Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of TIG Welded Joints in Hot-Rolled 410S Stainless Steel Plate
Literature Overview
The paper by Yang Ruicheng, Meng Wei, Shu Jun, and Hu Tianlei, published in Materials for Mechanical Engineering (Vol. 33, No. 11, 2009, pp. 72–75), examines the microstructure and performance of TIG-welded joints in hot-rolled 410S martensitic stainless steel plate. 410S is a high-strength martensitic stainless steel specified by ASTM A240, with a carbon content of 0.15–0.30% and chromium content of 11.5–13.5%. It is commonly used in applications requiring elevated strength, moderate corrosion resistance, and good machinability, such as valve bodies, pump shafts, and fasteners. The study was supported by the Gansu Provincial Key Laboratory of Nonferrous Metal New Materials.
Core Technical Findings
Microstructural Analysis
The welded joint exhibits a complex microstructural evolution driven by the austenitizing and subsequent martensitic transformation during the welding thermal cycle:
Weld Metal: The weld zone displays a coarse columnar dendritic solidification structure with pronounced epitaxial growth from the fusion boundary. The primary phase is columnar austenite, with interdendritic δ-ferrite distributed along the columnar grain boundaries. The δ-ferrite volume fraction is measured at 15%–20%, which is within the recommended range for preventing hot cracking in stainless steel welds. The δ-ferrite forms due to the Cr-depletion of the solidifying austenite at the dendrite tips, shifting the local composition into the austenite-ferrite two-phase region.
Heat-Affected Zone (HAZ): The HAZ undergoes a complete M→γ→M transformation cycle. The original martensite in the base metal dissolves into austenite during the austenitizing stage of the thermal cycle and re-transforms to martensite during cooling. A critical observation is that the fine, granular martensite originally distributed in grain boundary bands is replaced by coarse, blocky martensite in the HAZ. This transformation results in a coarser microstructure with potentially higher hardness and reduced toughness.
| Region | Primary Phases | δ-Ferrite Content | Microstructural Feature |
|---|---|---|---|
| Base Metal | Martensite + granular M in grain boundary bands | N/A | Fine, tempered martensite |
| HAZ | Blocky martensite | N/A | Coarse, untempered martensite |
| Weld Metal | Columnar austenite + interdendritic δ-ferrite | 15%–20% | Coarse dendritic structure |
Mechanical Properties
The welded joint demonstrates tensile strength equal to or exceeding that of the base metal, with good plasticity. This is somewhat unusual for martensitic stainless steel welds, where the HAZ often exhibits reduced toughness due to the formation of untempered martensite. The adequate ductility suggests that the welding parameters were selected to limit the extent of the high-temperature HAZ, or that the base metal composition provides sufficient toughness even after the M→γ→M transformation.
Intergranular Corrosion Resistance
A particularly important finding is the intergranular corrosion susceptibility ranking: HAZ > base metal > weld metal. The HAZ exhibits the highest susceptibility due to the sensitization of the coarse grain boundaries during the austenitizing stage. Chromium carbides (primarily Cr₂₃C₆) precipitate at grain boundaries when the material is held in the sensitization temperature range (approximately 500–800°C), depleting the adjacent matrix of chromium and rendering it susceptible to intergranular attack. The weld metal, with its relatively higher δ-ferrite content, shows the lowest susceptibility, as δ-ferrite is generally more resistant to intergranular corrosion than austenite.
Engineering Practice Implications
Welding Procedure Optimization
For 410S stainless steel welding, the following practices are recommended based on the findings of this study:
- Heat input control: Maintain low to moderate heat input to limit the extent of the sensitization-affected HAZ. A heat input range of 5–12 kJ/mm is typically appropriate for plate thicknesses of 5–20 mm.
- Interpass temperature: Control interpass temperature below 200°C to avoid prolonged exposure to the sensitization range.
- Preheating: Avoid preheating, as it increases the time spent in the sensitization temperature range. If preheating is necessary for thick sections, keep it below 100°C.
- Post-weld heat treatment: Stress-relief annealing at 650–750°C for 1–2 hours can temper the martensite and reduce residual stresses. However, this temperature range overlaps with the sensitization range, so the treatment must be carefully controlled to avoid excessive sensitization. A solution treatment at 1050–1100°C followed by rapid quenching and subsequent tempering may be more effective for critical applications.
Inspection and Quality Control
- Visual and dimensional inspection: Ensure proper weld profile and absence of surface defects.
- Magnetic particle testing (MT): Essential for detecting surface and near-surface cracks in the ferritic/martensitic structure.
- Ultrasonic testing (UT): Recommended for detecting internal defects such as lack of fusion and porosity.
- Microstructural examination: Cross-sectional metallographic examination of the HAZ is critical for assessing grain coarsening and intergranular carbide precipitation.
- Intergranular corrosion testing: ASTM A262 Practice A (65% boiling HNO₃) or Practice E (oxalic acid) should be performed on the HAZ to confirm acceptable corrosion resistance.
Key Questions and Reflections
The study raises an important question about the trade-off between strength and corrosion resistance in martensitic stainless steel welds. The HAZ, which undergoes the most severe microstructural changes, is also the most susceptible to intergranular corrosion. This suggests that optimizing the welding process to reduce HAZ damage may require accepting a slight reduction in strength. Additionally, the study does not address the long-term creep behavior of the welded joint at elevated temperatures, which is relevant for applications such as steam turbine components where 410S may be used.
Summary
This study provides a comprehensive characterization of TIG-welded 410S stainless steel joints, revealing the complex interplay between microstructural evolution, mechanical properties, and corrosion resistance. The identification of the HAZ as the critical region—both for mechanical performance and intergranular corrosion susceptibility—underscores the importance of careful heat input control and appropriate post-weld heat treatment. For engineers working with martensitic stainless steels, this work reinforces the need for a holistic approach to welding quality that considers not only strength and toughness but also long-term corrosion performance.