Welding Operation and Deformation Control of Heat Exchanger Tube Sheet Cladding

Literature Overview

This 2010 publication from Kaifeng Air Separation Group Co., Ltd., authored by Kong Bianli and Zhou Lixin, addresses a critical engineering challenge in the fabrication of large-diameter air separation columns and heat exchangers: the overlay welding of corrosion-resistant layers onto carbon steel tube sheets while controlling geometric distortion and residual stress. Tube sheets in cryogenic air separation units typically require austenitic stainless steel (304L or 321) overlay layers of 1.5 to 3.0 mm thickness to resist corrosion from trace contaminants and to provide thermal cycling resistance. The authors focus on the practical welding sequence, current parameters, and distortion mitigation strategies used in production environments, which makes this paper highly relevant to shop-floor engineers who must balance productivity with dimensional accuracy.

Core Technical Content

The study investigates the welding operation sequence for tube sheet cladding, which is a multi-pass, multi-directional process due to the large number of tube holes and the complex geometry of the tube sheet. The authors emphasize that the welding sequence is the single most influential factor in controlling distortion. A symmetric, progressive welding pattern radiating from the center outward is recommended to distribute heat input evenly and minimize angular and bow distortion. The overlay welding is typically performed using submerged arc welding (SAW) or gas metal arc welding (GMAW) with flux-cored or solid stainless steel wire, depending on the required thickness and production rate.

The following table summarizes the typical process parameters discussed:

Deformation Analysis and Control Measures

The primary deformation modes observed in tube sheet cladding are angular distortion, bow distortion, and local out-of-plane waviness. Angular distortion occurs because the thermal expansion of the overlay layer during welding is constrained by the cooler base metal, generating compressive residual stresses in the overlay and tensile stresses in the base. Upon cooling, this mismatch drives the tube sheet edges upward. Bow distortion results from asymmetric heat input distribution across the tube sheet diameter.

The authors propose several control measures:

1. Symmetric welding sequence: Welding should proceed in a spiral or concentric ring pattern from the center outward, ensuring that adjacent weld beads are welded in opposite directions to cancel angular distortion.
2. Back-step welding: Instead of welding continuously from one end to the other, the welder should weld in short segments stepping back toward the start, which reduces longitudinal residual stress.
3. Mechanical restraint: Rigid clamping fixtures or welding tables with adjustable supports can constrain the tube sheet during welding, though this increases residual stresses and must be balanced against distortion reduction.
4. Post-weld mechanical flattening: For large tube sheets, controlled hammering or hydraulic pressing after welding can correct minor waviness.
5. Heat input management: Reducing current and increasing travel speed lowers peak temperatures and the size of the heat-affected zone, thereby reducing distortion magnitude.

Engineering Practice Integration

From my experience in pressure vessel fabrication, the tube sheet cladding problem is one of the most challenging operations because the part is large, heavily constrained by the vessel shell and heads, and must meet tight flatness tolerances (typically within 1.5 mm over 1000 mm). In practice, I have found that the welding sequence must be planned in conjunction with the vessel assembly sequence. If the tube sheet is welded to the shell before cladding, the shell acts as a massive heat sink and restraint, increasing residual stresses but reducing distortion. If cladding is performed before assembly, distortion is more freely developed but can be corrected more easily with mechanical methods.

A critical quality control point is the bond strength test between the overlay and the base metal. Per NB/T 47002 and ASME VIII Div. 1, the bond strength must be verified by a tensile or peel test, and the overlay must fail cohesively within the overlay layer rather than at the interface. In my experience, incomplete bonding is most often caused by inadequate cleaning of the base metal surface before the first pass, or by excessive interpass temperature leading to base metal dilution and brittle intermetallic formation.

Key Reflections

The paper's greatest strength is its practical orientation: it does not merely present experimental data but provides actionable welding procedures that can be directly implemented on the shop floor. However, I would note that the study could benefit from quantitative residual stress measurements (using X-ray diffraction or hole-drilling methods) to validate the effectiveness of the proposed control measures. In modern fabrication, we increasingly rely on finite element simulations to predict distortion and optimize welding sequences before any actual welding is performed. The lessons from this paper remain valid as a foundational reference for developing such simulation models.

In summary, this publication provides valuable practical guidance for controlling distortion during tube sheet overlay welding, and its recommendations regarding symmetric welding sequences, heat input management, and mechanical restraint remain applicable in contemporary pressure vessel fabrication.