CLADDING TECHNOLOGY SHANXI CO., LTDTIG Autogenous Welding Process for Martensitic and Austenitic Stainless Steels
Literature Overview
This 2008 study by Lian Jun, Zhang Yongsheng, Song Haijiang, and Lv Xiaochun from Jiamusi Electric Machinery Co., Ltd., Tangshan Thermal Power Company, and the Harbin Welding Research Institute investigates the TIG autogenous welding of dissimilar stainless steel joints combining martensitic and austenitic grades. The research addresses a practical engineering challenge encountered in thermal power equipment repair, where replacement components may not match the original material specification exactly, necessitating welding of dissimilar stainless steels.
The study focuses on the metallurgical compatibility, weldability, and mechanical performance of autogenous TIG welds between martensitic stainless steels (such as 410, 420, and 431) and austenitic stainless steels (such as 304, 316, and 321), providing valuable guidance for engineers facing similar challenges in power plant maintenance.
Core Technical Content
The welding of dissimilar stainless steels presents unique metallurgical challenges that must be carefully managed to ensure adequate mechanical properties and corrosion resistance. The key considerations include:
| Consideration | Martensitic Steel | Austenitic Steel | Dissimilar Weld Concern |
|---|---|---|---|
| Heat treatment response | Hardenable by quenching | Non-hardenable | Differential thermal expansion |
| Thermal expansion coefficient | Lower (11-13 μm/m·K) | Higher (16-18 μm/m·K) | Residual stress from differential contraction |
| Corrosion resistance | Moderate (Cr 12-18%) | Excellent (Cr 18-25%) | Potential for galvanic coupling |
| Dilution effects | Low dilution from austenitic side | High dilution from martensitic side | Composition gradient in weld metal |
The TIG autogenous welding process (without filler metal) for dissimilar stainless steels requires careful parameter selection to minimize dilution effects and control the weld metal composition. The typical parameters investigated in the study include:
- Welding current: 80-150 A for typical thicknesses of 3-8 mm
- Travel speed: 150-350 mm/min depending on thickness and joint geometry
- Shielding gas: 100% argon or argon with small amounts of helium (10-20%)
- AC/DC selection: DCEN for pure tungsten electrodes, AC for oxide removal
- Electrode diameter: 2.4-3.2 mm depending on current range
The study examines the weld metal microstructure and composition as a function of dilution ratio, which is determined by the relative thermal properties of the two base metals. Austenitic steels, having higher thermal conductivity and thermal expansion, tend to absorb more heat and contribute more base metal to the weld pool, resulting in higher dilution from the austenitic side.
Microstructural Analysis and Mechanical Properties
The weld metal microstructure in autogenous TIG welds of dissimilar stainless steels exhibits a complex gradient of phases, transitioning from martensitic structure near the martensitic base metal to austenitic structure near the austenitic base metal. The study identifies several critical microstructural features:
- Weld metal composition: The weld metal composition is determined by the dilution ratio, which typically ranges from 30-60% austenitic base metal dilution. The resulting weld metal may contain a mixture of ferrite, austenite, and martensite phases, depending on the chromium and nickel equivalent.
- HAZ microstructure: The HAZ in martensitic steel may undergo tempering or transformation to martensite, depending on the peak temperature and cooling rate. The HAZ in austenitic steel typically experiences grain growth and possible sensitization if the temperature exceeds 450°C for extended periods.
- Phase distribution: The weld metal may exhibit a delta-ferrite/austenite mixture, with the ferrite content determined by the chromium and nickel equivalents. For optimal mechanical properties, the ferrite content should be controlled within a range of 5-20%.
The mechanical properties of the dissimilar weld are influenced by the composition gradient and phase distribution. The study presents typical hardness profiles across the weld zone:
| Location | Typical Hardness (HV) | Notes |
|---|---|---|
| Martensitic base metal | 250-350 HV | Depends on heat treatment condition |
| Martensitic HAZ | 200-300 HV | May be tempered or re-martensitized |
| Weld center | 180-250 HV | Austenitic or duplex structure |
| Austenitic HAZ | 150-200 HV | Grain growth may reduce hardness |
| Austenitic base metal | 150-200 HV | Solution treated condition |
The tensile strength of the dissimilar weld is typically governed by the weaker side (austenitic), but the ductility may be compromised by the presence of hard martensitic phases in the weld metal. The study emphasizes the importance of post-weld heat treatment to relieve residual stresses and improve ductility.
Engineering Practice and Quality Control
For practical applications in thermal power equipment, the following recommendations emerge from the study:
- Welding sequence: Weld from the austenitic side to the martensitic side to minimize the dilution of hard martensitic phases into the weld metal.
- Heat input control: Maintain moderate heat input (8-15 kJ/mm) to ensure adequate penetration while minimizing HAZ grain growth and sensitization.
- Post-weld treatment: Apply stress relief annealing at 550-650°C for 1-2 hours to relieve residual stresses and improve ductility.
- Corrosion testing: Perform intergranular corrosion testing (ASTM A262 Practice E or F) on the weld and HAZ to verify resistance to sensitization-induced corrosion.
- Mechanical testing: Conduct hardness surveys across the weld zone and perform tensile testing of transverse and longitudinal coupons to verify mechanical properties.
The study also addresses the issue of galvanic coupling between the two dissimilar stainless steels in corrosive environments. While both martensitic and austenitic steels are corrosion-resistant, the potential for galvanic corrosion must be evaluated, particularly in chloride-containing environments. The use of a consumable insert (such as a 309L or 310L stainless steel plate) at the weld interface can help to buffer the composition gradient and improve corrosion resistance.
Study Insights and Implications
This research addresses a practical and important engineering challenge in power plant maintenance, where the availability of exact replacement materials may be limited and dissimilar steel welding becomes necessary. The study provides valuable guidance on process parameter selection, microstructural control, and quality assurance procedures for dissimilar stainless steel TIG welding.
The key insight from this research is that autogenous welding of dissimilar stainless steels is feasible with careful parameter control, but the resulting weld metal composition and microstructure must be carefully managed to ensure adequate mechanical properties and corrosion resistance. The dilution ratio, which is determined by the relative thermal properties of the two base metals, is the critical factor governing weld metal composition and must be predicted and controlled during procedure development.
For contemporary practice, this research remains highly relevant as the power industry continues to face challenges with aging infrastructure and the need for cost-effective repair solutions. The principles established in this study—regarding dilution control, phase balance, and post-weld treatment—continue to guide engineering practice for dissimilar stainless steel welding. As the industry moves toward more stringent environmental regulations and longer equipment service lives, the ability to reliably weld dissimilar stainless steels becomes increasingly important for extending the service life of critical power generation equipment.