A COMPARISON OF THE CREEP-RUPTURE BEHAVIOR OF NEW AND SERVICE EXPOSEDLOW-ALLOY STEELS

Creep rupture tests have been carried out on 1Cr1/2Mo and 1/2Cr1/2Mo1/ 4V steels in the new (normalized and tempered) and in service exposed conditions. The results showed that the new materials were significant ly stronger than the service exposed materials in tests of short durat ion, owing to the transient strengthening effects of carbon redistribu tion and carbide precipitation. With increasing test time the stress r upture strengths of the two conditions converged, as a stable microstr ucture was established in the new materials. The extrapolation of the new material data can lead to an underestimation of the longterm stren gth, continuing the steep/decrease in strength for the new material be yond the region in which experimental data are available. It is sugges ted that the stress rupture curve will in reality have a sigmoidal sha pe, as the results for new and service exposed materials must eventual ly come together. For the 1Cr1/2Mo steel, the creep damage developed i n the longest tests depended on the condition. New material exhibited creep pore formation after relatively small creep strains of around 1% whereas the service exposed material exhibited no cavitation at creep strains of 5% after 10000 h testing. It was not possible in laborator y creep tests of 10000 to 20000 h duration to reproduce the creep dama ge typically found in Components of this steel after prolonged service .