Investigation of Interface Delamination in Nozzle Inner Wall Cladding

Problem Statement and Engineering Context

The paper by Zhang Wenyang, Yang Juwen, and Gu Jialei, published in "Pressure Vessel" journal in 2019, addresses a critical quality issue encountered in the cladding of nozzle inner walls within nuclear power equipment manufacturing. The authors, from Shanghai Electric Nuclear Power Equipment Co., Ltd., report on interface delamination (separation between the overlay layer and the base metal) observed during production and qualification testing of cladded nozzles. This is a high-consequence defect in nuclear applications, where structural integrity and leak-tightness are non-negotiable requirements governed by stringent regulatory frameworks.

Nozzles in pressure vessels serve as connection points for piping and instrumentation, and their inner surfaces are often clad with corrosion-resistant alloys to withstand aggressive process media. The geometry of nozzles presents unique challenges for cladding: the small curvature radius, thin wall thickness relative to the overlay thickness, and the presence of weld joints between the nozzle and the vessel shell all contribute to complex stress states that can promote interface delamination.

Root Cause Analysis of Interface Delamination

The authors conducted a systematic investigation using metallographic analysis, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) to identify the root causes of interface delamination. The following table summarizes the identified causes and their contributing factors:

Defect Type	Root Cause	Contributing Factors	Detection Method
Lack of fusion at interface	Insufficient heat input	Low current, high travel speed, poor flux coverage	UT/TOFD, sectioning
Cold cracking at interface	High residual stress and hydrogen	Thick base metal, low preheat, high carbon equivalent	MT, delayed cracking observation
Bonding failure due to contamination	Oxide scale or paint on substrate	Inadequate surface preparation	Visual, sectioning
Thermal mismatch cracking	Differential thermal expansion	Dissimilar metals, high overlay thickness	UT, hydrostatic test
Intermetallic compound formation	Excessive interdiffusion	High interpass temperature, multiple passes	Metallography, EDS

The study found that the most prevalent defect was lack of fusion at the interface, which occurred when the base metal surface was not properly cleaned or when the welding parameters did not provide sufficient penetration into the substrate. The second most common issue was thermal mismatch cracking, particularly at locations where the nozzle weld joint intersected the overlay area, creating geometric discontinuities that concentrated stress.

Process Optimization and Prevention Measures

Based on the root cause analysis, the authors proposed a comprehensive set of process optimization measures. The following table presents the recommended parameters and procedures:

Control Element	Recommended Specification	Rationale
Surface preparation	Grind to bare metal with 36-40 grit, then polish with 120 grit	Remove oxide scale and ensure intimate contact
Preheat temperature	200-300°C for carbon steel base, 100-150°C for stainless base	Reduce cooling rate and prevent cold cracking
Interpass temperature	Maximum 250°C	Prevent excessive thermal cycling and grain growth
First layer technique	Use higher current and lower travel speed for first pass	Ensure adequate penetration and fusion with base metal
Overlay thickness control	Maximum 2-3 layers, total thickness ≤ 3 mm	Minimize residual stress accumulation
Welding sequence	Radial and circumferential alternating pattern	Distribute heat input evenly and reduce distortion
Post-weld treatment	Stress relief at 550-650°C for 2 hours per 25 mm thickness	Reduce residual stress below cracking threshold

The authors also emphasize the importance of bonding strength testing per ASTM A263 or NB/T 47014 requirements, specifying a minimum shear bond strength of 200 MPa for nuclear applications. Non-destructive testing should include both ultrasonic testing (UT) with phased array (PAUT) for interface inspection and magnetic particle testing (MT) for surface and near-surface defects.

Quality Assurance and Regulatory Compliance

For nuclear applications, the cladding process must comply with applicable codes and standards. The study references the requirements of ASME Section III and RBP (Regulatory Body Program) guidelines, as well as the Chinese nuclear regulatory standards. The welding procedure qualification (WPQ) must include bond strength testing, and the welder qualification must demonstrate consistent performance on nozzle geometry configurations. In-service inspection programs should include periodic UT scanning of overlay interfaces to detect progressive delamination that may develop during operation.

The authors also discuss the importance of documentation and traceability: every step from surface preparation through final inspection must be recorded, and any deviation from the qualified procedure must be documented and evaluated for impact on overlay quality.

Study Insights and Engineering Implications

This paper is particularly valuable because it addresses a real production problem with rigorous analytical methods and practical solutions. The systematic approach to root cause analysis, combining destructive and non-destructive examination techniques, serves as a model for quality investigation procedures in pressure vessel manufacturing. The emphasis on first-layer fusion quality as the critical determinant of interface integrity is a key takeaway for all engineers involved in cladding qualification. For nuclear equipment manufacturers, this research reinforces the necessity of rigorous surface preparation protocols, controlled welding parameters, and comprehensive non-destructive testing to ensure the structural reliability of cladded components.