High carbon, Cr – Ni alloys are used for tubing in refinery olefin furnaces. During furnace operation, such tubes might be exposed to thermal shock and fatigue loading, which may result in cracking and/or complete rupture. The objective of this study was to develop and validate a testing procedure for evaluation and ranking the susceptibility to thermal fatigue cracking in tubing of such Cr – Ni alloys.
The proposed thermal fatigue testing procedure utilizes the Gleeble™ thermo-mechanical simulator and was developed to closely match extreme service conditions. Test samples with notched gauge section were subjected to repeating thermal cycles under fixed displacement to restrict the sample expansion and contraction, during heating and cooling, and simulate high level of structural restraint. The temperature range of thermal cycling was between 1110°F (600°C) and 2040°F (1115°C) with heating rate of 10,000°F/hr (1.54°C/s) and cooling rate of 83,462°F/hr (12.88°C/s).
The following parameters were recorded and used for ranking the thermal fatigue cracking susceptibility: number of cycles to failure, stress vs. time integral, maximum experienced tensile and compressive stresses, and change in gauge section area. All failed test samples were subjected to fracture surface analysis using scanning electron microscopy. The non-failed samples were examined for notch cracks using binocular microscope at up to x100 magnification. The gauge section of all tested samples was characterized with light optical and scanning electron microscopy to correlate crack nucleation and propagation to susceptible microstructural constituents.