Failure analysis of drop-dregs defect in the bulging and breaking connecting rod of 36MnVS4 non-quenched and tempered steel

Abstract

This paper analyzes the drop-dregs defect phenomenon during the expansion fracture of 36MnVS4 non-quenched steel connecting rods. Using optical metallographic microscope, scanning electron microscope, energy dispersive spectrometer, and Vickers hardness tester, the differences in inclusions, microstructure, and properties between drop-dregs and non-drop-dregs connecting rods are compared to explore the reasons for drop-dregs defect of connecting rods. The research shows that the non-metallic inclusions in 36MnVS4 non-quenched steel connecting rods are mainly MnS; the standard deviations of the minimum interface spacing Dmi of MnS inclusions in 1^# (drop-dregs) and 2^# (non-drop-dregs) samples are 6·5 and 14·2 respectively; the distribution uniformity of inclusions in 1^# sample is better, and it is not the main cause of drop-dregs defect; the average number density of MnS inclusions in 1^# sample is 970/mm², with an equivalent diameter of 1·16 μm; the density of inclusions in 2^# sample is 873/mm², with an equivalent diameter of 1·34 μm; the density of inclusions in 1^# sample is high, while the equivalent diameter is small, indicating that the inclusions in 1^# sample are more fine and uniform. The microstructure of the connecting rod is ferrite + pearlite, and the percentages of ferrite in 1^# and 2^# samples are 37·1% and 32·9% respectively; the grain size of ferrite is 7·5–8·0 and 7·0–7·5 respectively. 1^# Sample has fine and uniform MnS particles, which are more likely to act as nucleation centers for intracrystalline ferrite formation, promoting ferrite nucleation and increasing ferrite content. This causes the ferrite content to exceed the standard upper limit requirement. At the same time, microcracks were found in the decarburization layer of 1^# sample. Therefore, the reason for the drop-dregs defect of the connecting rod is: The microcracks in the decarburization layer of 1^# sample are prone to trigger "abnormal cracking". During the crack propagation process, the excessive ferrite content enhances the plasticity and toughness of the alloy, resulting in plastic fracture during the expansion process, which affects the expansion speed and direction of the crack. Eventually, a significant height difference is generated at the intersection of the cracks, causing shear fracture and inducing secondary cracks, resulting in the falling off of the rod.