Interpass Cooling Time and Arc Starting Method Effects on Surfacing Quality

Literature Overview

This paper by Bao Yang, Zhou Jianping, Xu Yan, and Li Bingru (2018), published in Hot Working Technology, investigates two critical process parameters affecting the quality of ring-shaped metal surfacing: interpass cooling time and arc starting method. The study was supported by the National Natural Science Foundation of China (51365053) and the Xinjiang Uyghur Autonomous Region Youth Science and Technology Innovation Talent Training Program (gn2015yx008). The authors from Xinjiang University combined numerical simulation with experimental validation to provide comprehensive insights into surfacing process optimization.

Core Technical Approach

Ring-shaped surfacing, as applied to pipe components, pressure vessel heads, and circular flanges, presents unique challenges due to the continuous geometry and heat accumulation effects. Unlike straight welds, the circular path creates a closed thermal circuit where heat from successive passes continuously accumulates, leading to progressive temperature elevation and potential quality degradation.

Numerical Simulation of Heat Accumulation

The authors employed finite element thermal simulation to model the temperature distribution during multi-pass ring surfacing. The simulation revealed that as the number of surfacing layers increases, the peak temperature and the base temperature (interpass temperature) both rise progressively. This heat accumulation effect is more pronounced in ring geometry compared to straight welds because:

• The heat from the trailing end of the weld path feeds forward to the leading end
• The continuous circular path prevents natural heat dissipation at weld ends
• Each successive pass starts at a higher base temperature than the previous pass

Effect of Interpass Cooling Time

The experimental results clearly demonstrated that increasing the interpass cooling time between successive passes improves surfacing quality. The recommended approach is to progressively increase cooling time as the number of layers increases:

The progressive cooling strategy compensates for the cumulative heat buildup and maintains the base temperature within an acceptable range for each subsequent pass.

Effect of Arc Starting Method

The study compared three arc starting methods for ring surfacing:

1. Fixed starting point - always starting at the same location
2. Rotating starting point - shifting the start position for each pass
3. Continuous welding without stopping

The rotating arc starting point method significantly reduced welding defects at the arc start and arc end locations. This is because:

• Fixed starting points create localized heat concentration at the start/end zone
• The arc start typically produces incomplete fusion and potential undercuts
• The arc end (crater) is susceptible to cracking due to rapid cooling
• Rotating the start point distributes these defects around the circumference

Process Parameters and Quality Indicators

Engineering Practice Integration

For pipe surfacing operations, particularly in the context of corrosion-resistant overlay or hardfacing of pipe internals, the findings of this study have direct practical implications:

1. Cooling time management: In production environments, implementing a progressive cooling schedule is straightforward. Thermocouples or infrared thermometers can be used to monitor base temperature, and operators should be instructed to maintain interpass temperatures below specified limits (typically 200-300°C for most surfacing alloys).
2. Start point rotation: For ring surfacing on pipes, the rotating start method requires careful planning of the circumferential weld layout. The start point should be shifted by a calculated increment for each pass to ensure uniform distribution of start/end locations.
3. Quality control checkpoints: The study's emphasis on arc start and end quality highlights the importance of visual inspection at these critical locations. Non-destructive testing (MT or PT) should be focused on start/end zones where defects are most likely to occur.
4. Process documentation: The interpass temperature records and start point locations should be documented as part of the welding procedure specification (WPS) and welding procedure qualification record (WPQR) for traceability.

Study Insights and Reflections

The combination of numerical simulation and experimental validation in this study provides a robust technical foundation for optimizing ring surfacing processes. The progressive cooling time approach is particularly practical and easy to implement in field conditions, requiring only basic temperature monitoring equipment.

The rotating arc start method represents a simple yet effective process improvement that requires no additional equipment, only operator training and procedure modification. In my experience with pipe surfacing operations, the arc start and end zones are consistently the weakest links in terms of quality, and this study provides clear evidence for implementing systematic start point rotation.

A practical consideration not explicitly addressed in the paper is the interaction between cooling time and production efficiency. While longer cooling times improve quality, they also extend production time. The optimal balance must be determined through cost-benefit analysis, considering the consequences of quality failures (rework, inspection costs, potential service failures) versus the cost of extended production time.