CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Medium-Thickness Stainless Steel in Laser-MIG Hybrid Welding
Literature Overview
This study by Chen Zhiwei and colleagues from Harbin Institute of Technology (Weihai) and Harbin Institute of Technology, published in Laser & Optoelectronics Progress in 2020, investigates the microstructure and mechanical properties of medium-thickness stainless steel welds produced by laser-MIG hybrid welding. The research is supported by the National Key R&D Program of China (Grant No. 2018YFB1107900), the Shandong Provincial Key R&D Program (Grant No. 2018GGX103026), and the Shandong Provincial Natural Science Foundation (Grant No. ZR2017MEE042). The work addresses the metallurgical challenges associated with welding stainless steel, which is widely used in pressure vessels, heat exchangers, and other corrosion-resistant applications.
Core Technical Content
Stainless steel, particularly austenitic grades such as 304 and 316, is extensively used in pressure vessel fabrication due to its excellent corrosion resistance, high-temperature strength, and formability. However, welding stainless steel presents several metallurgical challenges, including the risk of sensitization (chromium carbide precipitation), hot cracking, and the formation of intermetallic phases.
Laser-MIG Hybrid Welding Process
The laser-MIG hybrid welding process combines the deep penetration of the laser beam with the high deposition rate of the MIG arc, offering several advantages for stainless steel welding:
- Reduced heat input: The lower total heat input compared to standalone MIG welding reduces the risk of sensitization and minimizes the formation of detrimental phases in the HAZ.
- Narrower HAZ: The concentrated energy input produces a narrower HAZ, reducing the volume of material exposed to high temperatures and minimizing the impact on the base metal properties.
- Improved weld geometry: The deep, narrow weld profile reduces the number of passes required for medium-thickness plates, improving productivity and reducing distortion.
- Controlled dilution: The process geometry allows for better control of the dilution ratio, which is critical for achieving the required corrosion resistance in the weld metal.
Microstructural Analysis
The microstructure of laser-MIG hybrid welds in stainless steel is characterized by the following features:
| Region | Microstructure | Characteristics |
|---|---|---|
| Weld metal | Columnar and equiaxed grains | Fine grain size, low carbon content |
| Fusion line | Transition zone | Possible precipitation of intermetallic phases |
| HAZ (coarse grain) | Recrystallized grains | Potential for sensitization if heat input is high |
| HAZ (fine grain) | Partially recrystallized | Minimal sensitization, retained base metal properties |
| Base metal | Unchanged | Retains original microstructure and properties |
Key Technical Parameters
| Parameter | Typical Range | Influence |
|---|---|---|
| Laser power | 2–6 kW | Controls penetration depth |
| MIG current | 100–200 A | Controls deposition rate |
| MIG voltage | 18–26 V | Controls arc stability and droplet transfer |
| Travel speed | 400–1200 mm/min | Controls heat input per unit length |
| Plate thickness | 6–20 mm | Determines number of passes |
| Wire composition | 308L, 316L, or 347L | Controls weld metal composition and corrosion resistance |
| Shielding gas | Ar or Ar/CO₂ | Protects weld pool from oxidation |
| Preheat temperature | 0–100 °C | Controls cooling rate and reduces cracking risk |
Mechanical Properties
The mechanical properties of laser-MIG hybrid welds in stainless steel are generally superior to those of conventional MIG welds:
| Property | Base Metal (304) | MIG Weld | Laser-MIG Hybrid Weld |
|---|---|---|---|
| Tensile strength (MPa) | 520–720 | 450–550 | 500–600 |
| Yield strength (MPa) | 205–310 | 180–250 | 200–280 |
| Elongation (%) | 40–55 | 45–60 | 40–55 |
| Hardness (HV) | 150–180 | 130–160 | 145–175 |
Corrosion Resistance
Corrosion resistance is a critical requirement for stainless steel pressure vessels and heat exchangers. The study evaluates the corrosion resistance of laser-MIG hybrid welds through the following tests:
| Test Method | Standard | Purpose |
|---|---|---|
| Salt spray test | ASTM B117 | Evaluates general corrosion resistance |
| Intergranular corrosion test | ASTM A240 / GB/T 4334 | Evaluates sensitization resistance |
| Pitting corrosion test | ASTM G48 | Evaluates localized corrosion resistance |
| Crevice corrosion test | ASTM G44 | Evaluates crevice corrosion resistance |
| Electrochemical testing | ASTM G5 | Evaluates corrosion potential and rate |
The lower heat input of laser-MIG hybrid welding reduces the risk of sensitization in the HAZ, resulting in improved intergranular corrosion resistance compared to conventional MIG welds. The weld metal composition, controlled by the selection of the filler metal and the dilution ratio, also plays a critical role in determining the corrosion resistance of the joint.
Defect Analysis and Countermeasures
| Defect Type | Root Cause | Prevention Measures |
|---|---|---|
| Sensitization | High heat input, prolonged exposure to 500–800 °C | Minimize heat input, use low-car |