Effects of new rare earth additions on properties and microstructure of WC-14TiC-8Co cemented carbide

Abstract

Rare earth (RE) elements have been widely used in the smelting-casting metallurgy process of iron and steel as well as non-ferrous metals such as copper, aluminium, etc., but few successes have been reported in the powder metallurgy field. By the end of the 1980s, great progress for the application of RE to cemented carbide had been achieved in China [1-4]. The General Research Institute for Non-Ferrous Metals (GRINM) has developed new patented RE additions, which can steadily improve the bending strength, fracture toughness, room and high temperature hardness (800 °C), wear resistance and impact resistance of cemented carbide. The cutting lifespan of the RE hard metals can be extended by more than twice that of normal alloys. This paper reports the beneficial comprehensive effects of RE on the properties and microstructure of WC-14TiC-8Co alloy. The results are part of our systemetic investigation. The alloys with (WC-14TiC-8Co-RE) and without (WC-14TiC-8Co) RE were made by the vacuum sintering process. The RE/S ratio for WC-14TiC8Co-RE was more than 9 (by weight). Two kinds of specimens were tested and compared with each other. The dimensions of the bending specimens were 5 mm x 5 mm x 30 mm. The type of lathe tool used for the turning test was 31303C, and an Instron 1342 tester was used to determine the bending strength. The morphology, phase composition and structure of the alloys were analysed by scanning electron microscopy (SEM; JSM-840) with energydispersive spectroscopy (EDS), scanning transmission electron spectroscopy (STEM; H-800) with EDS, and transmission electron microscopy (TEM; JEM-2000FX) with EDS and electron energy loss spectroscopy (EELS). The properties of WC-14TiC-8Co and WC14TiC-8Co-RE are shown in Table I, which shows that the hardness and density of WC-14TiC-8CoRE are slightly improved and the bending strength is increased by about 10% compared to WC-14TiC8Co. The wear resis/ance was measured by turning 38CrNi3MoVA steel (VB = 0.35 mm, a f = 1 mm, f = 0.2 m m r 1). Fig. 1 shows that the wear resistance of WC-14TiC-8Co-RE is bettet than that of WC-14TiC-8Co. In order to test the impact property of the alloy, four cannelures (axial surface slots) were milled on a cylindrical steel specimen. The lathe tool was impacted four times while the specimen rotated. The results are shown in Table II, which shows that the impact resistance of WC14TiC-8Co-RE is rauch better than that of WC14TiC-8Co. The impact life is increased more than four times at turning speeds between 95 and 120 mmin -1. Thin-film samples of WC-14TiC-8Co-RE alloy were observed by STEM and TEM. The RE phase was found in (Ti, W)C solid solution and at the interface of several solid solutions (Fig. 2). It exists as spherical particle of diameter 20-400 nm. The composition of the RE phase was semi-quantitatively analysed by EDS. The results are shown in Table III. EELS analysis revealed that, besides these elements, there was a certain amount of oxygen in the RE phase.