EVALUATION OF CREEP-FATIGUE DAMAGE INTERACTION IN HK40 ALLOY

In order to establish design criteria for materials which may sustain creep-fatigue damage, the creep rupture and creep-fatigue behavior of a high-carbon centrifugal cast steel was investigated at three different temperatures of 800, 900, and 1000-degrees-C, using HK-40 alloy which is a typical furnace tube material for fuel cell plant reformers and so on. The strain waveforms used for the creep-fatigue tests consisted of triangular waveforms-pp waves (fast-fast waves), cc waves (slow-slow waves), pc waves (fast-slow waves), and cp waves (slow-fast waves)-and a trapezoidal waveform holding the peak strain at the tension side. The applicability of various creep-fatigue interaction damage assessment methods were evaluated with particular emphasis on the life fraction rule (LFR) employed in ASME Section III, Boiler & Pressure Vessel Code Case N-47 and the strain range partitioning method (SRP). As it turned out that through the LFR life evaluation of HK-40 alloy subjected to strain cycling with holding at the tension side was well interpreted, design criteria for reformer tubes were established by applying the LFR to creep-fatigue life evaluation.