Cleavage initiation in a bainitic steel

Abstract

AB3:Haurasmurtuman ydintymistä bainiittisessa teräksessä tutkittiin pyyhkäisyelektronimikroskoopin avulla tutkimalla Charpy ja CT näytteitä, jotka oli murrettu transitionlämpötila-alueella. Tarkoituksena oli saada selville, mikä mikrorakenteen piirre ydintää haurasmurtuman ja samalla mitata ydintymiskohdan koordinaatit. Tieto ydintymismekanismeista ja koordinaateista yhdistettynä elementtimenetelmällä suoritettuun analyysiin mahdollistaa paikallisten jännitysten selvityksen ja haurasmurtuman mallintamisen, mikä ei kuulu tämän diplomityön puitteisiin. Koska transitiolämpötila-alueella esiintyvä sitkeysarvojen suuri hajonta joutuu ydintymiskohdan ja -mekanismin tilastollisesta luonteesta koekappaleita tutkittiin riittävä määrä luotettavan kuvan saamiseksi. -

Scanning electron microscopy was used to identify the cleavage initiation sites and measure their coordinates in Charpy and Compact Tension specimens which were fractured in the ductile-to-brittle transition region. -It was found that the temperature has a significant effect on cleavage initiation mechanisms but the mechanisms were same in both specimen types. At -90° C many different cleavage initiation mechanisms were present but the most importants were small inclusions and large, elongated MnS inclusions. As the temperature increased the importance of the large, elongated MnS inclusions was emphasized. At -30° C it was found that the small inclusion do not initiate cleavage. -At all temperatures several cleavage initiation sites were sometimes found. As the temperature increased the number of cleavage initiation sites decreased. It was found that there is no correlation between the number of the initiation sites and the absorbed energy in Charpy test or between the number of the initiation sites and the fracture toughness. -The temperature also affected the coordinates of initiation sites. At low temperature the sites were homogeneously distributed along the specimen width. As the temperature increased the sites tended to situate more and more in the middle of the specimen width. As the temperature increased the distance from the notch to the site increased in Charpy specimens which can be seen as higher absorbed energy because there was more ductile crack growth. In CT specimens it was found that the lower bound value of fracture toughness varies linearly with the distance from the fatigue precrack to the site.