Welding Current Effects on TIG Welded 12Cr18Mn8Ni5N Austenitic Stainless Steel Joints

Literature Overview

This study, published in Thermal Processing Technology in 2015 by Zheng Nansong and colleagues from AVIC South Aviation Industry and Nanchang Hangkong University, investigates the effects of welding current on the microstructure and mechanical properties of TIG welded joints of 12Cr18Mn8Ni5N austenitic stainless steel. The research addresses a practical engineering challenge in the fabrication of aircraft and aerospace components, where the selection of welding parameters is critical to achieving the required mechanical performance and corrosion resistance. The material 12Cr18Mn8Ni5N is a manganese-nitrogen strengthened austenitic stainless steel, which offers improved strength and reduced density compared to conventional nickel-based austenitic stainless steels.

Material Characteristics and Welding Considerations

12Cr18Mn8Ni5N is a precipitation-strengthened austenitic stainless steel that contains approximately 18% Cr, 8% Mn, 5% Ni, and 0.15–0.25% N. The manganese and nitrogen content provide solid solution strengthening, while the chromium content ensures adequate corrosion resistance. The material exhibits a fully austenitic microstructure in the solution-treated condition, with a yield strength of approximately 450–550 MPa and a tensile strength of 600–750 MPa.

The welding of 12Cr18Mn8Ni5N presents several challenges, including the risk of solidification cracking, sensitization, and the formation of intermetallic phases in the HAZ. The high manganese content can promote the formation of MnS inclusions, which act as crack initiation sites during solidification. The nitrogen content can also affect the weld metal composition and the formation of nitrides, which can influence the mechanical properties and corrosion resistance of the welded joint.

Welding Current and Process Parameter Optimization

The study systematically varies the TIG welding current from 100 A to 250 A, with other parameters held constant, to investigate the effect of current on the weld bead geometry, microstructure, and mechanical properties. The welding current is a primary control variable in TIG welding, as it directly determines the heat input, penetration depth, and dilution ratio. Higher currents produce deeper penetration and wider beads, but can also increase the risk of burn-through and excessive dilution.

The results demonstrate that the welding current has a significant effect on the weld bead geometry, with higher currents producing deeper and wider beads. The dilution ratio also increases with current, as the higher heat input melts more base metal into the weld pool. This increased dilution affects the weld metal composition, particularly the chromium and manganese content, which can influence the microstructure and mechanical properties of the welded joint.

Microstructural Analysis and Phase Evolution

The microstructural analysis of the TIG welded joints reveals distinct differences between the weld metal, HAZ, and base metal. In the weld metal, the microstructure is characterized by columnar dendrites with interdendritic segregation of Mn and S, which can promote the formation of MnS inclusions. The cooling rate in the weld metal is typically 10–50 K/s, which is fast enough to produce a fine grain structure but not fast enough to completely suppress the formation of intermetallic phases.

In the HAZ, the microstructure undergoes a thermal transformation, with the region closest to the weld experiencing temperatures above the austenitization temperature. This region undergoes complete austenitization followed by rapid cooling, resulting in a fine grain structure with some retained austenite. The region farther from the weld experiences lower peak temperatures, resulting in partial austenitization and a mixed microstructure of austenite and ferrite.

The mechanical properties of the welded joints are strongly influenced by the welding current. Lower currents produce welds with higher hardness and strength but lower ductility, while higher currents produce welds with lower hardness and strength but higher ductility. The optimal welding current is determined by the balance between strength and ductility required by the application, as well as the need to minimize distortion and residual stresses.

Mechanical Properties and Performance Evaluation

The tensile testing results show that the welded joints exhibit a U-shaped curve of strength versus welding current, with the lowest strength occurring at intermediate currents. This behavior is attributed to the interaction between the dilution ratio and the microstructure, where intermediate currents produce a weld metal composition and microstructure that is suboptimal for strength. The hardness profile across the cross-section reveals a gradient from the weld metal to the base metal, with the HAZ exhibiting the highest hardness due to the formation of fine grain structures and retained austenite.

The impact testing results demonstrate that the welded joints exhibit good toughness across the range of welding currents, with the Charpy V-notch impact energy remaining above 50 J at -40 °C. This indicates that the material retains its low-temperature toughness after welding, which is critical for aerospace applications where the component may be exposed to cryogenic temperatures.