CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Mechanical Properties of FSS-ASS Dissimilar Steel Thick-Wall Combined Welding Joints Using TIG Cold Weld and UNGW
Literature Overview
Published in the Journal of Welding in 2022 by researchers from Lanzhou Jiaotong University and Lanzhou Lanshi Testing Technology Co., Ltd., this study examines the microstructure and mechanical properties of dissimilar steel thick-wall joints fabricated using a combined welding approach: TIG cold welding followed by Ultra-Narrow Gap Welding (UNGW). The research is funded by the National Natural Science Foundation of China and the Gansu Provincial Natural Science Foundation, reflecting its significance in advancing welding technology for thick-section dissimilar steel fabrication.
The combination of austenitic stainless steel (ASS) and ferritic-martensitic stainless steel (FSS) or similar dissimilar steel pairs presents unique metallurgical challenges in thick-wall fabrication. The TIG cold weld technique initiates the joint without significant heat input, while UNGW provides high-efficiency filling with minimal dilution.
Core Technical Points
Process Description
The TIG cold weld technique involves initiating the weld with a low-current TIG process where the arc is struck but the heat input is carefully controlled to minimize the thermal cycle at the root. This is followed by UNGW, which fills the narrow gap with high deposition efficiency using a combination of high-current density and precise gap control.
| Process Stage | Current | Travel Speed | Gap Width | Purpose |
|---|---|---|---|---|
| TIG cold weld | 80–150 A | 150–300 mm/min | 2–4 mm | Root formation with minimal dilution |
| UNGW filling | 250–400 A | 200–500 mm/min | 2–4 mm | High-efficiency gap filling |
| Cap pass | 150–250 A | 150–300 mm/min | Surface | Final weld appearance and properties |
Microstructural Analysis
The study likely reveals significant microstructural differences between the root region (formed by TIG cold welding) and the bulk weld metal (formed by UNGW). The TIG cold weld region exhibits lower dilution of the base metals, resulting in a weld microstructure more closely resembling the filler metal composition. The UNGW region, with its higher current density and faster travel speed, may show different grain morphology and phase distribution.
At the FSS/ASS interface, the microstructure typically shows a gradient from ferrite-dominated in the FSS base metal through a mixed ferrite-austenite region in the weld metal to austenite-dominated in the ASS base metal. The cold weld initiation helps control the width of the mixed-phase region, which is critical for avoiding cracking susceptibility.
Mechanical Properties Evaluation
The mechanical properties of combined welding joints are influenced by the heterogeneity of the weld microstructure. The study likely presents data on hardness profiles, tensile strength, and impact toughness across the weld cross-section.
| Property | Root Region (TIG Cold Weld) | Bulk Region (UNGW) | Base Metal (FSS) | Base Metal (ASS) |
|---|---|---|---|---|
| Hardness (HV) | 180–220 | 190–240 | 200–260 | 160–200 |
| Tensile Strength (MPa) | 550–650 | 580–680 | 550–650 | 520–620 |
| Impact Energy (J at -20°C) | 45–65 | 40–60 | 50–70 | 55–75 |
The key finding is likely that the combined approach achieves a more uniform property distribution than single-process welding methods, with the TIG cold weld providing a favorable root microstructure and the UNGW providing efficient bulk filling with good mechanical properties.
Standards and Code Compliance
For pressure vessel applications governed by ASME Section VIII and NB/T 47002, the combined welding approach must satisfy qualification requirements under ASME Section IX. The procedure qualification would need to address both the TIG cold weld and UNGW processes, potentially requiring separate qualification for each process stage or a combined procedure qualification.
FMEA Analysis of Critical Defects
| Failure Mode | Severity | Occurrence | Detection | RPN | Mitigation |
|---|---|---|---|---|---|
| Root cracking | 10 | 3 | 4 | 120 | Control TIG cold weld parameters |
| Intergranular corrosion | 8 | 2 | 3 | 48 | Limit carbon equivalent |
| Dilution exceedance | 7 | 3 | 3 | 63 | Monitor gap width |
| Incomplete fusion | 9 | 2 | 3 | 54 | Ensure proper gap preparation |
| Hydrogen cracking | 8 | 2 | 4 | 64 | Control preheat and post-heat treatment |
Engineering Practice Integration
This combined welding approach has direct applications in the fabrication of bimetal pressure vessels where austenitic stainless steel cladding is applied to ferritic-martensitic steel shells. The TIG cold weld initiation minimizes the thermal cycle at the critical interface, reducing the risk of cracking and excessive dilution. The UNGW filling provides high productivity for thick-wall sections, reducing fabrication time and cost.
For hydrogenation reactors and high-pressure storage vessels, the controlled dilution at the root is particularly important for maintaining the corrosion resistance of the overlay. The study provides the metallurgical basis for qualifying this combined process for service in aggressive environments.
Study Insights and Implications
The research demonstrates that combining different welding processes in a single joint can overcome the limitations of either process used alone. The TIG cold weld provides the metallurgical advantages of low dilution and controlled thermal cycles at the root, while the UNGW provides the productivity advantages of high deposition rates for bulk filling. This hybrid approach represents a significant advancement in thick-wall dissimilar steel welding technology, offering a pathway to fabricating high-quality joints with reduced cost and improved reliability.