Welding Process Research on Tube Sheet Cladding

Literature Overview and Core Research Question

Tube sheet cladding is a critical fabrication step in the manufacture of heat exchangers, condensers, and reactors where the tube sheet serves as the boundary between the corrosive process fluid and the structural shell. The cladding layer must provide corrosion resistance while maintaining sufficient mechanical strength and bond integrity under cyclic thermal and pressure loading. This study focuses on the welding process development for stainless steel cladding on carbon steel tube sheets, addressing the challenges of achieving uniform cladding thickness, minimizing dilution, and ensuring full metallurgical and mechanical bond across the entire tube sheet surface including the tube hole areas.

Process Development and Key Parameters

The study evaluated several cladding methods for tube sheet applications: submerged arc welding (SAW) with strip cladding, gas metal arc welding (GMAW) overlay, and plasma transferred arc (PTA) cladding. Each method presents distinct advantages and limitations for the geometry and functional requirements of a tube sheet.

For large-diameter tube sheets exceeding 1200 mm, the study recommends a hybrid approach: SAW strip cladding for the main body to achieve rapid and economical deposition of a thick corrosion-resistant layer, followed by PTA or GMAW repair welding around individual tube holes to ensure adequate cladding coverage and minimum remaining thickness after tube hole drilling.

Dilution Control and Bond Quality

Dilution is the most critical parameter governing the corrosion resistance of the cladding layer. For 304 or 316 stainless steel cladding on carbon steel, the dilution rate must be controlled below 15% to maintain sufficient chromium and nickel content in the weld metal. The study found that the first pass of cladding experiences the highest dilution (25-35%), while subsequent passes stabilize at 8-12%. Therefore, a minimum of three passes is recommended, with the first pass serving primarily to create a metallurgical bond and the final passes achieving the target composition.

Bond strength testing revealed that the mechanical bond between the carbon steel substrate and the stainless steel cladding layer is primarily governed by the dilution rate and the weld metal chemistry at the interface. A dilution rate of 10-20% produces an optimal interfacial composition with adequate iron content to ensure ductility and fracture resistance, while maintaining sufficient alloy content for corrosion protection. Bond strength tests (shear and tensile) consistently showed values exceeding 200 MPa for properly executed cladding, well above the minimum requirements of GB/T 150 and ASME VIII Div.1.

Defect Analysis and Quality Control

The most common defects observed in tube sheet cladding include lack of fusion at the substrate-cladding interface, porosity in the first pass, and undercut at the tube hole boundaries. The study emphasizes the importance of thorough surface preparation of the carbon steel substrate—grinding to bare metal with a minimum 15° toe angle at the edge—to ensure proper wetting and fusion. Preheating to 150-200°C is recommended for tube sheets thicker than 30 mm to reduce the risk of hydrogen-induced cracking at the interface.

Non-destructive testing protocol for tube sheet cladding should include magnetic particle inspection (MT) of the cladding surface and interface area, ultrasonic testing (UT) for bond quality verification, and radiographic testing (RT) for critical areas such as the shell-to-tube sheet joint. The study recommends UT bond testing in accordance with NB/T 47013.3 as the primary method for verifying full-bond integrity across the cladding area.

Engineering Practice Integration

In practice, the study's findings have been validated in the fabrication of shell-and-tube heat exchangers for petrochemical and power generation applications. A case study involving a 1800 mm diameter tube sheet clad with 316L stainless steel demonstrated that the hybrid SAW-PTA approach achieved uniform cladding thickness of 2.5 ± 0.3 mm across the entire surface, with dilution rates controlled below 12% in the final pass. The resulting cladding layer passed intergranular corrosion testing (ASTM A923 Practice E) and ferrite content analysis (30-80% ferrite by the magnetic method) without any measurable defects.

The study concludes that tube sheet cladding is a mature but detail-sensitive process where the success depends on rigorous procedure qualification, precise dilution control, and comprehensive post-cladding inspection. Engineers should prioritize procedure qualification testing per NB/T 47014 or ASME IX to establish validated parameter ranges before production fabrication.