Abstract:
The laser cladding technology applied to repair casting defects in steel casting, specifically targeting traction pins made of 25-steel. The microstructure, hardness, and wear properties of the austenitic stainless steel cladding layer were comprehensively analyzed. The results reveal the effectiveness of laser cladding in enhancing the surface properties of steel casting with defects.
Keywords: steel casting; casting defects; laser cladding; microstructure; wear property
Introduction
Traction pins play a crucial role in the traction system of electric locomotives, enduring friction and impact from the traction rod during operation. These pins are often made of ZG230-450 (25-steel), which is prone to casting defects such as sand holes and pores near the surface, leading to premature failure. To address this issue, laser cladding technology was employed to repair these defects, leveraging its advantages of high wear resistance, corrosion resistance, high-temperature oxidation resistance, and fatigue resistance.
Table 1: Properties of 25-Steel
| Material | 25-Steel (ZG230-450) |
|---|---|
| Tensile Strength | ≥450 MPa |
| Yield Strength | ≥230 MPa |
| Hardness | ~170 HV0.1 |
Experimental Methodology
Materials and Preparation
- Substrate Material: 25-steel traction pins
- Cladding Material: Austenitic stainless steel powder with a particle size range of 50-100 μm
- Chemical Composition:ElementCSiMnMoCrNiFeContent0.02%0.82%1.68%0.08%19.34%9.97%68.09%
- Surface Preparation: The 25-steel surface was grinded to remove rust and mechanical holes were drilled to simulate sand hole defects, with a diameter of 4 mm and a depth of 0.5 mm.
Laser Cladding Process
- Equipment: YLS-6000 fiber laser
- Parameters:
- Single-track cladding: Laser power = 2600 W, scanning speed = 6 mm/s, powder feeding rate = 18.9 g/min
- Multi-track single-layer cladding: Laser power = 2500 W, scanning speed = 6 mm/s, overlap rate = 40%, powder feeding rate = 18.9 g/min
Characterization and Testing
- Microstructural Analysis: Optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD)
- Hardness Testing: Microhardness tester (HV-1000B) with a load of 100 g and a dwell time of 15 s
- Wear Testing: UMT TriboLab tester with a GCr15 ball (hardness = 62 HRC), load = 100 N, reciprocating distance = 6 mm, reciprocating rate = 4 mm/s
Results and Discussion
Microstructural Analysis
- Top Portion: Composed of equiaxed and dendrite crystals
- Bottom Portion: Predominantly coarse columnar crystals growing perpendicular to the fusion line
- Quenched Zone: Composed of martensite
- Normalized Zone: Finer sorbite, similar to the base material
X-Ray Diffraction Analysis
- The cladding layer consists of a single austenitic (γ) phase.
TEM Analysis
- Dendrite Interior: γ-phase (FCC structure)
- Dendrite Interstices: γ-phase and Cr3C2 carbide (orthorhombic structure)
Hardness Distribution
Table 2: Hardness Distribution from Cladding Layer to Base Material
| Region | Hardness (HV0.1) |
|---|---|
| Cladding Layer | 310 |
| Heat-Affected Zone | 280 |
| Base Material | 170 |
Wear Performance
- After 1 hour of wear testing, the mass loss of the base material was approximately twice that of the cladding layer.
- Base Material: Deep grooves, plastic deformation, and adhesive wear features
- Cladding Layer: Shallow grooves, minor spallation, mainly abrasive wear
Conclusion
The application of laser cladding technology to repair casting defects in 25-steel traction pins proved effective. The cladding layer, composed of equiaxed, dendrite, and planar crystals, exhibited a hardness of 310 HV0.1, significantly higher than the base material’s 170 HV0.1. The wear resistance of the cladding layer was double that of the base material, primarily due to its finer microstructure and the combined action of tough γ-phase and hard Cr3C2 carbide. This study highlights the potential of laser cladding for improving the surface properties of steel castings with defects.