Toughness measurements of a Cr martensitic high alloy steel susceptible to clinking

Abstract

‘Clinking’ is an audible fracture that occurs during the cool down and reheating of as-cast high alloy materials. When this process occurs, audible fracture can be heard and observed as large transverse cracks that propagate through large slabs. This causes high material losses and major disruption to processing operations. Given the fracture is known to be brittle, this research is aimed at developing a way to predict the onset of clinking through the application of fracture mechanics. Linear elastic and elastic–plastic fracture mechanics were both used to assess the fracture behaviour. The stress state during cool down and reheating was estimated through finite element analysis using a three-dimensional finite element model. Tensile tests were conducted to obtain the stress–strain characteristics to be used in the fracture analysis. Charpy tests were completed to assess the relative toughness dependent on temperature across the temperature range for which the high alloy steel is susceptible to clinking. Four C(T) specimens were tested at a room temperature. Despite showing little ductile crack propagation on the fracture surface, the fractured samples did not meet the Linear Elastic Fracture Mechanics (LEFM) validity criterion but did meet the ${\text{J}}_{\text{c}}$ validity criterion. This allows a minimum ${\text{J}}_{\text{c}}$ value of 118 N/mm to be attributed to the onset of unstable fracture. Conversion into a ${\text{K}}_{\text{Jc}}$ gives $164\text{MP}\sqrt{\text{m}}$ , which gives a minimum critical crack length of 138 mm for the onset of brittle fracture. Charpy tests showed a pronounced increase in the energy for fracture between 20 ℃ and 300 ℃ which is in line with practical observations, where the onset of clinking is reduced with a higher reheat temperature.