CLADDING TECHNOLOGY SHANXI CO., LTDResearch on Laser-MIG Hybrid Welding of High-Strength Offshore Steel
Literature Overview
This 2023 study by Chen Tianwei, Wang Xiaoyong, Xie Keng, Bai Ni, Peng Hongbing, Leng Chen, and Xiong Zhaohui from Jiangsu University, published in Ordnance Materials and Engineering, investigates laser-MIG hybrid welding of high-strength offshore steel. Funded by the Jiangsu Provincial Natural Science Foundation Youth Project (BK2020997), this research addresses the challenge of welding advanced high-strength steels used in offshore structures, where high productivity and excellent weld quality are both essential.
Core Technical Content
High-strength offshore steels (such as EH36, EH40, EH45, and higher grades) are used in offshore platforms, FPSOs, and subsea structures. These steels typically have yield strengths of 355–460 MPa and require careful welding to maintain their mechanical properties. The laser-MIG hybrid process combines the deep, narrow penetration of laser welding with the high deposition rate of MIG welding, offering several advantages:
| Process Parameter | Laser Component | MIG Component | Combined Effect |
|---|---|---|---|
| Power | 5–20 kW | 15–30 kW (arc) | 20–50 kW total |
| Penetration | Deep, narrow | Shallow, wide | Deep, moderate width |
| Deposition rate | Low | High (5–10 kg/h) | High (8–15 kg/h) |
| Heat affected zone | Narrow | Wide | Moderate |
| Welding speed | High (1–3 m/min) | Low (0.5–1.5 m/min) | High (1.5–3 m/min) |
| Porosity tendency | Low | Moderate | Low |
The hybrid process configuration studied includes:
- Laser power: 8–16 kW fiber laser
- Arc current: 180–280 A
- Welding speed: 1.5–3.0 m/min
- Wire feed speed: 4–8 m/min
- Wire diameter: 1.2 mm (ER70S-6 or equivalent)
- Shielding gas: Ar + 2% CO2
- Laser-arc distance: 1–3 mm
- Laser-arc angle: 5°–15° (laser leading)
Welding Process Analysis
The study examines several aspects of the hybrid welding process:
Penetration characteristics: The laser component provides deep keyhole penetration, while the arc component fills the wider groove. The combined effect produces a weld with good depth-to-width ratio, reducing the need for excessive groove preparation. For 20 mm thick plates, the hybrid process achieves full penetration in a single pass, compared to 2–3 passes required for conventional MIG.
Heat input distribution: The hybrid process delivers heat input in a concentrated manner, with the laser providing high energy density at the center and the arc providing distributed heat around the periphery. This results in a moderate overall heat input of 1.5–3.0 kJ/mm, comparable to conventional processes but with higher productivity.
Microstructure evolution: The rapid heating and cooling rates in the hybrid process produce fine-grained microstructures in the weld metal and heat affected zone. The study observes:
| Zone | Microstructure | Hardness (HV) |
|---|---|---|
| Fusion zone | Fine acicular ferrite + martensite | 250–350 |
| Coarse grain HAZ | Mixed ferrite + martensite | 300–400 |
| Fine grain HAZ | Fine ferrite + pearlite | 250–300 |
| Base metal | Ferrite + pearlite | 200–250 |
Mechanical properties: The hybrid welds achieve mechanical properties comparable to or better than base metal:
| Property | Base Metal | Hybrid Weld | Conventional MIG |
|---|---|---|---|
| Tensile strength (MPa) | 520–620 | 550–650 | 500–580 |
| Yield strength (MPa) | 355–460 | 400–500 | 350–420 |
| Elongation (%) | 18–22 | 16–20 | 18–22 |
| Impact energy @-40°C (J) | 47–67 | 40–60 | 30–50 |
Defect Analysis
The study identifies and analyzes common defects in laser-MIG hybrid welding:
| Defect Type | Frequency | Cause | Countermeasure |
|---|---|---|---|
| Lack of fusion | Low | Insufficient arc coverage | Increase arc current; optimize laser-arc distance |
| Porosity | Moderate | Gas entrapment from keyhole | Optimize shielding gas; control travel speed |
| Cracking | Low | High cooling rate; hydrogen | Preheat; use low-hydrogen consumables |
| Undercut | Low | Excessive heat at edges | Reduce laser power; adjust torch angle |
| Excessive spatter | Moderate | Arc instability | Optimize MIG parameters; ensure proper shielding |
The hybrid process shows lower defect rates compared to conventional MIG, primarily due to the stabilizing effect of the laser on the arc and the reduced heat input per unit length.
Connection to Pressure Vessel Fabrication
The findings of this study have significant implications for pressure vessel fabrication, particularly for vessels made from high-strength steels:
- Vessel shell welding: The high productivity of laser-MIG hybrid welding reduces fabrication time for large-diameter vessels, while maintaining the mechanical properties required by ASME VIII or GB/T 150.
- Thick-section welding: For thick-walled pressure vessels (such as hydrogenation reactors), the deep penetration capability of the hybrid process reduces the number of passes, improving efficiency and reducing residual stresses.
- HAZ control: The moderate heat input and fine microstructures produced by hybrid welding help maintain the toughness properties of high-strength steels, which is critical for vessels operating at low temperatures or under cyclic loading.
- Welding procedure qualification: The process parameters identified in this study provide a starting point for welding procedure qualification per ASME IX or NB/T 47014.
Process Optimization Parameters
The study identifies the following optimal parameter ranges for 20 mm thick EH40 steel:
| Parameter | Optimal Range | Rationale |
|---|---|---|
| Laser power | 10–14 kW | Sufficient penetration without excessive keyhole |
| Arc current | 200–240 A | Adequate deposition without excessive spatter |
| Welding speed | 2.0–2.5 m/min | Balance between productivity and quality |
| Laser-arc distance | 1.5–2.5 mm | Optimal interaction between laser and arc |
| Preheat temperature | 50–100°C | Reduce cooling rate for HAZ toughness |
| Interpass temperature | <200°C | Control microstructure evolution |
Key Questions and Reflections
Several important questions emerge from this research:
- How does the hybrid process perform for welding higher-strength grades (EH45, EH50, and above)?
- What are the long-term fatigue properties of hybrid welds under offshore loading conditions?
- Can the process be adapted for overlay welding of corrosion-resistant alloys on high-strength steel pressure vessels?
- What are the economic considerations for implementing hybrid welding in vessel fabrication shops?
The research demonstrates the potential of hybrid welding technology for advanced steel fabrication. The combination of laser and arc provides synergistic benefits that neither process can achieve alone.
Study Insights and Implications
This study represents an important advancement in welding technology for high-strength offshore steels. The laser-MIG hybrid process offers a compelling solution to the challenge of achieving both high productivity and excellent weld quality in thick-section fabrication. For pressure vessel manufacturers, the adoption of hybrid welding technology could significantly reduce fabrication costs while improving product quality and reliability.
The research also highlights the importance of process optimization for specific materials and applications. The parameter windows identified in this study provide a practical starting point for industrial implementation, while the detailed analysis of microstructure and properties supports welding procedure qualification. Future work should focus on extending these findings to other high-strength steels and investigating the long-term performance of hybrid welds under realistic service conditions. As offshore structures face increasingly demanding operating environments, advanced welding technologies like laser-MIG hybrid will play a critical role in ensuring structural integrity and safety.