Технические науки

Металлургия

Zhiguts Yu.Yu., Legeta Ya.P., Petrov O.P.

Uzhgorod National University, Ukraine

MATERIALS GOT WITH THE USE OF SHS

Introduction. SHS has been lately introduced into science and technology by A.G.Merzhanov and co-workers as a means of obtaining refractory high-hard alloys and phases by using the chemical heat as a result of synthesis.

Very high burning temperatures (several thousand degrees Centigrade) can be attained by combining aluminothermic reactions (producing elemental metals – Fe, Cr, V, Mo, W, by reducing their oxides with the aid of Al powder) with the oxygen-less burning of these metals in carbon powder. Such combined reactions were called “hybrid" processes [1]. Medium high heat evolution processes have been used for hard-fasing of castings with carbides and /or borides surface layers by the in-mould route [2]. In the present work height heat evolution processes have been employed in the production of medium size cutting tools in order to compensate high heat losses because of rather small volumes of reacting powder mixtures and also in order to simultaneously weld the carbides metal obtained on the steel holder of tools.

Theory and experimental. The “hybrid” high temperature processes devised can be described by the following reactions:

Stage 1 (thermit process − Eq. 1) and stage 2 (oxygen-less burning Eq. 2):

(Me’+Me”) O_m+2m/3 Al®Me’+Me”+m/3 Al₂O₃+Q₁ (1)

Me’+nC®Me’C_n+Q₂ (2)

Total: (Me’+Me”) O_m+2m/3 Al+nC®(Me’C_n +Me”)+m/3 Al₂O₃+(Q₁+Q₂ ) (3)

Here Me’ is the carbide-forming element (e.g. W) and Me” is the metal that is not combined with carbon but forms the plastic matrix which binds together the hard carbides Me’C_n. In usual carbide alloys Me” is cobalt. In “carbidostal” [3]. Co is replaced by alloyed tool steel, e.g. 12% Cr or high-speed stell (HSS). In Eq. 1 and 3 the ratio Me’/Me” is not specified. This ratio is very important because it influences the total heat evolution Q=Q₁+Q₂ and also the ratio carbidic phase/cobalt or tool steel matrix in the carbidic alloy or in “carbidostal”. In case of the synthesis of “carbidostal” surplus carbon must be added to the powder mixtures as per Eq. 2 because this extra amount is necessary to carburise the austenite+martensite metal matrix. When this matrix is alloyed with Cr or with W+Cr the dissolution of carbon in the liquid iron-based highly hard alloyed solution gives a small additional evolution of heat Q₃ which supplements the sum Q₁+Q₂.

A programme for computer aided calculations of Q₁, Q₂ and Q has been elaborated and has been used in ref. 3. In fig. we have shown the microstructure of "carbidostal" and of its steel matrix.

Fig. Microstructure of a "carbidostal" produced by the SHS (self-propagating hightemperature synthesis): a − massive WC carbides in an ultra high-carbon high-tungsten steel matrix (magnification x100); b − matrix with spheroidized complex carbides (magnification x400)

This "carbidostal" has been obtained by the "hybrid" process using Fe₂O₃, Cr₂O₃, WO₃ graphite and aluminium powder mixtures. Tungsten binds most of the carbon into large WC carbides, shown in fig. 1. Another part of tungstem binds carbon into small WC carbides forming the complex eutectic matric. This matrix may also contain small amounts of other carbides: Me₇C₃, Me₆W₆C₂ et al. In fig. we can see that these eutectic carbides have been strongly spheroidised during rheocasting, when sharp edges have been rounded up. The investigate this alloys (at the x-rey spectrum of this alloy obtained in Cu_Ka radiation) are shows only the WC and W₂C carbide phases seen in it. The iron-rich phase is a'-martensite obtained after self-quenching and triple tempering at 570⁰C. The hardness of the alloys obtained in such a new way is 72-75 Ra. Medium size cutting tools have been obtained by burning thermit+SHS powder mixtures in a highly refractory combustion chamber placed over a small refractory mould surmounting the preliminarily heated steel holder of the tool. The combustion chamber and the mould must be separated by a thin titanium sheet. The exothermic mixture is ignited by a small amount of Mg or Ti powder, which is ignited itself by an ordinary match. When burning reactions end the slag floats up and the extremely hot liquid phase burns through the titanium sheet and fills the ceramic mould, being thus automatically welded to the steel holder of the tool. Such technology excludes brazing and other operations, designed to join the carbide alloy to steel. A number of different types of tools for metal cutting and rock boring have been produced in such a novel way with good exploitation features in semi-industrial and laboratory conditions. The further work must be focused on augmenting the content of primary WC carbides in the "carbidostal" obtained, the partial replacement of WO₃ by TiO₂ and other subjects of investigations.

TiO₂ is much less prone to oxidise Al than WO₃, the thermit-type reaction being much less exothermic in case of replacement of W by Ti. Therefore such full replacement is impossible. Yet the SHS reaction Ti+C=TiC is very "hot", the adiabatic temperature of such an oxygen-less burning being 3200 K. More exact computations and experiments must reveal the extent of partial replacements of that type.

References:

1. Жуков А.А., Мержанов А.Г., Боровинская И.П. Использование СВС в литейном производстве//Литейное производство. M:. Машиностроение. 1984, № 11. − С. 2−3.

2. Жигуц Ю.Ю., Жуков А.О. Новітні технології виготовлення та зміцнення деталей із використанням СВС-процесів// Восточно-европейский журнал передовых технологий. − Харьков. − Техн. Центр. − 2007. − №1 (25). − С. 32−38.

3. Zhiguts Yu.Yu., Shurokov V.V. Carbide steels synthesized by metallothermy// Materials Science. Springer. New York. − 2005. − V. 41. №5. − P. 666−672.