Influence of cryogenic treatment on mechanical properties of cemented carbides
Tutor / director / evaluatorLlanes Pitarch, Luis Miguel
Document typeMaster thesis
Rights accessOpen Access
Cemented carbides, often referred to as hardmetals, are materials composed by two major phases: a carbide ceramic phase and a binder metallic one. The metallic binder maintains the ceramic grains together in an interpenetrated network structure. This heterogeneous microstructure confers to the material good mechanical properties such as high hardness, elevated fracture toughness and high compressive strength and wear resistance. Given their good mechanical properties, the main application of cemented carbides is cutting tools. In order to improve the mechanical properties of cemented carbides, some researches have proposed cryogenic treatment, on the basis that it has proven in steel tools to extend the life in service by increasing its wear resistance. In the present project a WC-Co (ultra-fine) and a WC-Ni (fine) cemented carbides grades were studied in order to determine the effect of the cryogenic treatment in their mechanical properties. To determine the influence of the soaking time at cryogenic temperatures on the mechanical properties, two batches of samples were subjected to 4 h and 9 h respectively. Vickers, Rockwell A hardness and bending tests were performed in the samples before and after cryogenic treatment. The fracture toughness was determined by Palmqvist method. Main results are now listed. First, hardness of both grades does not present significant changes after the cryogenic treatments. Second, fracture toughness improves for the WC-Co grade (around 12%) after 9 h at cryogenic temperature while it decreases for the same grade (around 5%) after 4 h at cryogenic temperature. On the other hand, toughness of the WCNi grade decreases in both cases. However, reliability of assessed changes is uncertain because of the invalid implementation of Palmqvist method. Changes in the transverse rupture strength were observed for both grades but cryogenic treatment do not induce those changes. WC-Co grade with 9 h held at cryogenic temperature decreases, and WC-Ni grade with 4 h held at cryogenic temperature increases. Fractographic inspection showed that pre-existent defects are responsible for low values of TRS in the Co grade. Concerning the Ni grade, no defects were observed under eac h condition and it is assumed that it breaks from intrinsic microstructural heterogeneities (grains repartition). In a word, the changes observed in fracture strength are not statically significant. Finally, the cryogenic treatment superficially increases the fracture toughness of the cobalt grade; however, does not have a significant impact in the hardness and TRS of both studied grade.