Papers by Keyword: Allotriomorphic Ferrite

Abstract: The influence of prior austenite grain size on the crystallography of allotriomorphic ferrite is investigated in a low carbon steel. The results show that as the prior austenite grain size decreasing, the fraction of allotriomorphic ferrites that do not keep K-S orientation relationship with any surrounding prior austenite grains is increased. It is observed that such ferrites usually form at the grain edges or grain corners. It is known that with the grain size decreasing, the fraction of grain edges and corners increases. It is suggested that the free energy of the defects at such nucleation sites is higher than that at grain faces, and the nucleation barrier of ferrite is lower. As a result, the possibility for the ferrite to form that does not have orientation relationship with all surrounding austenite grains is increased at such sites.

Abstract: A Fe-0.05C-2.94Mn-1.87Si steel is heat treated using a two-stage isothermal holding process to obtain allotriomorphic ferrite and bainite. Two kinds of allotriomorphic ferrite are obtained, one with only carbon partitioning and the other, alloying element partitioning. It is observed that the allotriomorphic ferrite stimulates the adjacent bainite to select the similar variant on the side where near K-S relationship is maintained between ferrite and prior austenite. The longer the border length of the allotriomorphic ferrite, the larger the stimulated bainite area. The statistical measurement shows that the alltriomorhpic ferrite with alloying element partitioning stimulates such bainite variant selection as well as that with only carbon partitioning.

Abstract: There is now a great deal known about the atomic mechanisms of solid–state phasetransformations, and this knowledge can be exploited to determine the distribution of crystalorientations. It is possible to estimate accurately, the crystallographic texture, transformationstrains and details of the microstructure, particularly in the context of steels. The concepts havenow been applied to design metallic alloys which compensate automatically for the residualstresses which develop in engineering components when they are cooled heterogeneously fromelevated temperatures. Such materials are now in commercial use and represent an innovationresulting directly from phase transformation theory.

Abstract: . A low carbon steel is austenitized and isothermally held at 680°C to form allotriomorphic ferrite and followed by a holding at lower temperature to form bainite. The morphology of allotriomorphic ferrite/bainite interfaces is studied using optical microscope. Three kinds of combination are observed: Type I: interface on one side is clear while on the other side, unclear; Type II: unclear on both sides; Type III: clear on both sides. Clear interface indicates a large difference in the orientation between the bainite and the ferrite, and unclear interface, a very small difference. The statistical counting shows that the ratio of Type I is about 80-82%, and that of Type II, 7-8%, and Type III, 9-11%. It is observed that this ratio does not change with the austenite grain size and bainite forming temperature. And the clear and unclear side of allotriomorphic ferrite may have different influence on the nucleation rate of bainite at allotriomorhic ferrite/prior austenite interface.

Abstract: Substantial differences – mainly in plasticity – were found based on statistical analysis of hot rolled bars mechanical properties. Investigations presented in the paper were related to the possibility of modification of continuous bar mills used now in order to improve and stabilise plastic properties determined by the energy of breaking. The paper presents results of laboratory investigations representing processes of conventional and normalising rolling of bars from S355 steel. The experimental analysis of both rolling processes comprised assessment of actual changes occurring in the microstructure of bars, deformed acc. to suggested parameters corresponding to conventional and normalising rolling. The investigations included also the assessment of accelerated cooling after rolling influence on the microstructure of finished products.

Abstract: This paper addresses the physical mechanisms of the transformation of deformed austenite into allotriomorphic ferrite and Widmanstätten ferrite. The possible implementation of deformation in currently available transformation models for allotriomorphic ferrite and Widmanstätten ferrite is discussed based on a series of deformation dilatometry experiments. It is concluded that a small amount of deformation already gives significantly faster transformation kinetics and a significant decrease in fraction Widmanstätten ferrite, and that the key to understanding the effect of deformation on transformation lies in the nucleation of allotriomorphic and Widmanstätten ferrite. For Widmanstätten ferrite also the growth needs further study.