Causes of Transverse Cracking in Brass Tubes

Brass Tubes include C44300 Tubes and C68700 Tubes.

Transverse cracking in brass tubes is a complex phenomenon often resulting from the interplay of material properties, manufacturing processes, environmental conditions, and mechanical stresses. Below is a detailed analysis of the potential causes:

1. Material-Related Factors

a. Composition and Microstructural Defects

• Excessive Zinc Content: Brass alloys with zinc content exceeding 40% (e.g., H65 or higher grades) become brittle due to increased β-phase (CuZn intermetallic compound) formation, reducing ductility and promoting crack initiation under stress.
• Impurity Segregation: Elements like lead (Pb), iron (Fe), or phosphorus (P) accumulating at grain boundaries weaken intergranular cohesion, leading to intergranular cracking.
• Coarse Grain Structure: Improper annealing processes can result in abnormally large grains, reducing fracture toughness and facilitating crack propagation along grain boundaries.

b. Phase Instability and Corrosion

• Dezincification Corrosion: In corrosive environments (e.g., chloride-rich water), selective leaching of zinc leaves a porous copper matrix, significantly reducing mechanical strength and initiating cracks under stress.
• Stress Corrosion Cracking (SCC): Brass is highly susceptible to SCC in environments containing ammonia, sulfides, or moist chlorine. Residual stresses combined with these agents trigger crack formation perpendicular to the applied stress (transverse direction).

2. Manufacturing and Processing Issues

a. Residual Stresses

• Cold Working: Processes like drawing, bending, or rolling induce residual stresses. Inadequate post-forming annealing (e.g., insufficient temperature or dwell time) leaves these stresses unrelieved, creating susceptibility to SCC or fatigue failure.
• Welding Defects: Rapid cooling in welded joints generates high thermal stresses, microcracks, or porosity, acting as crack initiation sites.

b. Improper Heat Treatment

• Incomplete Recrystallization: Insufficient annealing fails to eliminate work-hardened structures from cold working, leaving the material prone to stress-induced cracking during service.
• Non-Uniform Cooling: Rapid quenching or uneven cooling rates during annealing can create localized stress concentrations.

3. Environmental and Corrosion Effects

• Corrosive Media: Exposure to acidic solutions, seawater, or industrial atmospheres (e.g., containing NH₃, SO₂, or Cl⁻) accelerates dezincification or SCC.
• Cyclic Corrosion Fatigue: Combined action of cyclic mechanical loads (e.g., vibration, pressure fluctuations) and corrosive environments leads to accelerated crack growth, forming transverse fatigue cracks.

4. Mechanical and Design Factors

a. Overloading and Stress Concentration

• Excessive Bending or Impact: Sudden mechanical overload during installation or operation can cause immediate transverse fracture.
• Poor Design Features: Sharp bends, abrupt changes in wall thickness, or improperly designed joints create localized stress concentrations, acting as crack nucleation points.

b. Thermal Stress

• Thermal Expansion Mismatch: Differential thermal expansion between brass and connected materials (e.g., steel) generates cyclic thermal stresses, promoting fatigue cracking.

5. Failure Analysis and Prevention

Key Investigation Steps

• Fractography: SEM analysis of fracture surfaces to identify failure mode (e.g., ductile dimples, cleavage facets, or intergranular cracking).
• Metallography: Examination of grain structure, phase distribution, and dezincification depth.
• Residual Stress Measurement: Techniques like X-ray diffraction to map stress distribution.

Mitigation Strategies

• Material Selection: Use low-zinc brass (e.g., C23000, Admiralty brass) or alloys with corrosion inhibitors (e.g., arsenic, tin).
• Process Optimization: Implement stress-relief annealing (260–400°C for 1–2 hours) after cold working; avoid welding where possible.
• Environmental Control: Apply protective coatings (e.g., electroplating, polymer coatings) or avoid exposure to aggressive media.
• Design Improvements: Increase bend radii, add vibration dampeners, and ensure uniform wall thickness.

Conclusion
Transverse cracking in brass tubes typically arises from synergistic effects of material embrittlement, residual stresses, corrosive environments, and mechanical overloading. A systematic approach combining failure analysis, material optimization, and design adjustments is essential to address this issue effectively.

ASTM B111 ASME SB 111 Brass Heat exchanger Tube