DOE OSTI2019
It is established that intercrystallite corrosion of cutting-ceramic inserts arises when the chromium content in the peripheral zone of the grains is depleted by the deposition of chromium carbides at the grain boundaries, in the presence of stress due to the fast cooling of the inserts from high temperatures. This is especially apparent when 12Kh18N10Т, 40Kh13, 14Kh17N2, and other steels that are hard to machine are worked by a tool equipped with inserts of VOK-60M modified cutting ceramic. Research shows that the intercrystallite corrosion of cutting-ceramic inserts may be attributed to the stress arising in the surface layers of the grains as a result of carbide or nitride deposition. Heating cutting-ceramic inserts at 650–800°С lowers this stress and restores their resistance to intercrystallite corrosion. Research also shows that chromomanganese cutting-ceramic inserts are less susceptible to intercrystallite corrosion than are chromium–manganese–nickel inserts, while chromium–manganese–nickel cutting-ceramic inserts are less susceptible than are chromonickel inserts. The benefits of chromium–manganese–nickel cutting-ceramic inserts are especially pronounced with relatively brief machining of 12Kh18N10Т, 40Kh13, 14Kh17N2, and other such steels, in rigorous conditions, with a temperature of 600–650°С in the workpiece–tool contact zone. A new theory of the intercrystallite corrosion of VOK-60M modified cutting ceramic is based on the role of the kinetic factor at the relatively low tempering temperatures associated with the deposition of metastable carbide, although this does not lead to maximum change in free energy of the system but only to a relative change. In the cutting ceramic, this carbide may be chromium carbide (Cr, Тi){sub 23}C{sub 6}, which is greatly enriched with titanium. With increase in tempering temperature, the deposited carbide tends toward the stable composition Cr{sub 23}С{sub 6}. Prolonged holding at the tempering temperature also tends to favor thermodynamic equilibrium. In other word, the chromium carbide is gradually enriched to reach the stable composition Cr{sub 23}С{sub 6}. This process is more rapid at higher temperatures. Obviously, the variation in composition of the carbide deposited at the grain boundaries in VOK-60M cutting ceramic with change in temperature and holding time will affect the kinetics of electrochemical solution of the system consisting of the carbide in contact with a solid solution. Research shows that vacuum sintering and annealing markedly improve the thermal stability. For the cutting ceramic, this is explained not only by the removal of fusible impurities but also by the larger quantity of aluminum and other alloying elements that remain in the strengthening phase, rather than being bound in oxides. Sintering in vacuum ensures greater plasticity of cutting ceramic based on nickel and cobalt in cutting 12Kh18N10Т and 14Kh17N2 steel at high speeds, with high temperatures in the machining zone. In addition, on sintering in vacuum furnaces, the content of small nonmetallic inclusions in the basic structure is 5–7 times less than in regular cutting ceramic. That significantly improves the deformability of the ceramic inserts.