| Abstract: | In view of efforts to develop ferritic creep resistant steels for applications above 600°C the effect of fine precipitate particles on the creep behaviour of ferritic model steels was studied as a function of stress, temperature and particle distribution. The chosen model steels contained 20% Cr (by mass), up to 0.9% Nb and up to 0.1 % C to produce NbC volume fractions up to 0.8% with particle sizes of about 0.1 μm (order of magnitude). The alloys and structures are briefly described (NbC solubility, precipitation and ageing behaviour, recrystallization and grain growth, oxidation resistance) as well as the mechanical short-term behaviour. The creep behaviour was studied between 600°C and 800°C (with emphasis on 700°C) at strain rates between 10?11 and 10?6 s?1 with times to rupture up to 20000 h. The creep resistance of the model steels at 700°C (for a strain rate of 10?8s?1) increases with increasing NbC content from about 5 MN/m2 for the alloy without NbC to about 50 MN/m2 for the alloys with 0.6% or 0.8% NbC. The analysis of the obtained results is the subject of the second part of this report. |
