Oxidation resistance of aluminum chromium steels in agressive environments

The analyzes of operating conditions of high-temperature parts of fuel-burning devices of boiler units at thermoelectric power stations (TPS) has revealed that their operating temperature reaches 1100–1250 ° C. Since the nozzles of such devices are subjected to intense abrasive actions of coal dust and other fuel components and wear out quickly as a result of erosion processes, then Cr-Al steels with an optimal carbon content can also be used. From the economic point of view, it should be noted that nickel, the content of which in some types of steel reaches 20 %, belongs to expensive and scarce metals. Thus, its use in such quantities for alloying heat-resistant steels is unjustified and impractical, and it becomes possible to replace expensive heat-resistant chromium-nickel steels with cheap casting materials, for example, iron-based alloys with a high chromium content. The choice of an alloy with high oxidation resistance is necessary but insufficient criterion to ensure the reliability and durability of its operation. This is due to the fact that in the process of operation, the products are subjected to periodic heating and cool-ing, that means thermal cycles. In this case, an unequal temperature field arises in the volume of the metal, which contributes to the accumulation of thermal tension. Such tensions can exceed the permissible norms under these conditions, as a result plastic deformation develops in the metal, which subsequently leads to its destruction. To achieve the goal, the work investigated the effect of alloying elements of chromium and aluminum on the heat resistance of iron-based alloys in superheated air with the addition of water vapor and carbon dioxide. The main chemical element in steels of this class is carbon, which has a negative effect on oxidation resistance; therefore, this fact must be taken into account when choosing a heat-resistant steel to manufacture products operating at high temperatures and aggressive environments. A decrease in the rate of oxidation of alloys at high temperatures is achieved due to the formation of a dense protective oxide layer on their surfaces, in which the processes of diffusion transfer of metal and oxygen ions are significantly complicated. This is obtained by adding appropriate amounts of chromium and aluminum to the steel. When the content of aluminum is more than 4 %, the rate of its burnout is practically constant, and the average content of aluminum in steel at any moment of the test time submits to linear relationship.