Fractographic research of destruction of 34CrNi1Mo steel gear shaft

Abstract

Goal. The purpose of this research work is to establish the causes and mechanisms of destruction of gear shaft made of 34CrNi1Mo steel, to provide recommendations to prevent this type of product damage. Methodology. The gear shaft made of 34CrNi1Mo steel destroyed during operation was studied. The fracture surface of the investigated steel was studied by macroanalysis and scanning electron microscopy using a SEM-100У microscope at a magnification of 1500-10000 times. Microstructural analysis of 34CrNi1Mo steel was carried out on Neophot-21 optical microscope at x250, 300; 800 magnification. Microhardness was measured according to the standard method on a PMT-3M microhardness tester at a 50 g loading. Results. The causes and nature of the operational destruction of the gear shaft made of steel grade 34CrNi1Mo were analyzed. The fatigue nature of fracture was determined. The unsatisfactory quality of the surface of the experimental product was established: shells, pores, scratches, chips were found on the surface. The destruction began in the place of the geometric stress concentrator (surface defect – shell) due to metal chipping. Originality. The features of the destruction of the gear shaft were determined: at the break, there is a significant distance between the fatigue grooves, which indicates a high crack propagation rate, the fatigue grooves are weakly expressed, irregular, which indicates the limited possibilities of steel to plastic deformation. The fractographic surface of the fracture is shiny and consists of cleavage facets, covered with coarse scars, fan-shaped diverge from the focus, has a coarse-grained structure, typical for brittle fracture. The brittle nature of the fracture was confirmed by scanning electron microscopy. The surface microrelief contains cleavage facets, microcracks, and intergranular spall cells. Non-metallic inclusions of MnS, which are additional structural stress concentrators, have been revealed. Microcracks originate near non-metallic inclusions; at the boundaries of former austenite grains, especially at their triple junctions; at the interface between matrix and secondary phases. An increase in microhardness from 2500 to 4750HV after heating to 840–860°C with subsequent cooling in air was established, which can be explained with the probable hardening of the experimental steel, which, along with an increase in hardness, leads to a decrease in steel ductility. Practical value. It is recommended to use steel with a low sulfur content, additional gear shaft surface treatment to prevent the formation of stress concentrators in places of surface discontinuities; compliance with the nominal temperature regime also prevents spontaneous hardening of the product material.