CLADDING TECHNOLOGY SHANXI CO., LTDKey Points and Precautions for Stainless Steel TIG Welding
Literature Overview
This 2006 publication in Hot Working Technology provides a practical guide to stainless steel TIG welding, covering material selection, process parameters, defect prevention, and quality assurance measures. While the paper does not focus specifically on cladding or bimetal applications, its content is foundational for understanding the metallurgical behavior of stainless steels during TIG welding, which is directly relevant to overlay welding and clad plate fabrication.
Core Technical Content
Stainless steel TIG welding presents unique challenges compared to carbon steel welding, primarily related to chromium depletion in the heat-affected zone (HAZ), sensitization leading to intergranular corrosion, and the high susceptibility to hot cracking in austenitic grades. The paper systematically addresses these issues through proper consumable selection, heat input control, and post-weld treatment.
Critical Process Parameters for Common Stainless Steel Grades
| Grade | Welding Current (A) | Travel Speed (mm/min) | Shielding Gas | Preheat (deg C) | Post-Weld Treatment |
|---|---|---|---|---|---|
| 304 | 80–160 | 100–250 | 100% Ar or Ar/2% He | Not required | Solution anneal if sensitized |
| 316 | 80–160 | 100–250 | 100% Ar or Ar/2% He | Not required | Solution anneal if sensitized |
| 321 | 100–180 | 80–200 | 100% Ar | 150–250 | Stabilization anneal at 870–900 deg C |
| 347 | 100–180 | 80–200 | 100% Ar | 150–250 | Stabilization anneal at 870–900 deg C |
Key Technical Points
1. Consumable Selection and Compatibility
The paper emphasizes the importance of matching the filler metal to the base material. For 304 stainless steel, ER308L or ER308 is typically used, while ER316L is specified for 316 grades. The low-carbon variants (L grades) are preferred to minimize sensitization risk. For overlay applications, this principle extends to selecting overlay alloys that maintain the required corrosion resistance despite dilution from the base metal.
2. Heat Input Control
Excessive heat input in austenitic stainless steels promotes chromium carbide precipitation at grain boundaries (sensitization), reducing corrosion resistance. The paper recommends keeping the heat input below 25 kJ/mm for 304 and 316 grades. In overlay welding, this translates to controlling the interpass temperature below 150 deg C and using lower amperage with higher travel speed to minimize the time spent in the sensitization range of 450–850 deg C.
3. Back Purging and Shielding
The paper stresses the critical importance of back purging with argon to prevent oxidation of the root side. For overlay welding on clad plates, this principle extends to protecting the exposed clad surface during adjacent weld operations. Incomplete back purging results in a scale layer that can act as a source of inclusions and reduce the effective thickness of the corrosion-resistant layer.
Defect Analysis and Countermeasures
| Defect Type | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Intergranular corrosion (IGC) | Sensitization in HAZ | Intergranular corrosion test (ASTM A923 Practice E) | Use L-grade filler; control heat input; post-weld stabilization anneal |
| Hot cracking | Sulfur/phosphorus segregation; low ductility trough | Visual inspection; RT | Add Ni to filler; control travel speed; use pulsed TIG |
| Tungsten inclusion | Arc wandering; electrode contamination | RT; MT | Dress tungsten regularly; maintain proper stick-out; use AC for aluminum |
| Porosity | Contaminated surface; inadequate shielding | RT; UT | Thorough surface cleaning; verify gas flow; use back purging |
| Excessive dilution | High heat input; too many passes | Metallographic analysis; hardness mapping | Reduce amperage; increase travel speed; use cold wire feeding |
Application to Cladding and Bimetal Products
In the context of bimetal manufacturing, the stainless steel TIG welding principles described in this paper are directly applicable to several critical operations:
1. Clad Plate Edge Welding: When fabricating vessels from clad plate, the edges of the clad layers must be welded to prevent corrosion attack at the exposed interface. The paper's guidance on heat input control and filler selection is essential for maintaining the integrity of the cladding layer at these edge welds.
2. Overlay Welding Cap Passes: In multi-pass weld overlay procedures, the final cap pass is often executed by TIG to achieve a smooth, low-dilution surface. The paper's emphasis on precise arc control and gas shielding is directly relevant to producing a high-quality cap layer.
3. Repair Welding: Post-fabrication repairs on clad surfaces require careful TIG technique to avoid damaging the underlying cladding. The paper's discussion of low heat input techniques and proper consumable selection provides a practical framework for repair procedures.
Study Insights and Practical Implications
The 2006 publication reflects a mature understanding of stainless steel welding technology in China. The systematic approach to defect prevention—combining proper consumable selection, process parameter optimization, and rigorous inspection—aligns well with modern quality management practices in pressure vessel fabrication.
One particularly valuable insight from the paper is the emphasis on operator skill as a critical factor in TIG welding quality. Unlike fully automated processes, TIG welding requires skilled manipulation of the torch and filler rod. In overlay welding applications, where dilution control is paramount, operator skill directly impacts the corrosion resistance of the final product. This underscores the importance of comprehensive welder qualification programs per NB/T 47014, which should include specific qualification tests for overlay welding procedures.
The paper also highlights the importance of material traceability. For bimetal products, the chemical composition of both the base plate and the cladding layer must be verified before welding begins. Any deviation from the specified composition can significantly affect the welding behavior and the final corrosion resistance of the product.
In summary, this practical guide serves as an essential reference for engineers and welders involved in stainless steel TIG welding operations, whether for structural welding, overlay cladding, or repair work on bimetal pressure vessels. The principles of heat input control, consumable compatibility, and rigorous quality assurance remain as relevant today as they were at the time of publication.