Hardness Distribution and Impact Toughness of Steel-Based Overlay Composite Plates

Literature Overview

This research, conducted by Zhang Ziling, Lu Liwei, Li Yiquan, and colleagues from Hunan University of Science and Engineering, in collaboration with Zhuhai Gree Electric Appliances and the Hunan Provincial New Energy Vehicle Industry-Academia-Research Innovation Center, investigates the mechanical properties of steel-based overlay composite plates, with particular focus on hardness distribution and impact toughness. Published in 2020 and supported by the National Natural Science Foundation of China and Hunan Provincial Natural Science Foundation, this work addresses the metallurgical challenges inherent in producing high-performance composite plates through overlay welding processes. The study provides valuable insights into the relationship between overlay process parameters, microstructural evolution, and mechanical performance, which are critical for the design and qualification of composite plate components used in demanding applications such as new energy vehicles and industrial equipment.

Fundamental Metallurgical Considerations

Steel-based overlay composite plates are produced by depositing a wear-resistant or corrosion-resistant alloy layer onto a structural steel substrate through welding processes such as submerged arc welding (SAW), gas metal arc welding (GMAW), or flux-cored arc welding (FCAW). The resulting composite plate exhibits a gradient of properties from the hard, wear-resistant overlay to the tough, ductile base metal, with a critical transition zone in between that determines the overall mechanical performance of the component.

The hardness distribution across the composite plate is governed by the dilution ratio between the overlay material and the base metal, which is influenced by the welding process, wire or powder composition, and welding parameters. A lower dilution ratio results in a harder overlay with better wear resistance but potentially reduced toughness at the interface. Conversely, a higher dilution ratio produces a softer overlay with better toughness but reduced wear performance. The optimal dilution ratio is a balance between these competing requirements and is typically in the range of 10 to 30 percent for most applications.

The impact toughness of the composite plate, particularly at the overlay-base interface, is a critical parameter for applications subject to impact loading or thermal cycling. The transition zone, where the microstructure changes from the overlay microstructure to the base metal microstructure, is the most vulnerable region for crack initiation and propagation. The toughness of this region is influenced by the cooling rate, which determines the grain size and phase composition, and by the presence of brittle intermetallic phases or excessive carbon segregation at the interface.

Experimental Results and Analysis

The study presents detailed hardness profiles across the thickness of the composite plate, measured at multiple locations to assess uniformity. The hardness of the overlay layer typically ranges from 350 to 550 HV, depending on the alloy composition and process parameters, while the base metal hardness is in the range of 150 to 250 HV. The transition zone exhibits a gradual decrease in hardness from the overlay to the base metal, with the rate of decrease being a function of the dilution ratio and the number of overlay passes.

Impact toughness measurements, conducted using Charpy V-notch tests at various temperatures, reveal that the impact energy of the composite plate is significantly lower than that of the base metal, particularly at low temperatures. The overlay layer, being a hard, high-carbon or high-chromium alloy, has inherently lower toughness than the structural steel base metal. The impact energy of the transition zone is intermediate, reflecting the mixed microstructure and the presence of residual stresses. The study identifies that the impact toughness can be improved by optimizing the welding parameters to reduce the dilution ratio, using a transition layer with intermediate composition, and applying post-weld heat treatment to relieve residual stresses and refine the microstructure.

Process Optimization and Microstructural Control

The study demonstrates that the hardness distribution and impact toughness can be significantly improved through careful process optimization. The use of a multi-pass welding sequence with a low-dilution first pass and a higher-dilution subsequent passes can create a graded transition zone that balances hardness and toughness. The selection of filler metal composition is also critical: a filler metal with lower carbon and chromium content for the first pass reduces the formation of brittle phases at the interface, while a higher-carbon, higher-chromium filler for subsequent passes builds up the hard, wear-resistant overlay.

Post-weld heat treatment is another critical factor. A stress relief anneal at 600 to 700 degrees Celsius for 2 to 4 hours can reduce residual stresses and improve the impact toughness of the transition zone by promoting the transformation of retained austenite and tempering of martensite. However, excessive heat treatment temperature or duration can lead to softening of the overlay layer and reduced wear resistance, so the heat treatment parameters must be carefully optimized for each specific application.

Engineering Applications and Quality Assurance

The findings of this study have direct applications in the manufacture of composite plate components for new energy vehicles, where lightweight, high-strength, and wear-resistant materials are essential for performance and safety. The hardness distribution and impact toughness data provide the basis for designing composite plate components that meet the stringent mechanical requirements of automotive applications. The study also emphasizes the importance of quality assurance procedures, including hardness profiling, impact testing, and microstructural examination, to ensure that the composite plate meets the specified performance requirements.

The research highlights the challenge of achieving a balance between wear resistance and toughness in overlay composite plates, which is a fundamental materials engineering problem that requires a deep understanding of the process-structure-property relationships. The use of a transition layer and post-weld heat treatment are practical solutions that can be implemented in industrial production to improve the mechanical performance of composite plates without significantly increasing manufacturing costs.

Study Insights and Reflections

This research provides a comprehensive analysis of the hardness distribution and impact toughness of steel-based overlay composite plates, offering valuable guidance for engineers designing and manufacturing composite plate components. The key insight is that the mechanical performance of the composite plate is not determined by the overlay material alone but by the entire structure, including the transition zone and the base metal, and that careful process control is essential to achieve the desired balance of properties. The study also underscores the importance of microstructural characterization in understanding the relationship between processing parameters and mechanical performance, which is a fundamental principle in materials engineering. For engineers working in the field of composite plate manufacturing, this research provides a practical framework for optimizing the welding process, filler metal selection, and post-weld heat treatment to produce high-performance composite plates that meet the demanding requirements of modern industrial applications. The collaborative approach between academia and industry, as exemplified by the partnership between Hunan University of Science and Engineering and Zhuhai Gree Electric Appliances, demonstrates the value of translating fundamental research into practical engineering solutions that drive innovation in the manufacturing sector.