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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.

Badania właściwości mechanicznych wybranych hartowanych bainitycznie gatunków stali przeznaczonych na elementy maszyn rolniczych

Łabęcki M.

Prace Przemysłowego Instytutu Maszyn Rolniczych

1999

Vol. 44, nr 2

11-13

Praca zawiera wyniki badań właściwości mechanicznych pięciu stosowanych w maszynach rolniczych stali (65, N7E, 65 G, 55S2, 50 HS), w których dzięki zastosowaniu hartowania bainitycznego z przemianą izotermiczną uzyskano strukturę bainitu. Badania wykazały możliwość uzyskania struktury bainitu przy grubości detalu 5 mm dla stali 65G i 15 mm dla stali 50 HS i 55S2. W porównaniu ze strukturą martenzytu odpuszczonego przy tej samej twardości, stale o strukturze bainitu posiadają zbliżone właściwości wytrzymałościowe oraz ponad dwukrotnie lepsze właściwości plastyczne.

Results of investigations of mechanical properties of five steel grades used in construction of agricultural machinery are presented in the article. These steel grades were first bainicticly hardened with isothermal process. The investigations indicated on the possibility of obtaining of bainite structure when the thickness of machine element is 5 mm for the 65G steel grade and 15 mm for the steel grades 50 Hs and 55S2. In comparison with the tempered martensite structure of the same hardness the steels of bainite structure have similar strength and over double better plastic properties.