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

Literature Overview

This study, conducted by Wu Shaojun, Guo Peng, Zuo Pengpeng, and Wu Xiaochun from Shanghai University's State Key Laboratory for Advanced Metals and Materials and Shanghai University's School of Materials Science and Engineering, was published in Mechanical Engineering Materials in 2020. The research was supported by the National Key R&D Program (2016YFB0300402) and the Guangdong Provincial Key R&D Program (2020B010184002). The work investigates the influence of welding current on the microstructure and mechanical properties of TIG weld joints in 4Cr5Mo2V steel, a high-strength low-alloy steel widely used in power generation equipment such as boiler tubes, pressure vessel components, and heat exchanger tubesheets.

Material Background and Welding Challenges

4Cr5Mo2V steel is a precipitation-hardening martensitic steel containing approximately 0.4% C, 5% Cr, 2% Mo, and 1% V. This composition provides excellent high-temperature strength, creep resistance, and oxidation resistance, making it suitable for service temperatures up to 600°C. However, the high carbon and alloy content also makes the steel highly susceptible to cold cracking and sensitization during welding.

The welding challenges associated with 4Cr5Mo2V steel include:

The TIG welding process is particularly suitable for welding 4Cr5Mo2V steel due to its low heat input, precise arc control, and the ability to use filler metals with tailored compositions to mitigate cracking susceptibility.

Experimental Design and Key Findings

The study systematically varied the TIG welding current over a range of 80–220 A while maintaining other welding parameters constant, including:

• Shielding gas: pure argon at 15–20 L/min
• Travel speed: 60–100 mm/min
• Torch angle: 15–20° from vertical
• Preheat temperature: 250°C
• Interpass temperature: ≤250°C

The authors conducted metallographic examination, microhardness testing, tensile testing, impact testing, and intergranular corrosion testing on the weld joints. The key findings can be summarized as follows:

Microstructural Evolution with Welding Current

At lower welding currents (80–120 A), the weld metal exhibits a fine-grained martensitic structure with minimal prior austenite grain growth. The heat affected zone (HAZ) shows a narrow tempered martensite region with limited carbide precipitation. As the welding current increases to 160–220 A, the weld metal microstructure coarsens, with the formation of retained austenite and increased carbide precipitation at grain boundaries. The HAZ width expands significantly, and the coarse-grained HAZ region develops a mixed microstructure of martensite, bainite, and retained austenite.

Mechanical Properties

The tensile strength of the weld metal decreases from approximately 950 MPa at 80 A to 820 MPa at 220 A, reflecting the coarsening of the microstructure and the increased retained austenite content. The impact toughness (Charpy V-notch at 20°C) shows a more pronounced decline, dropping from 85 J at 80 A to 42 J at 220 A. This reduction in toughness is attributed to the increased carbide precipitation and the formation of brittle martensite in the coarse-grained HAZ.

Intergranular Corrosion Resistance

The intergranular corrosion testing (ASTM A263 Method E) reveals that welds produced at currents above 160 A exhibit intergranular attack in the HAZ, indicating sensitization due to chromium carbide precipitation at grain boundaries. Welds produced at currents below 120 A show good resistance to intergranular corrosion, with only minor attack observed at the fusion line.

Engineering Practice Integration

The findings of this study have direct implications for the welding of 4Cr5Mo2V steel components in pressure vessel and heat exchanger fabrication. The following recommendations emerge from the literature:

1. Welding current selection: For 4Cr5Mo2V steel TIG welding, a current range of 80–140 A is recommended to minimize HAZ sensitization and maintain adequate impact toughness. The specific current should be selected based on the joint geometry and the required weld penetration.
2. Multi-pass welding strategy: When welding thick sections of 4Cr5Mo2V steel, a multi-pass approach with low current per pass should be adopted to control the thermal cycle and prevent excessive heat input accumulation.
3. Post-weld heat treatment: A PWHT at 700–750°C for 2 hours per 25 mm of thickness is essential to relieve residual stresses, temper the martensitic weld metal, and restore the intergranular corrosion resistance of the HAZ.
4. Non-destructive testing: Given the susceptibility of 4Cr5Mo2V welds to cracking, rigorous NDT including UT (per NB/T 47013) and PT (per JB/T 4730) should be performed after welding and after PWHT.

Study Insights and Implications

This research provides a quantitative basis for selecting TIG welding parameters for 4Cr5Mo2V steel, which is critically important for the fabrication of high-temperature pressure vessels and heat exchangers in the power generation industry. The systematic investigation of welding current effects demonstrates that even within the seemingly narrow range of TIG welding parameters, significant variations in microstructure and properties can occur.

For engineers involved in bimetal pressure vessel fabrication, the study underscores the importance of understanding the interaction between welding parameters and material properties. The findings suggest that the welding procedure specification (WPS) for 4Cr5Mo2V steel should include strict limits on welding current, travel speed, and interpass temperature to ensure consistent weld quality. Additionally, the study highlights the need for post-weld inspection protocols that account for the potential for sensitization and cracking in high-alloy steels.

In summary, this literature offers valuable guidance for the TIG welding of 4Cr5Mo2V steel, emphasizing the critical role of welding current in determining weld microstructure, mechanical properties, and corrosion resistance. The recommendations derived from this study should be incorporated into welding procedure development and qualification for components made of this high-strength alloy.