Spalling Mechanism of Stainless Steel Strip Electrode Submerged Arc Hardfacing on Nuclear Island Main Equipment

Literature Overview

This paper by Yang Chengdong, Ru Xiangkun, and Tang Weibao from Shanghai Electric Nuclear Power Group Co., Ltd., published in Pressure Vessel (2023, Vol. 40, No. 6, pp. 22–26), addresses a critical quality issue in nuclear power plant equipment manufacturing: the spalling (delamination) of stainless steel strip electrode submerged arc hardfacing layers on low-alloy steel base materials. The research is conducted at the Shanghai Nuclear Equipment Welding and Inspection Engineering Technology Research Center, reflecting the direct relevance of the findings to nuclear equipment fabrication and qualification.

Core Technical Approach

The study employs a multi-scale investigation approach, combining macroscopic observation of the spalling phenomenon with microscopic analysis of the transition zone microstructure, residual stress measurement, and fracture mechanics reasoning. The investigation identifies both internal factors (microstructural features) and external factors (residual stress) that contribute to the spalling failure mechanism.

Key Findings Summary

Interpretation of Technical Findings

Internal Factors: Microstructural Analysis

The transition zone between the low-alloy steel base material and the stainless steel hardfacing layer exhibits a hardened layer approximately 25 μm thick. This layer consists of lath martensite with high hardness but low plasticity. The formation of this hardened layer is attributed to the rapid heating and cooling cycle experienced by the base metal during the submerged arc welding process. The high carbon equivalent of the low-alloy steel base material, combined with the dilution from the stainless steel weld metal, creates a local composition and thermal cycle that favors martensitic transformation.

The Type II boundary identified in the transition zone is a metallurgical interface where the grain structure changes abruptly. This boundary represents a region of structural discontinuity where the crystallographic orientation, grain size, and phase composition differ significantly from both the base metal and the weld metal. Such boundaries are inherently weaker than the bulk material and serve as preferential paths for crack initiation and propagation.

The presence of gas pores within the hardened layer further compounds the problem. These pores act as stress concentrators, reducing the effective load-bearing cross-section and providing nucleation sites for crack initiation under tensile stress. The combination of a brittle martensitic microstructure, a weak Type II boundary, and porosity creates a highly susceptible zone for delamination.

External Factors: Residual Stress Analysis

The residual stress at the hardfacing interface increases progressively with the number of welding passes. This is a direct consequence of the cumulative thermal cycling effect: each additional pass subjects the previously deposited layers to a new thermal cycle, generating additional thermal stresses that are partially locked in as residual stresses upon cooling.

At 6 passes, the residual stress reaches 380 MPa, which appears to constitute a critical threshold for spalling initiation. The Type II boundary, already weakened by the hardened layer and porosity, acts as the crack source under this elevated residual stress. The crack then propagates along the Type II boundary, leading to complete delamination of the hardfacing layer.

Failure Mechanism Synthesis

The spalling failure is a multi-factor phenomenon:

1. The hardened layer with lath martensite provides a brittle zone with limited crack resistance.
2. The Type II boundary serves as a pre-existing weak plane at the interface.
3. Gas pores within the hardened layer act as stress concentrators and crack nucleation sites.
4. The cumulative residual stress from multiple welding passes provides the driving force for crack propagation.
5. When the residual stress exceeds the fracture resistance of the weakened interface (approximately at 380 MPa for 6 passes), catastrophic delamination occurs.

Engineering Practice Integration

Implications for Nuclear Equipment Manufacturing

Nuclear island main equipment, including reactor pressure vessels, steam generators, and primary coolant piping, often requires stainless steel hardfacing overlays for corrosion resistance or gasket sealing. The findings of this study have direct implications for the welding procedure qualification and production control of such components.

Key process control measures that can be derived from this research include:

• Limiting the number of hardfacing passes to reduce cumulative residual stress below the critical threshold
• Implementing interpass temperature control to moderate thermal gradients and reduce residual stress accumulation
• Applying post-weld stress relief to reduce interface residual stress before the component enters service
• Optimizing welding parameters (current, voltage, travel speed) to minimize the thermal input and reduce the extent of the hardened layer in the base metal
• Selecting appropriate consumables with lower carbon equivalent to reduce the tendency for martensitic transformation in the transition zone

Quality Control Considerations

For nuclear equipment fabrication, the detection of hardfacing layer spalling is critical for safety and regulatory compliance. Non-destructive testing methods such as ultrasonic testing (UT) and phased array ultrasonic testing (PAUT) can be employed to detect delamination at the hardfacing interface. The study's identification of the Type II boundary and hardened layer as key failure features provides valuable information for calibrating NDT inspection criteria and acceptance standards.

Study Insights and Implications

This paper provides a rigorous mechanistic understanding of hardfacing layer spalling that goes beyond simple empirical observations. The clear identification of both internal (microstructural) and external (residual stress) factors, along with the quantification of the critical residual stress threshold (380 MPa at 6 passes), offers actionable guidance for process optimization and quality control.

For engineers involved in nuclear equipment welding, the key lesson is that hardfacing layer integrity is not solely a function of weld metal quality but is critically dependent on the metallurgical compatibility at the interface and the residual stress state. The Type II boundary, while difficult to eliminate entirely, can be managed through careful control of the welding thermal cycle and post-weld heat treatment. The cumulative effect of multiple welding passes on residual stress accumulation underscores the importance of pass sequencing and interpass temperature control in multi-layer hardfacing operations.