PAW-TIG Composite Welding of 316L Stainless Steel

Literature Overview

This paper, published in Ordnance Materials and Engineering (Volume 42, Issue 6, 2019, pages 29–33) by Cao Runping, Wang Keyong, and Qi Yongtian from Baotou Vocational and Technical College and Hefei Vocational and Technical College, investigates the composite welding of 6 mm thick 316L stainless steel using a combination of plasma arc welding (PAW) and tungsten inert gas welding (TIG). The research is supported by the Anhui Provincial Key Research Project of Natural Science in Higher Education Institutions (KJ2019A1118) and the Inner Mongolia Autonomous Region Higher Education Scientific Research Project (NJZY16460). The study demonstrates full penetration through 6 mm plate without groove preparation, achieving single-face welding with double-sided formation.

Core Technical Content and Key Findings

Process Principle and Configuration

The PAW-TIG composite welding process combines the high-energy-density plasma arc for deep penetration with the stable, low-heat-input TIG arc for surface refinement and bead shaping. The plasma arc serves as the primary energy source for melting and penetration, while the TIG arc provides:

• Surface tension regulation for improved bead formation
• Additional shielding gas coverage for enhanced protection
• Heat input modulation for microstructure control
• Stabilization of the plasma arc during welding

Weld Geometry and Penetration

The funnel-shaped cross-section is characteristic of plasma arc welding, where the constricted arc creates a narrow, deep penetration profile. The ability to achieve full penetration through 6 mm plate without groove preparation is a significant productivity advantage over conventional TIG welding, which typically requires V-groove preparation for thicknesses exceeding 3–4 mm.

Microstructural Analysis

The weld metal microstructure consists primarily of austenite with a small fraction of delta ferrite. This is the desired microstructure for 316L stainless steel welds, as:

• Austenite provides excellent corrosion resistance and ductility.
• Delta ferrite (typically 5–15%) prevents hot cracking by absorbing sulfur and phosphorus segregants and providing a nucleation site for austenite during solidification.
• The "funnel" cross-section indicates uniform solidification from the surface inward, which promotes a homogeneous microstructure.

The HAZ is reported as narrow, which is advantageous because:

• Less material is exposed to sensitization temperatures (450–850°C)
• Less distortion is generated due to reduced heat-affected volume
• The narrow HAZ preserves more of the base metal's original properties

Mechanical Properties

The mechanical properties of the weld joint are reported as good, with the following characteristics:

• Tensile strength: Adequate for structural applications
• Impact toughness: Satisfactory for service conditions
• Hardness: Uniform distribution across the weld joint
• Weldability: Excellent, with no cracking or porosity observed

The 100% RT (radiographic testing) qualification confirms the absence of internal defects such as porosity, slag inclusions, incomplete fusion, and lack of penetration.

Process Analysis and Engineering Implications

Comparative Process Evaluation

Process Parameter Optimization

The optimal process parameters for PAW-TIG composite welding of 6 mm 316L stainless steel involve balancing the plasma arc and TIG arc parameters:

• Plasma arc current: Primary control for penetration depth
• Plasma arc voltage: Controls arc length and energy density
• Plasma gas flow rate: Determines arc stability and penetration
• TIG arc current: Controls surface formation and bead width
• TIG arc voltage: Controls arc length for surface refinement
• Travel speed: Must be synchronized with both arcs
• Shielding gas flow rate: Combined protection from both arcs

Defect Analysis and Countermeasures

Integration with Engineering Practice

The PAW-TIG composite welding process is particularly relevant for several industrial applications:

• Chemical processing equipment: 316L stainless steel is widely used in chemical reactors, heat exchangers, and piping systems where corrosion resistance is critical.
• Food processing equipment: The single-pass, no-groove approach reduces post-weld cleaning and finishing requirements.
• Pharmaceutical equipment: The excellent surface quality and absence of internal defects support hygienic design requirements.
• Marine applications: 316L's superior corrosion resistance in marine environments benefits from the high-quality welds achievable with PAW-TIG.

Quality Assurance Considerations

For production implementation, the following quality assurance measures should be established:

1. Welder qualification: Welders must be qualified on both plasma and TIG processes and on their combined operation.
2. Weld procedure qualification: A WPS (Welding Procedure Specification) must be qualified per applicable codes (ASME Section IX, AWS D1.6, or EN ISO 15614).
3. Non-destructive testing: 100% RT inspection is recommended, supplemented by PT or MT for surface defect detection.
4. Corrosion testing: Intergranular corrosion testing (ASTM A262 Practice E) should be performed to verify resistance to sensitization.
5. Mechanical testing: Tensile, bend, and impact tests should be conducted on qualification coupons.

Process Development Roadmap

For organizations considering adoption of PAW-TIG composite welding:

1. Phase 1: Conduct feasibility trials on coupon specimens to establish baseline parameters.
2. Phase 2: Perform weld procedure qualification testing per applicable codes.
3. Phase 3: Train and qualify welders on the combined process.
4. Phase 4: Implement pilot production on representative components.
5. Phase 5: Scale to full production with ongoing quality monitoring.

Key Questions and Reflections

Several questions arise from this study that warrant further consideration. First, the long-term corrosion resistance of PAW-TIG welds in aggressive environments (such as seawater or chloride-containing industrial solutions) should be evaluated through accelerated corrosion testing. Second, the effect of the dual-arc process on sensitization and intergranular corrosion susceptibility needs systematic investigation, as the combined heat input from two arcs may create a wider sensitization zone than expected. Third, the scalability to thicker sections (8–12 mm) requires investigation, as the current study is limited to 6 mm plate.

The funnel-shaped weld cross-section, while aesthetically pleasing, may have implications for stress distribution at the weld toe. The sharp transition from the narrow weld root to the wider surface bead could create stress concentration, which should be evaluated for fatigue-critical applications.

Study Insights and Implications

This paper demonstrates that the strategic combination of plasma arc and TIG welding offers a compelling solution for welding 316L stainless steel without groove preparation. The key insight is that the two processes complement each other: the plasma arc provides the energy density for deep penetration, while the TIG arc provides the stability and surface quality that plasma welding alone may lack. This complementary approach exemplifies the principle of process hybridization, where combining two established processes can achieve performance that neither process can achieve independently.

For engineers working with austenitic stainless steels, the PAW-TIG composite process represents a significant productivity improvement without compromising weld quality. The elimination of groove preparation reduces fabrication time and material waste, while the single-pass approach reduces heat input and distortion. The 100% RT qualification confirms that the process produces sound, defect-free welds suitable for critical applications.