Crack Cause Analysis of B10 Copper Alloy Plate-Made Tee Fittings

Literature Overview

This paper, published in Materials Development and Application (2025, Vol. 40, No. 2, pp. 109-114) by Chen Liyang and colleagues from the 725th Research Institute of China Shipbuilding Industry Corporation, investigates the cause of cracking in B10 copper alloy plate-made tee fittings during the forming process. The study employs a comprehensive failure analysis methodology including chemical composition analysis, mechanical property testing, metallographic examination, scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), and fractography. The research identifies grain boundary weakening caused by Cu-enriched phases or precipitates as the primary cause of cracking.

Technical Background: B10 Copper Alloy and Tee Fitting Fabrication

B10 copper alloy (also known as C70600 or similar designations depending on the standard) is a high-strength copper-nickel-zinc alloy used in marine and naval applications due to its excellent corrosion resistance and mechanical properties. The alloy typically contains approximately 1% nickel and 1% zinc, which contribute to its enhanced strength and corrosion resistance compared to pure copper or brass.

The fabrication of tee fittings from B10 copper alloy plate involves several critical process steps:

1. Blanking: Cutting the required blank from the plate using shearing, laser cutting, or waterjet cutting.
2. Forming: Shaping the blank into the tee geometry through spinning, pressing, or stamping.
3. Welding (if applicable): Joining formed sections using appropriate welding processes.
4. Heat treatment: Stress relieving or solution treatment to restore mechanical properties.

Cracking during the forming process is a serious quality issue that can lead to scrap material, production delays, and increased costs. Understanding the root cause of cracking is essential for preventing recurrence.

Failure Analysis Methodology

The authors employed a systematic failure analysis approach following established metallurgical investigation protocols:

Chemical Composition Analysis

The chemical composition of the B10 copper alloy was analyzed to verify conformance to the specified alloy designation. Any deviations in composition, particularly in the nickel and zinc content, could affect the alloy's mechanical properties and susceptibility to cracking.

Mechanical Property Testing

Tensile tests and hardness measurements were conducted on samples from the plate and from the cracked tee fitting to assess:

• Tensile strength and elongation of the base material.
• Hardness distribution across the plate and at the crack location.
• Comparison between plate properties and formed fitting properties to identify any property degradation during forming.

Metallographic Examination

Metallographic analysis was performed on polished and etched samples to examine:

• Grain structure and grain size of the base material.
• Grain boundary characteristics including the presence of precipitates or segregated phases.
• Deformation microstructure in the formed region to assess strain distribution.

Scanning Electron Microscopy (SEM) and EDS Analysis

SEM fractography was used to examine the crack surface morphology, while EDS was used to analyze the chemical composition of features observed on the fracture surface. This combination provides both morphological and compositional information critical for determining the cracking mechanism.

Key Findings

Crack Morphology

The analysis revealed several characteristic features of the crack:

• The crack exhibits clear branching, indicating that the crack propagation was not a simple, single-path event but involved complex interaction between multiple crack fronts.
• The crack shows intergranular propagation characteristics, meaning it propagated along grain boundaries rather than through the grain interiors.

These morphological features are diagnostic of a specific cracking mechanism:

• Intergranular cracking is associated with grain boundary weakening, which can be caused by segregation of impurities, precipitation of brittle phases at grain boundaries, or environmental factors such as stress corrosion cracking.
• Branching is characteristic of cracks that propagate under conditions of low stress intensity, where the crack front interacts with microstructural features such as grain boundaries, inclusions, and precipitates.

Grain Boundary Weakening

The primary finding of the study is that Cu-enriched phases or precipitates at grain boundaries in the original plate caused grain boundary weakening, which was the main cause of cracking during the forming process.

This finding has several important implications:

1. The root cause lies in the plate manufacturing process, not the forming process. The grain boundary weakening was present in the original plate before forming began. The forming process merely provided the mechanical stimulus (plastic deformation and associated stresses) that triggered the crack initiation and propagation.
2. The Cu-enriched phases are likely a result of solidification segregation. During the casting of the B10 copper alloy ingot, copper tends to segregate to the last-solidifying regions of the dendrites. If the subsequent homogenization heat treatment is insufficient, these Cu-enriched regions can persist and eventually precipitate as distinct phases at grain boundaries.
3. The presence of original defects in the plate provided nucleation sites for crack initiation. The study notes that local defects in the original plate (such as shrinkage pores, inclusions, or processing defects) served as stress concentrators that facilitated crack initiation.

Evidence from EDS Analysis

The EDS analysis of the grain boundary regions confirmed the presence of Cu-enriched phases or precipitates. The chemical composition of these phases differed from the bulk alloy composition, with higher copper content and lower nickel and zinc content. This is consistent with the expected segregation behavior of copper during solidification.