I. L. Yakovleva

Peculiarity of structure and crystallography of plastic deformation of lath martensite in structural steels

Abstract The structure of a martensite packet and cold plastic deformation within one packet, as occurring in the quenched of pseudosingle crystals of 15CrNi4 and 37CrNi3 structural steels, was studied by X-ray diffraction, scanning electron microscopy, and optical microscopy. By electron microscopic analysis it is shown that there are six variants of martensite crystals throughout the packet, in agreement with previous X-ray crystallographic results on the number of martensitic variants in a packet. The slip planes in pseudosingle crystals were found to be close to {112} M and {110} M , which are typical of the bcc lattice.

Dislocation structure of cementite in granular pearlite after cold plastic deformation

Cementite microstructure of the U8 steel with a granular pearlite structure has been investigated by the method of electron microscopy. It has been established that, at the early stages of deformation, carbide particles are deformed through the movement of stacking faults, which are characterized by an α[010] partial shift in the (001) planes of cementite. The Burgers vector, the slip plane [010](001) of the split dislocations forming pileups, and deformation bands have been determined using gb analysis. The stacking fault energy has been estimated in a (001) cementite plane: γsf ∼ 12.8 mJ/m2. With increasing degree of deformation, an additional slip has been shown to occur in cementite by the system [100](011).

Structural features of the behavior of a high-carbon pearlitic steel upon cyclic loading

The structural evolution upon high-cycle fatigue (tension with the magnitude of stress in a cycle below the macroscopic yield stress) of the hypereutectoid steel U10 (1.03 wt % C), in which pearlite of different morphology (fine-lamellar, coarse-lamellar, and partially spheroidized pearlite) was formed, has been investigated by scanning and transmission electron microscopy. Based on the fractographic analysis, features of fracture of these structural states have been considered. At a significant distance (10 mm) from the fatigue fracture, features of structural transformations caused by cyclic loading have been revealed. It has been shown that upon high-cycle fatigue in the steel U10 with structures of lamellar and partially spheroidized pearlite, substantial structural changes occur, namely, fragmentation and partial dissolution of cementite plates, and in fine pearlite, additionally, spheroidizing of cementite and polygonization of the ferritic component are observed. A dependence of the character of fracture on the type of structure formed upon fatigue loading has been established. In the steel with a nonequilibrium structure of unannealed fine pearlite, an enhanced elasticity modulus, as compared to other more stable structures (coarse-lamellar, annealed fine, and spheroidized pearlite), and a reduction in the magnitude of the elasticity modulus under the action of cyclic loading have been found. It has been established that the structural changes in fine pearlite of laboratory specimens of the steel U10 upon cyclic tension are qualitatively similar to those in a railroad wheel of the steel 65G under the service conditions.

Effect of austenite-decomposition temperature on bainite morphology and properties of low-carbon steel after thermomechanical treatment

Peculiarities of the bainite structure formed in low-carbon steel 07G2NDMBT during isothermal austenite decomposition, namely, the sizes of crystallites, their mutual orientation, and the presence of cementite precipitates, are considered. The temperature range of the formation of bainite with the subgrain structure was determined. The size of the austenite grain and degree of hot deformation were found to affect the transformation of bainite that occurs upon subsequent cooling and the submicrocrystalline bainite structure. We studied the structure and mechanical properties of a rolled sheet 16 mm thick, which was subjected to thermomechanical treatment (TMT) under plant conditions in accordance with optimum regimes. It was shown that the high structure dispersion of the steel subjected to TMT is due to not only the formation of bainite with the subgrain structure, but also the partial transfer of crystal-structure defects from hot-rolled austenite to the final bainite structure.

Microstructure and properties of low-carbon weld steel after thermomechanical strengthening

Optical metallography and transmission electron microscopy have been used to study the structure of the rolled sheets produced from the pipe steel of Kh90 class. The sheets were manufactured using different technological schemes of strengthening: direct quenching from rolling heating (QRH) followed by high-temperature tempering; and two-step thermomechanical treatment with rapid cooling to a desired temperature (TMT). A relationship between the nature of the structure and mechanical properties of the rolled sheet has been established. It has been shown that the two-step thermomechanical treatment has significant advantages over the quenching from the rolling heating, which is related to the formation of highly dispersed bainite with a subgrain structure upon TMT.

Effect of thermomechanical treatment on the resistance of low-carbon low-alloy steel to brittle fracture

Structure and mechanical properties of rolled plates (20–35 mm thick) of low-carbon low-alloy steel subjected to thermomechanical treatment (TMT) according to various regimes under laboratory and industrial conditions have been studied. Structural factors that favor obtaining high mechanical properties have been established. The retarding action of TMT on softening upon tempering has been revealed. The reasons for the decrease in the resistance to brittle fracture of the steel subjected to TMT, repeated quenching from the temperature of the furnace heating, and tempering have been determined.

Effect of cold plastic deformation on the structure of granular pearlite in carbon steels

Carbon steel with a structure of granular pearlite has been investigated by methods of scanning electron microscopy and transmission electron microscopy as a function of the degree of cold deformation. It has been shown that with increasing degree of deformation the microstructure of the ferritic constituent changes regularly from dislocation to cellular and to microbanded. It has been established that the structural state of the granular pearlite is stable upon deformation to 50%. An increase in the carbon concentration in the ferrite occurs. Fragmentation of carbide particles into blocks and their subsequent dissolution hawe been observed upon deformation exceeding 50%. Fine-dispersed globular carbides are formed in the ferrite matrix.

Structural aspect of delamination crack formation during the HTMT of steels with a ferritic structure

The fine structure, texture, and fracture of the ferritic 08Kh18T1 steel subjected to repeated hot rolling at temperatures of 800–1100°C and reductions of 35, 50, and 65% have been studied. When significant strains are reached, a layered subgrain structure forms in the steel, and {100} and {111} planes are predominantly located in the rolling plane. The mechanisms of delamination crack formation during rolling under HTMT conditions are considered. The development of a crack in a material with a layered structure is analyzed theoretically.

Possibilities of Nondestructive Testing of Physico-Mechanical Characteristics of Hypereutectoid Carbon Steels with the Structures of Isothermal Austenite Decomposition

The possibilities of estimating the hardness, the wear resistance under abrasive action and sliding friction, the impact strength, and the internal stresses of the U9 steel (with 0.94% of carbon) with the initial fine-lamellar pearlite structure annealed at 650°C during 2–600 min on the basis of magnetic and electrical characteristics, readings of an eddy-current instrument, and parameters of electromagnetic-acoustic conversion are studied. A comparative analysis of the magnetic, electromagnetic, strength, and tribological characteristics of the U10 and U15 hypereutectoid steels (with 1% and 1.53% of carbon, respectively) subjected both to isothermal treatment at 330–650°C (with formation of pearlite and bainite structures) and to additional short-time annealing at 650°C is carried out.

Computer simulation of carbon diffusion near b/2[010](001) dislocation in cementite

Partial contributions Ui to the activation energy for the diffusion of carbon atoms in the Fe3C lattice have been calculated. The Ui values have been compared upon the migration of carbon atoms in the ideal lattice, near the stacking-fault plane, and near the core of a partial edge dislocation with a Burgers vector b/2[010]. The most preferable ways of the migration of carbon atoms near the studied structural defects in the (001) cementite plane have been revealed. The values of the stacking-fault energy in this plane have been calculated. The possibility of splitting the dislocation with a Burgers vector b/2[010] into two partial dislocations has been shown.