Abstract Scope |
Introduction
High carbon Fe-Cr–Ni alloys designed for elevated temperature service in olefin furnaces are exposed to thermal fatigue and thermal shock loading during operation. Centrifugally cast tubes and statically cast fittings made of three different alloys were tested in the study. The objective was to develop and validate a testing procedure for evaluation and ranking the susceptibility to thermal fatigue and thermal shock cracking in Fe-Cr–Ni alloys, as well as investigate the failure mechanism and compare the cracking susceptibility of the tested alloys.
Technical Approach
The proposed thermal fatigue testing procedure utilized the GleebleTM 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 1110oF (600oC) and 2040oF (1115oC) with heating rate of 10,000oF/hr (1.54oC/s) and cooling rate of 83,462oF/hr (12.88oC/s). 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.
Results / Discussion
The test outputs included numbers of cycles to failure, change in gauge cross section area, failure stress, failure temperature, maximum experienced tensile and compressive stress, stress vs. time integral, failure location, and failure mode. These test outputs were used for quantification and ranking thermal fatigue / thermal shock cracking susceptibility.
The microstructure of tested alloys before and after testing was evaluated. The dendrite cell boundaries in all tested alloys were outlined with Cr-rich M7C3 and Nb-rich MC type carbides. Finely dispersed intergranular Cr-rich carbides were found in some of the tested alloys. The crack nucleation and propagation occurred preferentially along Cr-rich grain boundary carbides, while Nb-rich carbides also served as voids nucleation sites. The crack propagation mechanism involved potential dynamic recrystallization in the plasticly deformed zone in front of the crack tip, evidenced as fine grains formed within the austenitic matrix of the dendrite cells. It was found that thicker continuous carbides provide sites for preferential crack nucleation and propagation, while eutectic carbide morphology would delay the crack propagation process by diverting the cracks through the austenitic matrix.
It was also found that materials containing larger amount of intermetallic phases are more susceptible to thermal fatigue cracking. Al-containing alloys form NiAl and gamma prime precipitates that harden the grain interior and cause grain boundary strain concentration, which would further promotes grain boundary crack nucleation and propagation.
The casting process was also found affect the cracking susceptibility of the alloys is. The better cracking resistance in centrifugally cast alloys was related to smaller grain size and finer grain boundary carbides.
Conclusions
Susceptibility to thermal fatigue and thermal shock cracking of high carbon Fe-Cr–Ni alloys designed for working under elevated temperature was tested utilizing a procedure that replicated service conditions in olefin furnaces. The effect of alloy composition, microstructure, and casting process on the failure mechanism was evaluated. Crack nucleation and propagation occurred predominantly along interconnected Cr-rich carbides at the dendrite cell boundaries. The grain size, content and morphology of grain boundary carbides, intermetallic phases, and solidification microstructure were idenal as controlling factors in the thermal fatigue and thermal shock failure mechanism. |