FEATURES OF FRACTURE OF METAL ITEMS (HARDWARE) MADE OF STEEL 30KhGSA

The reasons for destruction of fasteners made of steel 30KhGSA are analyzed. It is shown that in case of destruction of bolts in the package connecting frame elements, damage of bolts results from fretting corrosion, attributed to mutual friction between bolts and elements of skin, belt and tape. Fretting corrosion develops in the sites where cadmium coating of bolts is damaged upon friction. The occurrence of fracture in the belt, skin and tape is promoted by break-in scuffing of the openings and corrosion damages (fretting corrosion) triggering the intergranular fracture, most severe in the skin and tape. The reason for damage arises from structural and technological factors: increased stresses present in the package, micro-displacements of the mating parts, violation of the cadmium coating, insufficient corrosion protection and environmental impact. Destruction of self-locking nuts made of 30KhGSA steel with anti-corrosion cadmium coating used in the assembly of the auxiliary gas turbine engine is attributed to their significant overheating, i.e., the impact of molten coating on the surface of loaded nuts and the penetration of the liquid metal into the base material along the grain boundaries. The use of cadmium coatings in operation of steel parts at a temperature exceeding (even briefly) the melting temperature of Cd (T_melt = 320.9°C), under conditions of contact with stressed metal due to the Rebinder effect, leads to a sharp loss of strength and premature failure of the part. Study of the destruction of galvanized bolts made of 30KhGGSA steel during the repeated-static testing revealed that several factors contribute to the onset of fatigue multiple site damage: presence of the rough risks; sandblasting, performed before galvanizing using large sand fractions, which contribute to the premature origin of cracks; areas with deposits formed upon coating with a loose adhesion of the layer. The presence of a large number of pits along with the fatigue grooves indicates to high load of the bolt material upon operation. The study was carried out using optical and scanning electron microscopy and X-ray microanalysis.

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