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Heat treatment and mechanical stability behaviour of medium-carbon TRIP-aided bainitic steel

Grajcar A.

Archives of Materials Science and Engineering

2008

Vol. 33, nr 1

5-12

Purpose: The aim of the paper is to determine the influence of the isothermal holding conditions in a range of bainitic transformation on the mechanical stability of retained austenite for medium-carbon TRIP-aided steel. Design/methodology/approach: The examinations were carried out on medium-carbon steel containing 0.55%C and 1.35%Si. The conditions of heat treatment consisted of isothermal quenching of the specimens to a temperature range of 250 to 550°C, where they were held for 600 and 1800 s. Tensile deformation of steel to the given strain equal 0.25, 0.5 and 0.75 of total elongation of samples was conducted in order to determine the kinetics of retained austenite transformation into martensite. The retained fraction of the γ phase was determined by the use of the quantitative X-ray phase analysis. Findings: Increasing the carbon concentration to 0.55% in TRIP-type steels makes possible to obtain very high strength properties without a deterioration of the ductility. The retained austenite of the 19% volume fraction can be obtained after the isothermal quenching of the steel to a temperature of 250°C. In these conditions, the matrix of the steel is the ferritic bainite. The size of regular grains of retained austenite is equal up to 3μm, while the rest of γ phase is present in a form of thin films between individual laths of bainite. Diversification of retained austenite form has a reflection in its mechanical stability, connected with two-stage kinetics of martensitic transformation of γ phase. Research limitations/implications: To determine with more detail the stability of retained austenite the knowledge of lattice parameter changes with an isothermal holding temperature is needed. Practical implications: The proposed heat treatment can be useful for manufacturing reinforced structural elements characterized by high strength and ductile properties in the automobile industry. Originality/value: The developed conditions of the heat treatment concern the medium-carbon TRIP-type bainitic steel, offering higher product of UTS UEl compared with usually investigated TRIP-type ferritic-bainitic steels.