ADVANCED NICKEL-BASED AND NICKEL-IRON-BASED SUPERALLOYS FOR CIVIL ENGINEERING APPLICATIONS

The use of high-temperature materials is especially important in power station construction, heating systems engineering, furnace industry, chemical and petrochemical industry, waste incineration plants, coal gasification plants and for flying gas turbines in civil and military aircrafts and helicopters. Particularly in recent years, the development of new processes and the drive to improve the economics of existing processes have increased the requirements significantly so that it is necessary to change from well-proven materials to new alloys. Hitherto, heat resistant ferritic steels sufficed in conventional power station constructions for temperatures up to 550℃ newly developed ferritic/martensitic steels provide sufficient strength up to about 600 - 620℃. In new processes, e.g. fluidized-bed combustion of coal, process temperatures up to 900℃ occur. However, this is not the upper limit, since in combustion engines, e.g. gas turbines. Material temperatures up to 1100℃ are reached locally. Similar development trends can also be identified in the petrochemical industry and in the heat treatment and furnace engineering. The advance to ever higher material temperatures now not only has the consequence of having to use materials with enhanced high-strength properties, considerable attention now also has to be given to their chemical stability in corrosive media. Therefore not only examples of the use of high-temperature alloys for practical applications will be given but also be contributed to some general rules for material selection with regard to their high-temperature strength and corrosion resistance.

Thermal and microstructural characterization of a novel ductile cast iron modified by aluminum addition

In high-temperature applications,like exhaust manifolds,cast irons with a ferritic matrix are mostly used.However,the increasing demand for higher-temperature applications has led manufacturers to use additional expensive materials such as stainless steels and Ni-resist austenitic ductile cast irons.Thus,in order to meet the demand while using low-cost materials,new alloys with improved high-temperature strength and oxidation resistance must be developed.In this study,thermodynamic calculations with Thermo-Calc software were applied to study a novel ductile cast iron with a composition of 3.5wt%C,4wt%Si,1wt%Nb,0‒4wt%Al.The designed compositions were cast,and thermal analysis and microstructural characterization were performed to validate the calculations.The lowest critical temperature of austenite to pearlite eutectoid transformation,i.e.,A1,was calculated,and the solidification sequence was determined.Both calculations and experimental data revealed the importance of aluminum addition,as the A1 increased by increasing the aluminum content in the alloys,indicating the possibility of utilizing the alloys at higher temperature.The experimental data validated the transformation temperature during solidification and at the solid state and confirmed the equilibrium phases at room temperature as ferrite,graphite,and MC-type carbides.