Interface Delamination in Nozzle Inner Wall Hardfacing

Literature Overview

The paper by Zhang Wenyang, Yang Juwen, and Gu Jialei (2019), published in Pressure Vessel (Vol. 36, No. 2, pp. 10-15), addresses a critical quality issue in nuclear power equipment manufacturing: interface delamination during inner wall hardfacing of safety end nozzles. The authors from Shanghai Electric Nuclear Equipment Co., Ltd. systematically analyze the microstructure and residual stress distribution near the fusion line between 18MND5 base material and 309L hardfacing layer, identifying the root causes of delamination and proposing effective countermeasures.

Problem Statement and Significance

Interface delamination in dissimilar metal hardfacing is one of the most serious defects in nuclear power equipment fabrication. Unlike conventional pressure vessels, nuclear components are subject to extremely stringent quality requirements, and any defect at a material interface can compromise the integrity of the entire component. The safety end of a reactor pressure vessel is a critical component that must withstand extreme pressure, temperature, and radiation conditions, making the integrity of its material interfaces paramount.

The delamination problem occurs specifically at the 18MND5/309L interface—the boundary between the low-alloy martensitic base material and the austenitic stainless steel overlay. This interface is inherently vulnerable due to the significant differences in thermal expansion coefficients, thermal conductivity, and microstructure between the two materials.

Root Cause Analysis

The authors conduct a systematic investigation using metallographic examination and residual stress analysis. The key findings reveal a chain of causation:

The analysis reveals that the martensitic transformation occurring in the heat-affected zone (HAZ) of the 18MND5 base material creates a wide band of hard, brittle microstructure adjacent to the fusion line. This martensitic zone has significantly reduced ductility compared to the base material, and when combined with high residual stresses from improper bead sequencing, it becomes a preferential path for crack initiation and propagation.

Microstructural Evolution at the Fusion Line

The fusion line region exhibits a complex microstructural gradient:

1. Base material side (18MND5): Fine-grained martensite with retained austenite, transitioning to coarser martensite near the fusion line.
2. HAZ: Widened martensitic band due to excessive dilution and rapid cooling, with high carbon and alloy enrichment.
3. Overlay side (309L): Austenitic matrix with delta ferrite, relatively ductile and corrosion-resistant.

The mismatch in ductility between the brittle martensitic HAZ and the ductile austenitic overlay creates a stress concentration that promotes delamination under thermal cycling or mechanical loading.

Proposed Countermeasures

The authors propose three complementary solutions, each addressing a different aspect of the problem:

1. Adjustment of Slag Retaining Ring Structure

The slag retaining ring (or backup ring) controls the molten pool geometry and prevents slag inclusions. By modifying its structure, the authors achieve better control over the weld bead profile and reduce the likelihood of incomplete fusion and slag entrapment at the interface.

2. Optimization of Weld Bead Arrangement

The weld bead sequencing is critical for controlling residual stress distribution. The authors propose a specific arrangement that ensures uniform heat input distribution and minimizes the peak residual stress at the fusion line. This involves careful planning of the welding direction, bead overlap, and interpass temperature control.

3. Change to TIG Hardfacing for the First 309L Layer

Perhaps the most significant recommendation is the switch from submerged arc welding (SAW) to tungsten inert gas (TIG) welding for the first layer of 309L deposition. The rationale is compelling:

• TIG welding provides significantly lower heat input compared to SAW, reducing base metal dilution.
• The precise arc control of TIG allows for better management of the fusion line geometry.
• Lower heat input reduces the width of the martensitic HAZ in the 18MND5 base material.
• The reduced dilution ensures that the first layer composition remains closer to the intended 309L composition, maintaining its austenitic character.

Engineering Practice Integration

The findings of this study have direct implications for the welding procedure specifications (WPS) used in nuclear power equipment fabrication. Key process parameters that should be controlled include:

The study also highlights the importance of post-weld heat treatment (PWHT) in relieving residual stresses and tempering the martensitic HAZ. However, the authors note that PWHT alone is insufficient if the fundamental issues of dilution and bead arrangement are not addressed.

Reflections

This paper is a textbook example of systematic engineering problem-solving applied to a high-consequence manufacturing challenge. The approach—identifying the problem, analyzing the root cause through metallurgical examination, and proposing targeted countermeasures—is exactly the methodology that should be applied to any quality issue in critical equipment fabrication. The emphasis on the first layer process (TIG instead of SAW) is particularly noteworthy, as it demonstrates that sometimes the most effective solution is not to change the material but to change the process. For nuclear power equipment manufacturers, the lessons of this study should be incorporated into qualification programs and quality assurance procedures to prevent recurrence of interface delamination.