Welding Metallurgy of Heat Resisting Steels

Abstract

Heat resisting steels are normally alloyed with chromium, silicon, aluminium and often also with nickel, with chromium being the most important element with regard to the scaling resistance. There exist ferritic, ferritic-austenitic und austenitic heat resisting steels. The latter contain higher amounts of nickel, normally in the range of approx. 4–35%. A comprehensive survey of the effect of alloying elements and scaling behaviour in different corrosive media is given by Rapatz [8], Houdremont [9], Fontana and Green [233] and Sedrics [390]. Table 24 shows some examples of the chemical composition requirements and scaling temperature limits of heat resisting steels. Compared to the corrosion resisting steels (see Tables 4, 12, 14, 16, 18, 20 and 22) not only are the silicon and aluminium contents increased but also the carbon content is often increased. If the ferritizing effect of 1% silicon and 1% aluminium is considered, these alloys will sometimes show high delta ferrite contents which in turn will lead to a strong tendency towards coarse grain formation at elevated operating temperatures. The ferritic heat resisting steels No. 1–3 thus show a strong tendency towards embrittlement which often appears only after cooling to ambient temperature. The presence of austenite counteracts coarse grain formation and such steels show less embrittlement than the ferritic grades No. 1–3. During operation, the fully austenitic heat resisting grades Nos. 5–9 also become coarse grained, however, they retain their toughness when cooled to ambient temperature because of their austenitic structure.