CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of K360 Wear-Resistant Steel Weld Overlay Alloy Layer
Literature Overview
This paper by Deng Hanzhong, Sun Yuanzhang, Liu Shaoping, Zhang Weiqiang, Kang Shaoguang, and Cao Wenzhi from Liaoning Technical University and China Coal Zhangjiakou Coal Machinery Co., Ltd., published in the journal Mechanical Engineering Materials in 2008, investigates the microstructure and mechanical properties of the weld overlay alloy layer deposited on K360 wear-resistant steel. K360 is a high-strength, wear-resistant steel widely used in mining and construction equipment, and the overlay layer is intended to further enhance its wear resistance for severe service conditions.
Core Technical Content
The study focuses on the metallurgical characteristics of the overlay alloy layer and its relationship to wear performance. The authors employed a combination of metallographic examination, X-ray diffraction, microhardness testing, and wear testing to characterize the overlay deposits.
Material System and Process Parameters
The overlay welding was performed using submerged arc welding (SAW) with a specific alloy wire designed to deposit a hard, wear-resistant layer on the K360 substrate. The base metal composition of K360 typically includes:
| Element | C | Si | Mn | Cr | Mo | Ni |
|---|---|---|---|---|---|---|
| Content (wt%) | 0.25–0.35 | 0.3–0.6 | 1.2–1.6 | 0.5–1.0 | 0.2–0.4 | 0.3–0.6 |
The overlay wire composition was designed to include higher carbon and alloying elements to promote the formation of hard carbides in the weld metal.
Microstructural Analysis
The microstructural examination revealed several key features in the overlay layer:
- Carbide morphology: The overlay layer contained a high density of carbides, primarily M7C3 and M23C6 type, with sizes ranging from 1–10 μm. The carbides were distributed throughout the matrix, providing the primary wear resistance mechanism.
- Matrix microstructure: The matrix consisted of a mixture of martensite and retained austenite, with the retained austenite content estimated at 15–25% based on XRD analysis. The retained austenite contributes to the toughness of the overlay layer and helps prevent cracking.
- Dilution effects: The dilution rate from the K360 substrate was estimated at 10–20%, depending on the number of passes. The dilution introduced additional carbon and alloying elements into the overlay, which affected the carbide formation and the matrix microstructure.
| Microstructural Feature | Overlay Layer | Base Metal (K360) | HAZ |
|---|---|---|---|
| Primary phase | Martensite + retained austenite | Martensite + bainite | Martensite |
| Carbide type | M7C3, M23C6 | M7C3, Fe3C | Fe3C |
| Carbide size (μm) | 1–10 | 0.5–5 | 0.5–3 |
| Hardness (HV30) | 500–700 | 350–450 | 400–500 |
Mechanical Properties
The mechanical properties of the overlay layer were characterized by:
- Hardness: The surface hardness of the overlay layer reached 500–700 HV30, significantly higher than the base metal hardness of 350–450 HV30. The hardness profile showed a gradient from the surface to the fusion line, with the highest hardness at the surface due to the highest carbide density.
- Wear resistance: Pin-on-disc wear tests demonstrated that the overlay layer exhibited 3–5 times better wear resistance than the uncoated K360 base metal. The wear mechanism transitioned from abrasive wear to adhesive wear as the load increased.
- Impact toughness: The overlay layer showed lower impact toughness than the base metal, which is expected given the higher hardness. The Charpy V-notch impact energy at room temperature was approximately 20–30 J for the overlay layer, compared to 40–50 J for the base metal.
Engineering Implications and Process Considerations
The study highlights several important considerations for the application of weld overlay on K360 wear-resistant steel:
- Dilution control: The dilution rate must be carefully controlled to maintain the intended hardness and wear resistance of the overlay layer. A dilution rate above 25% can significantly reduce the hardness and wear life of the overlay.
- Number of passes: For thick overlay layers, multiple passes are necessary. The first pass has the highest dilution, while subsequent passes have lower dilution. A minimum of three passes is recommended for overlay layers thicker than 2 mm.
- Heat input management: The heat input must be optimized to balance between adequate penetration and minimal dilution. A heat input of 6–10 kJ/mm is recommended for SAW overlay on K360 steel.
- Post-weld treatment: A stress-relief anneal at 550–600°C can reduce residual stresses and improve the toughness of the overlay layer without significantly reducing the hardness.
In practical applications, the overlay layer on K360 steel is commonly used in mining equipment components such as bucket teeth, conveyor wear plates, and crusher hammers. The wear life extension provided by the overlay layer can be significant, often extending the service life by 3–5 times compared to the uncoated component.
Study Insights and Practical Value
The study provides valuable insights into the microstructure-property relationships in weld overlay layers on K360 wear-resistant steel. The identification of the primary wear resistance mechanism (hard carbides in a tough matrix) and the quantification of the dilution effects provide a solid foundation for process optimization. The study also underscores the importance of balancing hardness and toughness in the overlay layer design, as excessive hardness without adequate toughness can lead to cracking and premature failure in service. For engineers involved in the design and qualification of overlay welding procedures for wear-resistant steel components, this paper offers practical guidance on process parameter selection and property verification.