Welding Process Research for Tube Plate Surfacing

Literature Overview

The paper by Wang Hengxiang, published in Henan Chemical Engineering in 2010, presents a systematic study of tube plate surfacing welding processes, with emphasis on the metallurgical challenges encountered during the surfacing operation and the process solutions developed to achieve quality weld deposits. Conducted at Luoyang Longhui Petrochemical Engineering Co., Ltd., this research addresses the practical difficulties of surfacing thick tube plates in petrochemical applications, where the overlay must provide corrosion resistance while maintaining structural integrity. The paper discusses weldability analysis, the problems encountered during surfacing, and the successful welding process that was ultimately developed, concluding that careful analysis of welding conditions and operating environment, combined with reasonable process measures, can yield high-quality weld joints.

Weldability Analysis of Tube Plate Surfacing

The weldability of tube plate surfacing is influenced by several factors unique to this application. Tube plates are typically thick, low-alloy carbon steel components with high carbon equivalent values, which makes them susceptible to cold cracking during welding. The application of a corrosion-resistant overlay introduces additional complexities related to dilution, residual stress, and microstructural transformation at the dissimilar metal interface.

The weldability analysis should consider the following aspects:

The concept of the low plasticity zone (LPZ) is particularly important in tube plate surfacing. The LPZ is a narrow region adjacent to the fusion line where the microstructure consists of a mixture of austenite and martensite, with reduced plasticity and increased susceptibility to cracking. This zone forms because the cooling rate in the LPZ is intermediate between the weld metal (which solidifies as austenite) and the base metal HAZ (which transforms to martensite or bainite). The LPZ is the most vulnerable region for cracking in dissimilar metal welds, and its formation and properties are strongly influenced by the welding parameters, base metal composition, and heat input.

Process Challenges and Solutions

The paper identifies several key problems encountered during tube plate surfacing:

1. Cracking in the first surfacing layer: The first layer experiences maximum dilution and the highest residual stress concentration. Cracking can occur in the weld metal itself (hot cracks or cold cracks) or at the fusion line (LPZ cracking). The solution involves using a transition layer with a composition that bridges the gap between the base metal and the final overlay composition, typically by using a wire with moderate alloy content for the first pass and increasing the alloy content in subsequent passes.
2. Excessive dilution: In thick tube plates, the dilution rate can be high because the large heat capacity of the base metal absorbs significant welding heat, and the deep penetration of the first pass mixes substantial base metal into the weld pool. The solution involves controlling the welding parameters to limit penetration depth, using flux-cored wires that provide higher deposition rates and better alloy retention, and applying multiple thin passes rather than fewer thick passes.
3. Residual stress and distortion: The thermal stress from surfacing a large area on one side of a thick plate creates a through-thickness stress gradient that can cause warping. The solution involves using a balanced welding sequence, applying mechanical restraints, and possibly performing post-weld stress relief annealing if the application allows.
4. Porosity: Gas porosity can occur due to inadequate gas coverage, contamination of the base metal surface, or excessive carbon in the weld metal that generates CO gas. The solution involves ensuring proper gas flow rate and shielding gas coverage, thorough surface preparation, and controlling the carbon content of the consumable wire.

Successful Welding Process Parameters

Based on the research findings, the following welding process parameters were identified as successful for tube plate surfacing:

The choice between CO2-only and mixed gas shielding depends on the specific requirements. CO2 provides deeper penetration and better wetting but results in greater alloy loss and more spatter. Mixed gas (such as Ar/CO2 at 80/20 or 75/25) provides better surface profile, less spatter, and higher alloy retention, but requires more expensive gas and may result in shallower penetration.

Microstructural Analysis and Metallurgical Considerations

The microstructure of the surfacing weld joint evolves from the base metal through the HAZ, the LPZ, and into the weld metal. Understanding this microstructural gradient is essential for predicting the mechanical properties and long-term performance of the joint.

The base metal HAZ of 16MnR or similar low-alloy steel typically transforms to a mixture of martensite, bainite, and ferrite, depending on the cooling rate. The hardness in the HAZ can reach 300-400 HV, which may be acceptable for the base metal but creates a hardness mismatch with the softer overlay. The LPZ, as mentioned earlier, is characterized by a mixed austenite-martensite structure with reduced ductility.

The weld metal of the austenitic overlay, if properly composed, should consist primarily of austenite with some delta ferrite (typically 5-20% ferrite is desirable to prevent hot cracking). The ferrite content can be estimated using the Schaeffer diagram or the DeLong diagram based on the weld metal composition. Excessive ferrite reduces corrosion resistance, while insufficient ferrite increases hot cracking susceptibility.

Key Questions and Reflections

An important question is the adequacy of the welding procedure qualification testing for this type of surfacing application. Standard welding procedure qualification tests (such as those per ASME Section IX or ISO 15614) may not fully capture the unique challenges of dissimilar metal surfacing on thick plates. Engineers should consider supplementing standard qualification testing with additional tests specific to the application, such as corrosion testing of the overlay, fatigue testing of the interface, and dimensional accuracy verification.

Another reflection concerns the role of welding operator skill in achieving consistent results. Tube plate surfacing requires the operator to maintain consistent travel speed, torch angle, and wire stick-out over large areas, often in challenging positions. Operator fatigue and inattention can lead to variations in bead geometry, dilution, and defect formation. Investment in operator training and the use of mechanized or semi-automated welding systems can improve consistency and reduce the risk of defects.

The paper's conclusion that careful analysis of welding conditions and reasonable process measures can yield quality weld joints is encouraging but somewhat generic. In practice, achieving quality surfacing requires not only the right process parameters but also rigorous quality control, proper consumable management, and experienced personnel. The margin for error is small, particularly when stringent NDT requirements are imposed.

Study Insights and Implications

This paper reinforces the principle that successful tube plate surfacing is achieved through a systematic approach that integrates metallurgical understanding, process design, and quality control. The identification of the low plasticity zone as a critical failure-prone region is particularly valuable for engineers who need to design surfacing procedures that minimize cracking risk at the dissimilar metal interface. The emphasis on multi-layer surfacing with a transition layer strategy is a practical solution that balances corrosion resistance requirements with metallurgical compatibility. For petrochemical engineering companies, this research underscores the importance of investing in thorough welding procedure development and qualification before committing to large-scale surfacing operations, as the cost of rework or failure far exceeds the cost of proper upfront engineering.