CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Invar Steel and 06Cr19Ni10 Steel Laser Welding and TIG Welding Study Note
Literature Overview
This study compares the microstructural evolution and mechanical properties of Invar steel (Fe-36Ni) and 06Cr19Ni10 stainless steel (equivalent to 304L) joints fabricated using laser welding and TIG welding. Invar steel, with its exceptionally low coefficient of thermal expansion (CTE ≈ 1.2×10⁻⁶/K), is used in applications requiring dimensional stability under thermal cycling, such as precision instruments, aerospace structures, and thermal management components. The combination of Invar with 06Cr19Ni10 stainless steel creates a bimetallic system that combines dimensional stability with corrosion resistance, making it attractive for specialized pressure vessel components, heat exchanger tubes, and cryogenic equipment.
Core Technical Findings
Welding Challenges of Invar-06Cr19Ni10 Joints
The dissimilar welding of Invar steel and 06Cr19Ni10 stainless steel presents unique challenges:
- Thermal expansion mismatch: The CTE difference (Invar: 1.2×10⁻⁶/K, 06Cr19Ni10: 17.3×10⁻⁶/K) creates significant residual stresses during cooling
- Chemical segregation: The high nickel content of Invar (36%) and the chromium-nickel balance of 06Cr19Ni10 create complex solidification behavior
- Phase transformation: Invar steel undergoes a martensitic transformation at approximately 100°C, which can occur during welding and introduce additional stresses
- Intermetallic formation: The potential for brittle Fe-Ni intermetallic phases at the fusion boundary
Laser Welding vs. TIG Welding: Microstructural Comparison
| Feature | Laser Welding | TIG Welding |
|---|---|---|
| Heat Input (kJ/mm) | 0.5–1.5 | 3.0–6.0 |
| HAZ Width (mm) | 0.5–1.0 | 3.0–5.0 |
| Weld Width (mm) | 3.0–5.0 | 8.0–12.0 |
| Penetration Depth (mm) | 5.0–8.0 | 3.0–5.0 |
| Aspect Ratio (Depth/Width) | 1.5–2.5 | 0.4–0.6 |
| Cooling Rate (°C/s) | 500–2000 | 50–200 |
The laser welding process produces a narrow, deep weld with minimal HAZ, while TIG welding produces a wider, shallower weld with a more extensive HAZ. The high cooling rate of laser welding results in finer grain structures and reduced δ phase formation, but may promote martensitic transformation in the Invar side.
Microstructural Evolution in the Weld Zone
Laser Weld Zone:
- Centerline: Fine equiaxed austenite grains with some martensite in the Invar-rich region
- Fusion boundary: Transition zone with mixed austenite-ferrite structure
- HAZ (Invar side): Retained austenite with some martensite due to rapid cooling
- HAZ (06Cr19Ni10 side): Fine austenite grains, minimal sensitization
TIG Weld Zone:
- Centerline: Coarse columnar grains with significant δ phase (5–15% area fraction)
- Fusion boundary: Widened transition zone with sensitization susceptibility
- HAZ (Invar side): Coarse austenite grains, possible martensite formation
- HAZ (06Cr19Ni10 side): Sensitized zone with carbide precipitation along grain boundaries
Mechanical Property Results
| Property | Base Metal (Invar) | Base Metal (06Cr19Ni10) | Laser Weld | TIG Weld |
|---|---|---|---|---|
| Tensile Strength (MPa) | 550–620 | 520–580 | 500–560 | 480–540 |
| Yield Strength (MPa) | 250–290 | 220–260 | 230–270 | 210–250 |
| Elongation (%) | 30–35 | 40–45 | 32–38 | 35–40 |
| Hardness (HV) | 200–230 | 170–200 | 200–230 | 180–210 |
| Impact Energy @ 20°C (J) | 80–100 | 100–120 | 85–105 | 90–110 |
| Intergranular Corrosion (ASTM A262 Practice E) | Pass | Pass | Pass | Marginal |
The laser weld exhibits superior mechanical properties and corrosion resistance compared to the TIG weld, primarily due to the reduced δ phase formation and minimized sensitization. However, the TIG weld offers better ductility due to the more homogeneous microstructure and lower residual stresses.
Process Parameter Analysis
Laser Welding Parameters
| Parameter | Typical Value | Influence |
|---|---|---|
| Laser Power (kW) | 2.0–4.0 | Higher power increases penetration |
| Travel Speed (mm/min) | 500–1500 | Higher speed reduces heat input |
| Focal Position (mm) | -2 to +2 | Affects spot size and penetration |
| Shielding Gas (Ar) | 15–20 L/min | Prevents oxidation |
| Preheat (°C) | 100–200 | Reduces residual stress, prevents cracking |
TIG Welding Parameters
| Parameter | Typical Value | Influence |
|---|---|---|
| Arc Current (A) | 100–180 | Higher current increases penetration |
| Arc Voltage (V) | 12–18 | Higher voltage increases weld width |
| Travel Speed (mm/min) | 150–300 | Higher speed reduces heat input |
| Shielding Gas (Ar) | 15–20 L/min | Prevents oxidation |
| Preheat (°C) | 150–250 | Reduces residual stress, prevents cracking |
Filler Metal Selection
The selection of filler metal is critical for dissimilar welding:
- ER309L (32Cr-25Ni): Common choice for Invar-304L joints, provides good ductility and corrosion resistance
- ERNiCr-3 (35Ni-20Cr): Used when higher nickel content is required to match Invar
- ERNiMo-13 (60Ni-9Mo-9Cr): Used for high-temperature applications requiring enhanced creep resistance
Engineering Practice Integration
Application in Pressure Vessel Fabrication
Invar-06Cr19Ni10 bimetallic components are used in:
- Cryogenic equipment: Thermal expansion compensation in cryogenic storage tanks and transfer lines
- Precision instruments: Dimensional stability in high-temperature environments
- Aerospace structures: Thermal management components requiring corrosion resistance
- Nuclear components: Thermal shields and heat exchangers requiring dimensional stability
Quality Control and Inspection
| Inspection Method | Acceptance Criteria | Frequency |
|---|---|---|
| Visual examination | No cracks, porosity, or undercut | 100% |
| RT (Radiographic Testing) | No defects > 2 mm or > 10% of weld width | 100% |
| UT (Ultrasonic Testing) | No delamination or lack of fusion | 100% |
| MT (Magnetic Particle Testing) | No surface or near-surface cracks | 100% |
| Hardness test | No excessive hardening in HAZ | Per batch |
| Intergranular corrosion test | Pass ASTM A262 Practice E | Per batch |
| Tensile test | Meets code requirements | Per batch |
Standards Compliance
- GB/T 150: Pressure vessel design and fabrication
- NB/T 47002: Fabrication of pressure vessels
- NB/T 47014: Welding procedure qualification
- ASME VIII Div.1: Construction of pressure vessels
- ASME IX: Qualification of welding procedures
- JB/T 4730: Non-destructive testing methods
Key Questions and Reflections
The comparison of laser and TIG welding for Invar-06Cr19Ni10 joints reveals a fundamental trade-off: laser welding offers superior microstructural quality and corrosion resistance but requires precise parameter control and may introduce martensitic transformation in the Invar side. TIG welding offers better ductility and tolerance to parameter variations but risks sensitization and δ phase formation. For critical pressure vessel applications, the choice between laser and TIG welding should be based on the specific service conditions (temperature, pressure, corrosion environment) and the applicable code requirements.
A practical consideration is the effect of welding sequence on residual stress distribution. For multi-pass welds, the sequence of passes should be planned to minimize distortion and residual stress. For dissimilar joints, the weld should be directed from the more ductile material (06Cr19Ni10) toward the less ductile material (Invar) to minimize the risk of cracking.