Grade P92 has been designed to have a tempered martensitic microstructure in initial state.However

it is also susceptible to the formation of small amounts of ferrite under certain conditions

especially in heavy-wall components.Extensive microstructural investigations were carried out on a broad collection of samples

and the effect of the presence of a small amount ferrite microconstituent in the tempered martensitic matrix on short and long term mechanical properties was examined.The results show two types of ferritic microconstituent

differing in origin and properties

are identified in P92 tubular products.Stable ferrite

also called delta ferrite

forms during solidification and remains in the microstructure after cooling down to room temperature if the ferrite and the austenite forming alloying elements are not properly balanced.In addition

another form of ferritic microconstituent is formed during relatively slow cooling down from the austenitisation temperature by decomposition of austenite into ferrite and carbide.Samples without and with 8% of ferritic microconstituent have been studied and compared

no significant changes were observed.Particularly for the long term creep properties with the creep rupture times up to 70 000 h

it has been shown that the creep rupture data are within the scatter band around the master curve of the ECCC data assessment.Additionally

aguideline for determining ferritic microconstituent content using the comparison method has been developed and is now established and published as a VdTV data sheet.