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Centrifugally cast tubes of heat resisting alloys are widely used in reformers and other high temperature chemical industries. In the present paper, the creep behavior of two different HP-40 alloys modified with a low level addition of Nb is investigated. The study focuses on damage localization and on damage quantification. Creep tests are carried out at 980, 1030 and 1050°C with different stress levels, in the range of 20 to 50 MPa. A qualitative and quantitative analysis of creep damage is carried out that needed a special procedure which is detailed. As for the results, emphasis is put on the effect of the stress level and on the difference between the two alloys.