Cold Isostatic Pressing Effect on the WC-15 wt.% Co Hard Alloy Strength

Abstract

The effect of cold isostatic pressing of WC-15Co powder blanks on the sintered alloy strength in threepoint bending was evaluated. The specimens treated or nontreated by cold isostatic pressing (CIP) before sintering were analyzed by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD), strength and Vickers and Rockwell hardness test equipment. The strength of CIP-treated specimens in three-point bending was found to be 2410 MPa, which is 13% higher than that of the CIP-nontreated ones and 30% higher than the standardized strength of a VK15 hard alloy of a similar composition. The hardness was shown to be the highest for the specimens CIP-treated before sintering, while average WC grain sizes remained unchanged compared to the CIP-nontreated ones. The predominant crystal modification of cobalt in the CIPtreated specimens was revealed to be HCP-Co, in contrast to the CIP-nontreated ones, where cobalt was mainly in the metastable FCC modification. It is shown that in the specimens CIP-treated before sintering, the cobalt content on the fracture surface is 3-4 times higher than for the nontreated ones, and the fracture crack propagates mainly along the WC matrix phase–Co binder interface. In the CIP-nontreated specimens, the crack propagates in the Co volume, and WC grains on the fracture surface are covered with cobalt interlayers of different thicknesses. Cold isostatic pressing has been proposed as a requisite additional operation of standard hard alloy technology since already at the pressing stage, it provides essential conditions for enhancing the strength of sintered specimens.