| dc.description.abstract | High-cycle, low-cycle fatigue tests and fatigue crack propagation measurements were conducted on SA533B1 steels, reactor pressure vessel (RPV) steels, with four levels of sulfur content (0.0062 wt%~ 0.035 wt%) at room temperature and 300 ℃. Efforts were made to study the effects of sulfur content on the fatigue degradation of RPV steels. Fractographic and microstructural examinations by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were performed to investigate the fatigue crack initiation and propagation mechanism. To make a comparison of different crack measurement techniques, fatigue crack length measurements were concurrently taken on the same specimen by an alternating current potential drop technique and a crack opening displcement gauge. Fatigue crack opening levels were also measured to characterize the fatigue crack closure effect on fatigue crack growth behavior.
The results of high-cycle fatigue (HCF) limit were observed to be ~650 MPa, around the yield strength of SA533B1 steel, and showed little or no dependence on sulfur content at room temperature. At 300℃, the fatigue limits were slightly higher than those tested at room temperature, notably for the steel with a sulfur content 0.006 wt% whose fatigue limit at 300℃ was up to 702 MPa. The low-cycle fatigue (LCF) limit showed little or no dependence on sulfur content, but significantly depended on the test temperature. Fatigue life could be predicted either by means of the strain-life equation or the SWT (Smith, Watson, and Topper) parameter for specimens under low-cycle fatigue conditions. The strain-life equation for SA533B1 steel in air was irrespective of the sulfur content, but dependent on the test temperature. HCF life was significantly affected by the inclusions which are composed of O, Al, S, Ca, and Mn. The inclusions were randomly distributed on the surface or in the interior of the specimen. Fractographic examination results suggested that inclusions near the specimen surface served as the crack initiation site for a majority of the fatigued specimens tested at room temperature. For those tested at 300℃, some cracks were identified to initiate around the inclusions in the interior of the fatigued specimens. However, the effects of inclusion on the LCF are less significant than HCF. No LCF specimens were observed to initiate at/around the inclusions.
The fatigue resistance of SA533B1 steel subjected to HCF and LCF tests at 300℃ was improved by the combined effects of dynamic strain aging (DSA) and grain size reduction. DSA and grain size reduction increase the steel strength, accordingly improve the HCF and LCF limits at 300℃. But, concurrently, the precipitation of carbides/nitrides in SA533B1 steel leads to a decrease in the steel strength. The grain size reduction and the precipitation compete with each other. The grain size reduction is dominant and the net effect is reflected in an increase in hardness value.
The crack growth rate measurements for steels of different sulfur contents tested at room temperature and 300℃ can be characterized by the same material constants C = 1.98*10-8 and m = 2.54 for Paris law, where da/dN is in mm/cycle and ?K in MPa . It implies that fatigue crack growth rate has little or no dependence on sulfur content and test temperatures (25℃, 150℃ and 300℃). But fatigue crack growth rate becomes faster, when crack propagation runs parallel to the sulfide orientation. Fatigue crack closure became evident till the stress ratio (R value) was decreased to the values smaller than 0.13. Thus, for the tests at the stress ratio of R = 0.2, it is not necessary to calibrate the ?K value. The results of crack opening measurement show that the levels of crack closure significantly depend on the loading history and plastic zone size ahead of crack tip. The phenomenon of crack closure was observed to be more significant for the specimens with a through crack than those with a surface crack.
Smaller subgrains and a greater number of precipitates were identified with the specimens tested at 300?C by SEM and TEM examinations. No clear relationship was found between fatigue damage and the mis-orientation changes of cell walls (or subgrain boundaries) in the fatigued samples of SA533B1 steel. | en_US |