Defect Analysis and Remediation of Overlay Welding Layers on Thick-Walled Nozzles of High-Pressure Heat Exchangers

Literature Overview

This paper by Cheng Chuan-yu, published in Chemical Engineering Equipment and Piping (2026, Vol. 63, No. 3), addresses a critical engineering problem encountered in the construction of high-pressure heat exchangers for residue hydrocracking units. The study focuses on cracking defects observed in the INCONEL 625 overlay weld layer applied to the bevel surfaces of thick-walled nozzles, and systematically investigates the root causes and proposes corrective measures. The work originates from Sinopec Guangzhou Engineering Co., Ltd., reflecting the practical challenges faced in large-scale petrochemical project execution.

Background and Engineering Context

In residue hydrocracking and hydrotreating units, high-pressure heat exchangers operate under severe conditions involving elevated temperatures, high pressures, and hydrogen-containing atmospheres. To resist high-temperature H₂-H₂S corrosion, austenitic stainless steel overlay layers are typically applied to the inner surfaces of heat exchanger shells and tubesheets. Additionally, nickel-based alloy overlays (such as INCONEL 625) are deposited on the top of heat exchanger nozzles to facilitate field welding with corresponding austenitic stainless steel piping. The thick-walled nature of these nozzles, combined with the aggressive service environment and the metallurgical complexity of dissimilar metal joining, creates a highly susceptible condition for weld defects.

Core Technical Content and Defect Analysis

The paper systematically examines four primary domains that may contribute to overlay layer cracking:

Overlay welding process parameters — including heat input, travel speed, interpass temperature, and welding sequence.
Post-weld heat treatment (PWHT) — the thermal cycle applied after overlay welding, which affects residual stress relief and microstructural evolution.
Non-destructive testing (NDT) procedures — the timing, methods, and acceptance criteria applied to the overlay welds.
Surface grinding and finishing — the mechanical preparation of the weld surface after welding, which may introduce residual stresses or expose subsurface defects.

Root Cause Analysis Framework

The author employs a systematic approach to identify contributing factors. The following table summarizes the key defect mechanisms identified:

Defect Domain	Potential Root Cause	Mechanism	Consequence
Welding process	Excessive linear heat input	Overheating of base metal, grain coarsening in HAZ	Cracking susceptibility increase
Welding process	Inadequate interpass temperature control	Rapid cooling, high residual stress	Cold cracking in overlay layer
PWHT	Insufficient soak time or temperature	Incomplete stress relief, retained austenite instability	Delayed cracking
NDT	Inadequate coverage or method selection	Missed subsurface defects	Undetected cracks propagate in service
Surface grinding	Excessive material removal, grinding burns	Surface hardening, micro-cracks introduced	Stress concentration, crack initiation

Metallurgical Considerations

The INCONEL 625 overlay alloy (UNS N06625) is a nickel-chromium-molybdenum base alloy with excellent resistance to oxidation and corrosion at elevated temperatures. However, when applied to thick carbon steel or low-alloy steel nozzle bases, several metallurgical challenges arise:

Dilution effects: During multi-pass overlay welding, each subsequent pass dilutes the previous overlay layer with base metal. The first pass typically exhibits the highest dilution rate, which can significantly alter the composition and properties of the deposit.
Residual stress: Thick-walled nozzles experience significant拘束 (constraint) during welding due to the geometry and base metal properties. The high residual stresses in the overlay region, combined with the thermal cycling, create conditions favorable for cracking.
Hydrogen-induced cracking: The base metal of thick-walled nozzles may contain trapped hydrogen, which can diffuse into the overlay weld during welding and cause delayed cracking.

Remediation Strategy

The paper proposes a comprehensive repair scheme that includes:

Defect removal: Complete removal of the defective overlay layer using mechanical methods (grinding) or thermal cutting, followed by thorough NDT verification of the repair area.
Revised welding procedure: Optimized welding parameters with controlled heat input, proper preheating, and a modified multi-pass sequence to minimize dilution and residual stress.
Enhanced PWHT: Extended soak time at appropriate temperatures to ensure complete stress relief and microstructural stabilization.
Improved NDT protocol: Application of multiple NDT methods (RT, UT, PT) at critical stages to ensure comprehensive defect detection.
Controlled surface finishing: Careful grinding with monitoring of removed depth and surface quality to avoid introducing new defects.

Engineering Practice Implications

This study carries significant practical value for engineers involved in the construction and maintenance of high-pressure equipment in petrochemical applications. Several key lessons emerge:

WPS qualification must account for base metal thickness: Standard welding procedure qualifications may not adequately represent the conditions encountered on thick-walled nozzles. Procedure-specific qualification tests should be conducted.
PWHT is not optional: For thick-walled overlay welds, post-weld heat treatment is essential, not merely recommended. The thermal cycle must be carefully controlled.
Surface preparation is critical: The final surface condition of the overlay weld significantly affects its long-term performance, particularly in corrosive environments.

Key Questions and Reflections

One question that arises from this study is whether the selection of INCONEL 625 as the overlay material is always optimal for this application. Alternative nickel-based alloys, such as INCOLOY 825 or Hastelloy C-276, may offer different combinations of corrosion resistance and weldability. The choice should be guided by the specific service conditions, including temperature, pressure, and corrosive medium composition.

Another consideration is the role of welding sequence in managing residual stresses. For thick-walled nozzles, a symmetric welding sequence or the use of stress-relief grooves may be beneficial. The paper does not appear to address this aspect in detail, which represents an area for further investigation.

Study Insights and Reference Value

This paper provides a valuable engineering reference for practitioners dealing with overlay welding defects on thick-walled components. The systematic approach to defect analysis and the comprehensive remediation strategy demonstrate good engineering methodology. The study reinforces the principle that overlay welding quality is a system property, dependent on the integrated performance of process parameters, post-weld treatment, inspection, and surface finishing. Engineers should adopt this holistic perspective when addressing similar challenges in their own projects.