Crack initiation kinetics in upset cylindrical samples

Introduction. The article is devoted to the crack occurrence analysis in a powder body, and identification of the basic laws of this process in the freely upset material. The work objective is to study kinetics of crack initiation and propagation in freely upset powder materials, and microstructure of alloy and iron powders in a variety of areas in order to determine the conditions for defect-free deformation of the material under free upsetting. The analysis of the crack development in a powder body is of great practical importance, and therefore, the investigation of this problem is essential for the fabrication of accepted parts. Materials and Methods. New powder materials from the Swedish company Höganäs which are characterized by the improved deformation characteristics are considered. Besides, these materials differ in occurrence of alloy elements, such as Ni and Mo which affect cracking. Microstructures of the upset samples are analyzed using the metallographic microscope. Research Results. Cracks in the upset materials are classified as cleavage cracks. The crack origin is determined by the absence of factors for the development of the sample metal component plasticity due to the occurrence of defects and pores in its body. It is established that rapid cracking is confirmed by the nature of the crack when it scarcely has extension into the sample material, and, moreover, ends at an angle to the area for development. Discussion and Conclusions. The analysis of the microcrack development allows identifying morphological structural differences of the central and fringe regions of the powder material. The peripheral area is characterized by different values of grains and increased content of micropores, and the directivity of clusters of micro-discontinuities and non-metallics, in the line of the former physical interface of the metal powder particles.