Study on Microstructure and Properties of TIG Overlay Welding 308L Stainless Steel on Q235 Steel

Literature Overview

This research examines the microstructure and mechanical properties of a weld overlay joint formed by gas tungsten arc welding of 308L austenitic stainless steel onto Q235 carbon steel. The study is relevant to engineers designing and fabricating bimetallic components where a corrosion-resistant stainless steel surface layer is required on a low-cost carbon steel substrate. The authors investigate the effect of welding parameters on dilution, microstructure, hardness distribution, and corrosion resistance of the overlay, providing practical guidance for weld procedure development and quality assurance.

Welding Process and Parameters

The overlay is deposited using gas tungsten arc welding with a 308L stainless steel filler wire of 2.4 mm diameter. The welding parameters are optimized to minimize dilution while maintaining adequate fusion and penetration. The study evaluates three parameter sets: low heat input, medium heat input, and high heat input, with corresponding variations in current, voltage, and travel speed.

The low heat input condition produces the lowest dilution rate and the best corrosion resistance, but the weld metal is more susceptible to lack of fusion and porosity due to the rapid cooling rate. The high heat input condition produces the highest dilution rate and the poorest corrosion resistance, but the weld metal has better fusion and lower porosity. The medium heat input condition provides the best balance of dilution control, fusion quality, and mechanical properties.

Microstructural and Property Analysis

The microstructure of the overlay metal varies significantly with dilution rate. At low dilution below 20 percent, the weld metal is predominantly austenitic with less than 5 percent delta ferrite, providing excellent corrosion resistance and ductility. At medium dilution around 25 percent, the ferrite fraction increases to 10 to 15 percent, and the hardness increases due to the higher carbon and manganese content from the base metal. At high dilution above 30 percent, the ferrite fraction exceeds 20 percent, and the weld metal becomes susceptible to cracking and corrosion.

The hardness distribution across the overlay shows a gradient from the fusion boundary to the weld surface. At the fusion boundary, the hardness is approximately 220 HV due to the high dilution and the formation of martensite in the heat-affected zone. At the weld surface, the hardness decreases to 160 to 180 HV depending on the dilution rate. The tensile strength of the overlay metal ranges from 550 to 650 MPa, with elongation decreasing from 35 to 20 percent as dilution increases. The intergranular corrosion test performed according to ASTM A262 Practice E shows that the low dilution condition passes the test without any evidence of intergranular corrosion, while the high dilution condition shows severe intergranular attack.

Engineering Practice and Recommendations

For engineers specifying TIG overlay welding of 308L on Q235 steel, the key recommendation is to limit the dilution rate to below 25 percent to ensure adequate corrosion resistance. This can be achieved by using low heat input parameters, multiple thin passes, and a filler wire with slightly higher chromium and nickel content than the standard 308L composition. The preheat temperature should be kept below 100 degrees Celsius to minimize dilution and reduce the risk of cracking in the carbon steel base. The interpass temperature should be maintained below 150 degrees Celsius to prevent grain growth and maintain fine microstructure in the overlay.

The study also highlights the importance of proper surface preparation and cleaning before welding. Any oxide, grease, or contamination on the base metal surface can lead to porosity and reduced fusion. The welding area should be cleaned with a wire brush and solvent, and the workpiece should be preheated uniformly to reduce thermal shock and minimize residual stress. Post-weld heat treatment at 1050 degrees Celsius followed by water quenching can homogenize the microstructure and reduce residual stresses, but must be carefully controlled to avoid sensitization of the stainless steel phase.

Conclusion

This study provides comprehensive data on the microstructure and properties of TIG overlay welding 308L stainless steel on Q235 carbon steel. The findings demonstrate that the dilution rate is the primary factor controlling the corrosion resistance and mechanical properties of the overlay, and that careful control of welding parameters is essential to achieve the desired performance. Engineers should use the dilution rate data and property correlations presented here as a basis for weld procedure qualification, and should always verify the corrosion resistance of the overlay through appropriate testing before placing the component into service.