ABSTRACT
The use of high-temperature materials is especially important in power station construction, heating systems engineering, furnace industry, chemical and pet rochemical industry, waste incineration plants, coal gasification plants and for flying gas turbines in civil and military air crafts 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. Hith erto, heat resistant ferritic steels sufficied in conventional power station con structions for temperatures up to 550°C, newly developed ferritic/martensitic steels provide sufficient strength up to about 600 - 620°C. In new processes, e. g. fluidised-bed combustion of coal, process temperatures up to 900°C occur. How ever, this is not the upper limit, since in combustion engines, e. g. gas turbines, material temperatures up to 1100°C are reached locally. Similar development trends can also be identified in the petrochemical industry and in 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 hightemperature strength properties; considerable attention now also has to be given to their chemical stability in corrosive media. If temperatures exceed 550°C, the mechanical properties of the heat-resistant ferritic steels are drastically reduced, especially the creep strength. The low creep strength of the ferritic materials at high temperatures is largely attributable to the relatively high mobility of the at oms in the body centred cubic lattice. As Fig. 4.1 clearly shows that using mate rials with a face centred cubic lattice provides a significantly higher creep strength [1].The 3 materials on the left in Fig. 4.1 are body centred cubic, while those on the right are face centred cubic. The greater high-temperature strength is largely attributable to the more densely packed face centred cubic lattice with the atoms having lower mobility. Classic examples of this are the nickel-alloyed austenitic stainless steels with approximately 16-25 wt% chromium and 8 - 20 wt% nickel. A further increase in the high-temperature strength properties of these auste nitic steels can be obtained by solid solution hardening and certain other mecha nisms. Elements other than the alloying components iron, chromium and nickel are necessary for this purpose, for instance Mo, W, Co, Ti, Al, Nb. The austenitic stainless steels and also the iron-based alloys have the disadvantage of a very limited solubility for these elements. This can be compensated by increasing the
Figure 4.1: 100,000h-creep strength of some steels and Ni-base alloys
nickel content and hence producing extremely highly alloyed materials contain ing practically no iron. High-temperature materials are thus frequently based on the element nickel, since this produces a face centred cubic lattice and a high solubility for those alloying elements that promote high-temperature strength properties and corrosion resistance.
