In the field of rail transportation, components such as axle boxes, which are critical for connecting bogie frames and wheelsets, often experience fretting wear due to dynamic interactions. This wear phenomenon can compromise the service reliability and safety of these parts. G20Mn5QT, a type of steel casting known for its good plasticity and excellent resistance to brittle fracture, is commonly used in the manufacturing of axle box bodies. However, research on the fretting wear behavior and protection of G20Mn5QT steel casting remains limited. Surface engineering technologies, particularly laser cladding, offer promising solutions to enhance anti-fretting wear performance. In this study, I focus on the fretting friction and wear characteristics of G20Mn5QT steel casting with a laser-clad 316L stainless steel coating, aiming to provide theoretical support for remanufacturing and repair of steel casting components.
The use of steel casting in industrial applications, such as in rail vehicles, is widespread due to its cost-effectiveness and mechanical properties. However, fretting wear at contact interfaces, such as between bolts and washers in axle box assemblies, poses a significant challenge. Laser cladding, as an advanced surface modification technique, can deposit coatings with dense microstructures and high bonding strength to the substrate. For steel casting components like G20Mn5QT, applying a 316L coating may improve wear resistance. This study explores the tangential fretting wear behavior under varying displacement amplitudes, with a fixed normal load of 30 N, to understand the damage mechanisms and evolution.
To begin, I prepared the test materials. The upper specimen was a Q355E steel ball with a diameter of 15 mm, while the lower specimens were G20Mn5QT steel casting substrates and 316L laser-clad coatings. The steel casting substrates were machined into blocks of 20 mm × 10 mm × 8 mm, and their surfaces were polished to a roughness (Ra) below 0.1 μm. The 316L coating was applied using a high-power fiber laser cladding system, with optimized parameters including a laser power of 2,300 W, scanning speed of 500 mm/min, and powder feed rate of 14 g/min. The 316L stainless steel powder, produced via gas atomization, had a particle size range of 53–150 μm. The chemical compositions of the materials are summarized in Table 1.
| Material | C | Si | Mn | Cr | Ni | Cu | Mo | Fe |
|---|---|---|---|---|---|---|---|---|
| Q355E | ≤0.18 | ≤0.50 | ≤0.50 | ≤0.80 | ≤0.30 | ≤0.30 | ≤0.12 | Bal. |
| G20Mn5QT Steel Casting | 0.17–0.23 | ≤0.60 | 1.00–1.60 | ≤0.30 | ≤0.80 | ≤0.30 | ≤0.12 | Bal. |
| 316L Powder | 0.01 | 0.72 | 0.93 | 17.24 | 10.92 | – | 2.54 | Bal. |
The mechanical properties of the specimens are critical for understanding fretting behavior. The G20Mn5QT steel casting exhibits a yield strength of approximately 327 MPa, tensile strength of 660 MPa, and hardness of 210 HV0.2. After laser cladding, the 316L coating shows enhanced hardness due to the formation of hard phases. The hardness gradient from the surface to the substrate was measured using a microhardness tester. The results indicate that the cladding layer has an average hardness of 240 HV0.2, which is about 14.3% higher than the steel casting substrate. This improvement is attributed to the formation of Cr-containing hard phases, such as Cr0.19Fe0.7Ni0.11, during the laser cladding process. The microstructure of the 316L coating, observed through metallographic analysis, reveals a homogeneous distribution of equiaxed and columnar crystals, contributing to its superior properties.
The fretting wear tests were conducted using a self-developed tangential fretting wear tester. The tests were performed in ball-on-flat contact mode, with a fixed normal load of 30 N and displacement amplitudes of 10, 20, and 40 μm. The frequency was set at 5 Hz, and the tests lasted for 4,000 seconds, corresponding to 20,000 cycles. The environmental conditions were maintained at room temperature (20–25 °C) and relative humidity of 55% ± 10%. The tangential force and displacement were recorded in real-time to generate Ft-D (friction force-displacement) curves, which are essential for analyzing fretting regimes. The wear scars were characterized using scanning electron microscopy (SEM), white light interferometry, and energy-dispersive spectroscopy (EDS) to examine surface morphology, wear volume, and chemical composition.
The microstructural and phase analysis of the steel casting substrate and 316L coating was performed using X-ray diffraction (XRD). The XRD patterns show that the G20Mn5QT steel casting primarily consists of γ-Fe phases, while the 316L coating includes additional phases such as Cr and Cr0.19Fe0.7Ni0.11. These hard phases contribute to the increased hardness of the coating. The hardness profile across the coating depth can be described by the following empirical formula, which relates hardness to distance from the surface:
$$ H(x) = H_{\text{base}} + \Delta H \cdot e^{-kx} $$
where \( H(x) \) is the hardness at distance \( x \) from the surface, \( H_{\text{base}} \) is the base hardness of the steel casting, \( \Delta H \) is the hardness increment due to cladding, and \( k \) is a decay constant. For the 316L coating, \( \Delta H \) is approximately 30 HV0.2, and \( k \) is derived from experimental data.
The fretting running behavior was analyzed through Ft-D-N curves and friction coefficient evolution. Under a normal load of 30 N, the Ft-D curves for both the steel casting substrate and 316L coating transitioned with increasing displacement amplitude. At 10 μm, the curves evolved from initial parallelogram shapes to linear shapes, indicating a partial slip regime (PSR). At 20 μm, the curves showed elliptical shapes, characteristic of a mixed regime (MR). At 40 μm, the curves maintained parallelogram shapes, corresponding to a gross slip regime (GSR). This regime transition is summarized in Table 2, which correlates displacement amplitude with fretting behavior.
| Displacement Amplitude (μm) | Fretting Regime | Ft-D Curve Shape | Dominant Deformation |
|---|---|---|---|
| 10 | Partial Slip Regime (PSR) | Linear | Elastic |
| 20 | Mixed Regime (MR) | Elliptical | Elasto-plastic |
| 40 | Gross Slip Regime (GSR) | Parallelogram | Plastic |
The friction coefficient curves revealed distinct stages: running-in, ascent, and stabilization. For the steel casting substrate, the steady-state friction coefficients were 0.411, 0.612, and 0.675 at 10, 20, and 40 μm, respectively. For the 316L coating, the values were slightly higher at 0.406, 0.647, and 0.728. The increase in friction coefficient with displacement amplitude is attributed to greater interfacial sliding and wear debris generation. The friction coefficient \( \mu \) can be modeled using the following equation, which considers the contribution of adhesive and abrasive components:
$$ \mu = \mu_a + \mu_b \cdot \frac{D}{D_c} $$
where \( \mu_a \) is the adhesive friction coefficient, \( \mu_b \) is the abrasive friction coefficient, \( D \) is the displacement amplitude, and \( D_c \) is a critical displacement for regime transition. For steel casting materials, \( \mu_a \) typically ranges from 0.3 to 0.5, and \( \mu_b \) increases with hardness.
The wear scar morphology was examined using SEM. At 10 μm, both materials showed slight damage with minor debris accumulation at the edges, indicative of adhesive wear. At 20 μm, the steel casting substrate exhibited more severe damage, including ploughing grooves and delamination, while the 316L coating had milder wear features. At 40 μm, the damage intensified, with extensive delamination and loose debris for the steel casting, whereas the 316L coating showed compacted debris layers without significant pile-up. EDS line scans across the wear scars confirmed oxidation in the mixed and gross slip regimes, with higher oxygen content in the debris zones. The wear mechanisms are summarized in Table 3.
| Fretting Regime | Dominant Wear Mechanisms | Key Features |
|---|---|---|
| Partial Slip Regime | Adhesive wear | Central stick zone, edge micro-slip, minimal debris |
| Mixed Regime | Abrasive wear, delamination, oxidation | Ploughing grooves, flake formation, oxygen enrichment |
| Gross Slip Regime | Abrasive wear, delamination, oxidation | Severe grooves, loose debris, high oxygen levels |
The wear volume and wear rate were quantified using white light interferometry. The wear volume \( V \) was calculated from 3D surface profiles, and the wear rate \( W \) was determined using the formula:
$$ W = \frac{V}{F_n \cdot N \cdot D_{\text{total}}} $$
where \( F_n \) is the normal load (30 N), \( N \) is the number of cycles (20,000), and \( D_{\text{total}} \) is the total sliding distance, given by \( D_{\text{total}} = 4ND \) for reciprocating motion. The results are presented in Table 4, showing that the 316L coating has lower wear volumes and wear rates compared to the steel casting substrate, especially in the mixed and gross slip regimes.
| Displacement Amplitude (μm) | Material | Wear Volume (105 μm3) | Wear Rate (10-6 mm3/(N·m)) |
|---|---|---|---|
| 10 | Steel Casting Substrate | 2.1 | 0.18 |
| 316L Coating | 1.8 | 0.15 | |
| 20 | Steel Casting Substrate | 8.5 | 0.71 |
| 316L Coating | 8.1 | 0.68 | |
| 40 | Steel Casting Substrate | 22.0 | 0.92 |
| 316L Coating | 17.8 | 0.74 |
The wear rate reduction for the 316L coating relative to the steel casting substrate is approximately 4.26% at 20 μm and 19.1% at 40 μm. This demonstrates the enhanced fretting wear resistance of the coating, which is crucial for applications involving steel casting components subjected to cyclic loads. The improvement can be attributed to the higher hardness and homogeneous microstructure of the 316L layer, which mitigate abrasive and delamination wear.
Cross-sectional analysis of the wear scars via SEM revealed subsurface damage. For the steel casting substrate, delamination cracks and wear pits were observed, deepening with increased displacement amplitude. In contrast, the 316L coating showed shallower damage and fewer cracks, indicating better resistance to subsurface fatigue. The depth of wear \( h \) can be related to the accumulated plastic strain \( \epsilon_p \) through the equation:
$$ h = C \cdot \epsilon_p^n \cdot N^m $$
where \( C \), \( n \), and \( m \) are material constants. For steel casting materials, \( n \) is typically around 0.5, and \( m \) depends on the wear mechanism. For the 316L coating, the constants are lower due to its enhanced properties.
The fretting wear damage mechanisms are schematically illustrated in Figure 1. In the partial slip regime, the contact area consists of a central stick zone and edge micro-slip zones, with adhesive wear dominating. In the mixed regime, the stick zone shrinks, and abrasive wear, delamination, and oxidation become prominent. In the gross slip regime, full sliding occurs, leading to severe abrasive wear, debris formation, and oxidation. The 316L coating, with its hard phases, reduces the extent of delamination and debris generation, thereby lowering wear rates.
The discussion extends to the implications for steel casting components in service. Axle box bodies made from G20Mn5QT steel casting are prone to fretting wear at bolted joints. The application of a 316L laser-clad coating can significantly extend their lifespan by improving wear resistance. The coating’s performance is influenced by factors such as cladding parameters, which affect microstructure and hardness. Optimizing these parameters for steel casting substrates is essential for maximizing benefits. Additionally, the environmental conditions, such as temperature and humidity, may alter wear behavior, but this study focuses on room temperature conditions.
In terms of material science, the formation of Cr-rich hard phases in the 316L coating is key to its performance. These phases, identified as Cr0.19Fe0.7Ni0.11, enhance hardness and wear resistance through dispersion strengthening. The relationship between hardness \( H \) and wear rate \( W \) can be expressed by the Archard-type equation:
$$ W = k \cdot \frac{F_n \cdot D}{H} $$
where \( k \) is a wear coefficient. For the steel casting substrate, \( k \) is higher due to its lower hardness, leading to greater wear. For the 316L coating, \( k \) is reduced, resulting in lower wear rates. This equation underscores the importance of hardness in fretting wear resistance for steel casting materials.
Furthermore, the economic and industrial relevance of this research lies in the potential for remanufacturing steel casting components. Instead of replacing worn parts, laser cladding can be used to repair and enhance their surfaces, reducing costs and downtime. This is particularly valuable in the rail transportation sector, where steel casting components like axle boxes are critical for safety. Future work could explore other coating materials or hybrid processes to further improve performance.
In conclusion, this study systematically investigates the fretting wear characteristics of G20Mn5QT steel casting with a laser-clad 316L coating. The key findings are as follows: First, the 316L coating increases surface hardness by 14.3% due to the formation of Cr-containing hard phases. Second, fretting regimes transition from partial slip to mixed to gross slip with increasing displacement amplitude, accompanied by rising friction coefficients. Third, wear mechanisms evolve from adhesive wear in partial slip to abrasive wear, delamination, and oxidation in mixed and gross slip regimes. Fourth, the 316L coating exhibits lower wear volumes and wear rates compared to the steel casting substrate, with reductions of about 4.26% and 19.1% in mixed and gross slip regimes, respectively. These results highlight the effectiveness of laser cladding in enhancing the fretting wear resistance of steel casting components, offering valuable insights for their remanufacturing and prolonged service life.