Wear behaviour of tool steels with added (WTi)C particles

Abstract

The volume fraction of hard second phase particles in tool steels is limited by the formation of large interconnected carbides on solidification that embrittle the alloy. In this work, a novel carbide master alloy, FeWTiC, which contains (WTi)C particles, was used to increase the volume fraction of carbide in an H21 hot work die steel (0.3%C, 9%W, 3%Cr). Additions of up to 5 vol.% (WTi)C were successfully made to the vacuum melted steel, producing a homogeneous distribution of (WTi)C throughout the microstructure. TEM and fracture testing confirmed that a good bond existed between the (WTi)C and martensitic matrix. Wear testing was undertaken using pure sliding against an M2 tool steel counterface, in a block on ring configuration, in the load range of 54–254 N and a sliding speed of 0.98 m/s. At 54 N, an increase in volume fraction of (WTi)C gave a reduction in wear coefficient, associated with predominantly oxidational wear, with associated ploughing of the tool steel surface by oxide particles. At higher loads the addition of (WTi)C did not result in reduced wear rates. In addition to oxidational wear, delamination occurred, frequently initiated at the matrix/M 6C particle interface. Fracture of the (WTi)C became more common as the load increased. A 1% (WTi)C marginally reduced the depth of deformation compared to the H21, but further additions had little additional effect. Detailed TEM indicated the complex interaction between oxide formation and surface deformation of the tool steel. The relationship between wear mechanism and microstructure is discussed.