Microstructure and Residual Stress in S355 Steel Laser-MIG Hybrid Welding Joints

Literature Overview

This paper by Yan Chunyan and colleagues from Hohai University, published in the Transactions of the China Welding Institution (Volume 41, Issue 6, 2020, pp. 12-18), investigates the microstructure, hardness distribution, temperature field, and residual stress in laser-MIG hybrid welds of 12 mm thick S355 structural steel plates. The research was supported by the National Natural Science Foundation of China (Grant No. 51804097) and the Central University Basic Scientific Research Business Special Fund (2017B17614). The study employs both experimental metallographic analysis and finite element simulation using SYSWELD software, with a laser power of 9 kW and three welding speeds of 1.0, 1.5, and 2.0 m/min.

Core Technical Findings

Heat Source Model

The authors developed a dual-ellipsoid plus three-dimensional conical composite heat source model to describe the energy distribution of the laser-MIG hybrid welding process. This model is particularly suitable for low-alloy high-strength steels, where the laser provides deep penetration while the MIG arc contributes heat input for wider fusion and improved weld geometry. The dual-ellipsoid component represents the laser energy distribution, while the conical component accounts for the MIG arc heat contribution, creating a more realistic thermal profile than single-source models.

Microstructure and Hardness

The key experimental findings are summarized below:

The coarse-grained heat affected zone (CGHAZ) consistently exhibited martensitic transformation across all welding speeds, which is attributed to the rapid cooling rates inherent in laser-assisted hybrid welding. This martensitic transformation in the CGHAZ of S355 steel is a significant concern because S355 is a low-carbon structural steel that normally does not form martensite under conventional welding conditions. The high thermal gradients from the laser source elevate cooling rates well above the critical transformation rate, resulting in hard and brittle microstructures that may be susceptible to hydrogen-induced cracking.

Residual Stress Analysis

The SYSWELD simulation results reveal important trends in residual stress distribution:

The equivalent residual stress levels are relatively high, with stress concentration occurring at the HAZ location. As welding speed increases, the peak values of equivalent residual stress, longitudinal residual stress, transverse residual stress, and through-thickness residual stress all rise. However, a notable transition occurs between 1.5 and 2.0 m/min: the tensile stress peak increases only slightly, while the compressive stress peak rises significantly. This suggests a redistribution of the stress state at higher welding speeds, potentially shifting the dominant failure mechanism from tensile cracking to compressive buckling or fatigue initiation.

Engineering Practice Implications

Implications for Pipe and Fitting Fabrication

For steel pipe and fitting fabrication, the findings carry several practical implications. S355 steel is widely used in pressure piping systems governed by ASME B31.3 and EN 10216-2, as well as in structural applications. The presence of martensite in the CGHAZ at hardness levels exceeding 350 HV raises concerns about weldability, particularly regarding:

1. Hydrogen-induced cracking susceptibility, as hardness above 350 HV is a common threshold for crack sensitivity in low-alloy steels
2. Reduced toughness in the CGHAZ, which may compromise impact performance requirements specified in standards such as API 5L or EN 10216
3. Elevated residual stresses that may accelerate fatigue crack initiation in cyclically loaded piping systems

Process Optimization Recommendations

Based on the study findings, the following process optimization strategies can be recommended:

• Preheating: Application of preheat (typically 80-120 °C for S355) can reduce cooling rates and suppress martensite formation in the CGHAZ
• Post-weld heat treatment (PWHT): Stress relief annealing at 550-650 °C can reduce residual stresses by 60-80% and temper the martensitic microstructure
• Welding speed selection: The transition observed between 1.5 and 2.0 m/min suggests that 1.5 m/min may represent a more favorable operating point for balancing productivity and stress levels
• Interpass temperature control: Maintaining interpass temperatures between 150-250 °C can further moderate cooling rates

Standards Compliance Considerations

When applying laser-MIG hybrid welding to S355 pipe components, the following standards requirements should be considered:

The hardness exceeding 350 HV in the CGHAZ may not comply with certain specification requirements that limit HAZ hardness to below 350 HV or to the base metal hardness plus a specified increment. Engineers must carefully evaluate whether the hybrid welding process requires additional post-weld treatment to meet applicable code requirements.

Key Questions and Reflections

The study raises an important question regarding the balance between productivity and metallurgical quality in laser-MIG hybrid welding. While the high welding speeds of 1.5-2.0 m/min offer significant productivity advantages over conventional MIG welding alone, the resulting martensitic microstructure and elevated residual stresses may require compensatory measures that partially offset these gains. For pipe fabrication, where joint integrity is critical for pressure containment, the metallurgical consequences must be carefully weighed against productivity benefits.

Another reflection concerns the applicability of the dual-ellipsoid plus conical heat source model to other hybrid welding configurations. While the model was validated for S355 steel, its transferability to other materials such as stainless steels or high-temperature alloys requires further investigation. The thermal conductivity, specific heat, and phase transformation behavior differ significantly across material families, and the model parameters may need recalibration for each application.

Summary

This literature provides valuable insights into the metallurgical and mechanical behavior of laser-MIG hybrid welds in S355 structural steel. The formation of martensite in the CGHAZ at hardness levels above 350 HV, combined with elevated residual stresses that increase with welding speed, highlights the need for careful process parameter selection and post-weld treatment when applying hybrid welding to structural steel pipe and fitting fabrication. The transition in stress component behavior between 1.5 and 2.0 m/min welding speeds offers a useful indicator for process optimization. Engineers working with S355 steel piping systems should consider these findings when evaluating hybrid welding as a productivity-enhancing technology, ensuring that metallurgical quality and code compliance are maintained alongside manufacturing efficiency gains.