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2 Journal of Achievements in Materials and Manufacturing Engineering

2 2006

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Influence of hot and warm deformation on austenite decomposition

100%

Jandová D.

Journal of Achievements in Materials and Manufacturing Engineering

2006

tom Vol. 18, nr 1-2

375--378

Purpose: The substructure of austenite influences phase transformations during the austenite decomposition and consequently the final properties of the steel. Design/methodology/approach: Steel 0.5C-1Cr-0.8Mn-0.3Si was processed using the thermo-mechanical cycling simulator. Different methods of the thermo-mechanical processing were applied including austenitization at 950°C, compression deformation at 950°C or 650°C and isothermal dwell at temperatures in the range (350°C÷450°C). Microstructure was investigated using light and transmission electron microscopy. Findings: It was demonstrated that straining in austenitic region accelerated the ferrite and pearlite transformations. Bainite reaction depended on the temperature of austenite deformation, the strain level and the temperature of isothermal dwell. Hot deformation slightly accelerated the transformation to upper bainite and retarded the transformation to lower bainite. Warm deformation resulted in mixture structures containing pearlite, ferrite and bainite; bainitic reactions were accelerated. Fine ferritic grains, pearlitic nodules and clusters of individual ferrite/carbide units enclosed with martensitic matrix were observed in heavy strained parts of specimens. Practical implications: Different morphologies of ferritic structures which can occur in the wrought steel can result in deterioration of mechanical properties. This fact has to be taken into account in numerical simulations of thermo-mechanical processing of low alloy steels. Originality/value: Of this paper consists in elucidation of the processes taking place in heavy strained austenitic structure during its isothermal decomposition at temperatures in bainitic region.

Refinement of steel microstructure by free-forging

51%

Jandová D. , Divišová R. , Skálová L. , Drnek J.

Journal of Achievements in Materials and Manufacturing Engineering

2006

tom Vol. 16, nr 1-2

17--24

Purpose: Refinement of austenitic steel and low alloyed C-Mn-Si-Nb steel microstructure by free-forging. Design/methodology/approach: In this study severe hot deformation was applied on low alloyed 0.2C-1.5Mn-2Si-0.05Nb steel and austenitic 0.07C-18Cr-2Mn.-10Ni steel. Steels were processed in a full-hydraulic press. Different temperatures of preheating, dwells and strain levels were applied. Microstructure was characterised using optical metallography, scanning and transmission electron microscopy. Findings: After optimization of processing fine grained structures with convenient phase composition were obtained. Multiple free-forging followed by recrystallization annealing was successfully used for refinement of austenitic Cr-Ni steel and improvement of mechanical properties. While free-forging of low alloyed C-Mn-Si-Nb steel still has not satisfied expected increase of strength and ductility. Research limitations/implications: Free-forging usually results in formation of very heterogeneous structures. In submitted study relatively fine grained and uniform structures were obtained. Grain size below a few micrometers can not be probably achieved using above mentioned technology. Originality/value: Obtained results can be used for development of forging technology in service conditions.