In this thesis work the influence of alloying elements: chromium, aluminum, carbon, titanium and REM on the complex of foundry, mechanical and special properties and the structure of heat-resistant steels has been examined and the optimal content of these elements has been determined, pro-viding the best combination of technological and operational characteristics.
For the manufacture of heat-resistant cast parts, which operate in the conditions of high tem-peratures and aggressive environments, steels with the following chemical composition,% by weight: C = 0,25...0,35; Cr = 25...32; Al = 1,2...3,5; Ti = 0,25...0,50; Si <1,0; Mn <0,8; P <0,025; S <0,025 have been developed.
It has been established that in order to achieve high technological properties, long and efficient operation of products under extreme conditions in alloys the ratio [% Cr] / [% Al] = 6 ... 10 must be kept, that is, with chromium content of 25% in steel aluminum should be 3.5 ... 4.5%, and for steel with 30% of chromium - 2,0 ... 3,0% of aluminum. The carbon content of all alloys should be kept within the limits of 0,25 ... 0,35%, and the titanium - 0,25 ... 0,50%.
X-ray studies and microchemical tests of an oxide film which is formed on the surfaces of products during their operation in the conditions of high temperatures and aggressive environments have found that it consists of oxides of aluminum, chromium and titanium and small quantity of ferrous oxide. The oxide film is formed on the surface of a product for several tens of minutes at the beginning of its operation and in future protects the product under extreme conditions due to the in-crease the amount of aluminum oxide in it. Such film has the highest protective properties and practi-cally determines the product lifetime.
The research of mechanical properties at high temperatures and the thermal stability of created steels has established that the choice of the steel grade for the production of heat-resistant products should be based mostly on the configuration of a product, the thickness of the wall of a cast part, its overall dimensions and the foundry properties of the recommended steels should be taken into account.
It has been established that for temperatures above 1100 ° C, medium carbon chromium-aluminum steels of the ferrite class have much higher oxidation resistance and resistance to growth than chromium nickel steels of austenitic class, although the strength and plasticity of the latter are somewhat higher if compare with chromium-aluminum steels. This confirms the expediency of replac-ing expensive chromium-nickel steels with cheap chrome-aluminum for the production of cast parts, which work in high temperature and aggressive environments without external loads.
The prospect of expanding the limits of the use of recommended steels for the production of heat-resistant products with the use of thermo-mechanical treatment of cast sections by pressing and forging has been determined.
To operate in conditions of intense wear, a new nickel-free high abrasion-resistant chromium-manganese cast iron with high operational and satisfactory foundry properties of chemical composi-tion,% mass: 2,8...3,2 С; 18,0...20,0 Cr; 3,5...4,5 Mn; 0,6...0,8 S; P <0,05; S <0,05 has been developed. To increase wear resistance of cast iron up to 20...25% it is advisable to microalloy it with titanium within the limits of 0,1...0,5%, vanadium – 0,5...0,8% or antimony – 0,1...0,2% and modify with boron in the range of 0,005...0,020% or REM – 0,1...0,25% (by additive). In order to achieve the maximum hardness of metal and wear resistance of products from the recommended chromium-manganese cast irons, they must be quenched in the air at temperatures of 900...950 °C.
A data bank has been created on the influence of chromium, aluminum, carbon, titanium and REM on the technological and operational properties of the proposed alloys. The program has been developed and tested for calculation of charge for melting of alloys with high chromium content and the prediction of the quality of the melt in the melting unit, based on the results of the first chemical analysis and the temperatures of its overheating in the melting unit and pouring into molds.
The technological processes of melting of the proposed heat-resistant and wear-resistant alloys in induction and arc furnaces with various lining and manufacturing of small and large-sized thin-walled castings from these alloys by casting in one-time three-D sandy-clay forms and special methods of casting: in shell and metal forms, for models that are hot or gasified and centrifugal casting. Cast-ings can be made from alloys from a few tens of grams to several hundreds of kilograms of different geometry and with different wall thicknesses. For implementation of melting processes, the corre-sponding technological instructions have been developed.
