Microstructure and Crack Resistance of Surfacing Alloy Layer on K360 Steel

Literature Overview

The study by Sun Yuanzhang and colleagues, published in the Journal of the China Coal Society (2008, Vol. 33, No. 9, pp. 1067-1071), investigates the microstructure and crack resistance of a surfacing alloy layer deposited on K360 super wear-resistant steel plate using CO2 gas shielded welding (GMAW) with RD-YD450(Q) wire. The research was conducted jointly by Liaoning Technical University and China Coal Zhangjiakou Coal Mining Machinery Co., Ltd., reflecting a strong industry-academia collaboration focused on practical mining equipment applications.

Material Background

K360 is a Japanese super wear-resistant steel plate with a hardness of approximately 360 HBW, widely used in mining equipment for components subject to severe abrasion and impact. The steel typically contains elevated carbon and alloying elements (Cr, Mo, Mn) to achieve its high hardness through a martensitic microstructure. Surfacing such high-hardness substrates presents significant challenges due to the high hardenability and residual stress levels that can promote cracking.

Welding Process and Experimental Methodology

The surfacing was performed using CO2 gas shielded arc welding (GMAW) with RD-YD450(Q) surfacing wire. The experimental methodology included:

• Optical microscopy (OM) for general microstructural characterization
• Scanning electron microscopy (SEM) for detailed microstructural and fracture analysis
• Transmission electron microscopy (TEM) for nanoscale structural analysis
• X-ray diffraction (XRD) for phase identification
• Rigid restraint butt crack test for crack resistance evaluation

The rigid restraint butt crack test is a severe condition test that imposes maximum restraint on the weld, simulating the worst-case stress conditions encountered in thick-section or heavily constrained components.

Microstructural Analysis

The surfacing layer microstructure consists of martensite with dispersed carbide particles, as confirmed by XRD and TEM analysis. The rapid cooling inherent to GMAW surfacing promotes the formation of hard, non-equilibrium martensitic structures. The carbide phases identified include M7C3 and possibly M23C6, depending on the chromium content of the RD-YD450(Q) wire. The TEM analysis likely revealed nanoscale precipitates within the martensite laths, contributing to the overall hardness of the surfacing layer.

Crack Resistance Analysis

The study identifies the cracks observed in the surfacing layer as delayed cracks (hydrogen-induced delayed cracking). This is a critical finding because delayed cracks do not appear immediately after welding but develop over hours or days as hydrogen diffuses through the weld metal and accumulates at microstructural traps. The primary cause is identified as excessive restraint stress during the welding process.

The study demonstrates that preheating the K360 substrate to an appropriate temperature effectively prevents crack formation. The recommended preheating temperature is not explicitly stated in the abstract but would typically be in the range of 200-300°C for a steel of this hardness and alloy content, depending on the section thickness and welding parameters.

Engineering Practice Implications

For engineers working with K360 or similar high-hardness wear-resistant steels, this study provides critical guidance on surfacing crack prevention. The key lessons include:

1. Preheating is essential: The high hardness and alloy content of K360 steel create conditions favorable to hydrogen-induced delayed cracking. Preheating reduces the cooling rate, promotes hydrogen diffusion and escape, and reduces the residual stress level.
2. Wire selection matters: The RD-YD450(Q) wire was specifically designed for surfacing high-hardness substrates, with a composition that balances hardness and crack resistance. The "(Q)" designation likely indicates a low-hydrogen or low-carbon variant.
3. Process monitoring: Because delayed cracks develop over time, post-weld inspection should include a waiting period (typically 24-72 hours) before final NDT examination.
4. Multi-pass strategies: For thick overlays, maintaining interpass temperatures and using back-step welding sequences can reduce restraint stress.

Key Reflections

The identification of delayed cracking as the dominant failure mode in K360 surfacing is particularly important because it challenges the common assumption that crack prevention is achieved solely through immediate post-weld inspection. In practice, components may appear crack-free immediately after surfacing but develop critical cracks during subsequent handling, transport, or early service. This has significant implications for quality assurance protocols in mining equipment maintenance.

The study also highlights the importance of substrate properties in determining surfacing process parameters. K360 steel's high hardness and alloy content create a fundamentally different welding environment compared to conventional carbon steel, requiring tailored preheating, wire selection, and post-weld treatment strategies. Engineers should not apply generic surfacing procedures to high-hardness substrates without specific process qualification.

This work provides a practical framework for crack-free surfacing of super wear-resistant steels, combining fundamental understanding of crack mechanisms with actionable process recommendations that can be directly implemented in mining equipment maintenance operations.