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Bainitic reaction and microstructure evolution in two normalized and tempered steels designed for service at elevated temperatures

Ławrynowicz Z.

Advances in Materials Science

2017

Vol. 17, nr 4(54)

22--36

In the present work conventional heat treatment like normalizing (bainitic microstructure) and tempering of the alloys has been performed. The materials used in this study were two steels, one the laboratory prepared experimental low alloy Cr-Mo steel in comparison to typical commercial 10CrMo9-10 steel. The determined carbon concentrations of the residual austenite at the different temperatures of bainite transformation supports the hypothesis that the growth of bainitic ferrite occurs without any diffusion with carbon being partitioned subsequently into the residual austenite. It was found that bainitic reaction has stopped when average carbon concentration of the untransformed austenite is close to the T0 line and supports formation of bainitic ferrite by a shear mechanism, since diffusionless transformation is not possible beyond the T0 curve. Normalized samples were air cooled down to room temperature before tempering at various temperatures in the range of 500-750°C. Samples have been austenitized at 980°C for 0.5 hour air cooled and tempered at 500, 550, 600, 650, 700 and 750°C for 1 hour. After heat treatment, the assessment in the microstructure and phase precipitation was made using the samples prepared for metallographic and transmission electron microscope (TEM) on thin foils analysis. Quantitative X-ray analysis was used to determine the retained austenite content after heat treatment like normalizing and tempering and the total volume fraction of the retained austenite was measured from the integral intensity of the (111)γ and (011)α peaks. The changes observed in the microstructure of the steel tempered at the higher temperature, i.e. 750°C were more advanced than those observed at the temperature of 500°C. Performed microstructural investigations have shown that the degradation of the microstructure of the examined steel was mostly connected with the processes of recovery and polygonization of the matrix, disappearance of lath bainitic microstructure, the growth of the size of M23C6 carbides, and precipitation of the secondary M2C precipitates. The magnitude of these changes depended on the temperature of tempering.

Mikrostruktura i własności blach grubych ze stali C15V

Dryja M., Lis A., Kolan C.

Hutnik, Wiadomości Hutnicze

2013

Vol. 80, nr 5

376--379

W pracy przedstawiono mikrostrukturę i własności blach grubych ze stali C15V. Analizowano blachy o grubości od 7÷16 mm. Mikrostruktura stali jest bainityczna oraz ferrytyczno-bainityczna. Wskazuje to, że po zakończeniu procesu walcowania blachy zostały poddane przyspieszonemu chłodzeniu. Mikrostruktura blach jest drobnoziarnista. Wyniki badań stereologicznych przedstawione w postaci histogramów dodatkowo potwierdzają istnienie drobnej struktury. We wszystkich przypadkach zmierzona wielkość ziarna wahała się od 0,5 μm do 9 μm. Blachy o grubości 7, 8 mm posiadają największe wartości parametrów wytrzymałościowych, natomiast twardość kształtuje się w zakresie 220÷243 HV30. Dobre własności plastyczne określone w statycznej próbie rozciągania w połączeniu z drobnym ziarnem skutkują wysoką energią łamania, która wynosiła od 158÷204 J.

The article presents the microstructure and the properties of heavy plates made of steel C15V. Plates of 7÷16 mm were analyzed. The microstructures present bainite and ferrite-bainite structures after accelerated cooling after the process of rolling. The microstructure of plates is fine-grained, which was proved by the study on the size of grain estimation. Moreover, the results of the stereology study illustrated by the histograms also confirm the existence of fine-grained structure. In all cases the measured size of a grain was from 0,5 to 9 μm. The plates of the thickness of 7, 8 mm have the highest mechanical parameters, whereas the hardness was on the level of 220÷243 HV30. Good plastic properties, defined in the static tensile test, combined with fine grains result in high impact strength, which amounted from 158 to 204 J.