Interface Delamination of Nozzle Inner Wall Hardfacing in Nuclear Power Equipment

Literature Overview

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

Root Cause Analysis

The interface delamination in dissimilar metal hardfacing is a complex failure mode involving multiple interacting factors. The authors identified two primary contributing mechanisms:

The 18MND5 steel is a low-alloy steel specifically designed for nuclear reactor pressure vessel applications. It contains approximately 0.15% carbon, 1.0% manganese, and 0.5% nickel, giving it good toughness and weldability. However, when 309L stainless steel is deposited directly onto this substrate without proper process control, the dilution rate can become excessive. The resulting microstructure at the fusion line includes a wide zone of martensite, which is hard and brittle, creating a preferential path for crack propagation.

Microstructural Analysis

The paper provides detailed metallographic examination of the fusion line region, revealing that the martensitic zone width is directly related to the dilution rate and cooling rate. When the dilution rate exceeds acceptable limits, the carbon equivalent at the fusion line increases significantly, promoting martensite formation even in the nominally austenitic weld metal. This martensitic zone, combined with high residual tensile stresses from the thermal cycling of successive weld passes, creates conditions favorable for interface delamination.

The weld pass arrangement is particularly critical in inner wall hardfacing of nozzles because of the geometric constraints. The confined space limits the number of passes that can be applied and the angles at which they can be deposited. If the passes are arranged in a way that concentrates heat input at the same fusion line location repeatedly, the thermal cycling effect amplifies residual stresses and promotes crack initiation.

Proposed Countermeasures

The authors proposed three complementary solutions, each addressing a specific aspect of the problem:

1. Adjustment of slag ring structure: Modifying the slag ring geometry to better control the arc and improve heat input distribution, reducing localized thermal stress concentrations.
2. Optimization of weld pass arrangement: Redesigning the pass sequence to distribute heat input more evenly across the weld area, preventing excessive thermal cycling at any single location.
3. Switching to GTAW for the first 309L layer: Using tungsten inert gas welding for the initial pass to achieve lower heat input and reduced dilution, creating a more favorable metallurgical interface before subsequent passes are applied.

The GTAW process for the first layer is particularly effective because it provides precise heat input control, resulting in a narrower fusion zone and lower dilution rate. This creates a metallurgical interface with a narrower martensitic zone and lower residual stress, providing a sound foundation for subsequent hardfacing passes.

Engineering Practice and Quality Control Implications

For nuclear power equipment manufacturing, the interface delamination problem carries severe consequences because it compromises the structural integrity of pressure boundaries. The FMEA approach applied here demonstrates the value of systematic failure analysis in identifying root causes and implementing effective countermeasures. The combination of microstructural analysis and residual stress measurement provides a comprehensive understanding of the failure mechanism.

In terms of quality control, this paper highlights the importance of:

• Monitoring dilution rates through spectroscopic analysis of the first weld pass.
• Controlling heat input within specified limits for each welding process.
• Implementing weld pass sequencing that minimizes thermal cycling at critical interfaces.
• Performing post-weld inspection, including dye penetrant testing and ultrasonic testing, to detect interface defects before they propagate.

The case study from Shanghai Electric Nuclear Equipment demonstrates that even in highly regulated nuclear manufacturing environments, process optimization is an ongoing challenge that requires continuous metallurgical understanding and process refinement. The proposed solutions represent practical improvements that can be implemented without major equipment changes, making them highly applicable to existing manufacturing facilities.

Study Insights and Reflections

This paper exemplifies the rigorous approach required for nuclear-grade manufacturing, where every weld must be qualified and every potential failure mode must be understood and controlled. The interface delamination problem is not unique to nuclear applications but is particularly critical in this context due to the safety implications. The authors' systematic approach, combining microstructural analysis with residual stress measurement and process optimization, provides a template for addressing similar dissimilar metal welding challenges in other high-integrity applications. Engineers working on nuclear power equipment should recognize that the first weld pass in any overlay application is the most critical, as it establishes the metallurgical interface that determines the long-term integrity of the entire overlay system.