The present paper is aimed at the study of the kinetics of Mn, Si, Cr partitioning in 0.2wt%C-Si2Mn2CrMoVNb TRIP-assisted steel under the annealing at 770 oC and 830 oC to be within the intercritical temperature range. The work was fulfilled using SEM, EDX, dilatometry, and hardness measurements. It was found that under heating a redistribution of the alloying elements between ferrite and austenite took place. Specifically, silicon partitioned to ferrite while chromium diffused to austenite with distribution coefficient values of 1.12-1.21 (KSi) and 0.75-0.86 (KCr). Manganese was found to partition to a much greater extent resulting in a distribution coefficient of KMn=0.38-0.50 and 2.6 times higher concentration in austenite as compared to ferrite. As annealing temperature raised from 770 oC to 830 oC the elemental partitioning was accelerated, followed by the decrease in manganese content in austenite (by 1.44 time) and ferrite (by 1.34 time) caused by an increase in austenite volume fraction. Silicon featured uneven distribution within ferrite to be accumulated at the “martensite/ferrite” interface and near ferrite grain boundaries, while manganese was concentrated in MC carbides. The recommendation for annealing holding was formulated based on elemental partitioning kinetics.

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The present article is aimed at studying the austenite transformation kinetics and tensile properties of constructional 0.2 wt%C-Si2Mn2CrMoVNb TRIP-assisted steel subjected to isothermal holding in the subcritical temperature range (350-650 °C with the step of 50 °C) after intercritical annealing at 770 °C. The study was fulfilled using optical microscopy (OLYMPUS GX-71), electron scanning microscopy (JEOL JSM-), dilatometric analysis, tensile testing, Vickers hardness measurements. The critical temperatures of the steel were found to be Ac1=750-760 °C and Ac3=930 °C. The results showed that austenite demonstrated increased stability to pearlite and bainite transformations with an incubation period of decades of seconds at any of the mentioned temperatures. The bainitizing treatment at 400 °C with holding of 300-600 s resulted in ferrite/bainite/retained austenite structure with precipitates of nanosized carbide (V,Nb)C providing an improved combination of mechanical properties as compared to direct quenching (YS=548-555 MPa, UTS=908-1000 MPa, total elongation=16-18 %, PSE index=14.6-18.0 GPa%, YS/UTS ratio=0.55-0.60). The contributions of different strengthening components were estimated in order to reveal the benefits of a multi-phase microstructure for constructional applications.

В статье описаны фазово-структурный состав и механические свойства низколегированной стали 60С2ХФА, подвергнутой обработке «Quenching and Partitioning» (Q-n-P). Режим обработки включал: а) аустенитизацию при 880°C; б) закалочное охлаждение до температуры «Quenching» (240, 200, 160°C) в ванне с расплавом сплава Вуда; в) выдержку при температуре «Partitioning» (270, 300°C) в ванне с расплавом ПОС-61 в течение 300-3600 с для перераспределения углерода из мартенсита в аустенит; г) окончательное охлаждение на спокойном воздухе. Установлено, что Q-n-Pобработка приводит к формированию мультифазной структуры, состоящей из мартенсита отпуска, бескарбидного нижнего бейнита и остаточного аустенита. Лучшее сочетание свойств достигается при закалке до 160-200°C с формированием 50-70% мартенсита и последующей выдержке

Purpose. This study investigates the possibility of improving the quality of grinding steel balls with a diameter of 100 mm by using Q-n-P heat treatment. Methodology. Steel grinding balls with a diameter of 100 mm, obtained by transverse screw rolling, were used as a material. The heat treatment consisted of balls quenching in a drum-type device according to various modes and subsequent tempering. The quenching modes provided the bulk temperature of the balls in the range of 120-240 оС, which is lower than the martensitic point Ms of the steel. After quenching, the balls were immediately tempered at 170-300 °C for 2-10 hours. The treated balls were visually examined, tested for hardness and impact resistance. The hardness was measured according to the Rockwell method by scale "C", the microhardness was measured with microhardness tester "Affri" at a load of 50 g. The impact resistance of the balls was evaluated by impact of a dropped load with impact energy of 6.8 kJ. Microstructural analysis was conducted using an optical microscope "Axiovert 40-M". The amount of residual austenite was determined using an "IV-Pro Rigaku" diffractometer in copper Ka-radiation. Findings. It is shown that, as a result of treatment of 100 mm diameter balls of chromium-manganese steel according the standard mode, the cracks occur on balls surface. Using Q-n-P heat treatment allows achieving higher hardness through the cross section (within 54-57 HRC) while quenching cracks are absent on the balls. The Q-n-P-treated balls have high impact resistance under repeated impacts with energy of 6.8 kJ. Originality. For the first time the authors investigated the feasibility of using the Q-n-P heat treatment for steel grinding balls of large diameter made of steel with increased hardenability. It has been determined that Q-n-P-processing allows to significantly increase (by 10 HRC) the bulk hardness of 100 mm diameter balls, while retaining their high impact resistance in tests with a dropped load. The obtaining of such properties is pro-vided by the relaxation of quenching stresses and the formation of a two-phase martensitic-austenite structure with an increased amount (25-30 %) of retained austenite. The latter becomes possible due to the process of partitioning carbon from martensite to austenite with enrichment of the latter to 1.12 % C. Practical value. The advisability of Q-n-P-heat treatment in the production of steel grinding balls with higher bulk hardness is shown.