Zhuojin Pipe Fitting Co., LtdPost-Weld Heat Treatment Effects on K360 Steel Hardfacing Layer Microstructure and Properties
Literature Overview
This paper by Deng Hanzhong and Wang Dayong from Liaoning Technical University, published in the Journal of the China Coal Society in 2011 (Vol. 36, No. 6, pp. 1036-1040), investigates the effect of post-weld heat treatment on the microstructure and properties of a hardfacing layer deposited on K360 wear-resistant steel plate. The hardfacing was performed using flux-cored wire with CO2 gas protection, and the post-weld treatment consisted of tempering at approximately 400°C. The authors conducted metallographic examination, hardness testing, impact toughness testing, and abrasive wear testing on both the as-welded and heat-treated conditions.
Core Technical Content
K360 steel is a high-strength, high-hardness wear-resistant steel commonly used in mining and material handling applications. The base material has a high carbon equivalent, which makes it susceptible to cold cracking during welding. The hardfacing process using flux-cored wire with CO2 shielding provides a practical solution for repairing worn surfaces while maintaining the structural integrity of the component.
The as-welded hardfacing layer exhibits a microstructure consisting of martensite, network carbides, block carbides, and a small amount of retained austenite. This microstructure results from the rapid cooling of the deposited metal during the welding process. The high hardness of the as-welded layer is primarily due to the martensitic transformation, but the high hardness comes at the expense of impact toughness.
| Condition | Microstructure | Hardness | Impact Toughness | Wear Resistance (relative to base) |
|---|---|---|---|---|
| As-welded | Martensite + network carbides + block carbides + retained austenite | High | Poor | 1.46× base material |
| After 400°C tempering | Tempered troostite + carbides | High | Improved | 1.32× base material |
Microstructure Evolution During Tempering
The tempering treatment at 400°C causes significant microstructural changes in the hardfacing layer. The untempered martensite decomposes into tempered troostite (a fine mixture of ferrite and cementite), which retains high hardness while improving ductility. The network carbides, which are prone to causing intergranular cracking, are partially dissolved and spheroidized, reducing the risk of brittle fracture.
The block carbides are also modified by the tempering treatment. Some of the block carbides dissolve and re-precipitate as finer, more uniformly distributed particles. This refinement improves the toughness of the deposited layer without significantly reducing the hardness.
The retained austenite in the as-welded layer may partially transform to martensite during tempering, or it may remain stable depending on its composition. The stability of retained austenite is influenced by the carbon and alloying element content, which determines the Ms and Mf temperatures of the austenite.
Wear Mechanism Analysis
The wear resistance of the hardfacing layer is governed by the interaction between the hard carbide phases and the matrix. In the as-welded condition, the high hardness of the martensitic matrix provides excellent resistance to abrasive penetration. However, the poor toughness of the untempered martensite makes the layer susceptible to cracking under impact loading, which can lead to catastrophic failure.
After tempering, the wear resistance decreases slightly (from 1.46× to 1.32× the base material), but the improvement in toughness provides a more balanced combination of hardness and ductility. The tempered troostite matrix retains sufficient hardness to resist abrasive wear while providing the ductility necessary to accommodate plastic deformation without cracking.
The engineering significance of this finding is that the heat-treated condition offers a more reliable performance under real service conditions, where impact loading and thermal cycling are inevitable. The slight reduction in wear resistance is more than compensated by the substantial improvement in toughness, which extends the service life of the repaired component.
Engineering Practice Implications
The study provides clear guidance for the repair of K360 steel components in the field. The recommended procedure is:
- Prepare the worn surface by grinding or machining to remove loose material and expose sound metal.
- Apply the hardfacing deposit using flux-cored wire with CO2 shielding, maintaining appropriate heat input to avoid excessive dilution.
- Perform post-weld tempering at approximately 400°C to optimize the microstructure and mechanical properties.
- Inspect the repaired surface for cracks, porosity, or other defects using visual examination or magnetic particle testing.
The tempering treatment can be performed using induction heating, oxy-fuel torch heating, or in a furnace. For large components that cannot be moved to a furnace, induction heating is the preferred method because it provides localized heating with minimal distortion.
Study Insights and Reflections
The work by Deng and Wang demonstrates the importance of post-weld heat treatment in optimizing the performance of hardfacing deposits. The as-welded condition provides the highest wear resistance, but the poor toughness makes it unsuitable for applications involving impact loading. The tempering treatment achieves an optimal balance between hardness and toughness, which is essential for reliable service performance.
One important consideration is the selection of tempering temperature. The study uses 400°C, which is appropriate for the K360 steel system. However, different base materials and hardfacing alloys may require different tempering temperatures to achieve optimal properties. Engineers must consult the material specifications and welding procedure specifications to determine the appropriate tempering temperature for each application.
The study also highlights the trade-off between wear resistance and toughness. In many industrial applications, the optimal solution is not the maximum wear resistance but rather the best combination of wear resistance, toughness, and fatigue resistance. This trade-off must be carefully evaluated for each specific application.
For engineers working on pipeline and pressure vessel repair, this study provides valuable insights into the optimization of hardfacing procedures. The post-weld heat treatment step, which is often overlooked in field repair operations, can significantly improve the long-term performance of the repair.