Fractographic analysis of fracture features of a slugger punch and profile bending roll made of Kh12MF steel

Abstract

A slugger punch and a roll forming roller made of C1.5Cr12MoV steel destroyed during operation are studied. It was established by spark spectrometry that the steel was up to the requirements for the chemical composition. Metallographic studies have shown that the steel structure represents martensite with carbides precipitates along the grain boundaries. Typical of this steel grade sulfides were found in the structure by the method of energy-dispersive X-ray spectral microanalysis. Fractographic analysis of fractures made it possible to determine the location of the initial crack and the subsequent fracture nature of the tool. The punch geometry discrepancy to the drawing is established. Its working surface has a one-sided slope. This fact led to unequal lateral stresses and brittle fracture of the punch support surface. Wear marks and transverse cracks were found on the roller working surface. They are caused by high contact stresses in conjunction with the difference in relative speeds of the roller surface and the workpiece. The propagation of main cracks in the radial direction of the roller led to its fatigue failure. Measurements of the punch and roller hardness by the Rockwell method revealed a deviation of the values from the normative documentation. It is found that the punch has extra hardness, which increases the fragility of the tool and reduces its resistance to impact loads. The working surface hardness of the roller is lowered, which leads to a decrease in the strength characteristics of the tool. Thus, the destruction of the tool was caused by the uneven action of operating stresses in combination with a violation of the recommendations on the tool hardness.