Formation conditions and genesis of the pearlitic ferrite fragmentation during pearlite transformation in proeutectoid steels.Wednesday (28.09.2016) 17:30 - 17:45 Part of:
The presence of orientation gradients in lamellar pearlite and the absence of them in proeutectoid ferrite in low carbon ferrite-pearlite steel in as-transformed state were established by means of EBSD [Takahashi et al., 2007]. Such gradients caused by the multiple local orientation changes can result in formation of large fragments inside the pearlitic colonies.
The aim of present investigation is to characterize formation conditions and genesis of pearlitic ferrite fragmentation. The lamellar pearlite and the degenerate pearlite fragmentation in proeutectoid steel were investigated by means of EBSD, T-EBSD and high-resolution SEM.
The rather large fragments on IPF orientation maps are visualized by colouring differences of distinct parts of the ferritic matrix in the same pearlitic colony. It follows that the pearlitic ferrite matrix, which is traditionally considered as monocrystal since classical analysis of Hillert [Hillert, 1962], consists of fragments caused by the development of long-range misorientations. Frequently these fragments are divided by low angle grain boundaries, but in some cases the neighboring fragments differ only in the gradual changes in colour. In general case these fragments are not flat, since interphase surfaces between ferrite and cementite are rather distorted because of local lamellar pearlite imperfections. Due to the fragments are not flat, such coincidences can occur in which the fragments in pearlite colony are indistinguishable on the IPF maps.
The established before correlation relation between local topographical curvature of pearlite colony and the development of fragmentation [Takahashi et al., 2007] is confirmed. The existence of such relation with other local imperfections of lamellar pearlite is shown by combination of EBSD and high-resolution SEM: discontinuities and morphology changes of cementite lamellas, arrangement faults.
The presence of fragmentation in degenerate pearlite with lamellar cementite particles and the absence of such fragmentation in degenerate pearlite with non-lamellar cementite particles are revealed. The ferrite of degenerate pearlite with non-lamellar cementite particles morphology undoubtedly appears as the product of pearlitic transformation. Therefore the fact of presence of unfragmented ferrite in the product of pearlitic transformation points at the absence of fragmentation in parent austenite grains. At the same time both the fragmentation of the degenerate pearlite with lamellar cementite particles and the fragmentation of the imperfect lamellar pearlite indicate their interrelation with cooperative growth of ferrite and cementite. There is a certain paradoxicality of the observed phenomena. On the one hand the fragmentation of pearlitic ferrite emerges at the growth stage and on the other hand it is revealed only in connection with local imperfections in pearlite.
Though the application of 2-d EBSD in combination with high-resolution SEM is rather effective, it should be noted that the revealing of the fragments during the 2-d investigation is impeded. Therefore the 3-d reconstruction of microstructure should be used for the investigation of such fragmentation aspects [Zaefferer et. al.].