16MnR Tube Sheet Strip Submerged Arc Cladding Process

Literature Overview

This technical paper by Hu Wei and Liu Rongjun from CNPC Second Construction Company, published in China Chemical Equipment in 2005, addresses the strip submerged arc welding (SAW) cladding process applied to 16MnR tube sheets, which are critical structural components in pressure vessels and heat exchangers operating under corrosive or high-temperature conditions. The study provides practical engineering guidance on process parameters, consumable selection, and quality control for achieving reliable metallurgical bonding and adequate overlay thickness on carbon-manganese steel tube sheets.

Core Technical Points

Strip submerged arc welding offers significant advantages for tube sheet cladding over conventional multi-pass processes, including higher deposition rates (typically 3-5 kg/h compared to 0.5-1.5 kg/h for manual methods), superior deposit uniformity, reduced spatter, and excellent slag protection that minimizes oxidation. The process is particularly well-suited for the large, flat surfaces of tube sheets where consistent overlay thickness and bonding quality are critical for subsequent tube installation and pressure containment.

The 16MnR base material, a low-alloy pressure vessel steel with approximately 160 MPa yield strength and good weldability, presents specific challenges for cladding due to the potential for dilution effects, hydrogen-induced cracking in the heat-affected zone, and residual stress development. The process design must address these concerns through appropriate preheating, interpass temperature control, and post-weld heat treatment where required.

Process Parameters and Procedural Requirements

The strip SAW cladding process for 16MnR tube sheets requires careful optimization of multiple parameters to achieve the desired overlay composition, thickness, and mechanical properties. The following table summarizes typical process parameters and their effects:

The strip composition selection is critical and depends on the service environment. For general corrosion resistance, 304 stainless steel strips are adequate, while 316L strips are preferred for chloride-containing environments. The strip thickness directly influences the dilution rate; thinner strips result in higher dilution and lower overlay purity, while thicker strips may not fully melt and can create bonding discontinuities.

Quality Control and Inspection Requirements

Quality assurance for strip SAW cladding of 16MnR tube sheets involves multiple inspection methods applied at various stages of the process. Pre-weld inspection includes verification of base material chemistry, surface preparation quality (grinding to bright metal with Ra below 12.5 μm), and confirmation of consumable certifications. In-process monitoring involves continuous observation of arc stability, flux coverage, and travel speed consistency.

Post-weld inspection requirements include: visual examination for surface defects, porosity, or incomplete melting; magnetic particle testing (MT) of the entire cladding surface to detect surface-breaking cracks; ultrasonic testing (UT) of the bond interface using contact methods or immersion techniques to verify metallurgical bonding across the full surface; and hardness testing to confirm adequate dilution control and overlay composition.

For pressure vessel applications governed by GB/T 150 or ASME VIII Division 1, the cladding must meet specific acceptance criteria including minimum bond strength (typically verified by UT showing continuous bonding), overlay thickness tolerance (usually ±0.5 mm or ±10% of nominal), and mechanical property requirements including hardness limits and, where specified, impact testing of the heat-affected zone.

Engineering Practice and Lessons Learned

In practical implementation, several challenges frequently arise during tube sheet cladding. Edge effects at the boundaries of the cladding area often result in reduced overlay thickness and incomplete bonding due to heat loss and inadequate flux coverage. This is mitigated by using backing plates or edge strips that extend beyond the cladding boundary and are subsequently trimmed.

Another common issue is the formation of unmelted strip pockets, particularly when processing thick strips or at the edges of the welding path. These create voids that compromise bonding integrity and can serve as initiation sites for fatigue failure. Countermeasures include optimizing current density to ensure complete strip melting, maintaining consistent travel speed, and using multiple thin strips rather than single thick strips for critical applications.

Residual stress management is essential for tube sheets that will be subsequently machined or tube-installed. Post-weld stress relief at 580-620°C for durations proportional to section thickness (typically 1 hour per 25 mm of thickness) is recommended for most applications. For tube sheets requiring hole-drilling after cladding, the sequence should be: cladding, stress relief, hole drilling, and final inspection.

Study Insights and Reflections

This work provides valuable practical guidance for engineers involved in pressure vessel fabrication, demonstrating that strip SAW cladding is a mature and reliable technology when properly implemented. The key success factors identified are meticulous surface preparation, precise process parameter control, comprehensive quality inspection, and appropriate post-weld treatment.

The economic advantages of strip SAW cladding over alternative methods such as explosive cladding or explosion-welded strip bonding are significant for large tube sheets, typically reducing fabrication costs by 30-50% while achieving comparable performance. However, the technology requires skilled operators and well-maintained equipment to achieve consistent results, making it most suitable for dedicated production facilities rather than occasional repairs.