A study of carbide removal mechanisms during quartz abrasion I: In situ scratch test studies

Abstract

In this paper scratch test simulations of carbide removal mechanisms in high chromium white cast irons during low stress abrasion by quartz abrasive are described. Scratch testing was performed inside the scanning electron microscope using individual grains of rounded quartz as scratch tools. Linear multipass scratches in the same path were made on preworn and deeply etched metallographic surfaces of hypereutectic white cast iron. The results of the sequential multipass tests on preworn surfaces showed that a gradual enlargement of pre-existing cavities in primary carbides occurs by fracture near the unsupported edges of the cavities. Short duration abrasion tests performed on metallographic surfaces showed that preferential removal of matrix material present in the pre-existing cavities present in some of the primary carbides resulted in small pits in these carbides. The large pits produced in large primary carbides by low stress quartz abrasion apparently are produced by a progressive series of small fracture events rather than by gross fracture in a single event. Gross carbide fracture was not produced during scratch testing even though the loads used were considerably in excess of those experienced during low stress abrasion. The rounding of carbides was shown to be due to microfracture of the sharp carbide edges left unsupported by the matrix. Quartz scratch tests on preworn surfaces produced no grooves on the carbides even at high loads, and the matrix between the carbides was protected by the protruding carbides. Scratch tests performed using fragments of carbide debris showed that a carbide can cut through another carbide. It is proposed that the grooves which are produced in the leading edges of carbides during quartz abrasion are caused by carbide debris.