Effect of Welding Current on Microstructure and Properties of 4Cr5Mo2V Steel TIG Weld Joints

Literature Overview

This study by Wu Shaojun, Guo Peng, Zuo Pengpeng, and Wu Xiaochun from Shanghai University, published in Materials in Mechanical Engineering (2020, Vol. 44, S2, pp. 97-102), investigates the influence of welding current on the microstructure and mechanical properties of TIG weld joints in 4Cr5Mo2V steel. The research was supported by the National Key R&D Program (2016YFB0300402) and the Guangdong Provincial Key R&D Program (2020B010184002). This work addresses a critical materials selection and process optimization challenge for high-strength alloy steel applications in demanding service conditions.

Material Background and Welding Challenge

4Cr5Mo2V steel is a high-strength, high-temperature alloy steel characterized by:

The high carbon equivalent and alloy content make this steel highly susceptible to cold cracking during welding, while the high hardenability creates challenges for achieving adequate toughness in the heat-affected zone. The welding current directly controls the heat input, which determines the thermal cycle experienced by the weld metal and HAZ, and consequently governs the resulting microstructure and properties.

Experimental Design and Results

The study examined three welding current levels: 70 A, 130 A, and 190 A, representing low, medium, and high heat input conditions respectively. The weld joint microstructure was characterized through optical microscopy and hardness testing across the entire weld cross-section.

Microstructural Evolution with Welding Current

The weld joint consists of five distinct zones: weld metal, fusion zone, fully quenched zone, partially quenched zone, and base metal. The following table summarizes the key findings:

Key Observations

1. At 70 A: The low heat input produces a narrow weld with a wide HAZ. The rapid cooling rate in the fully quenched zone produces fine tempered martensite with high hardness, but the extensive HAZ represents a large volume of potentially brittle material. The overall joint hardness is highest but the weldability margin is narrow.
2. At 130 A: This intermediate current produces the most favorable balance of properties. The HAZ width is minimized, the fusion zone is compact, and the microstructural uniformity is good. The tempering effect within the quenched zones is moderate, producing tempered martensite of appropriate fineness.
3. At 190 A: The high heat input produces a wide weld with coarse grain structure and extensive tempering. While the hardness is lowest, indicating good toughness potential, the coarse microstructure may compromise fatigue resistance and creep strength. The microstructural uniformity is improved due to the extended tempering effect.

Engineering Practice Recommendations

Based on the findings, the following recommendations emerge for practical welding of 4Cr5Mo2V steel:

• Preheat temperature: A preheat of 200-250°C is recommended to slow the cooling rate and reduce cold cracking susceptibility, particularly at lower welding currents.
• Interpass temperature: Maintain 250-300°C between passes to promote tempering of previously deposited weld metal.
• Post-weld heat treatment: A tempering treatment at 600-650°C for 2-4 hours is essential to relieve residual stresses and achieve the target toughness in the fully quenched zone.
• Current selection: For single-pass welds, 130 A represents the optimal balance. For multi-pass welds, use lower currents (70-100 A) for root and fill passes with the final cover pass at 130-150 A.

Metallographic Analysis Insights

The microstructural observations reveal important metallurgical principles:

• The fully quenched zone forms where the peak temperature exceeds the Ac3 temperature but the cooling rate exceeds the critical cooling rate for transformation. The tempering level depends on the peak temperature and the time spent above 500°C.
• The partially quenched zone exhibits a mixed microstructure of tempered martensite and untransformed pearlite/ferrite, reflecting the lower peak temperatures in this region.
• The weld metal grain size increases with welding current due to the higher heat input, which promotes grain growth during solidification and subsequent cooling.
• The hardness gradient across the joint is steepest at 70 A and most gradual at 190 A, with important implications for stress concentration and crack propagation behavior.

Study Reflections

This research provides valuable guidance for welding process development on high-strength alloy steels. The systematic variation of welding current as a single parameter allows clear identification of structure-property relationships. In engineering practice, however, multiple parameters interact simultaneously, and the optimal current must be considered in conjunction with travel speed, gas flow rate, and joint geometry. The findings underscore the importance of balancing hardness (strength) with toughness, and suggest that 130 A represents a practical starting point for process development on this alloy, subject to adjustment based on specific application requirements and post-weld heat treatment capability.