Research On Fatigue Behavior And Mechanism Of FV520B In Very High Cycle Regime

The centrifugal compressor impeller is the component with high rotation speed, and it suffers the load with high frequency and low amplitude caused by the wake flow, the unit vibration and other reasons. Therefore, the fatigue life requirement of impeller is higher than 107 cycles and into the very high cycle fatigue (VHCF) regime. The researches of VHCF behavior and mechanism of FV520B which is an important material of impeller support a long fatigue life design and the prevention of fatigue failure of centrifugal compressor impellers. And relevant researches support the determination of remanufacture critical threshold of centrifugal compressor impeller too. The mechanical characteristics of a centrifugal compressor impeller were analyzed, and the effects of aging temperature, specimen sizes, surface roughnesses, welding on VHCF behavior and mechanism of FV520B were further studied.The relevant mechanical characteristics of an ideal shrouded impeller and this impeller with local blade thinning caused by the non-uniform wear in a long-term operation were analyzed by the finite element method. This impeller is the first stage of the low pressure cylinder in H418 air separation air pressure unit. The results showed that:(1) The maximum static stress appeared in the middle of the blade root when the impeller was only suffered the centrifugal load. (2) The stress concentration mainly appeared in the middle of the blade leading edge when the speed of impeller was close to the rated speed, and this location was in accordance with the fatigue fracture site of this impeller. (3) The local thinning of blade leading edge close to the shroud caused by the wear was the "small mistiming", and the topology structures of mistuning impellers and the ideal impeller were essentially identical, so the natural frequencies and vibration modes of mistuning impellers seemed unchanged compare with the ideal impeller’s. (4) The dangerous location was the middle of blade leading edge and the maximum stress amplitude was 33.3MPa when the impeller resonated near the rated speed caused by the wake flow. The vibration mode of blade leading edge was similar to the bending vibration of a cantilever with a sliding mass at the other end.The effects of aging temperature on the VHCF behavior and mechanism of FV520B were researched. The fatigue properties of FV520B-I and FV520B-S which were obtained after different heat treatments up to 109 cycles were tested by the ultrasonic fatigue testing machine with a symmetric tension-compression load. The S-N curves were obtained, and the VHCF mechanisms were analyzed by observing fracture morphology and measuring characteristic area sizes. The fatigue behavior and mechanism of FV520B-I and FV520B-S were very different. The S-N curve of FV520B-I continuously declined and could be divided into two sloping lines becasue the crack initiation site transfered from the surface to the subsurface. The S-N curve of FV520B-S was far below FV520B-I’s, and there was a conventional fatigue limit even from 106 cycles. In the VHCF regime, most cracks of FV520B-I initiated from the subsurface inclusions, and a few cracks initiated from the surface matrix or subsurface soft ferrites. There was a close relationship between the fish-eye and the distance from the crack origin to the specimen surface. The subsurface crack transformed into the surface crack quickly when the crack origin was close to the specimen surface. The large difference of crack growth rate in two regions resulted in the fish-eye formation. The VHCF life increased with an increase of the GBF (granular bright facet) diameter, and the relationships of the VHCF life with the other characteristic area sizes were not evident. The initiation life of crack initiating from subsurface inclusion seemed not the main part of the VHCF life, and the VHCF life was mainly used up in the GBF formation. The inclusion diameter had some effects on the GBF diameter, and the GBF diameter significantly decreased with the obvious decrease of the inclusion diameter. The effects of inclusion shape on the prediction of fatigue strength were analyzed on the base of Murakami model by fracture mechanics. The prediction value involving the effects of inclusion shape was 2% higher than that of Murakami model, and the prediction value of FV520B-I VHCF strength by modified model was more close to the test result. The VHCF life of FV520B-I was fitted by the Paris law and the corrosion fatigue crack initiation life model. The fitting effect of the corrosion fatigue crack initiation life model was significantly better than that of the Paris law.The VHCF properties of FV520B-I large specimens were tested by the ultrasonic fatigue testing and the effects of specimen sizes on VHCF behavior and mechanism of FV520B-I were studied. The inclusion size increased with an increase of specimen size and the heat effect of large specimen seemed obvious. So the S-N curve of large specimens moved downward, and the fatigue life reduced under the same stress level and the fatigue strength at 109 cycles decreased too. But the VHCF mechanism was not change as the specimen size increases. Similar to the conclusions about small size specimens, there were no obvious relationships between the VHCF life of large specimens with the distance from the crack origin to the specimen surface, with the fish-eye diameter. The VHCF life basically increased with the decrease of inclusion diameter, and with the increase of the ratio of the GBF diameter to the inclusion diameter under the same load. For large specimens, the initiation life of crack initiating from subsurface inclusion was not the major part of the VHCF life, and VHCF life was usually used up in the GBF formation. The maximum inclusion sizes in the specimens with different sizes were predicted by statistics of extreme values. The fatigue strength was predicted by Murakami model and the modified model involving effects of inclusion shape, and the prediction values of above models especially that of the modified model were more close to the test results compare with the predictions of fatigue strength of small size specimens. The VHCF life of large specimens was fitted by the corrosion fatigue crack initiation life model, and the fitting effect was declined compare with the previous fitting of small size specimen test results.The fatigue properties of FV520B-I up to 109 cycles when the surface roughness is close to the actual surface roughness of impeller were tested by the ultrasonic fatigue testing machine with a symmetric tension-compression load, and the test results were compared with the previous two groups of test results of smooth specimens. The effects of surface roughnesses on the VHCF behavior and mechanism of FV520B-I were analyzed. As the surface roughness increases, the S-N curve moved downward, and the stress amplitude of transition from surface cracking to subsurface cracking was decreased and the conventional fatigue limit finally appeared. The growth mechanism of subsurface crack in the VHCF regime was independent of surface roughness. Compare with the prediction by Murakami model, the prediction value of surface fatigue limit when the deepest groove around the danger section of specimen is regarded as the single crack was closer to the test result. For the specimens of Ra≈0.05 and Ra≈0.2, the high cycle fatigue (HCF) life was less than the prediction value of Tanaka’s model at higher stress amplitude because the HCF crack insufficiently propagated. For the specimens of Ra≈0.6, the prediction value of Tanaka’s model coincided with the test result well because the HCF crack sufficiently propagated and the test results were more accurate. The failure mode of specimen was determined by the priority of crack propagation. The determination of specimen fracture site according to the parameter D*=NG/Ns was more reasonable due to the crack growth in the GBF is similar to the crack initiation.The VHCF behavior and mechanism of FV520B-I welding seams were further studied by funnel specimens with different surface roughnesses. The initiation modes of cracks in FV520B-I welding seams were more than that of FV520B-I, and some cracks initiated from the pore and (electrode coating) slag which were caused by the welding. HCF cracks usually initiated from the surface, and VHCF cracks usually initiated from subsurface complex nonmetallic oxides caused by a metallurgic reaction. The C enrichment in the GBF supported the GBF formation theory of "dispersive decohesion of spherical carbide". In the progress of GBF formation, the group behaviors of short cracks gradually evolved into the behavior of a single long crack. The surface roughness of the GBF was larger when the location was closer to the inclusion because of the higher ESFCs (effective short fatigue cracks) density caused by the larger stress concentration factor. FV520B-I smooth welding seams had obvious VHCF characteristics. The crack usually initiated from the subsurface when the fatigue life was greater than 107 cycles. As the surface roughness increases to Ra≈0.6, the S-N curve moved downward and cracks usually initiated from the surface. The surface roughness was not affected the initiation mechanism of VHCF subsurface cracks. The surface fatigue limit values predicted by Murakami model were much larger than the test results, and the HCF life of surface cracks could be well predicted by Tanaka’s model. The initiation life of crack initiating from an inclusion (pore) was not the major part of the VHCF life. The VHCF life increased with a decrease of inclusion (pore) diameter, and with an increase of the ratio of the GBF diameter to the inclusion (pore) diameter. The stress concentration factor was decreased and an increase of ESFCs density around an inclusion (pore) was decelerated as the inclusion (pore) diameter decreases, and the DESFC (dominant effective short fatigue crack) propagation length was increased as the GBF diameter increases.In short, the mechanical characteristics of a centrifugal compressor impeller were analyzed, and the fatigue load with high frequency and low amplitude and the dangerous location were determined. Above works provided references to the VHCF tests of FV520B. Through researching the VHCF behavior and mechanism of FV520B, the effects of heat treatments, specimen sizes, surface roughnesses and welding on VHCF behavior and mechanism of FV520B were further studied. Relevant works supported the estimation of residual life of centrifugal compressor impeller and the determination of remanufacture critical threshold of centrifugal compressor impeller. However, VHCF tests of dog bone specimens of base metal and welded joints are need to conducted in the future due to the constraints of time and experimental conditions.