Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of Fiber Laser-MIG Hybrid Welded Joints in Marine Steel DH36
Overview of the Research
This paper, published in the Transactions of the China Welding Institution in 2019 by Gao Yan and colleagues from Beijing University of Technology and the Shanghai Shipbuilding Technology Research Institute, investigates the microstructure, mechanical properties, and corrosion behavior of fiber laser-MIG hybrid welded joints in 12 mm thick DH36 marine corrosion-resistant steel. The study examines four different wire feed rates and their influence on weld formation, microstructure evolution, tensile properties, and electrochemical corrosion performance. Funded by the National Natural Science Foundation of China (Grant 51475006), this research addresses a critical need in marine and offshore engineering where DH36 steel is widely used for ship hulls, offshore platforms, and subsea piping systems.
Welding Process and Parameters
Fiber laser-MIG hybrid welding combines the deep penetration capability of laser welding with the high deposition rate and wide bead flexibility of MIG welding. For 12 mm thick steel plates, this hybrid approach offers significant advantages over conventional single-process welding, including reduced welding passes, lower heat input, and improved weld geometry.
| Parameter | Condition | Effect on Weld Width | Effect on Microstructure |
|---|---|---|---|
| Wire feed rate 7.5 m/min | Lowest studied | Narrower | Baseline acicular ferrite |
| Wire feed rate 8.5 m/min | Optimal formation | Moderate | Balanced microstructure |
| Wire feed rate 9.5 m/min | Higher | Wider | Increased acicular ferrite |
| Wire feed rate 10.5 m/min | Highest | Widest | Altered ferrite morphology |
Microstructural Analysis
The weld metal is composed primarily of acicular ferrite (AF), proeutectoid ferrite (PF), and a certain amount of bainite. The wire feed rate has a significant influence on the morphology and quantity of acicular ferrite, which is the most desirable microstructure for marine steels due to its excellent combination of strength and toughness.
Wire Feed Rate Effects on Microstructure
At 8.5 m/min wire feed rate, the weld formation is optimal with a well-defined bead profile. As wire feed rate increases, both the arc region and laser region weld widths increase. The increased deposition rate at higher wire feed rates alters the solidification conditions, affecting the nucleation and growth of acicular ferrite. Higher wire feed rates tend to produce more acicular ferrite with finer morphology, which is beneficial for impact toughness at low temperatures.
Mechanical Properties
The tensile strength of the welded joints is not significantly affected by wire feed rate, indicating that the joint strength is maintained across the entire parameter range studied. However, the elongation after fracture decreases with increasing wire feed rate. The maximum elongation of 16.5% was achieved at an intermediate wire feed rate, demonstrating an optimal balance between strength and ductility.
| Wire Feed Rate (m/min) | Tensile Strength (MPa) | Elongation (%) | Notes |
|---|---|---|---|
| 7.5 | Relatively stable | Moderate | Lower corrosion resistance |
| 8.5 | Relatively stable | Higher | Best weld formation |
| 9.5 | Relatively stable | Moderate | Increased AF content |
| 10.5 | Relatively stable | Lower | Widest weld profile |
Corrosion Performance
Polarization curve testing reveals that the self-corrosion current density is minimized at a wire feed rate of 7.5 m/min, indicating the lowest corrosion tendency. This is an important finding because DH36 steel is specifically selected for its enhanced resistance to atmospheric and marine corrosion. The wire feed rate that produces the best mechanical properties (8.5 m/min) does not coincide with the wire feed rate that produces the best corrosion resistance (7.5 m/min), creating an engineering trade-off that must be carefully managed.
The difference in corrosion performance across wire feed rates can be attributed to variations in microstructure. Lower wire feed rates may produce a more uniform grain structure with fewer microsegregation zones, resulting in a more homogeneous electrochemical potential across the weld surface and reduced galvanic corrosion susceptibility.
Engineering Practice Integration
For marine and offshore piping applications where DH36 steel is specified, the selection of wire feed rate must balance competing requirements. If corrosion resistance is the primary concern (as in splash zone or submerged applications), a wire feed rate of 7.5 m/min should be preferred. If weld formation quality and mechanical ductility are prioritized, 8.5 m/min is the recommended setting. In practice, engineers should consider the specific service environment, the expected corrosion protection system (coating, cathodic protection), and the mechanical loading conditions to make an informed parameter selection.
This research also highlights the importance of hybrid welding for thick-section marine structures. The ability to weld 12 mm plates with fewer passes reduces the risk of interpass defects and improves overall joint quality. The process should be further validated through full-scale qualification testing in accordance with classification society requirements such as DNV-ST-F101 or ABS standards.
Study Insights and Reflections
The most notable finding of this research is the decoupling of optimal mechanical properties from optimal corrosion resistance with respect to wire feed rate. In practice, this means that a single wire feed rate cannot simultaneously optimize all performance criteria, and engineers must make deliberate trade-offs based on the specific application requirements. The 16.5% maximum elongation achieved is respectable for a welded joint in DH36 steel, which typically has base metal elongation of 20-25%, indicating that the hybrid welding process introduces a modest but acceptable reduction in ductility.
The observation that tensile strength is relatively insensitive to wire feed rate while elongation is sensitive is consistent with the metallurgical understanding that strength in low-carbon steels is primarily determined by carbon and alloy content, while ductility is more strongly influenced by microstructure morphology and grain size. The hybrid welding process appears to maintain a relatively stable chemical composition in the weld metal across the wire feed rate range, while the solidification conditions that govern microstructure are more sensitive to the process parameters.
In conclusion, this research provides valuable guidance for the application of fiber laser-MIG hybrid welding to DH36 marine steel. The identification of optimal wire feed rates for specific performance criteria, combined with the detailed microstructural analysis, equips welding engineers with the knowledge needed to develop qualified welding procedures for marine and offshore applications.