Advanced Materials for Advanced Heat Exchangers

A variety of new austenitic and nickel based alloys have been developed in the 1980s and 90s, many of which are finding application in advanced energy conversion plant. The paper briefly examines the historical background to these new materials. They can be traced back to the simple austenitic stainless steels of the 1930s, in both their wrought and spun cast forms, and in the development, in the late 1940s, of super stainless alloys such as Incoloy 800. Most of these alloys have been developed outside of the boiler plant materials programmes aimed at conventional power generation systems. These programmes have largely focused on low alloy ferritic, bainitic, and martensitic superheater tube materials, intended for use at fairly modest temperatures and in moderate corrosive conditions. The advanced alloys, covered in the paper, are used in much more aggressive conditions, in terms of temperatures and in the severity of corrosion. Both are characteristic of novel energy conversion processes requiring advanced forms of heat exchanger. Examples of alloys from Europe, America, and Japan are used to describe some of the thinking behind the development of these new materials. The background to commercial and near commercial energy conversion systems, which require advanced heat exchangers, is also described. These include indirect fired gas turbines; recuperative gas turbines of various types, fluidised bed combustion, coal gasification, and waste incineration. To optimise strength over a given temperature range, each of the new alloys needs to utilise a specific type of precipitation or dispersion strengthening mechanism. In the older austenitic alloys the principal precipitation processes involved gamma prime and chromium carbides. The newer alloys utilise yttria for dispersion strengthening, and nitrides, carbo-nitrides, complex carbides, and copper rich phases for precipitation hardening. It is emphasised that the general fall off in stress rupture properties, with temperature, is governed by the ease of dislocation climb over strengthening precipitates. It is therefore possible to draw an ‘average’ stress rupture strength line for austenitic and nickel based alloys, the gradient of which is probably governed by the diffusion rate and elastic modulus of such materials. In order to maximise resistance to attack in advanced heat exchanger environments, the newer materials need higher levels of alloying elements. In general, chromium contents are very much greater, particularly where the attack involves a molten salt phase. However, at very high temperatures, to resist simple oxidation, the newer alloys need to contain significant amounts of aluminium, so that in that in some cases they form scales of alumina rather than chromia. This feature has been 80 made possible by the addition of active elements, so reducing the propensity to scale spallation. Recent progress with the understanding of coal ash corrosion is described. Two major problems remain to be overcome in a satisfactory manner. These are corrosion in coal gasification and waste incineration systems. In both processes there is now a reasonably good understanding of the complex mechanisms that lead to such high rates of corrosion at such comparatively low temperatures. The presence of high levels of chlorine creates new difficulties. This points to the need for development of purpose designed alloy systems and coatings. Spun cast production of high alloy tubing may give new opportunities in this respect.