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Toughness

toughness for carbide roll

The Palmqvist method uses the corner crack of a vickers hardness indentation to derive the fracture toughness. The critical strees intensity factor is defined as:
KIC=6.2(HV50/∑L)1/2 in MN/m3/2

When components in tools like carbide rolls are exposed to external loads, static or dynamic, mechanical stresses occur within the material. The mechanical strength and deformability of the material are therefore important. In many cases, particularly in carbide rolls, when dealing with shock loading, both these properties must be considered simultaneously. This forms the background to the term “toughness” which can be defined as “the ability to resist fracture”, i.e. a complete separation into at least two parts.

A method commonly used for determining the toughness of Cemented Carbides is the Palmqvist method. In this case, the fracture toughness of the material is represented by its critical stress intensity factor K1C.

The results of toughness tests show that this property increases with increasing binder content, and with increased WC grain size.

toughness for carbide rollHowever, different Cemented Carbides show large differences in toughness behavior. This is best explained by a close look at the microstructure. The types of fracture seen are cleavage fractures in carbide grains, grain boundary fractures between carbide grains and shear fractures in the binder. Generally, the amount of cleavage fractures increases with increased grain size and the amount of shear fractures with increased binder content. Expressed as fracture energy, the major contribution to toughness, is from the latter, i.e. the crack path through the binder.