Effect of initial notch orientation on fracture toughness in fail-safe steel

Abstract

A 0.4C-2Si-1Cr-1Mo steel bar with an ultrafine-elongated grain (UFEG) structures was produced by multi-pass warm caliber rolling. The test sample was machined from the rolled bar with 0°, 45°, and 90°rotation along the rolling direction, and a static three-point bending test was conducted at ambient temperature. The toughness anisotropy on the steel with UFEG structures were studied, including the crack propagation on the basis of the microstructural features. The strength and toughness decreased with an increase in the rotation angle along the rolling direction. The toughness decreased drastically, compared to the strength. The notch orientation dependence on toughness is due to differences in the spatial distribution of weak sites such as {100} cleavage planes and boundaries of elongated grains. For the toughness design in ultrafine-grained materials, it is essential to understand the spatial distribution of these weak sites as well as the grain size.

Influence of Carbon Content on Toughening in Ultrafine Elongated Grain Structure Steels

“…7) Even when we consider an intergranular fracture in the UFEG structure, a similar magnitude relationship may be established for the intergranular fracture stress; the intergranular fracture stress in the SD is low, as a result of the elongated grain shape and the good grain boundary continuity along the RD. 9,23) If the Tresca yield criterion is assumed in the process zone within the plastically yielding region of the notch root, the σt//RD and σt//SD in the process zone can be approximated as 2.6 × σy and 1.6 × σy, 7) respectively. The condition of the σt//SD < σt//45° < σt//RD eventually holds.…”

“…[4][5][6][7]21) Brittle fracture occurs when the maximum tensile stress (σt) at the notch root or crack tip exceeds the brittle fracture stress (σBF). The σt is proportional to the σy.…”

“…Slight differences between the σc//45° and σc//SD were also demonstrated from the static three-point bending test of the 0.39% C steel. 7) When the three-point bending test was performed for the specimens that were machined from the rolled bar with 0° (//RD), 45° (//45°), and 90° (//SD) rotation along the RD, the fracture energy (JF) of JF//RD, JF//45°a nd JF//SD were measured to be 5 184, 142, and 44 kJ/m 2 , respectively. The difference between the JF//45° and JF//SD was thus small.…”

“…[4][5][6] It was clear that the transverse grain size, the grain shape and the <110>//RD fiber texture in the UFEG structure were the dominant microstructural factors in controlling the delamination. [4][5][6][7] Furthermore, the carbon content may influence the microstructural features of the UFEG structures and the occurrence of delamination for the warm tempformed steels; however systematic investigation on this subject has not yet been conducted.…”

Delaminating Crack Paths in Ultrafine, Elongated Ferritic Steel

“…5) Therefore, ultrafine-grained (UFG) materials are increasingly attractive. [6][7][8][9][10][11][12] Recently, we have developed stronger and tougher steel by combining grain refinement and delamination to arrest cracks. 1,12) The steel has an ultrafine, elongated grain structure, and can arrest the propagation of a crack.…”