Superior strength of carbon steel with an ultrafine-grained microstructure and its enhanced thermal stability

Abstract

The paper presents the results of a study on the microstructure and mechanical properties of a mediumcarbon steel (0.45 % C) processed by severe plastic deformation (SPD) via high-pressure torsion (HPT). Martensite quenching was first applied to the material, and then HPT processing was conducted at a temperature of 350°C. As a result, a nanocomposite type microstructure is formed: an ultrafine-grained (UFG) ferrite matrix with fine cementite particles located predominantly at the boundaries of ferrite grains. The processed steel is characterized by a high-strength state, with an ultimate tensile strength over 2500 MPa. Special attention is given to analysis of the thermal stability of the microstructure and properties of the steel after HPT processing in comparison with quenching. It is shown that the thermal stability of the UFG structure produced by HPT is visibly higher than that of quenchinginduced martensite. The origin of the enhanced strength and thermal stability of the UFG steel is discussed.

Crystal Engineering for Mechanical Strength at Nano-Scale Dimensions

“…A similar report has been made for achievement of a tensile strength of 2.5 GPa for a medium carbon steel of 0.45 wt % carbon first quenched to body-centered tetragonal martensite and then processed by SPD via high-pressure torsion [24]; thus, adding deformation-induced phase transformation into the engineering tools available to some materials for improving strength properties. In this case, the strength was attributed largely to two sources: stronger grains via solid solution strengthening and an increased dislocation density, both strengthening terms being in σ 0 in Equation (2); and, because of grain boundary strengthening, in the k −1/2 term.…”

“…An increase in mechanical toughness has been attributed to nano-scale silicon crystal thin films, nanospheres, and nanopillar type materials [61] mentioned at the beginning of the current report [2,4]. Valiev et al [22][23][24] have described a full range of strength and related material properties that are improved at nano-scale dimensions. A sample report on the importance of taking material size into account in micro-scale deformation and fabrication processing of metal components is given in [62].…”

Effects of the Tempering and High-Pressure Torsion Temperatures on Microstructure of Ferritic/Martensitic Steel Grade 91

“…The high strength of martensite is due to several hardening mechanisms: solid solution, dislocations and grain boundaries. Short-term tempering is required to reduce internal stresses prior to deformation of the martensitic steel by HPT [ 28 ]. The T500 + HPT20 treatment resulted in striking grain refinement in the structure.…”

Nanostructured Low Carbon Steels Obtained from the Martensitic State via Severe Plastic Deformation, Precipitation, Recovery, and Recrystallization

“…Dobatkin et al deformed martensitic 0.14 wt% C and 0.1 wt% C‐B steels via equal‐channel angular pressing at 300 °C and proved the enhanced strength and more uniform microstructure after additional subsequent annealing as compared to the ferritic‐pearlitic counterparts as starting materials . Ganeev, Karavaeva et al investigated the deformation of martensitic 0.1 wt% C and 0.45 wt% C steels using HPT . A hardness increase as compared to the as‐quenched state and an ultimate tensile strength of 2.65 GPa was reported for 0.45 wt% C martensitic carbon steel after HPT deformation at 350 °C .…”

“…Ganeev, Karavaeva et al investigated the deformation of martensitic 0.1 wt% C and 0.45 wt% C steels using HPT . A hardness increase as compared to the as‐quenched state and an ultimate tensile strength of 2.65 GPa was reported for 0.45 wt% C martensitic carbon steel after HPT deformation at 350 °C . The effect of various deformation temperatures was only investigated in one of the studies and resulted in a maximum hardness of a 0.45 wt% C martensitic steel for a deformation temperature of 350 °C in the investigated temperature range from 300 to 450 °C .…”