Examination of a Damaged Slitting Knife using Scanning Electron Microscopy and Finite Element Simulations

Abstract: At voestalpine Precision Strip AB, the slitting process is used frequently. Slitting is a commonly applied manufacturing technique during sheet metal processing that aims to split large coils of sheet metal into narrower widths. The coiled sheets are uncoiled and transferred to a slitter, where they pass between rotating circular knives that produce long and straight cuts, splitting the wide sheet into desired widths. However, during operation, their slitting knives are occasionally damaged. This means that the knives must be replaced, which wastes both time and money.   The goal of this project is therefore to examine the damage of a degraded slitting knife. A scanning electron microscope (SEM) is used to analyse the fracture surface and the microstructure of the knife. Further analysis of the knife is a Vickers hardness test to determine the knife’s hardness. Moreover, finite element (FE) simulations of the slitting process are performed in FE software Abaqus to understand the stresses that the knives are subject to during operation. The results show that the Mises stress reaches a maximum value of around 2250 MPa, which is approximately 75% of the materials yield stress in compression.   The SEM images show clear signs of smooth cleavage zones in the fracture area, indicating that the knife has failed by a brittle fracture. Furthermore, the images reveal possible signs of fatigue. The hardness test proved that the knife is very hard with a hardness of 802,2 HV. Plenty of carbides were revealed by the microstructure test – mainly two types. MC vanadium carbides and M6C carbides mainly consisting of tungsten and molybdenum, which explains the hardness of the knife.   To draw further conclusions on the damage of slitting knives in the future, there needs to be proper documentation of the slitting process. This involves taking note of the knife setup, the sheet material and its thickness, as well as the lifetime of the knives as failure occurs.