Fracturing Behavior of Metalware Made of 30KhGSA Steel

Аннотация

The reasons for fracture of fasteners made of 30KhGSA steel were analyzed. It was found that, in the case of fracture of bolts in the package connecting the frame elements, the damage of the bolts is the result of the fretting corrosion due to the mutual friction between bolts and elements of skin, belt, and tape. Fretting corrosion develops in the locations of the cadmium coating damage of bolts in the process of friction. The break-in scuffing of the openings and corrosive damage (fretting corrosion) triggering intergranular fracture, which is the most severe in the skin and tape, contribute to the fractures in the belt, skin, and tape. The reason for fracture is structural and technological factors: high stresses in the investigated package, micro-displacement of the mating parts, violation of the cadmium coating, insufficient protection against corrosion and environmental impact. Fracture of the self-locking nuts made of 30KhGSA steel with the anticorrosion cadmium coating used during assembly of an auxiliary gas turbine engine is induced by their severe overheating—the impact of the melted coating on the surface of the loaded nuts and penetration of the liquid metal into the base material along the grain boundaries. Application of the cadmium coating for the operation of the steel parts at a temperature exceeding (even for a short time) the melting temperature of Cd (T_melt = 320.9°C) under the condition of contact with the stressed metal owing to the Rebinder effect leads to a dramatic loss of strength and premature failure of the part. Fracture of the galvanized bolts made of 30KhGSA steel was studied in the process of the repeated static testing. It was found that several factors contributed to the onset of the multiple fractures: rough scratch marks, sandblasting performed before galvanizing with the use of large sand fractions, dents which contribute to the premature origin of cracks, areas with deposits formed during coating with loose adhesion of the layer. The presence of a large number of pits along with the fatigue grooves indicates the operation of the material under high load. The investigation was carried out by the method of optical and scanning electron microscopy applying the X-ray microanalysis.