CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Heat Input on Microstructure and Mechanical Properties of 2205 Duplex Stainless Steel MIG Welded Joints
Literature Overview
This comprehensive study by Wang Jianjun, Wen Yanhui, Wu Tianhai, Li Guoping, Jiang Zhouhua, and Liu Chunming, published in the Journal of Materials and Metallurgy in 2016, investigates the influence of welding heat input on the microstructure evolution and mechanical properties of 2205 duplex stainless steel (DSS) MIG welded joints. The research was supported by the National Science and Technology Support Program (2012BAE04B01) and conducted collaboratively between Northeast University and Taiyuan Stainless Steel Co., Ltd. Duplex stainless steels have gained widespread adoption in oil and gas, chemical processing, and marine engineering due to their exceptional combination of strength, corrosion resistance, and resistance to chloride-induced stress corrosion cracking.
Core Technical Points
The 2205 duplex stainless steel contains approximately 22% Cr, 5% Ni, and 3% Mo, with a dual-phase microstructure consisting of austenite (γ) and ferrite (α) in roughly equal proportions. The ferrite equivalent number (FEN) and ferrite percentage are critical parameters that determine the balance between mechanical strength and corrosion resistance. During welding, the thermal cycle causes significant phase transformations that can shift the austenite-ferrite balance away from the optimal 40-60% range.
The study systematically examines the effects of varying heat input on:
- Phase composition and ferrite content in the weld metal and HAZ
- Grain size evolution and grain boundary character
- Mechanical properties including tensile strength, yield strength, and elongation
- Corrosion resistance as assessed by potentiodynamic polarization and intergranular corrosion tests
- The formation of detrimental phases such as σ-phase and χ-phase under excessive heat input conditions
Heat Input Effects and Phase Transformations
The following table presents the relationship between heat input and key metallurgical outcomes:
| Heat Input (kJ/mm) | Ferrite % (Weld) | Ferrite % (HAZ) | UTS (MPa) | Elongation (%) | Hardness (HV) |
|---|---|---|---|---|---|
| 0.8-1.2 | 55-60 | 45-50 | 620-650 | 32-35 | 240-260 |
| 1.2-1.8 | 50-55 | 40-45 | 600-630 | 35-38 | 230-250 |
| 1.8-2.5 | 40-50 | 35-40 | 580-610 | 38-42 | 220-240 |
| 2.5-3.5 | 25-40 | 30-35 | 550-590 | 40-45 | 210-230 |
| >3.5 | 15-25 | 25-30 | 520-560 | 42-48 | 200-220 |
The research demonstrates that at low heat inputs (below 1.0 kJ/mm), the weld metal tends to be ferrite-rich due to rapid cooling rates that suppress austenite formation. Conversely, at high heat inputs (above 2.5 kJ/mm), the prolonged time in the temperature range of 1200-800°C promotes austenite stabilization, potentially leading to excessive austenite content that compromises resistance to stress corrosion cracking.
Process Optimization and Standards Compliance
For 2205 DSS welding in accordance with NB/T 47014 and ASME IX qualification requirements, the following process parameters represent the optimal window:
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Heat input | 1.2-2.0 kJ/mm | Maintains 40-60% ferrite |
| Wire composition | ER319L or equivalent | Low carbon, balanced alloy |
| Shielding gas | 100% Ar or Ar + 1-2% O2 | Clean arc, minimal porosity |
| Interpass temperature | <150 °C | Prevents σ-phase precipitation |
| Preheat | Not required (<50 °C) | Avoids excessive austenitization |
| Travel speed | 300-500 mm/min | Controls heat input |
| Current | 180-230 A | Stable spray transfer |
A critical finding of this study is that the HAZ represents the weakest link in the welded joint, particularly regarding corrosion resistance. The sensitization of grain boundaries in the HAZ due to chromium carbide precipitation during the thermal cycle can significantly reduce pitting resistance. The study recommends limiting the time above 600°C to less than 5 minutes per pass to minimize sensitization effects.
Engineering Practice and Quality Control
For pressure vessel fabrication involving 2205 DSS, the following quality control measures are essential:
- Ferrite number measurement on macroetched cross-sections after welding to verify phase balance.
- Intergranular corrosion testing (ASTM A263 Practice 3) on both weld metal and HAZ samples.
- Hydrogen-induced cracking (HIC) and sulfide stress corrosion (SSC) resistance testing per NACE MR0175/ISO 15156 for sour service applications.
- Mechanical property testing including tensile, impact, and hardness surveys across the weld cross-section.
The study's findings have direct implications for the fabrication of hydrogenation reactors and heat exchangers where 2205 DSS is commonly specified for its superior resistance to chloride stress corrosion cracking compared to 316L austenitic stainless steels. The optimal heat input window identified provides a clear process control target for weld procedure qualification.
Study Insights and Reflections
This research provides a valuable quantitative framework for understanding the heat input-phase transformation relationship in duplex stainless steel welding. The systematic approach of correlating thermal parameters with microstructural and property outcomes offers a transferable methodology for process development. The practical significance for pressure vessel engineers is substantial, as the identification of the critical heat input range (1.2-2.0 kJ/mm) directly informs weld procedure specification and qualification testing. The collaborative nature of this research, bridging academic investigation with industrial application at Taiyuan Stainless Steel, exemplifies the ideal approach to translating fundamental metallurgical understanding into manufacturing capability.