Creep Fracture in Tempered Martensitic Steels

Creep strength enhanced ferritic (CSEF) steels which contain 9 to 12%Cr have become the steels of choice in many industrial high energy systems. The typical compositions of alloy types P91, P92 and E911 are compared with 12CrMoV in Table 1. These steels are selected for use in applications where high temperature creep can occur because of a favorable balance between cost and properties [1,2]. The excellent creep and fracture resistance are achieved because the microstructure, which, when properly processed, exhibits a tempered martensitic matrix containing a substructure with a high dislocation density and a fine dispersion of second phase precipitates. It has been pointed out that the typical alloy compositions for these steels frequently do not include recommendations for upper limits on trace or ‘other’ elements. At least for Grade 91 steels this situation is changing [3]. Because it is well established that inclusions and trace elements can lead to embrittlement it is likely that further controls will be needed to reduce the risk of in-service fracture in other CSEF steels.