CLADDING TECHNOLOGY SHANXI CO., LTDTIG Autogenous Welding Process for Martensitic and Austenitic Stainless Steel Joints
Literature Overview
This 2008 publication by Lian Jun, Zhang Yongsheng, Song Haijiang, and Lv Xiaochun from Jiamusi Electric Motor Co., Ltd., Tangshan Thermal Power Company, and the Harbin Welding Research Institute of the Mechanical Science Research Institute addresses the challenging problem of joining dissimilar stainless steels — specifically martensitic and austenitic grades — using autogenous TIG welding without filler metal. The application context is the repair and fabrication of components in thermal power stations where mixed stainless steel grades are commonly encountered in heat exchangers, turbine casings, and boiler components.
Core Technical Points
Dissimilar steel welding presents unique metallurgical challenges that are compounded when autogenous welding (no filler metal) is employed. The differing thermal expansion coefficients, melting temperatures, and solidification behaviors of martensitic and austenitic stainless steels lead to significant residual stresses, distortion, and potential cracking in the weld zone.
| Parameter | Martensitic Steel (e.g., 410, 420) | Austenitic Steel (e.g., 304, 316) | Dissimilar Joint Consideration |
|---|---|---|---|
| Thermal expansion coefficient | 12–13 × 10⁻⁶/K | 16–18 × 10⁻⁶/K | Differential expansion causes residual stress |
| Melting temperature | 1425–1450°C | 1400–1420°C | Lower melting side melts first |
| Thermal conductivity | 15–25 W/m·K | 14–20 W/m·K | Relatively similar, moderate thermal gradient |
| Solidification mode | Dendritic, ferrite-austenite | Dendritic, austenite-ferrite | Mixed solidification in weld zone |
| Susceptibility to cracking | High (cold cracking) | Low (hot cracking possible) | Cracking risk concentrated at martensitic side |
Process Analysis and Defect Control
The study identified several critical process parameters and their effects on weld quality. The welding current for autogenous TIG welding of dissimilar stainless steels typically ranges from 120–250 A depending on plate thickness, with a travel speed of 100–250 mm/min. The arc length must be maintained at 2–3 mm to ensure stable arc behavior and minimize spatter. The shielding gas flow rate should be 15–20 L/min with a trailing shield to protect the cooling weld from atmospheric oxidation.
The primary defect risks in this application are:
- Hot cracking in the weld centerline: The centerline of an autogenous dissimilar steel weld has a composition that is a mixture of the two base metals, which may fall within a cracking-prone composition range. The addition of ferrite to the weld metal is beneficial for preventing hot cracking, and the study notes that the weld centerline in martensitic-austenitic joints naturally contains a significant ferrite fraction due to the dilution of austenitic stabilizers (Ni, Mn) from the austenitic side.
- Cold cracking at the martensitic side: The martensitic steel side is susceptible to hydrogen-induced cold cracking, particularly if the base metal contains carbon above 0.2%. The study recommends preheating to 150–250°C and post-weld cooling control to reduce the cooling rate below 100°C/s in the martensitic side.
- Excessive residual stress and distortion: The differential thermal expansion between the two steel grades creates significant residual stresses at the weld zone. The study recommends post-weld stress relief annealing at 600–650°C for 1–2 hours to reduce residual stresses to acceptable levels.
Engineering Practice Integration
In the context of thermal power station components, the dissimilar steel TIG welding process described in this study is directly applicable to the repair of heat exchanger tubesheets, turbine casing welds, and boiler header connections where martensitic and austenitic stainless steels are joined. The autogenous welding approach eliminates the need for expensive filler metals and reduces the risk of filler metal-related defects such as porosity and lack of fusion. However, the process requires careful control of heat input and post-weld heat treatment to ensure acceptable mechanical properties and corrosion resistance.
For engineers working on bimetal pressure vessel fabrication, the principles established in this study extend to the welding of clad plate joints where the cladding layer (typically austenitic stainless steel) is joined to the base plate (typically carbon steel or martensitic steel). The same metallurgical challenges — differential thermal expansion, mixed solidification, and residual stress — must be addressed through careful process parameter selection and post-weld treatment.
Study Insights
This study demonstrates that autogenous TIG welding of dissimilar stainless steels is technically feasible with appropriate process control, but it requires a thorough understanding of the metallurgical interactions between the two base metals. The approach is particularly attractive for repair welding applications where matching the base metal composition is desirable and where the use of filler metal could introduce additional metallurgical complications. The study's emphasis on post-weld heat treatment as a critical step in achieving acceptable weld properties is a reminder that the welding process is only one part of the overall quality assurance strategy for dissimilar steel joints.