Welding Operation and Deformation Control in Heat Exchanger Tube Sheet Cladding

Literature Overview

Published in 2010 by Kong Bianli and Zhou Lixin from Kaifeng Air Separation Group Co., Ltd., this paper addresses one of the most technically challenging applications of weld overlay in pressure vessel fabrication: the cladding of stainless steel or nickel-based alloy layers on heat exchanger tube sheets. Air separation units require tube sheets that combine the strength and weldability of carbon or low-alloy steel with the corrosion resistance of austenitic stainless steel (typically 304 or 316L) against liquid oxygen, liquid nitrogen, and liquid air service. The paper focuses on the practical welding operations and, critically, the deformation control strategies required to maintain the dimensional accuracy of large-diameter tube sheets during multi-pass overlay welding.

Core Technical Content

Application Context and Requirements

Heat exchanger tube sheets in air separation service typically have diameters ranging from 800 mm to over 3000 mm, with thicknesses of 40–120 mm. The cladding layer—usually 304 or 316L stainless steel—must be applied to the channel side (liquid process side) with a thickness of 3–6 mm. The critical requirements include:

Requirement	Specification
Cladding material	304L or 316L austenitic stainless steel
Cladding thickness	3–6 mm
Base material	Q345R, 16MnR, or 15CrMo
Dilution limit	≤20% (per GB/T 150)
Bond strength	≥90 MPa (per GB/T 150)
Surface roughness	Ra ≤ 12.5 μm
NDT coverage	100% MT or PT; spot UT for bond

Welding Process and Operational Techniques

The welding methods commonly employed for tube sheet cladding include:

SAW (Submerged Arc Welding): Preferred for large-diameter tube sheets due to high deposition rate and deep penetration. Multi-wire or multi-arc SAW can achieve deposition rates of 8–15 kg/h. The process requires careful alignment and travel speed control to maintain uniform bead width and profile.
GMAW (Gas Metal Arc Welding): Used for edge areas, corners, and areas inaccessible to SAW. Provides good visual control and adaptability to complex geometries.
GTAW (Gas Tungsten Arc Welding): Employed for the first pass (root pass) to establish a clean, low-dilution bond between the base metal and the overlay.

Deformation Control Strategies

Deformation is the central challenge in tube sheet cladding. The thermal expansion mismatch between the austenitic overlay (higher thermal expansion coefficient, ~17 μm/m·°C) and the ferritic base metal (~12 μm/m·°C) generates significant residual stresses and distortion. The paper describes several control strategies:

Symmetric welding sequence: Welding in a spiral or radial pattern from the center outward, or in alternating segments, to balance thermal input and minimize warping.
Back-plate clamping: Rigidly clamping the tube sheet to a back-plate or fixture to constrain deformation during welding.
Staggered pass welding: Dividing the cladding area into sectors and welding them in a staggered sequence to allow thermal relaxation between passes.
Temperature monitoring: Using thermocouples to monitor surface and back-side temperatures, ensuring they remain within the specified preheat and interpass temperature ranges.
Post-weld stress relief: Applying controlled heat treatment (590–650 °C for carbon steel base) to relieve residual stresses after cladding is complete.

Defect Analysis and Countermeasures

Defect	Cause	Prevention
Edge cracking	High restraint; excessive thermal gradient	Reduce interpass temperature; use compatible filler; preheat edges
Center warping	Asymmetric heat input	Symmetric welding sequence; fixture clamping
Delamination	Insufficient penetration; oxide inclusion	Ensure proper surface prep; verify first-pass penetration
Excessive dilution	High heat input; poor groove design	Optimize arc parameters; use appropriate groove geometry

Engineering Practice Reflections

The experience documented in this paper is directly applicable to modern air separation and cryogenic pressure vessel fabrication. The emphasis on deformation control through welding sequence optimization and fixture design remains a fundamental principle. In current practice, finite element analysis (FEA) of thermal stresses is often employed to predict deformation before fabrication, but the empirical strategies described here—symmetric sequencing, back-plate clamping, and temperature monitoring—remain the backbone of practical shop-floor execution. The paper also highlights the importance of process qualification per NB/T 47014 or ASME IX, ensuring that the welding procedure produces consistent bond strength and minimal dilution across the entire tube sheet diameter. For engineers involved in cryogenic heat exchanger fabrication, this work provides a practical foundation for managing the thermal and mechanical challenges of large-scale overlay welding.