CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Ni Pipe Longitudinal Seam Plasma-TIG Hybrid Weld Joints
Literature Overview
Published in Hot Working Technology in 2016 and supported by the Gansu Provincial Science and Technology Major Project (Grant No. 145RTSA004), this research investigates the microstructure and mechanical properties of nickel pipe longitudinal seam weld joints produced by plasma-TIG hybrid welding. The collaborative study involves researchers from the State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals at Lanzhou University of Technology and Jinchuan Group Co., Ltd. This work addresses a critical welding challenge in the nickel industry, where high-purity nickel pipes are used in nuclear applications, chemical processing, and other demanding service environments.
Technical Background
Nickel and nickel alloys are widely used in applications requiring excellent corrosion resistance, high-temperature strength, and thermal stability. High-purity nickel pipes (such as those produced by Jinchuan Group) are commonly used in nuclear reactor coolant systems, chemical processing equipment, and other applications where material purity and weld integrity are critical. The longitudinal seam welding of nickel pipes presents unique challenges due to the material's high thermal conductivity, thermal expansion coefficient, and susceptibility to oxidation and porosity during welding.
Plasma-TIG hybrid welding combines the deep, narrow penetration of plasma arc welding with the wider weld pool and better bead geometry of TIG welding. This hybrid approach offers several advantages for nickel pipe welding:
- The plasma arc provides deep penetration with minimal heat input
- The TIG arc provides additional energy for complete fusion and better bead geometry
- The combined process allows for higher productivity than TIG welding alone
- The plasma arc's concentrated energy reduces the heat-affected zone
Process Configuration and Parameters
The plasma-TIG hybrid process configuration involves a coaxial arrangement where the plasma arc and TIG arc are aligned along the same axis. The plasma arc is typically positioned to lead the TIG arc, with the TIG arc following to fill the weld groove and improve bead geometry.
| Parameter | Typical Range | Effect on Weld Quality |
|---|---|---|
| Plasma Arc Current | 40–80 A | Controls penetration depth |
| TIG Arc Current | 100–180 A | Controls bead width and fill |
| Travel Speed | 5–12 mm/s | Controls heat input and penetration |
| Plasma Gas Flow Rate | 1.5–3.0 L/min | Controls arc stability and penetration |
| Shielding Gas Flow Rate | 15–25 L/min | Controls contamination and arc stability |
| Arc Length | 2–4 mm | Controls arc force and penetration |
| Pipe Diameter | 50–200 mm | Affects heat distribution and distortion |
| Wall Thickness | 3–10 mm | Affects penetration requirements |
Microstructural Analysis
The weld metal microstructure in plasma-TIG hybrid welded nickel pipe joints is characterized by a predominantly austenitic matrix with fine grain structure. The microstructure varies across the weld cross-section due to the different thermal cycles experienced by different regions of the weld.
Weld Metal Microstructure
- Centerline region: Ultrafine grain structure with cellular dendritic morphology. This region experiences the highest cooling rate due to the concentrated energy input from the plasma arc. Grain size is typically in the range of 50–100 micrometers.
- Side regions: Coarser grain structure with equiaxed dendritic morphology. This region experiences a lower cooling rate due to the wider heat input from the TIG arc. Grain size is typically in the range of 100–200 micrometers.
- Weld toes: Fine grain structure with some grain refinement due to the thermal cycle. This region may exhibit some delta ferrite formation if the cooling rate is sufficiently high.
Heat-Affected Zone (HAZ) Microstructure
The HAZ in nickel pipe welding is characterized by grain growth and possible carbide precipitation. The extent of grain growth depends on the peak temperature and the time at temperature. In plasma-TIG hybrid welding, the HAZ is typically narrower than in conventional TIG welding due to the higher energy density of the plasma arc.
| HAZ Region | Peak Temperature | Microstructural Features | Mechanical Properties |
|---|---|---|---|
| Fusion line | 1400–1500°C | Complete melting and resolidification | Similar to weld metal |
| Coarse grain HAZ | 1200–1400°C | Significant grain growth; possible carbide precipitation | Reduced ductility |
| Fine grain HAZ | 900–1200°C | Moderate grain growth; some recrystallization | Slightly reduced strength |
| Recrystallized HAZ | 500–900°C | Partial recrystallization; grain refinement | Improved ductility |
Mechanical Properties
The mechanical properties of plasma-TIG hybrid welded nickel pipe joints are critical for ensuring structural integrity in demanding service environments. The study evaluates tensile strength, yield strength, elongation, and hardness across the weld cross-section.
Tensile Properties
| Location | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) |
|---|---|---|---|
| Base metal | 550–650 | 300–350 | 40–50 |
| Weld metal (center) | 500–600 | 280–330 | 35–45 |
| Weld metal (side) | 520–620 | 290–340 | 38–48 |
| HAZ (coarse grain) | 530–630 | 290–340 | 35–45 |
| HAZ (fine grain) | 540–640 | 300–350 | 38–48 |
The tensile properties of the weld metal are slightly lower than the base metal, which is typical for nickel welding. The elongation values indicate adequate ductility for most applications. The HAZ properties are generally comparable to the base metal, indicating that the plasma-TIG hybrid process produces a narrow HAZ with minimal property degradation.
Hardness Distribution
The hardness distribution across the weld cross-section exhibits a characteristic profile with slightly lower hardness in the weld metal and HAZ compared to the base metal. The base metal hardness is typically in the range of HV 120–150, while the weld metal hardness is in the range of HV 100–130. This slight softening is attributed to grain growth and possible carbide dissolution during the welding thermal cycle.