Zhuojin Pipe Fitting Co., LtdMicrostructure and Crack Resistance of Hardfacing Alloy Layer on K360 Steel
Literature Overview
This study by Sun Yuanzhang, Deng Hanzhong, and colleagues from Liaoning Technical University and China Coal Zhangjiakou Coal Mining Machinery Co., Ltd. investigates the microstructure and crack resistance of a hardfacing alloy layer deposited on K360 super wear-resistant steel using CO2 gas shielded arc welding with RD-YD450(Q) wire. Published in the Journal of the China Coal Society (2008, Vol. 33, No. 9, pp. 1067-1071), the research addresses the practical challenge of hardfacing high-strength wear-resistant steels without introducing cracking defects.
Material and Process Details
K360 steel is a Japanese super wear-resistant steel plate used in mining and heavy industry applications. The hardfacing wire RD-YD450(Q) was selected for its compatibility with this base material. CO2 gas shielded arc welding was used as the surfacing process, which is common in industrial applications due to its equipment simplicity and consumable cost-effectiveness.
Microstructural Characterization
The hardfacing layer microstructure was characterized using optical microscopy, SEM, TEM, and XRD. The analysis revealed the typical microstructure of a high-carbon hardfacing alloy, including martensite and M7C3 carbides. The high carbon and alloy content in the deposit promotes the formation of hard phases, which are essential for wear resistance.
Crack Analysis
The most significant finding of this study is the identification of delayed cracking in the hardfacing layer. The rigid fixture butt crack test was used to evaluate crack resistance, and the results indicated that the primary cause of cracking was excessive restraint stress during welding.
| Crack Type | Cause | Countermeasure |
|---|---|---|
| Delayed cracking | Excessive restraint stress | Preheating to appropriate temperature |
The delayed nature of the cracking is characteristic of hydrogen-induced cracking in high-strength materials. The high carbon content of the hardfacing deposit promotes martensite formation, which is susceptible to hydrogen embrittlement. The restraint stress from the rigid fixture further exacerbates the cracking tendency.
Preheating as a Countermeasure
The study demonstrates that preheating the base material to an appropriate temperature before hardfacing can effectively prevent cracking. Preheating serves multiple functions:
- Reduces the cooling rate of the weld metal, promoting more favorable microstructural evolution and reducing martensite brittleness.
- Allows hydrogen to diffuse out of the weld metal during cooling, reducing the risk of hydrogen-induced delayed cracking.
- Reduces the thermal gradient and consequent residual stress between the weld metal and base material.
The optimal preheat temperature depends on the specific material combination and section thickness. For K360 steel with a high-carbon hardfacing deposit, a preheat temperature in the range of 150–250 °C is typically recommended, though the exact value should be determined through trial welding and crack sensitivity testing.
Engineering Practice Implications
This study has direct relevance to the hardfacing of high-strength wear-resistant steel components in mining and heavy equipment applications. Several practical lessons can be drawn:
- Preheating is not optional for high-strength base materials with high-carbon hardfacing deposits; it is a necessary process parameter to prevent delayed cracking.
- The choice of welding process (CO2 gas shielded arc in this case) and consumable (RD-YD450(Q) wire) must be compatible with the base material's mechanical properties and cracking susceptibility.
- Post-weld heat treatment may also be beneficial to reduce residual stresses and further improve crack resistance, though this must be balanced against potential softening of the hardfacing layer.
Study Insights and Conclusion
This research highlights the critical importance of cracking prevention in the hardfacing of high-strength wear-resistant steels. The identification of delayed cracking as the dominant failure mode and the demonstration that preheating is an effective countermeasure provide clear guidance for industrial practice. The study also underscores the need for careful selection of consumables and process parameters when hardfacing advanced high-strength steels, as the combination of high carbon content in the deposit and high strength in the base material creates a challenging welding environment. In conclusion, successful hardfacing of K360 steel requires a systematic approach that includes appropriate preheating, consumable selection, and process parameter optimization to ensure crack-free deposits with acceptable wear resistance.