Microstructure and Properties of H3Cr5WMoV Submerged Arc Surfacing Alloy Layer on Q235 Steel

Literature Overview and Background

The paper by Liu Shuofeng, published in the Journal of Anhui University of Technology (Natural Science Edition) in 2002 (Vol. 19, No. 4, pp. 263-265), investigates the microstructure, hardness, wear resistance, and thermal fatigue performance of an H3Cr5WMoV alloy layer produced by automatic submerged arc surfacing on Q235 steel substrates. The study compares the properties of the as-welded and tempered conditions, providing valuable data for the selection of post-weld heat treatment parameters in the repair of hot-work tooling and wear-prone components.

Core Technical Content and Metallurgical Analysis

H3Cr5WMoV is a high-speed tool steel characterized by its excellent hot hardness, wear resistance, and thermal fatigue resistance, making it an ideal surfacing material for components subjected to high-temperature wear conditions such as forging dies, hot rolling mill rolls, and extrusion dies. The alloy contains approximately 5% chromium, along with significant amounts of tungsten and molybdenum, which contribute to the formation of hard carbides (WC, W₂C, Mo₂C, and Cr7C3) that provide wear resistance and maintain hardness at elevated temperatures.

The substrate material Q235 is a common carbon structural steel with relatively low alloy content and moderate mechanical properties. The combination of a high-alloy tool steel surfacing layer on a low-alloy substrate creates a significant mismatch in thermal expansion coefficients and mechanical properties, which must be managed through careful control of welding parameters and post-weld heat treatment.

Microstructural Characteristics and Property Comparison

The study identified the following microstructural features and property differences between the as-welded and tempered conditions:

The as-welded microstructure consists primarily of lath martensite with retained austenite. The lath martensite forms due to the rapid cooling rate of the submerged arc surfacing process, which promotes diffusionless transformation from austenite. The retained austenite fraction depends on the cooling rate and the carbon and alloy content of the weld metal. While martensite provides high hardness, the as-welded condition is characterized by high residual stress and reduced toughness, making it susceptible to cracking under thermal cycling conditions.

Tempering transforms the brittle martensite into tempered martensite, which consists of fine, dispersed carbides within a ferrite matrix. This transformation reduces hardness slightly in some cases but significantly improves toughness and thermal fatigue resistance. In the specific case of H3Cr5WMoV, the tempering process allows for the precipitation of hard secondary carbides (particularly tungsten and molybdenum carbides) that contribute to the enhanced wear resistance and hardness observed in the tempered condition.

Process Parameters and Engineering Considerations

The automatic submerged arc welding process used in this study offers several advantages for surfacing applications:

• Consistent deposition: Automated travel speed and wire feed rate ensure uniform layer thickness and composition.
• High deposition rate: Submerged arc welding provides significantly higher deposition rates than manual processes, making it economical for large-area surfacing.
• Clean weld metal: The flux provides both shielding and alloying, producing weld metal with low gas content and minimal surface oxidation.
• Deep penetration: The process provides good fusion with the base metal, ensuring strong metallurgical bonding.

However, several engineering considerations must be addressed:

• Preheating: The high carbon equivalent of H3Cr5WMoV necessitates preheating to prevent cold cracking. A preheat temperature of 200-300 °C is typically recommended.
• Interpass temperature control: Maintaining interpass temperature below 300 °C prevents excessive grain growth and retained austenite formation.
• Post-weld heat treatment: Tempering at 560-620 °C is essential to relieve residual stress and improve toughness. Multiple tempering cycles may be required for thick surfacing layers.
• Layer thickness management: Building up the surfacing layer in multiple passes with controlled interpass temperatures ensures uniform microstructure throughout the layer thickness.

Study Insights and Practical Applications

This study provides valuable data for the repair and hardfacing of hot-work tooling in the steel industry. The finding that the tempered condition outperforms the as-welded condition in all measured properties is consistent with general metallurgical principles for high-carbon, high-alloy tool steels. The improvement in thermal fatigue resistance is particularly significant for applications involving cyclic heating and cooling, such as forging dies and hot rolling mill rolls, where thermal fatigue cracking is a primary failure mode.

The combination of submerged arc surfacing with appropriate post-weld tempering offers an economical and effective solution for extending the service life of wear-prone components. Engineers should note that the specific tempering temperature and duration must be optimized based on the thickness of the surfacing layer, as thicker layers require longer tempering times to achieve complete stress relief and uniform microstructure throughout the layer. The study also highlights the importance of considering the entire component system, including the base metal, when selecting surfacing materials and process parameters, as the mechanical and thermal mismatch between the high-alloy surfacing layer and the low-alloy substrate can lead to delamination or cracking if not properly managed through process control and heat treatment.