Application of Pre-deformation Method in Tube Sheet Cladding

Literature Overview

This 1996 publication in the journal Welding Technology by Xu Zailin and Sheng Likuan from Changzhou Chemical Machinery Factory in Jiangsu Province describes the application of a pre-deformation method to address challenges encountered during the cladding of tubesheets. Tubesheets, also known as tubesheet plates or channel covers, are critical components in heat exchangers, reactors, and pressure vessels that serve as the interface between the shell side and tube side. Cladding of tubesheets is performed to provide corrosion resistance on the tube side or shell side while maintaining the mechanical strength of the base material. The pre-deformation method is presented as a technique to mitigate residual stresses, distortion, and cracking that commonly occur during the cladding process.

Core Technical Analysis

The fundamental challenge in tubesheet cladding is the management of thermal stresses generated during the welding process. Tubesheets are typically thick plates, with thicknesses ranging from 30 mm to over 200 mm, and are often made of carbon steel or low-alloy steel. The cladding layer, typically stainless steel (such as 304 or 316), is deposited on one or both surfaces to provide corrosion resistance. The significant difference in thermal expansion coefficients between the base material and the cladding layer, combined with the large heat input required for thick plate cladding, generates substantial residual stresses that can lead to distortion, cracking, or delamination.

The pre-deformation method involves introducing controlled plastic deformation into the tubesheet before cladding is performed. This pre-deformation can be achieved through mechanical means such as rolling, pressing, or stretching, or through thermal means such as localized heating and controlled cooling. The purpose of pre-deformation is to create a residual stress state that counteracts the stresses generated during cladding, thereby reducing the net residual stress and minimizing distortion.

Pre-deformation Methods and Their Characteristics

Pre-deformation Method	Mechanism	Typical Deformation Level	Applicable Thickness (mm)	Effectiveness
Mechanical Rolling	Compressive plastic strain	0.5-2.0% elongation	30-150	High
Tensile Stretching	Tensile plastic strain	0.3-1.0% elongation	50-200	Moderate
Thermal Expansion	Differential thermal expansion	0.2-0.8% elongation	30-200	Moderate
Shot Peening	Surface compressive stress	-300 to -600 MPa	10-100	High (surface)
Indentation	Localized plastic deformation	Variable	20-150	Moderate

Residual Stress Management Strategy

The residual stress in a cladded tubesheet can be decomposed into three components: welding residual stress from the cladding process, thermal residual stress from the temperature differential between cladding and base, and mechanical residual stress from the constraint of the cladding layer to the base material. The pre-deformation method introduces an additional residual stress component that is designed to oppose the welding and thermal residual stresses.

The effectiveness of pre-deformation depends on the magnitude, direction, and distribution of the pre-introduced stress. For example, tensile pre-deformation of the tubesheet introduces a residual tensile stress in the base material, which is then partially relieved during cladding as the cladding layer contracts upon cooling. This contraction reduces the net tensile stress in the cladding layer, which is the primary driver of cladding cracking.

Engineering Implementation and Quality Control

The implementation of the pre-deformation method in tubesheet cladding requires careful planning and execution to ensure that the pre-deformation achieves the desired residual stress state without introducing defects such as wrinkles, cracks, or excessive thinning. The pre-deformation process must be controlled to maintain the dimensional accuracy of the tubesheet, particularly the flatness and hole position tolerances, which are critical for tube insertion and sealing.

Quality control of pre-deformed tubesheets involves measurement of the residual stress state before and after pre-deformation, typically using X-ray diffraction (XRD) or hole-drilling methods. The residual stress profile through the thickness should be characterized to verify that the pre-deformation has achieved the target stress distribution. After cladding, the residual stress should be measured again to verify the effectiveness of the pre-deformation in reducing the net residual stress.

Process Sequence for Pre-deformed Tubesheet Cladding

Step	Operation	Key Parameters	Quality Check
1	Base plate inspection	Thickness, flatness, material certification	Visual, UT, chemical analysis
2	Pre-deformation	Deformation method, magnitude, direction	Residual stress measurement, dimensional check
3	Surface preparation	Cleaning, degreasing, flux application	Visual, contamination test
4	Cladding welding	Process parameters, number of passes, interpass temperature	Visual, MT, UT
5	Post-weld inspection	Bond strength, hardness, mechanical properties	Bond test, HV, tensile test
6	Final stress measurement	Residual stress profile	XRD, hole-drilling

Application in Heat Exchanger and Reactor Manufacturing

Tubesheet cladding is a standard practice in the manufacturing of heat exchangers and reactors for the chemical, petrochemical, and power generation industries. The tubesheet must withstand high internal pressures while providing a reliable seal between the shell side and tube side. Cladding provides corrosion protection against aggressive process fluids such as acidic solutions, caustic solutions, and chloride-containing media.

The pre-deformation method is particularly valuable for large-diameter tubesheets with thick walls, where the residual stresses from cladding can be substantial. In such cases, the cladding layer may crack or delaminate if the residual stresses are not properly managed. The pre-deformation method provides a practical and cost-effective solution to this problem, as it requires no additional equipment beyond standard mechanical forming tools.

Comparative Analysis of Residual Stress Reduction

Tubesheet Thickness (mm)	Cladding