HPb59-1 brass pipe joint stress corrosion fracture defect, high-frequency induction annealing equipment can easily solve it

HPb59-1 lead brass pipe joint is an aircraft pipeline connector. The chemical composition of HPb59-1 lead brass is (mass fraction, %): Cu58.1, Pbl.58, and the remainder is Zn. The conventional forging temperature is 640~780℃. The forging temperature process of (680±10)℃-(780±10)℃ was used in production, and the pipe joint broke and failed. After analyzing the reasons for the fracture defects, it was found that high-frequency induction annealing equipment was used for heat treatment, which had good results and met the needs of the work.

The inspection found that the two phases of HPb59-1 brass have reached the β phase region when heated to 780°C ± 10°C. The β grains grow, the α phase is distributed along the crystal in a feather-like or massive shape, and there are fine needle-like α phases in the crystal. The pipe joints heated by this process showed serious cracking after turning and passivation. Fracture inspection showed that there was a three-pronged crack at the bottom of the φ14mm closed hole, and the crack on the side wall of the inner hole extended along the axial direction. When the crack at the bottom of the hole was cut open and observed, it was found that the grains in the macro section were coarse, and the inner section was pure copper color, indicating that dezincification occurred at the crack during pickling, and there was slight dezincification on the outside. Microscopic cracks are mostly intergranular cracks. The cracks have multiple sources and expand from the inside to the outside.

According to the Cu-Zn phase diagram, the analysis points out that although the forging rheology of the workpiece is good in the single-phase β zone, the β phase is easy to grow, and the coarse grain boundaries lead to a significant decrease in forging performance. When forging in the temperature range below the forging temperature (α+β)/β phase zone boundary, the proportion of α phase increases, but the thinner the α phase needles, the less conducive to forging. Therefore, the forging process is most suitable at a temperature slightly lower than the (α+β)/β phase zone boundary. HPb59-1 usually has a lower copper content and a better forging temperature of 720°C. Brass parts are prone to stress corrosion cracking under certain processing stress and working in corrosive media. Fracture analysis of HPb59-1 pipe joint shows that the cracking of this workpiece is a typical brittle intergranular fracture. Fracture observation found that cracking always occurred in the lower part and inner wall area of the closed hole where the stress concentration is the most serious, and the direction of the crack was perpendicular to the direction of tensile stress; sometimes cracks and cracking were not obvious during forging, but the existence of internal stress caused the workpiece to still appear during storage. Cracks and cracking phenomena, this cracking defect is caused by stress corrosion cracking damage caused by defects in the forging structure.

To sum up, the fracture of HPb59-1 pipe joint is caused by forging structural defects caused by high forging temperature, which triggers the initiation and expansion of stress corrosion cracks. The more serious the structural defects are, the more serious the stress corrosion cracking will be and lead to the breakage and damage of the workpiece.

To prevent and avoid the fracture of brass workpieces, we should start by improving the forging process. Analysis and production show that using high-frequency induction annealing equipment for forging at a temperature of 730°C-630°C is a suitable forging process for HPb59-1 lead brass parts. Production and processing show that the metallographic structure and mechanical properties of the workpiece processed according to this process meet the technical requirements, and no cracks and cracking defects are found. The production is stable and the operation is good.