SUS316L and 15CrMo Dissimilar Steel TIG Welding with Different Filler Metals

Literature Overview

This 2023 study by Li Yajie, Liu Tao, and colleagues from Jiangsu Institute of Technology, Jiangsu Bete Pipe Fittings Co., Ltd., and the Jiangsu Special Equipment Safety Supervision and Inspection Institute investigates the microstructure and mechanical properties of SUS316L/15CrMo dissimilar steel TIG welds using different filler metals. Funded by the National Natural Science Youth Fund (51601078) and the Jiangsu Provincial Government Overseas Scholarship Program (JS-2019-322), this work was published in Mechanical Engineering Materials. The research addresses a common industrial challenge in the fabrication of heat exchangers, piping systems, and pressure vessels where dissimilar steel joints are unavoidable.

SUS316L is a low-carbon austenitic stainless steel with excellent corrosion resistance, while 15CrMo is a low-alloy steel with good high-temperature strength and creep resistance. The combination of these two materials is common in power generation and petrochemical applications where different sections of a component experience different service conditions.

Core Technical Content and Analysis

Dissimilar Steel Welding Challenges

The SUS316L/15CrMo joint presents several metallurgical challenges:

The coefficient of thermal expansion for 316L (17.3 × 10⁻⁶/°C) is significantly higher than that of 15CrMo (12.5 × 10⁻⁶/°C). This difference creates substantial residual stresses during cooling, which can lead to cracking or distortion.

Filler Metal Selection and Effects

The study examines multiple filler metals to optimize the weld composition and properties. Common options for this dissimilar joint include:

The selection of filler metal directly influences the weld microstructure, mechanical properties, and service life. ER309L is the most commonly recommended filler for this combination due to its high nickel content, which maintains the austenitic structure despite dilution from the ferritic parent metal.

Microstructural Analysis

The weld metal microstructure depends on the dilution ratio and filler metal composition:

• With ER309L: The weld typically shows a mixed structure of austenite (70-85%) and delta ferrite (15-30%). The delta ferrite content is controlled by the nickel equivalent and chromium equivalent of the weld metal.
• With ER316L: Higher dilution from the 15CrMo side can lead to partial martensitic transformation, resulting in reduced ductility.
• With ER347: Similar to ER309L but with Nb stabilization, reducing the risk of intergranular corrosion in the HAZ.

The HAZ on the 316L side shows grain growth and potential sensitization if the heat input is excessive. The HAZ on the 15CrMo side may show tempering of the tempered martensite structure, reducing hardness.

Mechanical Properties

The weld metal typically has higher strength than both parent metals due to solidification strengthening. The elongation is generally lower than the 316L base metal but acceptable for most applications.

Engineering Practice Implications

Welding Procedure Development

For production welding of SUS316L/15CrMo joints, the following procedure elements are critical:

1. Pre-heat: 150-250°C to reduce cooling rate and minimize cracking risk in the 15CrMo HAZ.
2. Interpass temperature: Maintain below 250°C to avoid excessive grain growth in the 316L HAZ.
3. Heat input control: Limit to 1.5-2.5 kJ/mm to balance penetration and HAZ effects.
4. Weld geometry: Use a balanced weld bead to minimize dilution asymmetry.
5. Post-weld treatment: Stress relief at 620°C for 15CrMo side or solution treatment at 1050°C for 316L side, depending on the application requirements.

Code Compliance

For pressure vessel applications, the welding procedure must comply with applicable codes:

The qualification requirements for dissimilar welds are more stringent than for similar metal welds, requiring additional testing of the weld metal and HAZ properties at both parent material interfaces.

Key Reflections and Study Insights

This research provides practical guidance for engineers facing the common challenge of joining austenitic stainless steel to low-alloy steel in industrial applications. The systematic comparison of different filler metals demonstrates that filler metal selection is the most critical variable in achieving acceptable weld properties for dissimilar joints.

The findings reinforce the importance of understanding dilution effects and their impact on weld composition and microstructure. For engineers developing welding procedures for dissimilar steel joints, the key principle is to select a filler metal that produces a weld structure with adequate ductility and resistance to cracking under the expected service conditions.

The collaboration between academia, industry, and inspection authorities in this research exemplifies the best practice for welding technology development. The involvement of a pipe fitting manufacturer and a special equipment inspection institute ensures that the research findings are directly applicable to production and regulatory requirements. This approach provides a model for industry-academia cooperation in solving practical welding challenges.