An investigation of cobalt phase structure in WC–Co cemented carbides before and after deep cryogenic treatment

• Many studies indicate that wear resistance of cemented carbides and quality of workpiece surface machined can be improved by deep cryogenic treatment (DCT). But there exists a great difference in the strengthening effect.
• In this paper, the effect of deep cryogenic treatment (DCT) on the phase transformation of cemented carbides which have different amount of Co (12wt.% ~ 80wt.%) was investigated.
• Owing to the shield by WC particles, the diffraction peaks of binder phase Co are hardly shown in the X-ray phase analysis, In this paper, the selective electrolytic corrosion (SEC) technology was used to corrode WC particles. Therefore the diffraction of binder phase Co can be distinctly explored by XRD phase analysis.

The micro-morphology and crystal structure in WC–12 wt.%Co, WC–20 wt.%Co, WC–40 wt.%Co and WC–80 wt.%Co cemented carbides were investigated before and after deep cryogenic treatment (DCT) with scanning electron microscopy (SEM) and with the use of an X-ray diffractometer. In order to avoid the shielding of WC particles to diffraction peaks of binder phase Co in XRD phase analysis, a selective electrolytic corrosion (SEC) was used. The features of diffraction peaks about these cemented carbides before and after SEC were compared and analyzed. The results show that after DCT, there is no significant change in micro morphology, but the crystal structure of binder phase Co has an obvious change. Before SEC, the diffraction peaks of binder phase Co are only obvious in WC–80Co in XRD analysis. So, the change of diffraction peak intensity of binder phase Co before and after DCT is so small that it can hardly be observed except WC–80Co. But, after SEC it becomes obvious in all WC–Co cemented carbides. Both before and after DCT, ε-Co has a higher percentage in high-cobalt cemented carbides than that in low-cobalt cemented carbides. However, the effect of DCT is much more evident in low-cobalt cemented carbides than that in the high-cobalt ones.