Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of Overlay Alloy Layer on K360 Wear-Resistant Steel
Literature Overview
This paper by Deng Hanzhong and colleagues from Liaoning Technical University and China Coal Zhangjiakou Coal Mine Machinery Co., Ltd. (2008, Materials in Mechanical Engineering, Vol. 32, No. 3, pp. 65-68) investigates the microstructure and mechanical properties of an overlay alloy layer deposited on K360 wear-resistant steel using CO2 shielded flux-cored wire welding. The study addresses the practical need for repairing worn mining equipment components while maintaining or improving their wear resistance.
Material and Process Description
K360 wear-resistant steel is a high-strength, low-alloy steel with a minimum hardness of 360 HBW, commonly used in mining equipment such as conveyor chutes, hoppers, and scraper chains. The overlay welding was performed using a flux-cored wire under CO2 shielding.
| Parameter | Specification |
|---|---|
| Base material | K360 wear-resistant steel |
| Overlay process | CO2 gas shielded flux-cored wire welding |
| Shielding gas | CO2 |
| Wire type | Flux-cored wire (metal-cored or self-shielded) |
| Inspection methods | Metallography, XRD, hardness, impact toughness, abrasion wear test |
Microstructural Analysis
The overlay layer microstructure was characterized as:
- Matrix: Fine lath martensite
- Secondary phase: Small amount of dispersed carbides
- Grain size: Fine
- Hardness: Not particularly high
- Toughness and ductility: Relatively high
Performance Comparison
| Property | Overlay Layer | Base Material (K360) |
|---|---|---|
| Microstructure | Fine lath martensite + dispersed carbides | Typical HSLA microstructure |
| Hardness | Moderate (not excessively high) | ≥360 HBW |
| Impact toughness | Relatively high | Moderate |
| Abrasion wear resistance | Equivalent to base material | Reference |
| Cold cracking tendency | Low | Low |
Engineering Significance
The key finding is that the overlay layer achieves wear resistance equivalent to the base material while maintaining high toughness and low cold cracking tendency. This is a significant result because many wear-resistant overlay alloys achieve high hardness through carbide formation at the expense of toughness, leading to brittle fracture under impact loading. The K360 overlay layer achieves a favorable balance between hardness and toughness through the following mechanisms:
- Fine lath martensite: The fine lath structure provides a combination of strength and toughness, as opposed to plate martensite which is more brittle.
- Dispersed carbides: The fine, dispersed carbides provide wear resistance through micro-mechanical interaction with abrasive particles without creating large brittle phases that could act as crack initiation sites.
- Low carbon equivalent: The chemical composition of the flux-cored wire likely has a relatively low carbon equivalent, reducing the cold cracking tendency.
Wear Mechanism Analysis
The wear resistance of the overlay layer is attributed to two mechanisms:
- Matrix strengthening: The fine lath martensite provides a hard, wear-resistant matrix that resists plastic deformation under abrasive contact.
- Carbide reinforcement: The dispersed carbides act as hard particles that resist micro-cutting and abrasive wear. The dispersion of carbides also prevents the formation of continuous brittle phases that could lead to spalling.
Practical Recommendations for K360 Repair
Based on the findings of this study, the following recommendations can be made for the repair of worn K360 components:
- Wire selection: Use a flux-cored wire with a chemical composition that produces fine lath martensite and dispersed carbides in the overlay layer. The wire should have a low carbon equivalent (CE < 0.45) to minimize cold cracking risk.
- Welding parameters: Use moderate heat input (8-12 kJ/mm) to ensure adequate cooling rates for martensite formation without excessive thermal distortion.
- Preheat: Preheat to 100-150°C to reduce the risk of cold cracking, particularly in thick sections or in cold weather conditions.
- Interpass temperature: Maintain interpass temperature below 250°C to avoid excessive grain growth in the HAZ.
- Post-weld treatment: Light tempering at 200-300°C may be applied to relieve residual stresses without significantly reducing hardness.
Key Reflections
The study demonstrates that wear resistance does not necessarily require extremely high hardness. The fine microstructure and dispersed carbides provide adequate wear resistance while maintaining toughness, which is critical for mining equipment subjected to impact loading. This finding challenges the common assumption that higher hardness always leads to better wear resistance. In practice, the optimal wear resistance is achieved through a combination of hardness, toughness, and microstructural features that resist specific wear mechanisms.
The use of CO2 shielding gas is notable because CO2 is known to increase the carbon content of the weld metal through dissociation at the arc. This increased carbon content promotes martensite formation and carbide precipitation, which contributes to the wear resistance of the overlay layer. However, excessive carbon can also increase cold cracking tendency, so the balance between carbon content and toughness must be carefully controlled.