Application of Semi-Automatic GMAW in Stainless Steel Clad Plate Overlay Welding

Literature Overview

The paper by Zhou Junpeng, He Minggang, and Cui Shufen, published in Petrochemical Equipment (Vol. 47, Issue 6, 2018, pp. 65–68), reports on the application of semi-automatic gas metal arc welding (GMAW) for overlay welding of stainless steel clad plates, specifically the Q345R + S31603 composite plate system. This study is highly relevant to engineers working on pressure vessel fabrication, particularly in the petrochemical and oil and gas industries, where stainless steel clad plates are widely used to combine the strength of low-alloy steel with the corrosion resistance of stainless steel.

Core Technical Findings

The study analyzed the weldability and welding challenges of Q345R + S31603 stainless steel clad plates, determined the appropriate weld groove geometry, and selected ER309MoL and ER316L welding wires for the transition and face layers, respectively. The following table summarizes the key welding parameters and material selections:

Technical Interpretation of Key Points

Weldability Analysis of Q345R + S31603 Composite Plate

The Q345R + S31603 composite plate system presents several welding challenges:

• Dissimilar metal welding: The large difference in thermal expansion coefficients between Q345R (carbon steel) and S31603 (austenitic stainless steel) leads to significant thermal stresses during welding and cooling, which can cause cracking at the weld interface.
• Dilution control: During overlay welding, the base metal (Q345R) can dilute the stainless steel overlay, reducing the corrosion resistance of the clad layer. The dilution level must be controlled to ensure that the final composition of the overlay meets the required corrosion resistance specifications.
• Intermetallic compound formation: At the weld interface between carbon steel and austenitic stainless steel, brittle intermetallic compounds such as sigma (σ) and chi (χ) phases can form during solidification or subsequent heat treatment, reducing the toughness and ductility of the weld.
• Residual stress and distortion: The differential thermal expansion between the base and clad materials, combined with the thermal cycling of welding, leads to significant residual stresses and potential distortion of the weldment.

Weld Groove Geometry Selection

The selection of the weld groove geometry is critical for controlling dilution and ensuring proper fusion. For overlay welding of stainless steel clad plates, the groove should be designed to minimize the penetration into the base metal while ensuring adequate fusion with the clad layer. A shallow, wide groove is typically preferred to reduce the heat input into the base metal and limit dilution. The groove geometry should also be designed to accommodate the welding position and to ensure that the weld metal can be deposited uniformly across the entire overlay area.

Welding Material Selection

The selection of ER309MoL for the transition layer and ER316L for the face layer is based on the following considerations:

• ER309MoL (transition layer): This wire contains approximately 25% Cr and 13% Ni, with the addition of Mo for enhanced corrosion resistance. The high Cr and Ni content ensures that the transition layer is resistant to intergranular corrosion and stress corrosion cracking (SCC). The Mo addition improves resistance to pitting and crevice corrosion, which is particularly important in chloride-containing environments.
• ER316L (face layer): This wire contains approximately 18% Cr, 12% Ni, and 2.5% Mo, with a low carbon content (≤0.03%) to prevent sensitization. The face layer provides the final corrosion-resistant surface, and the low carbon content ensures that the overlay layer is resistant to intergranular corrosion even after exposure to elevated temperatures.

The use of a two-layer approach (transition + face) is a standard practice for overlay welding of dissimilar materials, as it provides a metallurgical buffer between the base metal and the final overlay, reducing the risk of cracking and ensuring adequate corrosion resistance.

Connection with Engineering Practice

The application of semi-automatic GMAW for stainless steel clad plate overlay welding is particularly relevant to the fabrication of pressure vessels and heat exchangers in the petrochemical and oil and gas industries. The following practical considerations are important:

1. Preheating and interpass temperature: The Q345R base plate may require preheating to reduce the risk of hydrogen-induced cracking, while the S31603 clad layer should be kept at a low temperature to prevent sensitization. A controlled preheating and interpass temperature regime is essential to balance these competing requirements.
2. Heat input control: The heat input should be kept to a minimum to reduce dilution and prevent the formation of brittle intermetallic compounds. This requires careful selection of welding current, voltage, and travel speed.
3. Welding sequence: The welding sequence should be designed to minimize distortion and residual stress. For large clad plate assemblies, a symmetric welding sequence is recommended to distribute the thermal input evenly across the weldment.
4. Post-weld inspection: The overlay weld should be inspected for porosity, lack of fusion, and intermetallic compound formation using non-destructive testing (NDT) methods such as ultrasonic testing (UT), magnetic particle testing (MT), and visual inspection (VT).

Key Questions and Reflections

An important question is how the semi-automatic GMAW process compares to other welding processes, such as manual GMAW, flux-cored arc welding (FCAW), and submerged arc welding (SAW), for overlay welding of stainless steel clad plates. Semi-automatic GMAW offers a good balance between productivity and weld quality, but the operator still has some control over the torch angle and travel speed, which can introduce variability. Fully automatic GMAW or FCAW may offer better consistency, but at the cost of reduced flexibility for complex geometries.

Another consideration is the long-term performance of the overlay weld under actual operating conditions. The corrosion resistance of the overlay layer depends not only on the chemical composition but also on the microstructure, which is influenced by the welding process parameters and post-weld treatment. Long-term exposure to corrosive media, such as chloride-containing solutions, can lead to pitting and crevice corrosion, which may initiate at microstructural features such as grain boundaries, inclusions, or phase boundaries. Therefore, the long-term corrosion resistance of the overlay weld should be evaluated through accelerated corrosion testing and field trials.

Study Insights and Implications

This paper provides a practical and valuable contribution to the field of stainless steel clad plate welding. The systematic approach—analyzing weldability, selecting welding materials, determining groove geometry, and validating the process through welding procedure qualification (WPQ)—is a model for engineering practice. The key takeaway is that semi-automatic GMAW is a viable and efficient process for overlay welding of stainless steel clad plates, provided that the welding materials, parameters, and procedures are carefully selected and controlled. For engineers working on pressure vessel fabrication, this study offers a proven methodology for achieving high-quality overlay welds with excellent corrosion resistance and mechanical properties. Future work should focus on the optimization of welding parameters for different clad plate thicknesses and geometries, the development of automated welding procedures for large-scale production, and the evaluation of long-term corrosion resistance under actual operating conditions.