Study into the effect of thermochemical treatment on VK6 and VK15 hard alloy performance

The study covers the effect of thermal diffusion saturation (TDS) of VK6 and VK15 hard alloys. A charge of 48.5 % K₄(Fe(CN)₆), 50 % Al₂O₃ (buffer substance), and 1.5 % NH₄Cl (activator) was used for TDS of the surface of samples (billets). K₄(Fe(CN)₆) yellow blood salt, B₄C borax, CuO copper oxide were used as a saturating element. Each container was loaded with 5 billets (each brand – VK6, VK15 – separately) and charge. Containers were sealed and subjected to heating up to 900 °C and exposure at a given temperature for 2 hours and up to 1100 °C with exposure for 4 hours. Then they were cooled together with the oven for 6 hours. After the TDS of VK6 and VK15 hard alloys, hardness was increased insignificantly with an increase in bending strength from 13.6 to 57 % in comparison with the initial state. The relationship between stress and relative longitudinal strain for VK6 and VK15 before and after TDS indicates an increase in the Young’s modulus after TDS. The resistance of samples during the cutting test increased up to 2 times. It is impossible to make a clear conclusion on wear resistance tests by turning, since there is a large variation in the amount of wear even on equally processed samples. It is also recommended to increase the number of passes in further wear resistance tests, since wear after the 1st pass is often not evident and associated with a defective surface layer. An increase in temperature from 900 to 1100 °C during thermal diffusion saturation increases the diffusion layer depth for the VK6 hard alloy, improves its performance due to fewer pores, inclusions and breaks in the surface layer, which can be explained by an increased diffusion intensity of various compounds (sodium tetraborate, potassium hexacyanoferrate, copper oxide) with and without activators, as well as impurity redistribution in the process of polymorphic transformation. The microhardness values of VK6 and VK15 samples after TDS as compared to initial samples. The best thermodiffusion saturation medium for VK6 and VK15 hard alloys, which increases their performance by 2 times: Al₂O₃ + NH₄Cl + K₄(Fe(CN)₆) at 900 °C. The fractographic analysis of sample fractures before and after treatment was performed to identify a general trend and make a reasonable choice of an optimal thermodiffusion saturation mode. It was found that the nature of fracture remains virtually the same (brittle fracture runs along grain boundaries) with an increase in the temperature of thermal diffusion saturation, but intergranular facets become smaller due to the brittle precipitates of tungsten carbide particles.

VK6 and VK15 hard-alloy billets and tetrahedral plates, thermal diffusion saturation, cutting wear, microstructure, particle size

1. Zhang Li, Wang Yuan-Jie, Yu Xian-Wang, Chen Shu, Xiong Xiang-Jin. Crack propagation characteristic and toughness of functionally graded WC—CO cemented carbide. Int. J. Refract. Met. Hard Mater. 2008. Vol. 26. No. 4. P. 295—300.

2. Colovcan V.T. Some analytical consequences of experiment data on properties of WC—Co hard metals. Int. J. Refract. Met. Hard Mater. 2008. Vol. 26. No. 4. P. 301—305.

3. Guo Zhixing, Xiong Ji, Yang Mei, Jiang Cijin. WC—TiCNi cemented carbide with enhanced properties. J. Alloys Compd. 2008. Vol. 465. No. 1-2. P. 157—162.

4. Kreimer G.S. Strengh of hard alloys. Moscow: Metallurgiya, 1971 (In Russ.).

5. Panov V.S., Chuvilin А.М. Technology and properties of sintered hard alloys and their products. Мoscow: MISIS, 2001 (In Russ.).

6. Bock H., Hoffman H., Blumenauer H. Mechanische Eigenschaften von Wolframkarbid-Kobalt legierugen. Technik. 1976. Bd. 31. No. 1. S. 47—51.

7. Gurland J. The fracture strength of sintered WC—Co alloys in relation to composition and particle spacing. Trans. Met. Soc. AIME. 1963. Vol. 227. No. 1. P. 28—43.

8. Suzuki H., Hayashi K. Strenght of WC—Co cemented carbides in relation to their fracture sources. Planseeber. Pulverment. 1975. Vol. 23. No. 1. P. 24—36.

9. Tokova L.V., Zaitsev A.A., Kurbatkina V.V., Levashov E.A., Sidorenko D.A., Andreev V.A. Features of the influence of ZrO 2 and WC nanodispersed additives on the properties of metal matrix composite. Russ. J. Non-Ferr. Met. 2014. Vol. 55. No. 2. P. 186—190.

10. Bogodukhov S.I. Materialovedenie. Мoscow: Machinostroenie, 2015 (In Russ.)..

11. Bondarenko V.A. Quality assurance and improvement of characteristics of cutting tools. Мoscow: Machinostroenie, 2000 (In Russ.).

12. Libenson G.A. Powder metallurgy processes. Мoscow: MISIS, 2001. Vol. 1 (In Russ.).

13. Kim C.S., Massa T.P., Rohrer G.S. Modeling the relationship between microstructural features and the strengh of WCCo composites. Int. J. Refract. Met. Hard Mater. 2006. Vol. 24 (1) P. 89—100.

14. Yamamoto T., Ikuhara Y., Watanabe T., Shirase F. High resolution microscopy study in Cr3 C2 -doped WC—Co. J. Mater. Sci. 2001. No. 36. P. 3885—3890.

15. Jaensson B.O. Die untersuchung von Verformungsersheinungen in hochfeste WC—Co Legierungeen mit Hilfeeines neuen localisierungsverfahrens fur die Abdruckelektronenmicroscopie. Pract. Metallogr. 1972. Bd. 9. No. 11. S. 624—641.

16. Bogodukhov S.I. Determination of modulus of elasticity of various materials by means of strain gauges. Vestnik Orenburgskogo gos. un-ta. 2014. No. 4. P. 289—295 (In Russ.).

17. Tret’yakov V.I. Nitriding of refractory metals. Moscow: Metallurgizdat, 1962 (In Russ.).

18. Redchenko D.S., Popov A.Yu. Method for processing superhard materials: Pat. 2440229 (RF). 2012 (In Russ.).

19. Sokolov A.G. Method for machining carbide tools: Pat. 2509173 (RF). 2014 (In Russ.).

20. Chekhovoi A.N., Bel’kov O.V., Prokopova T.I. Method of chemical-thermal treatment of products from hard alloy and steel: Pat. 2231573 (RF). 2004 (In Russ.).

21. Oskolkova T.N., Budovskikh Е.А. Method for surface hardening of wolfoamocobate hard alloy: Pat. 2398046 (RF). 2010 (In Russ.).

22. Hindrik E. Plate with a coating for cutting tools for turning steels: Pat. 2536014 (RF). 2014 (In Russ.).

23. Kabanov A.V., Fedorov S.V., Vislaguzov A.A., Pavlov M.D. Method of hardening of products from hard alloys: Pat. 2501865 (RF). 2013 (In Russ.).