Creep ratchetting is the accelerated damage experienced by a component under high-temperature (>0.4 Tm) creep conditions and fluctuating stresses. In the methanol production process, thick-walled (~15 mm) steammethane reformers consist of hundreds of vertical tubes operating at temperatures up to 950 °C and internal pressures of 2000-3500 kPa. Operating in creep conditions, these tubes are often subjected to fluctuating stresses in the form of plant shutdowns, local variations in temperature, and through-wall temperature gradients. As such, premature failure of reformer tubes has been experienced and is of particular interest to Methanex Ltd. NZ.

In the current work, a brief analysis of the as-cast and aged microstructures of Centralloy® G 4852 Micro-R reformer tube alloy was completed. Scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) techniques revealed as-cast microstructural phases consistent with those found in microalloyed reformer tube materials in industry and literature. Analysis of a macro-etched tube cross section revealed a 100% composition of columnar grains, indicating the removal of the equiaxed region during manufacturing or machining had occurred. Analysis of aged samples at 1050 °C for 1000 and 5000 hours revealed coarsening of the primary carbide network. Extensive precipitation and coarsening of smaller cube or needle shaped secondary carbides within the austenite matrix was observed. No clear difference in the structure, density, or distribution of precipitates was observed between the 1000-hour and 5000-hour aged samples.

As-cast specimens of Centralloy® G 4852 Micro-R were subjected to creep and creep ratchetting (C-CR) testing at 975 °C, stresses of 30, 36, and 42 MPa, and a ratchetting ratio (dwell to hold) of 1:6. At 30 and 36 MPa stress levels, creep ratchetting tests were completed at various dwell times (20, 30, 40 minutes), with a secondary stress of 6 MPa. The 42 MPa creep ratchetting tests were completed with a dwell time of 30 minutes, and varying secondary stresses (2, 4, 6 MPa). These two testing regimes were used to identify the relative effect of elevated stresses in a dwell period on the performance of Centralloy® G 4852 Micro-R. Post processing and interpretation of this data has been completed.

Further analysis of the collected data was completed using Robinson’s rule, Larson-Miller data, strain rate equations, the Omega method, the Gurson-Tvergaard-Needleman (GTN) model, and a proposed non-linear algebraic (NLA) equation. Finally, an assessment of the effectiveness of each technique on the modelling of creep ratchetting behaviour is provided.