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

Literature Overview

The paper by Wu Shaojun, Guo Peng, Zuo Pengpeng, and Wu Xiaochun from Shanghai University investigates the effect of welding current on the microstructure and mechanical properties of TIG welded joints of 4Cr5Mo2V steel. Published in Materials for Mechanical Engineering (Volume 44, Issue S2, 2020, pp. 97-102), the research was supported by the National Key R&D Program of China (2016YFB0300402) and the Guangdong Provincial Key R&D Program (2020B010184002). The authors conducted TIG welding experiments at three current levels—70 A, 130 A, and 190 A—and characterized the resulting microstructures and mechanical properties.

Core Technical Findings

Microstructural Zones

The welded joint consists of five distinct microstructural zones:

Weld zone (WZ): The fused region containing solidified filler and base metal.
Fusion zone (FZ): The transition region between weld metal and base metal.
Fully quenched zone (FQZ): The region in the HAZ that was heated above the Ac3 temperature and subsequently quenched to martensite.
Partially quenched zone (PQZ): The region in the HAZ that was heated between Ac1 and Ac3, resulting in a mixed microstructure.
Base metal (BM): The unaffected parent material.

Effect of Welding Current on Microstructure

Current (A)	Fully Quenched Zone	Partially Quenched Zone	HAZ Width	Weld Zone Grain Size	Overall Hardness
70	Fine tempered martensite	Fine tempered martensite + pearlite	Larger	Smaller	Highest
130	Moderate tempered martensite	Moderate tempered martensite + pearlite	Smallest	Moderate	Moderate
190	Coarse tempered martensite	Coarse tempered martensite + pearlite	Smallest	Largest	Lowest

Key Observations

At 70 A, the joint exhibited the finest microstructure and highest overall hardness, but the HAZ was relatively wide, indicating a longer thermal exposure.
At 130 A, the joint showed good microstructural uniformity with the smallest FZ and HAZ widths, representing an optimal balance between heat input and microstructural refinement.
At 190 A, the joint had the coarsest microstructure and lowest hardness, but exhibited the best microstructural uniformity due to the higher energy input promoting more homogeneous solidification.

Process and Standards Analysis

4Cr5Mo2V steel is a high-temperature alloy steel widely used in power plant boiler components, turbine parts, and hot-section piping. Its microalloyed composition (Cr, Mo, V) provides excellent creep resistance and high-temperature strength, but also makes it susceptible to weld cracking and HAZ softening during welding.

Welding Challenges for 4Cr5Mo2V Steel

Challenge	Mechanism	Mitigation Strategy
HAZ softening	Excessive tempering at high heat input	Minimize heat input; use low current, high travel speed
Cracking susceptibility	High carbon equivalent; martensitic transformation	Preheat; control cooling rate; post-weld heat treatment
Microstructural inhomogeneity	Non-uniform thermal cycles across HAZ	Optimize current to balance refinement and uniformity
Residual stress	Thermal contraction during cooling	Stress relief treatment; controlled welding sequence

Current Selection Guidelines

Based on the study findings, the following guidelines emerge for TIG welding of 4Cr5Mo2V steel:

Low current (70 A): Suitable when maximum hardness and fine microstructure are required, but the wide HAZ may be unacceptable in thick sections where distortion control is critical.
Medium current (130 A): Recommended for general applications where a balance between hardness, microstructural uniformity, and HAZ control is needed. This appears to be the optimal parameter for most engineering applications.
High current (190 A): May be used when microstructural uniformity is prioritized over hardness, such as in post-weld heat treated components where the initial hardness is less critical.

Engineering Practice Integration

Application to Pipeline Components

4Cr5Mo2V steel is used in high-temperature pipeline applications, including superheater tubes, reheater tubes, and steam lines in power plants. The welding quality of these components directly impacts the safety and reliability of the entire system. The findings of this study have direct implications for:

Welding procedure development: The optimal current of 130 A for 4Cr5Mo2V steel should be incorporated into welding procedure specifications (WPS) for pipeline fabrication.
Non-destructive testing (NDT): The microstructural differences between current levels may affect NDT signal characteristics, requiring calibration of UT and MT equipment for each welding condition.
Post-weld heat treatment (PWHT): The study indicates that higher currents produce coarser microstructures, which may require more aggressive PWHT cycles to achieve the required mechanical properties.

Study Insights and Reflections

This study provides valuable quantitative data on the relationship between welding current and microstructural evolution in a high-temperature alloy steel. The finding that 130 A represents an optimal balance is particularly useful for engineering practice, as it provides a clear target for welding procedure optimization. However, the study is limited in scope—it does not investigate the combined effect of current and travel speed, nor does it address the impact of welding position or joint geometry. Future work should extend these findings to multi-pass welding, where the interpass temperature and heat input accumulation can significantly alter the microstructural outcomes observed in single-pass welding. For engineers responsible for pipeline fabrication, this study reinforces the principle that welding current is not merely a process parameter but a critical variable that directly governs the metallurgical integrity of the final product.