Strip Electrode Surfacing Process for Q345R Tube Sheets

Literature Overview

The paper by Yan Shun and colleagues from Wenzhou Tianlong Light Industry Equipment Co., Ltd., published in Guangdong Chemical Industry in 2010 (Vol. 37, No. 3, pp. 214-215), documents a practical engineering challenge encountered during the implementation of strip electrode surfacing technology for corrosion-resistant overlay on pressure vessel tube sheets made of Q345R steel. The authors describe problems arising from excessive deformation during strip electrode surfacing when the overlay thickness is large, and present solutions that were developed through process optimization and engineering practice.

Technical Background

Strip electrode surfacing (also known as submerged arc strip welding) offers significant advantages over manual surfacing for tube sheet applications:

Higher deposition rate: Strip electrodes provide a larger cross-sectional area of filler metal, resulting in deposition rates several times higher than manual welding.
Improved consistency: The automated nature of the process ensures uniform weld bead geometry and composition.
Reduced labor cost: Automated processes reduce the dependence on skilled welders and improve productivity.
Better surface finish: Strip electrode surfacing produces smoother overlay surfaces that require less post-weld machining.

However, these advantages come with challenges, particularly regarding thermal input and deformation control. Strip electrode surfacing typically operates at higher current densities than manual processes, resulting in greater line energy and more severe thermal distortion of the base component.

Deformation Problem Analysis

The core problem identified in this study is the severe deformation of tube sheets during strip electrode surfacing when the overlay thickness exceeds a certain threshold. Tube sheets are critical components in pressure vessels, and excessive deformation can lead to:

Dimensional non-conformance: Warped or distorted tube sheets cannot be properly assembled with the vessel shell or tube bundle.
Tube hole misalignment: Deformation can cause misalignment of tube holes, leading to assembly difficulties or compromised tube-to-tubesheet joints.
Residual stress concentration: Severe deformation is accompanied by high residual stresses that can reduce the fatigue life of the component.
Component rejection: In extreme cases, the deformed tube sheet must be scrapped, representing a significant economic loss.

Root Cause Analysis Using FMEA Approach

Applying a Failure Mode and Effects Analysis (FMEA) framework to the deformation problem:

Failure Mode	Cause	Effect	Severity	Occurrence	Detection	RPN
Tube sheet warping	Excessive line energy	Dimensional non-conformance	9	7	5	315
Local buckling	Uneven thermal distribution	Component rejection	10	6	4	240
Tube hole distortion	Non-uniform deformation	Assembly failure	8	5	6	240
Cracking at overlay boundary	High residual stress	Reduced fatigue life	7	4	5	140

Process Solutions and Optimization

The authors developed several solutions to address the deformation problem, which can be summarized as follows:

Solution 1: Segmented Surfacing Strategy

Instead of surfacing the entire tube sheet surface in a single continuous operation, the overlay is divided into segments with controlled overlap. Each segment is completed before moving to the next, allowing thermal stresses to partially relax between passes. This approach requires careful planning of the surfacing sequence to ensure uniform coverage and minimize stress accumulation.

Solution 2: Backing Plate and Restraint

The use of rigid backing plates and mechanical restraint fixtures during surfacing helps control deformation by constraining the tube sheet movement. However, excessive restraint can induce high residual stresses, so the restraint system must be designed to allow controlled movement while preventing gross deformation.

Solution 3: Process Parameter Optimization

Reducing the welding current and increasing the travel speed can decrease the line energy per pass, reducing deformation at the cost of lower deposition rate. A multi-pass approach with lower heat input per pass may achieve the required overlay thickness with acceptable deformation.

Solution 4: Pre-Deformation Compensation

Based on the predicted deformation pattern, the tube sheet can be pre-shaped in the opposite direction before surfacing. This pre-deformation compensates for the expected distortion, resulting in a component that achieves the desired geometry after surfacing.

Parameter	Before Optimization	After Optimization	Improvement
Line energy per pass	High	Reduced by 20-30%	Lower deformation
Number of passes	Few	Increased	Better stress distribution
Overlay thickness per pass	Large	Reduced	Less local distortion
Travel speed	Low	Increased	Lower heat input
Deformation magnitude	Excessive	Within tolerance	Acceptable component
Scrap rate	High	Reduced	Economic benefit

Quality Control and Verification

The implementation of the optimized surfacing process requires comprehensive quality control:

Dimensional verification: Measure the flatness and roundness of the tube sheet after surfacing to confirm that deformation is within acceptable limits.
Overlay thickness measurement: Use ultrasonic thickness gauges to verify that the overlay meets the specified thickness throughout the surface.
Bond integrity testing: Perform peel tests or cross-section examinations to confirm that the overlay is properly bonded to the substrate without delamination.
Residual stress measurement: Use X-ray diffraction or strain gauge methods to measure residual stresses in the overlay and verify that they are within acceptable limits.
Corrosion testing: Verify that the overlay provides the required corrosion resistance through immersion testing or electrochemical evaluation.

Engineering Practice Integration

The experience documented in this paper is highly relevant to engineers working on pressure vessel tube sheet refurbishment and new construction. The key lessons are:

Process capability assessment: Before implementing strip electrode surfacing on a new component, conduct a process capability study to determine the achievable deposition rate, overlay quality, and deformation magnitude under specific conditions.
Process qualification testing: Perform weld procedure qualification (WPQ) tests on coupon specimens that simulate the actual component geometry and thickness to establish the process window before production welding.
In-process monitoring: Implement real-time monitoring of welding parameters and component temperature to detect deviations from the qualified procedure and make timely corrections.
Post-weld inspection: Conduct thorough dimensional and non-destructive testing after surfacing to verify that the component meets all specifications before release for assembly.

Study Insights and Implications

This paper provides a valuable case study in the practical implementation of advanced surfacing technology. The transition from manual to automated surfacing is a natural progression in modern manufacturing, but it requires careful attention to the differences in process behavior and the potential for new failure modes.

The deformation problem described here is not unique to tube sheet surfacing; it affects all automated welding processes that involve high heat input on thin-walled or geometrically constrained components. The solutions developed by the authors—segmented surfacing, backing plate restraint, parameter optimization, and pre-deformation compensation—are generalizable to other applications.

For engineers planning to adopt strip electrode surfacing technology, this paper serves as a cautionary tale about the importance of thorough process development before production implementation. The economic benefits of automated surfacing are substantial, but only if the process is properly controlled to avoid costly rework or scrap. A disciplined approach to process qualification, in-process monitoring, and quality verification is essential for successful implementation.