The 9%Cr-steels with boron and boron/cobalt additions were developed with excellent creep behaviour in the temperature range of about 600°C. The long-time creep exposure data (creep elongation, creep rate, creep rupture strength) obtained at 600 and 650°C under different stresses between 60 and 230MPa are presented. Testing times up to about 65.000h were accumulated so far. Some comparisons with the available creep data of other steels from this group (without boron additions) are shown. The microstructural investigations (mainly by electron microscopy methods: TEM/STEM/EDX, SEM) were carried out on the samples prepared after different degrees of creep deformation or after different ageing duration. Qualitative and quantitative analysis was focussed on the precipitation processes and on the matrix structure (tempered laths martensite, subgrains, free dislocations). The results were correlated to the creep behaviour of the material and the creep mechanisms were revealed. The evidence of the high microstructural stability of the new steels is shown. The reasons for the excellent creep strength and the sufficient (≥12%) creep ductility at the same time are discussed. The comparison with the creep behaviour of other steels from the 9-12%Cr group is provided.
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