To describe general trends in the variation of impact toughness and cold brittleness, we have studied two low-carbon, low-alloy, low-sulfur steels having a ferrite–bainite microstructure and similar in chemical composition. According to results of multiple impact bending tests in the temperature range of the ductile-to-brittle transition, the cold resistance (quantified by the fraction of the ductile component on fracture surfaces of specimens) of the steel containing less sulfur and carbon (0.002

The impact toughness, microstructure and break features of low-alloy steel weld samples produced by automatic submerged arc welding have been compared. It has been established that when a notch is applied along the weld axis, fracture occurs along large grains of grain-boundary ferrite 80–140 μm in size with an unfavorable cleavage planes orientation 001. The combination of such grains with non-metallic inclusions 3–5 μm in size facilitates the transcrystalline cleavage initiation. The effect of austenite grain size, crystallographic texture, and intergranular fracture on the impact toughness value is shown. Continuous chains of grain-boundary ferrite at the front of the main crack line up along the notch-parallel boundaries of columnar grains of axial orientation, which increases the tendency to transcrystalline cleavage. At the same time, with the lateral orientation of the columnar grains of the former austenite, the front line of the main crack intersects only certain separate areas of the grain-boundary ferrite. Offset notch results in an increase in both impact toughness and plasticity of weld metal.

The ambiguity of the splitting effect on X80 low-carbon microalloyed pipeline steels’ tendency towards brittle fracture prompted an experimental study of impact toughness scattering based on multiple Charpy impact tests in a temperature range from 20 °C to −100 °C. A fractographic analysis of a large number of fractured samples was carried out. The relationships between impact toughness, deformability and splitting characteristics were studied. A number of common features of three X80 low-carbon microalloyed pipeline steel fractures were revealed. It was experimentally established that the reason for the scattering of the impact toughness values during completely ductile fracture of specimens, as well as during fracture accompanied by the splitting formation, is the local inhomogeneity of plastic properties. The higher the susceptibility to the formation of splits for a particular steel, the lower the impact toughness. Using the electron backscatter diffraction (EBSD) technique, an uneven distribution of local plasticity in the plastic zone of impact-fractured specimens was established. A comparative analysis of specimens with equal impact toughness values at different test temperatures makes it possible to identify the mechanism of negative splitting influence compensation by the increased plasticity of certain specimen.

The degree of microstructural factors influences on the impact strength of four K60 strength class steels after simulating a thermal welding cycle in the coarse-grained area of the weld-affected zone is investigated. It has been shown that the differences between steels with different impact strength KCV–20 are mainly caused by the differences in the content and structure of titanium nitrites. In steels prone to brittle fracture, titanium nitrides act as sources of cleavage cracks origin. In the case of blocking the transition of the cleavage crack from nitride to the matrix, other less significant factors affect the impact strength. It is shown that high-angle grain boundaries stop cleavage cracks by limiting their size, which leads to a decrease in the probability of a crack crossing the grain boundary. The correlation of the impact strength with the volume fraction of the MA component particles and the residual austenite in the studied microstructures is weakly expressed. Such particles cannot be considered as localized sources of cleavage cracks. Differences in the level of Cr + Ni + Cu alloying in the studied steels, which are not characterized by the cleavage cracks origin on titanium nitride inclusions, can be considered as an important factor of impact strength. The mechanism of this influence may be associated with a change in the tendency to cleavage of the ferritic matrix.

A simulated coarse-grained heat affected zone microstructure formation mechanism is established in high strength low alloy steels using electron backscatter diffraction (EBSD). The governing effect of dispersion and ratios between different types of ferritic structural constituents on variation in impact strength is demonstrated. It is assumed from results of simulating a heat affected zone coarse-grained area that a reduction in welding energy input leads to a shift in ductile-brittle transition temperature towards a lower temperature.

The quantitative microstructure - impact toughness relationships in two batches of the same steel grade subjected to quenching and tempering (Q&T) have been established via characterization using EBSD technique and FIB visualization. The EBSD-based criterion for separation of structural constituents in microstructure of Q&T low carbon low alloy steels is proposed. Impact toughness differences between two steel batches subjected to nominally identical Q&T are caused by the changes in the volume fraction of structural constituents caused by various cooling conditions at quenching stage. High volume fraction of bainite containing more distorted bainitic ferrite and the highest amount of brittle cementite precipitates leads to the increase in strength and to the decrease in impact toughness.

In this paper, we study the morphology of the bainite component of the microstructure of rolled products and the mimic coarse-grained region of the thermal effect zone of K60–K65, 09G2S, and 08KhN2MFB low-carbon steels by methods of optical microscopy and electron backscatter diffraction. In the homogeneous microstructure of the former austenite—both in the coarse-grained heat-affected region and in rolled products—the density of high-angle grain boundaries is shown to be higher in lath bainite than that in granular bainite. In the case of a substantially inhomogeneous microstructure of the former austenite, the size effect of the initial grain structure on the effective grain size (high-angle boundaries) prevails over the change in the bainite morphology. The lower-temperature component, lath bainite, turns out to be coarser-grained (in the case of its formation from large-sized austenite grains) than granular bainite. The impact viscosity and cold resistance of a metal, the microstructure basis of which is a mixture of granular and lath bainite, increased at an increase in the density of high-angle boundaries, which in turn is determined by both the grain size of the initial austenite and the morphology of bainite.

— We have experimentally demonstrated continuity of hypoeutectoid and pearlite ferrites in aggregates in the microstructure of low-carbon, low-alloy ferrite–pearlite steel. Such ferrite aggregates of various origins have been repeatedly observed along with hypoeutectoid ferrite and pearlite grains. We propose that aggregates of hypoeutectoid and pearlite ferrites should be regarded as a characteristic microstructural feature and this should be taken into account in microstructural characterization.

Abstract The localization of impurity phases in ceramics created by direct nitridation of zirconium rolled metal was established. For ceramics synthesized at temperatures above the melting point of the metal, the impurity phases are localized along grain boundaries and at macrodefects. In a composite heterostructure of the composition ZrN –(ZrN x/α --solid solution of nitrogen in metal/ZrN x ) – ZrN, impurity phases are predominantly localized in an α-solid solution of nitrogen with their subsequent displacement to grain boundaries and macrodefects at the end of the nitridization process.

— Using commercial low-alloy steel as an example, we have studied the nature of the increased scatter in impact toughness in the temperature range of the ductile-to-brittle (D–B) transition by multiple impact toughness tests and examined the microstructure of the steel by electron backscatter diffraction (EBSD). The results demonstrate that the scatter in the impact toughness of the low-alloy steel in the case of fracture in the temperature range of the D–B transition is due to toughness nonuniformity in the plastic zone. To assess the local toughness nonuniformity of the plastic zone in the microstructure of particular specimens fractured in the range of the B–D transition, we evaluated the percentage of the total length of ductile/brittle microcracks. We have demonstrated conceptual feasibility of developing a method for separating the contributions of local plastic deformation preceding and accompanying fracture for particular specimens in the range of the D–B transition using EBSD.

The connection between occurrence of remarkable impact toughness scattering in ductile-to-brittle transition region and microstructure features of low carbon microalloyed steel was established by means of multiple impact toughness tests and electron backscatter diffraction microstructure measurements. The phenomenon of local inhomogeneity in ductility of plastic zone was established on the base of different microcracks nature revealed by electron backscatter diffraction. The residual microcracks occur in local embrittled regions of material as a result of cleavage at the early stage of fracture. Pre-strained regions with local ductility variations are fractured by the formation of splittings and by the ductile tearing. Pancaked parent austenite microstructure, various ferritic microstructures and martensite-austenite constituent along with the significant grain size variations can be considered as the sources for the occurrence of remarkable impact toughness scattering in low carbon microalloyed steels during fracture in ductile-to-brittle transition region.

New technique for quantification of M/A and retained austenite in microstructure of HSLA steels on the base of advanced EBSD data analysis and post-operational treatment was developed. Martensite-austenite (M/A) and retained austenite structural constituents were revealed using crystal distortions features. In contrast to highly distorted M/A structural constituent the retained austenite contains the same level of defects as the surrounding ferritic microstructure. Volume fraction and mean equivalent diameters of M/A and retained austenite were quantitatively determined.

It has been shown by means of EBSD techique that fracture of ferritic steel in ductile-brittle transition temperature region, along with the formation of previously discribed cleavage microcracks, results in the formation of ductile microcracks. It has also been shown that microstructure of plastic zones under brittle and ductile fracture components produced by the main crack propagation differ significantly. Better developed plastic zone under ductile fracture component protects steel from overstress. The plastic zone under brittle fracture surface, apparently, has a reduced local plasticity. Consequently, the cleavage microcracks formation precedes the fracture process. During the main crack formation such microcracks occur in steel microvolumes located both in front of its tip and in adjacent to its edges microvolumes. Further propagation of the main crack is realized in steel which already contains scattered cavities and reduces to ductile fracture of the connections between them.

The substructure of continuously cooled pearlitic transformation products in low carbon low alloy ferrite-pearlite steel was characterized by means of EBSD. The emergence of fragmentation in lamellar pearlite was confirmed. The presence of fragmentation in ferritic matrix of degenerate pearlite with lamellar cementite precipitates and absence of such fragmentation in the degenerate pearlite with non-lamellar cementite precipitates was established. The received experimental results allow to conclude that the fragmentation of degenerate pearlite with lamellar cementite precipitates originated on the growth stage.

The substructure and microstructure of pearlite in hypoeutectoid low-carbon steel 09G2S is studied using high-contrast images in a scanning electron microscope and electron backscattered diffraction. Acorrelation is established between local deviations from “ideality” in the structure of pearlite and the substructure of pearlitic ferrite. Fragments of pearlitic ferrite, whose development is connected with propagation of long-range misorientations, are seen. It is shown on the basis of visualization that the single-crystal matrix of the pearlitic ferrite consists of individual fragments. These fragments are not generally flat.

The efficiency of different techniques application for the investigation of orientation inhomogeneities in polycrystalline materials was studied using FEG SEM-FIB dual beam station equipped with EBSD. It is shown that for the visualization of pearlitic ferrite fragments it is more appropriate to acquire the images in secondary electrons induced by Ga ions. At the same time for the visualization of nano-sized particles it is more prospective to use images in forward scatter electrons in combination with IQ maps. It is established that pearlitic ferrite fragments are not flat. Complicated spatial configuration of orientation inhomogeneities in pearlitic ferrite is shown by means of 3-d reconstruction. The features of ferrites aggregation are revealed depending on pearlitic ferrite fragmentation criterion. The existence of long-range misorientations in the aggregation area of proeutectoid and pearlitic ferrites is shown.

The strength and plastic characteristics of thick ( d = 40–120 μm) amorphous wires made of a model Co-based alloy and fabricated by the Ulitovskii-Taylor method are studied. They are found to have stable strength and plastic characteristics along the length. The plasticity of the thick wires is high, and they can form a full knot and undergo a load of 0.5 ultimate tensile strength in this state. The surface state and fracture surfaces of the amorphous wires are analyzed by scanning electron and optical microscopy. The wires are found to have a smooth lustrous lateral surface almost free of defects and to retain stable geometrical parameters along their length. Zones with different positions and frequencies of shear bands can form on the wire surface depending on the type of deformation action. The fracture surfaces of the thick amorphous wires are specific: a venous zone consists of several large pronounced principal “veins” and a rare network of adjoining secondary veins.