Strip Cladding of Large Diameter Tube Sheets A Literature Study Note

Literature Overview

This paper addresses the practical challenges of applying strip cladding to large-diameter tube sheets, which are critical components in heat exchangers, reactors, and other pressure-containing equipment. Tube sheets with diameters exceeding 800 mm present unique difficulties related to thermal distortion, residual stress distribution, and cladding quality uniformity across large areas. The study presents process development, parameter optimization, and quality assurance strategies for strip cladding of carbon steel tube sheets with stainless steel overlay layers.

Technical Challenges of Large Diameter Tube Sheets

Large tube sheets differ fundamentally from smaller components in several respects. The thermal mass is greater, leading to slower heating and cooling rates, which can cause localized overheating near the cladding zone while distant regions remain cold. This thermal gradient induces significant residual stresses that may lead to distortion, cracking, or poor bond strength. The flatness tolerance of a large tube sheet, typically held to within 1 to 2 mm per meter, can be compromised by thermal distortion during cladding.

Process Strategy and Welding Sequence

The welding sequence is the most critical process variable for large tube sheets. The study proposes a spiral or segmented sequence that starts from the center and works outward, or vice versa, to distribute thermal input evenly. A center-outward sequence allows the inner region to cool while the outer region is being clad, reducing overall distortion. However, this approach may create a gradient in interpass temperature, requiring careful monitoring.

The author recommends dividing the tube sheet into concentric annular zones, each approximately 100 to 150 mm wide, and cladding each zone sequentially. Within each zone, the strip cladding is performed in a back-and-forth pattern with overlapping passes of 25 to 40 percent to ensure full coverage and uniform thickness. The overlap ratio is calculated based on the effective cladding width, which depends on the strip width, torch geometry, and arc parameters.

Welding Parameters and Equipment Configuration

Strip cladding typically employs submerged arc welding (SAW) or flux-cored arc welding (FCAW) equipment. For large tube sheets, dual-wire SAW is often preferred because it provides higher deposition rates and better arc stability. The study discusses the use of a wire diameter of 1.6 mm with a flux-cored strip or solid strip of 6 to 10 mm width, depending on the required overlay thickness.

The current range for strip cladding is typically 300 to 600 A, with voltage between 25 and 35 V. Higher currents increase penetration depth and bonding strength but also increase heat input and distortion risk. The study found that a current of 450 A with a voltage of 30 V provided an optimal balance for 304 stainless steel strip cladding on SA354 Grade 7 carbon steel tube sheets.

The flux or cored strip composition is critical for controlling dilution and preventing cracking. For carbon steel to stainless steel transitions, a low-carbon flux with controlled manganese and silicon content is recommended. The dilution rate, typically 20 to 40 percent for the first layer, decreases to 10 to 20 percent for subsequent layers as the overlay thickens.

Quality Assurance and Inspection

Quality assurance for strip-clad tube sheets requires a comprehensive inspection regime. Visual inspection is performed after each pass to check for surface defects, undercuts, and incomplete fusion. Magnetic particle testing (MT) is applied to detect surface and near-surface cracks, particularly at the strip edges where stress concentrations are highest. Ultrasonic testing (UT) is used to verify bond strength and detect subsurface defects.

The study references NB/T 47014 for weld procedure qualification and JB/T 4730 for non-destructive testing requirements. For pressure vessel applications, the overlay layer must be qualified in accordance with the relevant code provisions, including tensile testing, bend testing, and hardness measurement. The hardness of the overlay layer should not exceed the maximum allowable value for the specific alloy grade, typically 250 HV for 304 stainless steel.

Distortion Control Measures

Thermal distortion is the primary quality concern for large tube sheet cladding. The study recommends several mitigation strategies: preheating the entire tube sheet uniformly to reduce thermal gradients; using a backing plate with high thermal mass to absorb heat from the back side; and applying mechanical restraint devices at the tube sheet edges. The backing plate should be preheated to match the tube sheet temperature to prevent thermal shock at the interface.

A practical approach discussed in the paper involves using a water-cooled backing plate with adjustable coolant flow rates. By controlling the cooling rate, the operator can manage the thermal cycle and reduce distortion. However, excessive cooling can increase residual stress and cracking risk, so a balance must be struck. The author recommends a cooling rate of 0.5 to 2.0 degrees C per second for carbon steel substrates.

Study Insights and Implications

The paper provides valuable practical guidance for strip cladding of large tube sheets, particularly regarding welding sequence optimization and distortion control. The emphasis on systematic parameter development and quality assurance is commendable, as these aspects are often overlooked in favor of purely economic considerations.

One insight that stands out is the recognition that large tube sheet cladding is not merely a scaled-up version of small component cladding. The thermal, mechanical, and metallurgical behaviors change qualitatively with size, requiring fundamentally different process approaches. Engineers should not simply extrapolate parameters from smaller components without accounting for these size-dependent effects.

The paper could benefit from additional discussion on the economic aspects of large tube sheet cladding, including cost per unit area, productivity metrics, and the impact of rework on overall project costs. Nevertheless, the technical content is thorough and directly applicable to engineering practice.