CLADDING TECHNOLOGY SHANXI CO., LTDHDR Stainless Steel Pipe TIG Welding Process Study
Literature Overview
The paper by Chen Qianqing from Zhejiang International Maritime Vocational and Technical College, published in Welding Technology in 2006, addresses the TIG (GTAW) welding process for HDR (High-Density Resin-lined or High-Performance Duplex) stainless steel pipes. This work is significant in the context of marine and chemical processing applications where stainless steel piping systems demand high corrosion resistance and structural integrity.
Core Technical Content
Material Characteristics of HDR Stainless Steel
HDR stainless steel pipes are typically fabricated from austenitic stainless grades such as 304, 316, or duplex grades (2205) with enhanced mechanical properties and corrosion resistance. The material characteristics relevant to welding include:
| Property | Typical Value | Welding Relevance |
|---|---|---|
| Thermal conductivity | 14–16 W/m·K | Low → high heat concentration |
| Coefficient of thermal expansion | 16–18 × 10⁻⁶/K | High → residual stress |
| Carbon content | ≤ 0.03% (L grade) | Low → reduced sensitization risk |
| Cr content | 18–25% | Passivation and sensitization |
| Ni content | 8–22% | Stabilizes austenite |
| Yield strength | 205–450 MPa | Post-weld strength matching |
TIG Welding Process Parameters
The study examines optimized TIG welding parameters for HDR stainless steel pipe welding:
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Current type | DCEN | Tungsten erosion, deep penetration |
| Current range | 80–200 A (depending on thickness) | Adequate penetration without excessive heat |
| Travel speed | 150–400 mm/min | Balances penetration and bead width |
| Shielding gas | 100% Ar or 98% Ar + 2% O₂ | Inclusion of O₂ improves wetting and bead profile |
| Gas flow rate | 8–12 L/min | Adequate backside protection |
| Back purge | 100% Ar, 5–8 L/min | Prevents internal oxidation |
| Filler wire | ER308L or ER316L (matching grade) | Low carbon to prevent sensitization |
| Preheat | None (≤ 50°C) | Avoids sensitization in HAZ |
Microstructural Analysis
The weld metal microstructure of austenitic stainless steel TIG welds typically exhibits:
- Primary dendrites of austenite (γ) with columnar growth direction
- Secondary phases including δ-ferrite (controlled at 3–15% per DeLong ferrite number)
- Grain boundary precipitation of Cr₂₃C₆ if carbon content is excessive (sensitization)
The δ-ferrite content is critical: insufficient ferrite (< 3%) promotes hot cracking in the weld metal, while excessive ferrite (> 20%) leads to martensite formation during cooling and reduced corrosion resistance.
Process-Standard Analysis
Comparison with Other Joining Methods for HDR Pipes
| Method | Penetration | Dilution | Cost | Applicability |
|---|---|---|---|---|
| TIG (GTAW) | Moderate | Low | Moderate | Thin to medium wall |
| TIG + FCAW (hybrid) | High | Moderate | Low | Medium to thick wall |
| SAW (submerged arc) | High | Moderate | Low | Thick wall, flat position |
| ORB (orbital) | High | Low | High | Pipe girth welds, consistent |
| Laser + TIG | Very high | Low | High | Thin wall, high productivity |
Standards and Code Compliance
HDR stainless steel pipe welding must comply with:
- ASME B31.3 for process piping (material specifications, NDE requirements)
- ASME IX for welder qualification and WPS
- ASTM A268/A269 for seamless pipe material specifications
- EN 10217-7 for European stainless steel pipe specifications
- GB/T 12771 for Chinese stainless steel pipe standards
For pressure vessel applications, the welded joints must satisfy the requirements of ASME VIII Div.1 Appendix 12 or Div.2 Part 5 for stainless steel weld overlay and full-penetration welds.
Key Questions and Reflections
Several engineering challenges merit attention:
- Sensitization control: The heat-affected zone (HAZ) of austenitic stainless steels is susceptible to chromium carbide precipitation in the 450–850°C range, leading to intergranular corrosion (IGC). Post-weld heat treatment (PWHT) at 1050–1100°C followed by rapid cooling can dissolve precipitates, but this is often impractical for field-welded piping.
- Backside protection: Inadequate back purge leads to internal oxidation (scale formation), which reduces corrosion resistance and may cause stress corrosion cracking (SCC) in chloride environments. The backside weld appearance is a reliable indicator of purge adequacy.
- Distortion control: Stainless steel's low thermal conductivity and high thermal expansion coefficient lead to significant distortion in thin-walled pipes. Fixturing and welding sequence optimization are essential.
Study Insights and Implications
This work provides practical guidance for TIG welding of HDR stainless steel pipes in marine and chemical applications. The emphasis on backside protection, filler metal selection, and δ-ferrite control reflects the critical quality requirements of corrosion-resistant piping systems. For pressure vessel applications involving stainless steel clad or fully stainless construction, the principles outlined here extend to weld overlay qualification, where maintaining the specified alloy composition in the overlay layer requires careful control of dilution (typically limited to ≤ 10–15% base metal dilution per NB/T 47014).
The use of 2% O₂ addition to the shielding gas, while improving bead profile and wetting, must be carefully controlled to avoid excessive oxygen pickup in the weld metal, which can reduce corrosion resistance. A balance must be struck between weldability and final performance, particularly for applications exposed to chloride-containing environments where pitting and crevice corrosion are primary concerns.