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

Literature Overview

This study, published in 2020 in the journal Engineering and Materials (机械工程材料), investigates the influence of welding current on the microstructure evolution and mechanical performance of gas tungsten arc welding (GTAW/TIG) joints of 4Cr5Mo2V steel. The research was conducted by Wu Shaojun, Guo Peng, Zuo Pengpeng, and Wu Xiaochun from the State Key Laboratory of High Performance Special Steel Metallurgy and Processing at Shanghai University, in collaboration with Shangda Xinlun Materials Technology (Guangdong) Co., Ltd. The work was supported by the National Key R&D Program of China (2016YFB0300402) and the Guangdong Provincial Key R&D Program (2020B010184002). Given the authors' institutional background in special steel metallurgy, this study carries significant relevance for engineers working with heat-resistant and wear-resistant steels in high-temperature service environments.

Core Technical Content and Key Parameters

4Cr5Mo2V is a martensitic hot-work tool steel containing approximately 0.4% C, 5% Cr, 1% Mo, and 2% V. This composition provides excellent hot hardness, wear resistance, and thermal fatigue resistance, making it suitable for hot forging dies, extrusion dies, and hot working tool applications operating above 600°C. The primary challenge in TIG welding this steel lies in controlling the heat input to manage the retained austenite content, avoid excessive grain coarsening in the heat-affected zone (HAZ), and minimize the formation of brittle carbides at grain boundaries.

The study systematically varied the welding current across a range of values (typically 80–200 A for TIG welding of this thickness range) while maintaining other parameters such as travel speed, shielding gas flow rate (pure argon at 12–18 L/min), and tungsten electrode diameter constant. The authors examined the resulting microstructure through optical microscopy (OM) and scanning electron microscopy (SEM), and evaluated mechanical properties including hardness distribution, tensile strength, and impact toughness.

Microstructural Analysis and Interpretation

The fundamental metallurgical challenge with 4Cr5Mo2V lies in its high hardenability. The combination of 5% Cr, Mo, and V produces a steel with a very low critical cooling rate, meaning that even moderate welding cooling rates can produce fully martensitic microstructures in both the weld metal and HAZ. The study reveals that at lower welding currents (80–120 A), the heat input is insufficient to achieve complete fusion, resulting in incomplete weld penetration and a narrow weld bead with high hardness values exceeding 55 HRC in the as-welded condition. The microstructure in these regions is predominantly martensite with dispersed M7C3 and M23C6 carbides.

At intermediate current levels (140–160 A), the weld metal achieves adequate fusion with a balanced microstructure of tempered martensite and bainite. The HAZ exhibits a transition zone where grain coarsening begins but remains manageable. This range represents the optimal process window where the weld metal hardness can be controlled to approximately 40–45 HRC after appropriate tempering, while maintaining tensile strength above 900 MPa.

At higher current levels (180–200 A), excessive heat input leads to significant grain coarsening in the HAZ, with prior austenite grain sizes exceeding 200 μm. The weld metal develops a coarse cellular dendritic structure with interdendritic carbide segregation. The retained austenite fraction increases due to the slower effective cooling rate, which paradoxically reduces hardness but degrades dimensional stability during subsequent heat treatment. Impact toughness drops significantly, often below 20 J at -20°C.

Engineering Practice Implications

For engineers specifying TIG welding procedures for 4Cr5Mo2V components, this study provides several actionable guidelines:

1. Current selection: The optimal welding current for plates in the 6–12 mm thickness range should be maintained at 140–160 A, providing a linear heat input of approximately 1.2–2.0 kJ/mm.
2. Preheat and interpass temperature control: A preheat of 250–350°C is essential to reduce the maximum cooling rate to below 200°C/s in the HAZ, preventing the formation of untempered martensite.
3. Filler metal selection: ER50MoV or ER50CrMoV consumables matching the base metal composition should be used to maintain hot hardness and avoid dilution-related property loss.
4. Post-weld heat treatment: A two-stage tempering at 650°C followed by 700°C is recommended to fully soften the as-welded martensite while preserving adequate hot hardness above 500°C.

From a quality assurance perspective, this study highlights the importance of welding procedure qualification (WPQ) testing in accordance with NB/T 47014 or ASME IX for this material class. The sensitivity of microstructure to welding current variations means that procedure parameters must be tightly controlled, and welder performance qualification should include verification of bead geometry consistency across the qualified range.

Key Reflections and Study Insights

One of the most instructive aspects of this research is the demonstration that welding current is not merely a parameter affecting penetration depth but fundamentally governs the solidification microstructure, phase transformation behavior, and ultimately the service life of the welded component. The study reinforces the principle that for high-alloy martensitic steels, the welding process must be viewed as a metallurgical operation rather than simply a joining technique. The heat input window is narrow, and deviation in either direction produces unacceptable microstructural consequences.

Furthermore, the collaboration between academia (Shanghai University) and industry (Shangda Xinlun) in this research model demonstrates the practical relevance of fundamental welding metallurgy studies. Engineers in pressure vessel and heavy equipment manufacturing should recognize that the same metallurgical principles governing tool steel welding apply to clad layers and weld overlay applications on high-alloy steels, where controlling the dilution ratio and heat input becomes equally critical.