Overlay Welding Process for Waste Heat Boiler Tube Sheets

Literature Overview

The paper by Liu Baoxiang, published in China Chemical Equipment (Vol. 11, No. 1, 2009, pp. 43–44), addresses a specific and recurring engineering problem: cracking at the arc transition zones during overlay welding of waste heat boiler tube sheets. The author, representing Qingdao Soft Control Heavy Industry Co., Ltd., documents a defect analysis and process improvement effort that overcame the cracking phenomenon through systematic process optimization. This case study exemplifies the practical challenges encountered in chemical equipment repair and modification, where overlay welding is used to upgrade corrosion resistance or extend service life of existing components.

Problem Definition and Defect Analysis

The original overlay welding process produced cracks specifically at the arc transition areas—regions where the weld bead geometry changes from a flat overlay surface to the curved transition into the base material. This defect location is not random; it corresponds to zones of maximum geometric discontinuity where stress concentration factors are elevated. The cracking mechanism can be analyzed through a systematic approach:

Defect Parameter	Observation	Root Cause Analysis
Crack location	Arc transition zones (bead edges)	High geometric stress concentration
Crack morphology	Transverse, branching	Thermal stress + residual stress interaction
Crack initiation	Surface, near fusion line	Hydrogen embrittlement + tensile stress
Crack propagation	Intergranular and transgranular mixed	Combination of HAZ microstructural weakness and thermal cycling

The fundamental causes of cracking at transition zones include:

Thermal stress concentration: The abrupt change in cross-sectional geometry at the arc transition creates stress raisers that amplify thermal stresses during cooling.
Residual stress accumulation: Multi-pass overlay welding generates complex residual stress fields. The transition zones experience the superposition of stress from adjacent passes and the stress from the current pass.
Hydrogen embrittlement: Moisture from flux or contamination introduces hydrogen into the weld metal. Hydrogen diffuses to regions of high triaxial tensile stress—the transition zones—causing delayed cracking.
Microstructural vulnerability: The heat-affected zone adjacent to the transition area may exhibit coarse grain growth or retained austenite transformation, creating microstructural discontinuities that facilitate crack initiation.

Process Improvement Strategy

The improved process incorporated several key modifications following a PDCA (Plan-Do-Check-Act) approach:

Plan Phase - Process Design Modifications

Process Parameter	Original	Improved	Rationale
Preheat temperature	100°C	200–250°C	Reduce cooling rate, promote hydrogen diffusion
Interpass temperature	Not controlled	150–200°C	Maintain thermal continuity, reduce thermal shock
Welding current	High (fast deposition)	Moderate (controlled heat input)	Reduce thermal stress, improve wetting
Travel speed	High	Reduced	Increase bead overlap, smooth transitions
Layer sequence	Sequential	Spiral/circular pattern	Distribute residual stress uniformly
Post-weld treatment	None	Stress relief 550–600°C	Release residual stresses, stabilize microstructure

Do Phase - Implementation Details

The improved process emphasized several critical control points:

Gradual transition technique: Instead of abrupt bead termination at the transition zone, the welder was instructed to taper the bead width gradually, creating a smooth geometric transition that minimizes stress concentration.
Multi-layer, multi-pass strategy: A transition layer with intermediate composition was deposited between the base material and the final corrosion-resistant overlay, reducing dilution effects and providing a metallurgical buffer zone.
Weld sequence optimization: The welding sequence was modified to a balanced pattern that ensures symmetric thermal input and minimizes net distortion and asymmetric residual stress.
Flux and wire selection: Low-hydrogen flux or flux-cored wire was specified to minimize hydrogen ingress, and the filler metal composition was adjusted to provide adequate ductility in the transition layer.

Check Phase - Quality Verification

Post-improvement quality verification included:

Inspection Method	Purpose	Acceptance Criteria
Visual inspection (VT)	Surface crack detection	No visible cracks, uniform bead profile
Magnetic particle testing (MT)	Surface and near-surface defects	No indications at transition zones
Ultrasonic testing (UT)	Subsurface defects, bond integrity	No disbondment, no internal cracks
Hardness survey	Overlay hardness uniformity	Within specified range, no excessive softening at transition
Penetrant testing (PT)	Surface cracks, especially at edges	No linear indications

Act Phase - Standardization

The successful process was documented as a work instruction and incorporated into the quality management system, with defined acceptance criteria and operator qualification requirements.

Metallurgical Considerations

The tube sheet application in waste heat boilers presents unique metallurgical challenges. Tube sheets typically consist of carbon steel or low-alloy steel (such as 16Mn or 15CrMo) and must withstand:

High-temperature thermal cycling from hot flue gas on one side and cooler working fluid on the other
Internal pressure from the working fluid (water, steam, or process fluid)
Corrosive attack from combustion products (SO₂, HCl, acid dew point corrosion)
Mechanical loading from tube bundle insertion and operational vibration

The overlay welding must therefore produce a coating that is:

Corrosion resistant (typically Cr-Ni austenitic or duplex stainless steel)
Thermally stable at operating temperatures (up to 400–500°C for waste heat boilers)
Crack resistant under thermal cycling
Compatible with the base material in terms of thermal expansion and mechanical properties

The transition layer composition is critical. A typical transition layer might use a Ni-Cr base alloy (such as ENiCrFe-3 or equivalent) that provides intermediate thermal expansion and sufficient ductility to accommodate thermal mismatch between the ferritic base and austenitic overlay.

Engineering Practice Implications

This case study underscores several universal principles for overlay welding of thick-section components:

Geometry matters: Stress concentration at geometric discontinuities is often the primary driver of weld cracking, even when metallurgical factors are secondary. Process design must account for weld geometry, not just weld chemistry.
Thermal management is paramount: Preheat, interpass temperature control, and post-weld heat treatment are not optional—they are essential for preventing cracking in thick-section overlay welding.
Systematic defect analysis is indispensable: The PDCA approach applied here—identifying the defect, analyzing root causes, implementing targeted corrections, and verifying effectiveness—provides a replicable framework for addressing overlay welding problems.
Operator skill and technique are process variables: The gradual transition technique and welding sequence optimization require skilled operators who understand the metallurgical rationale behind their actions.

Study Insights and Reflections

The most valuable aspect of this paper is its demonstration of how a seemingly intractable cracking problem was resolved through systematic engineering analysis rather than trial and error. The insight that arc transition zones are inherently vulnerable to cracking due to geometric stress concentration, combined with the recognition that thermal management and welding sequence optimization can mitigate this vulnerability, represents practical engineering wisdom that transcends this specific application. For engineers working on overlay welding of large chemical equipment, this case reinforces the importance of thinking about the weld as a three-dimensional stress field rather than a simple material deposit.