CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Wear-Resistant Alloy Overlay Layer on K360 Wear-Resistant Steel
Background and Material Context
K360 wear-resistant steel is a high-strength, medium-carbon alloy steel widely used in mining, construction, and material handling applications where resistance to abrasion and impact is required. The base material typically has a hardness in the range of 360–400 HV, achieved through a combination of medium carbon content (approximately 0.25–0.35% C) and alloying elements such as manganese, chromium, and molybdenum. While the base material provides good general wear resistance, certain service conditions—such as severe abrasion, erosion, or impact-abrasion combined loading—require additional surface hardening that cannot be achieved by the base material alone. Weld overlay with a specialized wear-resistant alloy is the preferred solution for enhancing the surface properties of K360 steel components without altering the bulk mechanical properties.
This study note examines a research paper investigating the microstructure and mechanical properties of a wear-resistant alloy overlay layer deposited on K360 steel, with particular focus on the relationship between overlay composition, microstructure, hardness distribution, and wear performance under different tribological conditions.
Overlay Alloy Selection and Process Parameters
The overlay alloy studied was a high-chromium alloy with a composition designed to maximize carbide formation while maintaining sufficient toughness to resist impact loading. The following table summarizes the chemical composition of the overlay alloy and the process parameters used in the study.
| Parameter | Specification |
|---|---|
| Overlay alloy type | High-chromium alloy (Cr26 type) |
| Carbon content | 3.0–3.5% C |
| Chromium content | 25–28% Cr |
| Manganese content | 1.5–2.5% Mn |
| Deposition process | Submerged arc welding (SAW) |
| Number of passes | 4 passes |
| Deposition rate | 300–400 mm/min |
| Heat input | 0.8–1.2 kJ/mm |
| Interpass temperature | 100–150°C |
| Preheat temperature | 100°C |
The selection of submerged arc welding (SAW) was based on its high deposition efficiency, good shielding of the weld pool from atmospheric contamination, and the ability to achieve consistent dilution control through the use of appropriate flux coverage. The four-pass deposition strategy was designed to achieve a minimum overlay thickness of 4 mm while reducing base metal dilution to below 8% in the final pass.
Microstructural Analysis
Metallographic examination of the overlay layer revealed a microstructure consisting of hard carbide particles dispersed in a matrix of martensite and retained austenite. The primary carbide phases identified were M7C3 (Cr7C3 and Fe7C3) and M23C6, with M7C3 being the dominant phase due to the high chromium and carbon content of the alloy.
The carbide morphology was found to be strongly dependent on the cooling rate at each pass. In the first pass, deposited directly on the preheated K360 substrate, the cooling rate was relatively low (approximately 8–12°C/s), resulting in the formation of coarse, blocky carbides with sizes ranging from 5 to 15 μm. In the subsequent passes, the reheating of the previous pass reduced the cooling rate further, leading to even coarser carbides in the lower layers of the overlay. In the final (top) pass, the cooling rate was higher (approximately 15–20°C/s) due to the absence of underlying hot metal, resulting in finer carbides with sizes of 2–8 μm.
The matrix microstructure evolved from a fully martensitic structure in the lower passes to a mixed martensite-retained austenite structure in the upper passes. The volume fraction of retained austenite was estimated to be 15–25% in the top pass, increasing to 30–35% in the lower passes where the reheating effect was more pronounced. The retained austenite contributed to the toughness of the overlay layer but could transform to martensite during service cooling or subsequent welding operations.
The dilution zone at the K360/overlay interface was characterized by a mixed microstructure containing both base metal constituents and overlay alloy constituents. The dilution ratio in this zone was estimated to be 10–15%, resulting in a microstructure that was intermediate in hardness between the base metal (380 HV) and the overlay layer (550–600 HV). This dilution zone was identified as the weakest region in terms of both hardness and fracture toughness, and was the most likely location for crack initiation under impact loading.
Hardness and Wear Performance
The hardness distribution across the overlay layer was measured using Vickers microhardness testing with a 0.5 kgf load. The results are summarized in the following table.
| Depth from Surface (mm) | Hardness (HV) | Microstructure |
|---|---|---|
| 0–0.5 | 580–620 | Fine M7C3 carbides in martensite-retained austenite matrix |
| 0.5–1.5 | 540–580 | Medium M7C3 carbides in martensite matrix |
| 1.5–3.0 | 500–540 | Coarse M7C3 carbides in martensite matrix |
| 3.0–4.0 | 460–500 | Very coarse carbides, higher retained austenite |
| Interface zone | 400–450 | Mixed microstructure, 10–15% dilution |
| Base metal | 360–400 | Ferrite-pearlite with alloy carbides |
The wear performance was evaluated through dry sliding wear testing against a SiC counterface at various normal loads. The specific wear rate (volume loss per unit sliding distance and load) was found to be inversely proportional to the hardness of the overlay layer, consistent with the Archard wear equation. At a normal load of 20 N, the specific wear rate of the overlay layer was approximately 1.2 × 10⁻⁶ mm³/(N·m), compared to 4.8 × 10⁻⁶ mm³/(N·m) for the uncoated K360 base metal, representing a fourfold improvement in wear resistance.
However, the wear performance was found to be load-dependent. At low loads (5–10 N), the improvement in wear resistance was more modest (approximately 2–3 times), as the contact pressure was insufficient to fully engage the hard carbide particles. At high loads (30–50 N), the improvement was more pronounced (5–6 times), but the overlay layer exhibited increased susceptibility to spalling due to the high contact stresses exceeding the fracture toughness of the carbide-matrix composite.
Engineering Considerations and Practical Recommendations
The study's findings have several important implications for the practical application of wear-resistant overlay on K360 steel components.
- Minimum overlay thickness: A minimum thickness of 3 mm is recommended to ensure that the high-hardness zone extends sufficiently below the surface to resist wear without exposing the dilution zone. Thinner overlays are prone to premature failure through spalling of the dilution zone.
- Post-weld treatment: A low-temperature tempering treatment at 200–250°C for 1 hour can stabilize the retained austenite and reduce residual stresses without significantly reducing the hardness of the overlay layer. This treatment is particularly beneficial for components subject to cyclic loading or impact.
- Interface quality: The bond strength at the K360/overlay interface is critical for the long-term performance of the overlay. Visual inspection and magnetic particle testing of the interface are recommended to detect lack of fusion defects. The study found that a preheat temperature of 100°C and an interpass temperature of 100–150°C produced the best interface quality with no lack of fusion defects observed in 50 test specimens.
- Service temperature limitations: The overlay layer is suitable for service temperatures up to approximately 400°C. Above this temperature, the retained austenite begins to decompose, the carbides coarsen, and the hardness drops significantly. For higher temperature applications, a different overlay alloy with better high-temperature stability should be selected.
Summary and Conclusions
The study provides a detailed characterization of the microstructure and wear properties of a high-chromium wear-resistant alloy overlay deposited on K360 wear-resistant steel. The key findings are that the overlay layer achieves a hardness of 580–620 HV at the surface, representing a significant improvement over the 360–400 HV hardness of the base metal, and that the specific wear rate is reduced by a factor of 4–6 depending on the applied load. The microstructure of the overlay layer is a composite of hard M7C3 carbides in a martensite-retained austenite matrix, with the carbide size and distribution varying with depth due to the changing cooling rate during multi-pass deposition. The dilution zone at the interface represents the weakest region and should be avoided in the functional surface layer through appropriate overlay thickness design. The practical recommendations on minimum overlay thickness, post-weld tempering, and interface quality control provide actionable guidance for engineers applying this overlay technology to mining, construction, and material handling components. The study demonstrates that weld overlay is a cost-effective and reliable method for enhancing the wear resistance of K360 steel components, provided that the process parameters are carefully controlled to ensure adequate dilution management and interface quality.