The behavior of a crack in steel with the structure of a ferriticmartensitic composite is studied. The modes of heat treatment for obtaining a composite structure with alternating layers of ferrite and martensite are given. The kinetics of crack development during cyclic tests was studied. The obtained data show that the rate of development of the crack in the material with the structure of the composite is substantially lower than for the usual sorbite structure. It is shown that the frontal growth of the crack in the composite is carried out with “stops”, on which the matrix deforms and its stratification occurs in the direction normal to the front of the crack motion.

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The paper presents the results of a study of the surface fracture tested for bending at negative temperatures of a ferrite-martensite composite (FMC) obtained on the basis of K 02704 structural hypoeutectoid steel by atomic force microscopy. Metallographic studies of the sample were carried out on longitudinal sections. Investigations of microstructures were performed on optical microscopes Metam RV-22 and Neophot-21. Impact bending tests were carried out in accordance with ISO 148-1:2016. The quantitative assessment of martensite was carried out using the digital imaging program and amounted to 30%. A three-dimensional image of the fracture of the sample surface was obtained by atomic force microscopy (Nanoeducator II) in the semi-contact mode. An AFM study of the fracture surface showed that the data of optical and atomic force microscopy are in agreement. The three-dimensional AFM image of the surface fracture and its profiles, extracted in different areas, as well as the roughness parameters, show the presence of a dual line structure in steel.

The paper presents the results of a comprehensive study of micro-and nanostructures of steel by optical and atomic force microscopy methods, carried out in accordance with international standards. The studies were carried out on samples healed in two different ways: complete quenching and quenching from the intercritical interval. The object of the study was structural alloyed building steel 1.0481 with troost-martensitic and ferritic-martensitic structures. The chemical composition of the steel was controlled using method of optical emission spectrometer (Q8 MAGELLAN). Sample preparation for AFM investigations did not include etching. Microstructure studies were performed on optical microscopes Metam RV-22 and Neophot-21. Optical microscopy showed the presence of steel hardened from the intercritical interval in the structure of the sample, and a stitch structure consisting of ferrite and martensite. A quantitative assessment of the hardening phase (martensite) was carried out using the KOI-1 program, intended for the quantitative metallographic analysis of digital photographs of the microstructure of materials. According to stereometric analysis of the microstructure of this sample, the volume fraction of ferrite is 70 %. Three-dimensional images of the surface of the samples were obtained by atomic force microscopy (PHYWE Compact AFM) in the tapping mode in an air atmosphere. Next, one-dimensional (texture, roughness, waviness) and statistical parameters of the surface topography (arithmetic average roughness, root mean square roughness, asymmetry, excess) were determined. The method of counting cubes was used to estimate the fractal dimension. A comparative analysis of the data showed that a surface with a stitch structure is more developed and has higher roughness parameters and fractal dimension values than a surface with a homogeneous structure of troost-martensite.