Thermal Fatigue Crack Initiation and Propagation Behavior of Steels for Boiler

Abstract

Thermal fatigue tests were conducted for SB410 steel and SUS310S steel for boiler. A laboratory made thermal fatigue testing apparatus was used. This apparatus consisted principally of a heating device using oxygen and LPG gas, a temperature control device for the heated zone and a rapid cooling device for the specimen. During thermal fatigue tests the heating and cooling cycles were repeatedly loaded on the specimen placed on the specimen holder. City water was used as a cooling medium. The heating temperatures were 673K, 773k and 873K, and cooling water was sprayed on to the specimen surface through a nozzle. As it was difficult to measure the surface temperature of the specimen, small holes for a thermocouple were prepared to measure the temperature of the notch during the thermal fatigue tests. Thus the measured temperatures were used as the testing temperatures. The plate specimens with an electric discharged notch at Open image in new window Figure 1. Thermal fatigue test specimen the bottom of the mechanical U notch. The thicknesses of the specimens were 24mm and 12mm. The thermal fatigue crack initiation tests and thermal fatigue crack propagation tests were conducted up to 100 cycles for specimens with 24mm thick and 300 cycles for specimens with 12mm thick. Figure 1 shows the figure and the size of the thermal fatigue test specimen. The thermal fatigue crack length was measured at every ten cycles by use of a viewing microscope with magnification of 20 after interrupting thermal fatigue tests. The thermal fatigue cracks were examined by an optical microscope and thermal fatigue fracture surfaces were examined by a JEOL scanning electron microscope (JSM5500S). Figure2 shows crack propagation curves for SB410 steel with 12mm thick. The higher the testing temperature the smaller the number of heat cycles is. The higher the testing temperature the faster the crack propagation rate is. The same phenomenon was observed for SUS310S. The thermal fatigue crack propagation rate of SB410 steel was faster than that of SUS310S steel. Plural thermal fatigue cracks were observed on the bottom of the electric discharged notch for SB410 steel. The crack branching was observed on the specimen tested at 873K. Striation was observed on fracture surfaces of SB410 steel. Striation was predominant on fracture surfaces of SUS310S.The striation spacing per cycle ,S obtained from the measured striation spacing S versus K curve was well coincident with the da/dN K curve in the high crack propagation rate. It can be concluded that thermal fatigue crack of boiler steels propagate in association with striation. Open image in new window Figure2. Thermal fatigue crack propagation curves of SB410 steel with 12mm thick