Influence of Niobium on the Beginning of the Plastic Flow of Material during Cold Deformation
Investigations were conducted on low-carbon steel and the steel with same chemical composition with addition of microalloying element niobium. While tensile testing was carried out, the thermographic measurement was tacking place simultaneously. A specific behavior of niobium microalloyed steel was noticed. Test results have shown that, in the elastic deformation region, thermoelastic effect occurs, which is more pronounced in niobium microalloyed steel. Start of plastic flow in steel which is not microalloyed with niobium begins later in comparison to the microalloyed steel, and it is conducted so that, at the point of maximum stress, deformation zone is formed within which stresses grow. In steel microalloyed with niobium after proportionality limit, comes the occurrence of the localized increase in temperature and the occurrence of Lüders band, which propagate along the sample forming a deformation zone.
DETERMINING THE AMOUNT OF LÜDERS BAND
S. Rešković, I. Jandrlić, University of Zagreb, Faculty of Metallurgy, Sisak, Croatia, F. Vodopivec Institute of Metals and Technology, Ljubljana, Slovenia The article presents the results of measuring the amount of deformation during the formation and propagation of Lüders bands through the deformation zone. The amount of deformation was determined by applying the visioplasticity method with digital image correlation, and analyzing the captured images using the MatchID software package. Results show that during the formation of Lüders bands, the deformation of 0,004 mm/mm was achieved. Furthermore, it was determined that during the propagation of Lüders bands through deformation zone, the sample was deformed in the amount of 0,05 mm/mm.
Evaluation of Lapping Grains based on Shape Characteristics.
The shape and size of abrasive grains during lapping varies complicately and irregularly. In this study, the aspect ratio of grain size and the fractal dimension of the grain external form were used as shape characteristics of grain in order to evaluate grains with lapping time. The results obtained are summarized as follows : (1)The fractal dimension increases due to microscopic fracture and decreases due to macroscopic fracture or attritious wear. (2)The aspect ratio and the fractal dimension, which are grain shape characteristicsis of the macroscopic shape and the microscopic surface complication of grain respectively, can be used for evaluating grain shape variation during lapping.