THE PROJECT AIMS TO STUDY NEW TYPES OF LOWER COST FERRITIC STEELS DESIGNED TO OPERATE IN THE KIND OF ATMOSPHERE ENCOUNTERED IN COAL GASIFICATION PLANTS. THIS REQUIRES THAT UNDER MILDLY OXIDISING CONDITIONS THE STEEL IS RESISTANT TO CORROSIVE AND EROSIVE HIGH TEMPERATURE GAS ATTACK.
Steels have been developed containing chrome, aluminium, and in some cases titanium. These additions improve the metallurgical behaviour of the steel (eg fragility, durability) through control of the microstructure with precipitate formation. Titanium has been shown to improve corrosion resistance in steels, and although the process is not clearly defined, performance in very aggressive atmospheres is superior to chrome.
Results on research into ferrous materials containing aluminium have been gathered. The behaviour of steel containing chromium and aluminium with titanium were studied, in particular as they have interesting properties. The precipitation on an iron (2+) phase makes possible the control of their microstructure and affects their mechanical properties (eg shock susceptibility, fragility at 20 C). Alloys have been shown to have a good performance notably at 600 C in a sulphurous corrosive atmosphere. These materials have many industrial applications in, for example, coking plants, petrochemicals, high temperature chemical treatments or waste incinerators).
New types of lower cost ferritic steels designed to operate in oxidizing and sulphadizing atmospheres have been developed. This requires that under mildly oxidising conditions the steel is resistant to corrosive and erosive high temperature gas attack. Ferritic steels with good abrasion and corrosion features could be developed at a lower cost than conventional stainless steels. The innovation features the replacement of the nickel and a portion of the chromium contained in ferritic stainless steels with aluminium. Furthermore, stainless steel microstructures and properties are controlled by adding titanium and by carrying out appropriate thermal treatments during the manufacturing process.
A series of corrosion and erosion tests were carried out between 300 C and 600 C on 9% and 12% chromium steels alloyed with elements such as niobium, vanadium, tungsten and titanium, with additions of aluminium. Creep-corrosion interaction studies were carried out in aggressive atmospheres up to 600 C and at pressures between 0.1 and 6.77 MPa in order to test the viability of using ferritic alloys to replace more expensive austenitic materials such as 316 L steel alloy and 800 H. Aluminium steels subject to corrosion have a similar behaviour to 316 L.
Heat treatment was carried out between 500 C and 900 C in order to get recrystallised structures. The alloys had a poor ductability at room temperature. At high temperatures (600 C) the alloys had better mechanical properties than the existing ferritic steels and a fairly good behaviour against corrosion.
THE PRINCIPAL CORROSION MECHANISM IS THROUGH CARBURISATION AND SULPHURIZING, AND THIS IS INTENDED TO BE MINMISED BY THE ADDITION OF SMALL AMOUNTS OF AL TO THE FERRITIC STEEL.
THE EXPERIMENTAL WORK WILL CONSIST OF A SERIES OF COORROSION AND EROSION TESTS CARRIED OUT BETWEEN 300 AND 600 CELSIUS DEGREES ON 9% AND 12% CR STEELS ALLOYED WITH ELEMENTS SUCH AS NB,V,W AND TI, WITH ADDITIONS OF ALUMINIUM. CREEP-CORROSION INTERACTION STUDIES IN AGGRESSIVE ATMOSPHERES UP TO 600 CELSIUS DEGREES AND AT PRESSURES BETWEEN 0,1 AND 6,77 MPA WILL BE CARRIED OUT IN ORDER TO TEST THE VIABILITY OF USING FERRITIC ALLOYS TO REPLACE MORE EXPENSIVE AUSTENITIC MATERIALS SUCH AS 316 L STEEL OR ALLOY 800 H.
Funding SchemeCSC - Cost-sharing contracts