Effect of Stress Relief Annealing on Stress Corrosion Cracking of CuZn30 Alloy After Flow Forming M. Mutlu1,∗, A. Karakaş1, T. Yalçinkaya2 1 Repkon Machine and Tool Industry and Trade Inc., 34980 Şile, Istanbul, Türkiye 2 Department of Aerospace Engineering, Middle East Technical University Ankara 06800, Türkiye ∗ mehmet.mutlu@repkon.com.tr Keywords: CuZn30, flow forming, stress relief annealing, stress corrosion cracking. Cold work operations often induce residual stresses that can lead to stress-corrosion cracking for Cu-based alloys. Internal stress in alloys can be mitigated by employing stress-relieving annealing [1]. The duration and temperature of the annealing process depend on factors such as the extent of deformation, the alloy composition, and the allowable stress levels post-machining. In this study, CuZn30 material was subjected to flow forming at thickness reduction ratios of 55% and 75%. The flow-forming process introduced residual stresses that increased the susceptibility of copper alloys to stress corrosion cracking. To address this issue, stress relief annealing was applied to the formed parts. Annealing temperatures of 250 °C, 350 °C, 450 °C, and 550 °C were tested, with a 1-hour duration at each specified temperature. To evaluate the effectiveness of stress-relieving annealing in enhancing stress corrosion crack resistance, ammonia vapor test was conducted following the ASTM B858-06 standard [2]. The test was conducted using a test medium with pH values of 9.5 and 10.5, as specified in the standard. The test specimens were suspended in a sealed container for exposure to ammonia vapor and kept there for 24 hours. In the ammonia vapor test, no cracks were observed in any of the specimens at a pH value of 9.5. Cracks were only observed in the specimens which are only formed by flow forming (55% and 75% thickness reduction ratios) and annealed at 250°C after flow forming (55% thickness reduction ratio) with a pH of 10.5. Additionally, hardness measurements were carried to all specimens in order to see the effect of stress relief annealing. The hardness values of the specimens subjected to stress-relieving annealing at 250 °C after flow forming showed no decrease compared to the untreated specimens. This indicates that stress corrosion resistance can be improved without compromising hardness values. Overall, these findings demonstrate the effectiveness of stress-relieving annealing at 250 °C in reducing stress corrosion cracking in CuZn30 material formed by flow forming processes (%75 thickness reduction ratio) without compromising material hardness. References [1] Easteal, C. D. (1960). Stress corrosion cracking studies in cartridge brass [Master’s thesis, McGill University]. Department of Metallurgical Engineering. [2] ASTM, " B858-06(2018)-Standard Test Method for Ammonia Vapor Test for Determining Susceptibility to Stress Corrosion Cracking in Copper Alloys,"Annual Book or ASTM Standards, American Society for Testing and Materials, Vol. 02.01. 78
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