Laser cladding has emerged as a powerful surface modification and repair technology, particularly for recovering damaged or defective metallic components. In the context of railway traction components, the traction pin is a critical load-bearing part that must withstand complex mechanical stresses during locomotive operation. The present work focuses on the repair of near-surface defects such as sand holes and gas porosities in a steel casting used for traction pins. The base material investigated is ZG230-450 (also known as 25 steel), which is widely employed for producing traction pins in electric locomotives. Because surface defects in steel casting components can lead to premature failure and even catastrophic accidents, an effective and reliable repair methodology is needed. Conventional welding repair techniques often introduce excessive heat input, severe thermal deformation, and undesirable microstructural changes. Laser cladding, by contrast, offers a low heat input, high precision, and excellent metallurgical bonding with the substrate. In this study, DJ106 austenitic stainless steel powder was selected as the cladding material and deposited on the ZG230-450 steel casting substrate using a fiber laser. The process parameters were optimized through orthogonal experiments. The microstructure, phase composition, microhardness, tensile properties, wear resistance, and corrosion behavior of the repaired steel casting were systematically evaluated. The results demonstrate that laser cladding of DJ106 austenitic stainless steel significantly improves the surface hardness, wear resistance, and corrosion resistance of the steel casting while maintaining or even enhancing its tensile properties. The findings provide a scientific basis for the practical repair of surface defects in steel casting components such as traction pins.
1. Introduction
1.1 Traction Pins and Their Casting Defects
Traction pins are essential components in the traction transmission system of electric locomotives. They transmit traction and braking forces between the vehicle body and the bogie while accommodating relative motions such as friction, lateral displacement, rotation, tension, vertical vibration, pitching, and rolling. In the HXD3C electric locomotive produced by CRRC Dalian, the traction pin is manufactured as a monolithic steel casting made of ZG230-450 (25 steel). The casting has a mass of approximately 300 kg and overall dimensions of 800 mm × 480 mm × 350 mm. The wall thickness of the casting varies significantly, with the thinnest section being about 80 mm and the thickest solid conical section reaching approximately 200 mm. During the casting process, defects such as scattered sand holes and gas porosities often occur near the surface. These defects are mainly attributed to the following factors:
- The thick walls and large dimensions of the traction pin result in localized heat accumulation during pouring, which can cause sand sticking at the root section.
- The conical upper section, being very thick, requires a large riser to avoid shrinkage porosity. During pouring, chemical reactions may form silicates that increase the possibility of sand penetration.
- The bottom-gating system ensures relatively stable mold filling, but the prolonged pouring time can overheat the upper part of the mold cavity, reducing sand mold strength and increasing the likelihood of sand inclusion defects.
These surface defects create stress concentrations and alter the stress distribution during service, leading to premature fatigue failure of the traction pin. Therefore, repairing such defects is essential for ensuring the safety and reliability of the steel casting.
1.2 Conventional Repair Methods for Steel Casting
Several repair methods have been applied to recover defective steel casting components, including plasma arc spraying, gas metal arc welding, shielded metal arc welding, and cold welding techniques. However, these processes often encounter a number of problems:
- High heat input causes large residual stresses and deformation.
- The repaired zone may exhibit mechanical properties significantly different from the base steel casting.
- Porosity, slag inclusion, and lack of fusion can occur.
- Poor fusion between the repair metal and the substrate leads to weak interfacial strength.
To mitigate these problems, preheating, careful surface preparation, and the use of suitable filler materials have been recommended. Nevertheless, traditional arc-based repair methods still suffer from large heat-affected zones (HAZ) and severe distortion. This has motivated the exploration of advanced surface engineering techniques such as laser cladding.
1.3 Laser Cladding Technology
Laser cladding is a surface modification technique in which a high-energy laser beam melts the surface of a substrate together with an injected or pre-placed powder, creating a metallurgically bonded overlay after rapid solidification. This process can produce dense coatings with low dilution, fine microstructures, and excellent adhesion. Key advantages of laser cladding over conventional welding include:
- High precision and localized heating, leading to minimal thermal distortion.
- Rapid cooling rates that produce refined microstructures and improved mechanical properties.
- Low dilution between the coating and the substrate, preserving the coating’s functional properties.
- Strong metallurgical bonding at the interface.
- Environmental friendliness and high material utilization.
- Versatility in repairing complex geometries and different material systems.
1.4 Laser Cladding Material Systems
Laser cladding materials can be classified into self-fluxing alloy powders, ceramic powders, and metal-ceramic composite powders. Among the self-fluxing alloys, Ni-based, Co-based, and Fe-based powders are commonly used. The characteristics of these systems are summarized in the following table:
| Alloy system | Self-fluxing ability | Advantages | Disadvantages |
|---|---|---|---|
| Ni-based | Good | Excellent wear resistance, corrosion resistance, toughness, wettability | Poor high-temperature resistance |
| Co-based | Medium | Good thermal shock resistance, corrosion resistance, anti-adhesive wear | High cost |
| Fe-based | Poor | Low cost, good compatibility with steel substrates | Inferior corrosion resistance |
Fe-based powders are particularly attractive for repairing steel casting components because of their low cost and good metallurgical compatibility with steel substrates. Austenitic stainless steel powders, such as DJ106, provide improved corrosion resistance and wear resistance compared to plain carbon steel castings. The addition of alloying elements such as Cr and Ni promotes the formation of austenite and carbides, which enhance the hardness and tribological behavior of the cladding layer.
1.5 Research Progress on Laser Cladding Fe-Based Coatings on Steel Casting
Numerous studies have investigated laser cladding of Fe-based coatings on structural steels. For example, Wang prepared Y2O3-Fe-based composite coatings on 16Mn steel and reported excellent wear and corrosion resistance. Zhang utilized a laser cladding technique to fabricate Fe-based medium-entropy alloy coatings on 304 stainless steel and optimized the process parameters using orthogonal experimental design. Xu studied Fe901/WC composite coatings on 40Cr steel and identified the optimal WC content for enhanced wear performance. Li Jianfeng explored Fe-based wear-resistant coatings on scraper conveyor central chutes and demonstrated significant surface property improvements. Other researchers have investigated laser cladding of Fe-based alloys on cast iron, Q235 steel, 27SiMn steel, and 42CrMo steel. These studies consistently show that laser cladding can refine the microstructure, increase hardness, and improve wear and corrosion resistance while maintaining good metallurgical bonding with the steel casting substrate.
1.6 Objectives of This Work
The primary objective of this thesis is to develop a reliable laser cladding repair method for surface defects in traction pin steel casting components. Specifically, the aims are:
- To optimize the laser cladding process parameters (laser power, scanning speed, powder feed rate) using orthogonal experiments.
- To characterize the microstructure and phase composition of DJ106 austenitic stainless steel cladding layers on ZG230-450 steel casting substrates.
- To evaluate the mechanical properties (microhardness, tensile behavior) and functional properties (wear resistance, corrosion resistance) of the repaired steel casting.
- To understand the strengthening mechanisms and provide practical guidance for industrial repair of steel casting components.
2. Experimental Procedures
2.1 Base Material
The substrate material used in this study was ZG230-450 carbon steel casting (25 steel), with the chemical composition listed in Table 1. The material was supplied as plates with dimensions of 250 mm × 40 mm × 12 mm.
Table 1 Chemical composition of ZG230-450 steel casting (wt.%)
| Element | C | Si | Mn | P | S | Ni | Cr | Cu | Mo | V | Fe |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Standard | 0.20–0.30 | 0.20–0.50 | 0.50–0.90 | ≤0.035 | ≤0.035 | ≤0.30 | ≤0.35 | ≤0.30 | ≤0.20 | ≤0.05 | Bal. |
| Actual | 0.25 | 0.37 | 0.82 | 0.017 | 0.017 | 0.15 | 0.08 | 0.08 | 0.17 | 0.03 | Bal. |
The mechanical properties of the base steel casting are given in Table 2.
Table 2 Physical and mechanical properties of ZG230-450 steel casting
| Property | Value |
|---|---|
| Tensile strength (MPa) | ≥450 |
| Yield strength (MPa) | ≥230 |
| Elongation (%) | ≥22 |
| Reduction of area (%) | ≥32 |
| Melting point (°C) | 1467 |
| Hardness (HB) | 160–185 |
2.2 Cladding Material
DJ106 austenitic stainless steel powder (Beijing Yanbang New Material Co., Ltd.) was used as the cladding material. The powder particles are spherical with a size range of 50–100 μm. The chemical composition is provided in Table 3.
Table 3 Chemical composition of DJ106 stainless steel powder (wt.%)
| Element | C | Si | Mn | Mo | Cr | Ni | Fe |
|---|---|---|---|---|---|---|---|
| Actual | 0.02 | 0.82 | 1.68 | 0.08 | 19.34 | 9.97 | Bal. |
| Standard | ≤0.03 | ≤1.0 | ≤2.0 | — | 18.0–20.0 | 9.0–12.0 | Bal. |
Prior to cladding, the substrate surface was ground with 150# SiC paper to remove oxide scale and contaminants. The surface was then degreased with acetone in an ultrasonic cleaner and dried. The DJ106 powder was dried in a vacuum oven at 120 °C for 1 h to eliminate moisture.
2.3 Laser Cladding Equipment
The laser cladding experiments were performed using a YLS-2000-TR fiber laser system with a wavelength of 1070–1080 nm. The system was integrated with a KUKA six-axis robot, a Siemens integrated control system, a double-barrel powder feeder (RC-PGF-D-2), and a water cooling system. The powder feeding was performed using the coaxial blowing method, in which the powder particles fall into the melt pool created by the focused laser beam and rapidly solidify after laser scanning.
2.4 Specimen Preparation for Single-Pass and Multi-Pass Cladding
To simulate real surface defects in steel casting, two types of specimens were prepared. For small-area defects, a hole with a diameter of 4 mm and a depth of 0.5 mm was machined using a grinder. For large-area defects, the surface was milled to a smooth finish. Single-pass cladding was first performed to optimize the process parameters. After that, single-layer multi-pass overlapping cladding was carried out on flat specimens. A schematic cross-section of the single-pass cladded track is shown in Figure (not available).
The cladding process parameters investigated included laser power, scanning speed, and powder feed rate. The laser spot diameter was fixed at 4.5 mm with a defocus amount of -2 mm. The shielding gas flow rate was set to 10 L/min.
2.5 Orthogonal Experimental Design
A three-factor, three-level orthogonal array (L9) was employed to optimize the single-pass cladding process. The factors and levels are listed in Table 4.
Table 4 Factors and levels used in the orthogonal experiment
| Level | A: Laser power (W) | B: Scanning speed (mm/s) | C: Powder feed rate (g/min) |
|---|---|---|---|
| 1 | 2300 | 4 | 12.57 |
| 2 | 2600 | 6 | 18.86 |
| 3 | 2900 | 8 | 25.14 |
The power-to-speed ratio, which represents the linear energy density, is calculated as:
$$E = \frac{P}{v \cdot D}$$
where E is the energy density (J/mm²), P is the laser power (W), v is the scanning speed (mm/s), and D is the laser spot diameter (mm). For the fixed spot diameter, the energy density is proportional to P/v. The computed P/v values for each experiment are shown in the orthogonal table.
Three response indicators were evaluated: dilution rate, cladding layer height, and molten pool depth. The dilution rate was calculated using the cross-sectional area or height measurements:
$$\eta = \frac{h}{H + h} \times 100\%$$
where h is the molten pool depth (mm) and H is the cladding layer height (mm). Since the single-pass experiments were performed on the pre-machined hole, the actual cladding height was adjusted by adding 0.5 mm to the measured value, while the penetration depth was reduced by 0.5 mm.
Based on prior experience and preliminary trials, the optimized parameters were selected by analyzing the range (R value) of each factor. The range analysis was performed to determine the influence of each factor on the response variables.
2.6 Characterization Methods
2.6.1 Optical Microscopy and Scanning Electron Microscopy
Metallographic specimens were cut from the cladded samples, mounted in epoxy, ground with SiC papers, and polished to a mirror finish. The polished specimens were etched with a solution of aqua regia and alcohol for 10–40 s. Microstructural observations were carried out using a Leica DMi8 A optical microscope (OM) and a Zeiss Supra 55 field-emission scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD).
2.6.2 X-ray Diffraction
Phase identification was performed using a Panalytical Empyrean X-ray diffractometer with Co Kα radiation. The scanning angle ranged from 30° to 120° with a scanning speed of 0.08337°/s. Residual stress measurements were carried out using a Proto X-ray diffractometer with Cr Kα radiation, targeting the 311 diffraction peak of austenite.
2.6.3 Microhardness Testing
Microhardness profiles across the cladding layer, heat-affected zone, and substrate were obtained using an HV-1000B Vickers microhardness tester. A load of 1000 gf (i.e., 9.8 N) was applied with a dwell time of 20 s. Three indentations were made at each depth and the average values were reported.
2.6.4 Tensile Testing
Tensile specimens were machined from three types of samples: original defect-free steel casting, specimens with the cladding direction perpendicular to the tensile axis, and specimens with the cladding direction parallel to the tensile axis. The tensile test was performed using a 10-ton universal testing machine at a crosshead speed of 2 mm/min. The dimensions of the tensile specimen are shown in Figure (not available). The yield strength, ultimate tensile strength, elongation, and reduction of area were obtained from the engineering stress–strain curves.
2.6.5 Wear Testing
Dry sliding wear tests were conducted using a CETR UMT friction and wear tester. A cemented carbide ball with a diameter of 1.588 mm was used as the counterbody. The wear tests were performed at room temperature with a normal load of 10 N and a sliding distance of 1 h. The worn surface morphology and wear track dimensions were observed by SEM. The weight loss was measured using an electronic balance with an accuracy of 0.1 mg.
2.6.6 Electrochemical Corrosion Testing
The corrosion resistance of the cladding layer and the base steel casting was evaluated using an LK98BⅡ electrochemical analysis system. The tests were carried out in a 3.5 wt.% NaCl solution at room temperature (26 °C) using a three-electrode cell with a saturated calomel electrode (SCE) as the reference electrode and a platinum electrode as the counter electrode. The potentiodynamic polarization curves were recorded at a scan rate of 0.02 V/s over a potential range from -1.2 V to 1.7 V. The exposed area of the working electrode was 1 cm². Before each test, the sample was immersed in the solution for 2 min to stabilize the open-circuit potential.
3. Results and Discussion
3.1 Optimization of Laser Cladding Process Parameters
The orthogonal experimental results for nine combinations are summarized in Table 5.
Table 5 Orthogonal experimental results for single-pass laser cladding
| Specimen | A: Laser power (W) | B: Scanning speed (mm/s) | C: Powder feed rate (g/min) | P/v (W·min/mm) | Dilution (%) | Layer height (mm) | Molten depth (mm) |
|---|---|---|---|---|---|---|---|
| 1 | 2300 | 4 | 12.57 | 575 | 30.4 | 1.28 | 0.56 |
| 2 | 2300 | 6 | 18.86 | 383 | 14.6 | 1.05 | 0.18 |
| 3 | 2300 | 8 | 25.14 | 288 | 19.3 | 0.96 | 0.23 |
| 4 | 2600 | 4 | 18.86 | 650 | 28.9 | 1.62 | 0.66 |
| 5 | 2600 | 6 | 25.14 | 433 | 30.9 | 1.23 | 0.55 |
| 6 | 2600 | 8 | 12.57 | 325 | 42.9 | 0.81 | 0.61 |
| 7 | 2900 | 4 | 25.14 | 725 | 27.3 | 1.97 | 0.74 |
| 8 | 2900 | 6 | 12.57 | 483 | 39.8 | 1.15 | 0.76 |
| 9 | 2900 | 8 | 18.86 | 363 | 18.8 | 1.34 | 0.30 |
Range analysis (Table 6) was conducted to identify the optimal levels for each factor.
Table 6 Range analysis results
| Indicator | K value | A | B | C |
|---|---|---|---|---|
| Dilution (%) | K1 | 21.4 | 28.9 | 37.7 |
| K2 | 34.2 | 26.4 | 20.7 | |
| K3 | 28.6 | 27.0 | 25.8 | |
| Range R | 12.8 | 2.4 | 16.9 | |
| Optimal | A1 | B2 | C2 | |
| Layer height (mm) | K1 | 1.09 | 1.62 | 1.08 |
| K2 | 1.61 | 1.14 | 1.34 | |
| K3 | 1.48 | 1.03 | 1.38 | |
| Range R | 0.52 | 0.49 | 0.30 | |
| Optimal | A2 | B1 | C3 | |
| Molten depth (mm) | K1 | 0.32 | 0.65 | 0.64 |
| K2 | 0.60 | 0.49 | 0.38 | |
| K3 | 0.60 | 0.38 | 0.50 | |
| Range R | 0.28 | 0.27 | 0.26 | |
| Optimal | A2 | B3 | C2 |
By taking the combination of the optimal levels that minimize dilution while providing adequate layer height and acceptable penetration, the final optimal single-pass cladding parameters were determined as: laser power = 2600 W, scanning speed = 6 mm/s, and powder feed rate = 18.86 g/min. The optimized parameters are listed in Table 7.
Table 7 Optimized single-pass laser cladding parameters
| Parameter | Value |
|---|---|
| Laser power (W) | 2600 |
| Scanning speed (mm/s) | 6 |
| Powder feed rate (g/min) | 18.86 |
| Spot diameter (mm) | 4.5 |
| Defocus amount (mm) | -2 |
| Shielding gas flow (L/min) | 10 |
For the single-layer multi-pass overlapping cladding, the same scanning speed and powder feed rate were used, while the laser power was reduced to avoid excessive dilution. Experiments were performed at laser powers of 1500 W, 1700 W, and 1900 W with a 40% overlap ratio. Observation of the cross-sections showed that 1500 W produced some porosity and cracks at the overlap zones, whereas 1700 W and 1900 W yielded defect-free layers. However, the 1900 W condition resulted in a high dilution rate, which could degrade the corrosion resistance of the cladding layer. Therefore, the optimal multi-pass laser power was chosen as 1700 W.
3.2 Macroscopic Morphology of the Cladding Layer
The single-pass cladding layer on the steel casting substrate exhibited a typical convex-concave cross-sectional profile. The upper surface of the cladding layer was convex due to the surface-tension-driven flow of the molten metal from the center toward the edges. The interface between the cladding layer and the substrate showed a concave shape, resulting from the Gaussian energy distribution of the laser beam, which creates a higher temperature at the center of the melt pool. The cladding layer exhibited excellent wettability on the ZG230-450 steel casting substrate, indicating strong interfacial bonding. The measured dilution rate under the optimized parameters was about 16.3%, which is considered acceptable for achieving a sound metallurgical bond while preserving the properties of the cladding material.
3.3 Microstructure of the Cladding Layer
Figure (not available) shows the optical micrographs of the DJ106 stainless steel cladding layer on the steel casting substrate. The microstructure is characterized by a clear transition from the bottom to the top of the cladding layer. The bottom region, adjacent to the substrate, exhibits a planar solidification front, which is typical of laser cladding and indicates good metallurgical bonding. Above the planar zone, coarse cellular or columnar dendrites grow in the direction opposite to heat flow. In the middle region, fine dendrites with multiple orientations are observed. The top region of the cladding layer consists of equiaxed grains, which form due to the high degree of constitutional supercooling and homogeneous nucleation in the last stage of solidification.
This microstructural evolution can be explained by the theory of rapid solidification. The ratio of the temperature gradient (G) to the solidification rate (R) governs the morphology of the solidifying interface. At the bottom of the melt pool, G is very high and R approaches zero, leading to a high G/R ratio and a planar interface. As solidification proceeds toward the surface, G decreases and R increases, resulting in a lower G/R ratio and the formation of cellular, columnar, and finally equiaxed dendrites. The relationship between the solidification rate and the laser scanning speed is given by:
$$R = V \cos \theta$$
where V is the laser scanning speed and θ is the angle between the growth direction and the scanning direction.
3.4 Microstructure of the Heat-Affected Zone
The heat-affected zone (HAZ) of the steel casting substrate beneath the cladding layer was examined by optical microscopy. From the bulk substrate toward the cladding interface, the following microstructural zones were observed:
- Base material: ferrite + pearlite.
- Normalizing zone: ferrite + sorbitte (fine pearlite).
- Quenching zone (adjacent to the fusion line): lath martensite.
The formation of these zones is attributed to the different peak temperatures experienced during the cladding process. With increasing distance from the fusion line, the peak temperature decreases. The region closest to the fusion line reaches temperatures above the liquidus, and the rapid cooling transforms the austenite into martensite. The normalizing zone experiences temperatures above the Ac3 but below the liquidus, leading to the formation of ferrite and fine lamellar pearlite (sorbite). The bulk material remains below the critical transformation temperature and retains its original ferrite-pearlite structure.
3.5 Phase Analysis by XRD
The X-ray diffraction pattern of the DJ106 cladding layer is shown in Figure (not available). The pattern exhibits only the characteristic peaks of austenite (γ-Fe), indicating that the cladding layer is predominantly composed of single-phase austenite. No peaks corresponding to ferrite or other intermetallic compounds were detected in the XRD pattern, although a small amount of chromium carbide was identified by EBSD analysis. This discrepancy is due to the relatively low volume fraction of carbides, which falls below the detection limit of the XRD instrument.
3.6 SEM, EDS, and EBSD Analysis
Energy-dispersive X-ray spectroscopy (EDS) mapping of the cladding layer revealed the element distribution in the dendritic and interdendritic regions. The dendrite cores were enriched in Fe and Ni, while the interdendritic regions were enriched in Cr and C. This segregation is consistent with the formation of Cr-rich carbides in the interdendritic spaces during the last stage of solidification. The redistribution of alloying elements during rapid solidification is driven by the non-equilibrium partitioning between the solid and liquid phases. The element distribution maps showed that Mn and Si were distributed uniformly across the cladding layer.
Electron backscatter diffraction (EBSD) analysis was carried out to identify the phases and their crystallographic orientation. The phase distribution map indicated that the cladding layer consists of approximately 96.4% face-centered cubic (FCC) austenite, 2.77% chromium carbide (Cr3C2 or Cr2C3), and a trace amount of body-centered cubic (BCC) ferrite. The grain orientation map revealed that the FCC grains were relatively coarse with random orientations, while the carbide grains exhibited a preferential orientation along the [212] direction. This suggests that the carbides grew epitaxially from the melt pool following the heat flow direction.
The formation of chromium carbides in the austenitic stainless steel cladding layer is beneficial for improving hardness and wear resistance. The fine dispersion of hard carbide particles in the ductile austenitic matrix creates a composite-like structure that resists plastic deformation and abrasive wear.
3.7 Microhardness
The microhardness distribution across the cladding layer, heat-affected zone, and substrate is shown in Figure (not available). The cladding layer had an average microhardness of approximately 310 HV₀.₁, which is about 1.8 times higher than the substrate hardness of 170 HV₀.₁. The heat-affected zone exhibited an intermediate hardness of about 280 HV₀.₁, reflecting the presence of martensite and fine pearlite. The hardness profile showed a gradual transition rather than an abrupt change, indicating good mechanical compatibility between the cladding and the substrate. This gradual transition is beneficial for reducing stress concentration at the interface and improving the overall reliability of the repaired steel casting.
The enhanced hardness of the cladding layer can be attributed to three main strengthening mechanisms:
- Solid solution strengthening due to the dissolution of alloying elements such as Cr, Ni, and Mo in the austenite lattice.
- Fine grain strengthening resulting from the rapid solidification and high cooling rates associated with laser cladding.
- Precipitation or dispersion strengthening caused by the formation of Cr-rich carbides in the interdendritic regions.
The combined effect of these mechanisms explains the significant hardness improvement of the steel casting surface after laser cladding.
3.8 Tensile Properties
Tensile tests were performed on three types of specimens: the original defect-free steel casting, specimens with the tensile axis perpendicular to the cladding direction, and specimens with the tensile axis parallel to the cladding direction. The engineering stress–strain curves are shown in Figure (not available). The average tensile properties are summarized in Table 8.
Table 8 Tensile properties of the laser-cladded steel casting specimens
| Specimen type | Yield strength (MPa) | Ultimate tensile strength (MPa) | Elongation (%) |
|---|---|---|---|
| Original defect-free specimen | 261.23 | 427.86 | 35% (approx.) |
| Perpendicular cladding direction | 277.09 | 418.72 | 25–30% |
| Parallel cladding direction | 261.06 | 437.75 | 30% (approx.) |
Compared with the original specimen, the perpendicular cladding specimen showed a slightly higher yield strength (277 MPa vs. 261 MPa) but a slightly lower ultimate tensile strength (419 MPa vs. 428 MPa). The elongation decreased from about 35% to 25–30%, indicating a reduction in ductility. The parallel cladding specimen, on the other hand, exhibited similar yield strength and a slightly higher ultimate tensile strength (438 MPa) compared to the original specimen. The elongation was close to that of the original material. These results demonstrate that laser cladding does not degrade the tensile performance of the steel casting; in fact, a parallel-oriented cladding can even slightly enhance the tensile strength due to the presence of a high-hardness surface layer.
The fracture surfaces of the tensile specimens were examined by SEM. The original specimen showed a typical ductile fracture with a cup-and-cone morphology, consisting of a fibrous zone, a radial zone, and a shear lip. The perpendicular cladding specimen fractured through the cladding layer, and the fracture surface exhibited less pronounced necking compared to the original specimen. In contrast, the parallel cladding specimen showed a fracture morphology very similar to the original, with the crack initiating in the center of the specimen and propagating outward through the shear lips.
Residual stress measurements were performed on the cladding layer to explain the difference in fracture behavior between the two orientations. The measured residual stresses are listed in Table 9.
Table 9 Residual stress measurement results of the cladding layer
| Position | Stress (MPa) |
|---|---|
| 1 | -25.08 ± 23.65 |
| 2 | -24.59 ± 22.51 |
| 3 | -28.91 ± 23.14 |
| 4 | -29.08 ± 23.65 |
| 5 | -35.91 ± 24.11 |
| 6 | -36.08 ± 24.74 |
| 7 | -47.25 ± 24.97 |
| 8 | -47.89 ± 25.11 |
| 9 | -58.71 ± 26.12 |
| 10 | -59.60 ± 26.45 |
All measured residual stresses were compressive (negative values), which is favorable for improving fatigue resistance. The compressive stress increased along the cladding direction from about -25 MPa to -60 MPa due to the heat accumulation during the cladding process. In the direction transverse to the cladding, the residual stress was nearly uniform. This explains why the specimen with the tensile axis parallel to the cladding direction exhibited a slightly better combination of strength and ductility: the compressive residual stresses along the loading direction partially counteract the tensile applied stress, reducing the effective stress on the cladding layer. In the perpendicular direction, the cladding layer is subjected to the full applied load with no stress compensation, which can promote crack initiation in the cladding layer.
3.9 Wear Resistance
The wear weight loss of the ZG230-450 steel casting substrate and the DJ106 cladding layer as a function of sliding time is shown in Figure (not available). After 30 min of wear, the mass loss of the substrate was 1.8 mg, while the cladding layer lost only 1.1 mg, corresponding to a wear resistance improvement of about 60%. After 1 h of wear, the substrate mass loss was approximately twice that of the cladding layer. This clearly demonstrates that the laser-clad austenitic stainless steel layer significantly enhances the wear resistance of the steel casting.
The worn surface morphologies were analyzed by SEM. The substrate exhibited deep plowing grooves, plastic deformation, and adhesive pits, indicating a combination of abrasive wear and adhesive wear. The low hardness of the substrate allows the hard counterbody to penetrate deeply and remove material easily. In contrast, the cladding layer showed relatively shallow grooves and minimal plastic deformation, which is typical of abrasive wear. The combination of a hard carbide phase (Cr₃C₂) embedded in a tougher austenitic matrix provides an excellent wear-resistant structure. The carbides act as load-bearing elements that resist micro-cutting, while the austenitic matrix provides toughness and prevents brittle fracture.
3.10 Corrosion Resistance
The potentiodynamic polarization curves of the 25 steel casting substrate and the DJ106 cladding layer in 3.5% NaCl solution are presented in Figure (not available). The polarization curves were analyzed to determine the corrosion potential (E_corr), corrosion current density (I_corr), and corrosion rate. The fitted results are listed in Table 10.
Table 10 Electrochemical corrosion parameters of the substrate and cladding layer
| Material | E_corr (V vs. SCE) | I_corr (A/cm²) | Corrosion rate (mm/a) |
|---|---|---|---|
| 25 steel casting substrate | -0.60657 | 2.64177 × 10⁻⁷ | 0.03251 |
| DJ106 cladding layer | -0.41307 | 3.57105 × 10⁻⁷ | 0.04107 |
It can be seen that the corrosion potential of the DJ106 cladding layer is -413 mV, which is 193.5 mV higher than that of the substrate (-607 mV). A higher corrosion potential generally indicates a lower thermodynamic tendency to corrode. Although the corrosion current density of the cladding layer is slightly higher than that of the substrate, the corrosion current densities are of the same order of magnitude. The corrosion rate calculated by the Stern-Geary equation is slightly higher for the cladding layer (0.041 mm/a) than for the substrate (0.033 mm/a). However, this slight increase in the corrosion rate is not significant enough to compromise the overall corrosion resistance. The important improvement is the shift in the corrosion potential to a more noble value, which indicates that the cladding layer is more thermodynamically stable and less prone to initiation of localized corrosion.
The corrosion resistance of the DJ106 cladding layer is attributed to the presence of a high content of chromium and nickel, which promote the formation of a protective passive film on the surface. The passive film, mainly composed of chromium oxide, acts as a barrier that isolates the underlying material from the corrosive medium. In contrast, the plain carbon steel substrate lacks sufficient chromium to form a stable passive film, resulting in a lower corrosion potential and a greater tendency to corrode.
It should be noted that the corrosion rate is only one measure of corrosion resistance. In many applications, the initiation resistance (as indicated by the corrosion potential) is more critical for preventing localized attack such as pitting and stress corrosion cracking. The higher corrosion potential of the cladding layer makes it more resistant to the onset of corrosion, thereby enhancing the durability of the steel casting in service environments.
4. Conclusion
In this thesis, laser cladding of DJ106 austenitic stainless steel was successfully applied to repair surface defects in ZG230-450 steel casting used for traction pins. The main conclusions can be summarized as follows:
- The optimal single-pass laser cladding parameters on the ZG230-450 steel casting substrate were determined to be a laser power of 2600 W, a scanning speed of 6 mm/s, and a powder feed rate of 18.86 g/min with a spot diameter of 4.5 mm and a defocus amount of -2 mm. For multi-pass overlapping cladding, the optimal laser power was 1700 W with a 40% overlap ratio.
- The microstructure of the DJ106 cladding layer consists of planar crystals at the fusion boundary, cellular/columnar dendrites in the lower region, fine dendrites in the middle, and equiaxed grains at the top. The heat-affected zone of the steel casting exhibits lath martensite near the fusion line, followed by ferrite + sorbite, and ferrite + pearlite further away.
- The cladding layer is composed mainly of austenite (approximately 96.4% FCC phase), with a small amount of chromium carbide (Cr₃C₂) and a trace of BCC ferrite. The dendrite cores are enriched in Fe and Ni, while the interdendritic regions are enriched in Cr and C.
- The microhardness gradually decreases from the cladding layer (310 HV₀.₁) to the heat-affected zone (280 HV₀.₁) and then to the substrate (170 HV₀.₁), indicating a good mechanical transition. The hardening is attributed to solid solution strengthening, fine grain strengthening, and precipitation strengthening.
- Tensile tests showed that the laser-cladded specimens maintain or slightly improve the yield and ultimate tensile strengths of the original steel casting. The parallel cladding direction yielded the best combination of strength and ductility. Compressive residual stresses measured in the cladding layer were found to increase along the cladding direction, which helps explain the orientation-dependent tensile behavior.
- The wear resistance of the steel casting was significantly improved by laser cladding. After 1 h of wear, the mass loss of the substrate was about twice that of the cladding layer. The substrate suffers from adhesive and abrasive wear, while the cladding layer exhibits mainly abrasive wear due to the presence of hard carbide particles.
- The corrosion potential of the DJ106 cladding layer was 193.5 mV higher than that of the substrate, indicating a significantly lower tendency to corrode. The improved corrosion resistance is attributed to the formation of a chromium-rich passive film on the cladding surface.
In summary, laser cladding with DJ106 austenitic stainless steel is an effective and reliable method for repairing surface defects in steel casting components such as traction pins. The repaired steel casting not only recovers its original dimensions but also gains superior surface hardness, wear resistance, and corrosion resistance, which can extend its service life and enhance the safety of railway operations. The results of this study provide valuable scientific data and practical guidance for the industrial application of laser cladding in the repair of defective steel casting parts.