Shape effect of ultrafine-grained structure on static fracture toughness in low-alloy steel

Abstract

A 0.4C-2Si-1Cr-1Mo steel with an ultrafine elongated grain (UFEG) structure and an ultrafine equiaxed grain (UFG) structure was fabricated by multipass caliber rolling at 773 K and subsequent annealing at 973 K. A static three-point bending test was conducted at ambient temperature and at 77 K. The strength-toughness balance of the developed steels was markedly better than that of conventionally quenched and tempered steel with a martensitic structure. In particular, the static fracture toughness of the UFEG steel, having a yield strength of 1.86 GPa at ambient temperature, was improved by more than 40 times compared with conventional steel having a yield strength of 1.51 GPa. Furthermore, even at 77 K, the fracture toughness of the UFEG steel was about eight times higher than that of the conventional and UFG steels, despite the high strength of the UFEG steel (2.26 GPa). The UFG steel exhibited brittle fracture behavior at 77 K, as did the conventional steel, and no dimple structure was observed on the fracture surface. Therefore, it is difficult to improve the low-temperature toughness of the UFG steel by grain refinement only. The shape of crystal grains plays an important role in delamination toughening, as do their refinement and orientation.

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

“…In the TF(0°) sample, the tensile properties of the small plate specimens were similar to the results [3] of the JIS 14A specimens with a round cross section of 6 mm and a gage length of 30 mm. As expected, the TF samples exhibited an anisotropic tensile behavior due to the unique hierarchical anisotropic nanostructures, in contrast to the QT sample with an isotropic microstructure [14].…”

“…Rectangular bars of 10 9 10 9 55 mm 3 were machined for all specimens, and then a notch with a depth of a 0 = 5 mm and a root radius of q = 0.13 mm was introduced by electro-discharge machining with fine wire 0.1 mm in diameter. A notch tip in all test specimens was positioned at the center of the RD plane.…”

“…Since strengthening of materials reduces toughness [1][2][3], the idea for improving strength-toughness balance is always sought. It is reported that the refinement of crystal grains is an effective method for developing strength and toughness in metallic materials without the addition of alloying elements.…”

“…Toker et al [5] indicated the significant effect of anisotropy on the impact toughness of UFG niobium-zirconium alloys processed by equal-channel angular extrusion. We [3,6,7] developed stronger, tougher steel, i.e., fail-safe steel, by a microstructural design based on heterogeneity that was lacking in the traditional concept. In fail-safe steel, the strengthening is attributed to ultrafine-elongated grains with nanometer-sized carbides, and the toughening is attributed to extrinsic fracture mechanisms of the crackarrester type [3,[8][9][10], in which the propagation of a main crack associated with the heterogeneous microstructure with a strong a-fiber (RDkh101i) texture is arrested.…”

“…We [3,6,7] developed stronger, tougher steel, i.e., fail-safe steel, by a microstructural design based on heterogeneity that was lacking in the traditional concept. In fail-safe steel, the strengthening is attributed to ultrafine-elongated grains with nanometer-sized carbides, and the toughening is attributed to extrinsic fracture mechanisms of the crackarrester type [3,[8][9][10], in which the propagation of a main crack associated with the heterogeneous microstructure with a strong a-fiber (RDkh101i) texture is arrested. Namely, the property of toughness is sensitive to not only the size of crystal grains but also their orientation and shape because the fracture behavior is different by these microstructural parameters [2-7, 10, 11].…”