness uniformity: A grid of hardness measurements should be performed across the laser alloyed surface to verify uniform hardness distribution.
- Adhesion testing: Scratch testing or micro-indentation testing should be performed to verify that the laser alloyed layer is well-bonded to the underlying overlay layer.
Study Insights and Independent Reflection
This research demonstrates the significant potential of laser alloying as a surface modification technique for improving the performance of overlay layers in severe service conditions. The key advantage of laser alloying over conventional surface hardening techniques (such as carburizing, nitriding, or induction hardening) is the ability to achieve extremely fine microstructures and high hardness values without significant distortion or residual stress.
One important insight is that laser alloying is not merely a surface hardening technique but a true surface modification process that can fundamentally alter the microstructure and composition of the overlay layer surface. This means that laser alloying can be used to tailor the surface properties of the overlay layer to meet specific service requirements, rather than simply increasing hardness.
Another key insight is the importance of process parameter optimization. The laser alloying process is highly sensitive to process parameters, and small variations can lead to significant changes in the resulting microstructure and properties. This sensitivity requires careful process qualification and quality control to ensure consistent results in production environments.