Evolution of microstructure in 100Cr6 steel after cooling from a thixoforming temperature to bainitic transformation ranges

A new concept for the isothermal heat treatment of thixo-elements, consisting of controlled cooling from a semi-solid metal processing (SSM) range, was proposed. 100Cr6 steel (0.97% C, 1.4% Cr, 0.4% Si, 0.4% Mn bal. Fe) after forging was used as the starting material. The DSC analysis was used to determine the liquid phase fraction vs temperature in the solidus–liquidus range. The temperatures and kinetics of bainite transformation after cooling the steel in the semi-solid state were calculated based on the Chester and Bhadeshia models. The steel was heated up to 1425 °C in order to obtain about 25% of the liquid fraction. Then it was cooled in oil at three different temperatures: 135 °C, 235 °C and 335 °C, at which the samples were held for 5 h. The microstructure of the samples annealed at 135 °C consisted of globular grains (the average size of 323 µm), where coarse needles of martensite were observed. They were surrounded by an eutectic mixture of chemical composition different from that of the globular grains. The electron diffraction pattern (SAEDP) showed reflections from α′-Fe and from Fe3C carbides. The X-ray diffraction confirmed the presence of martensite, austenite and carbides to the amount of 74.5%, 22% and 3.5% respectively. The average hardness of samples was 735HV10, while the compression strength attained 3810 MPa at the plastic strain of 8.6%. The samples cooled down to 235 °C also showed globular grains surrounded by the eutectic mixture. The TEM studies allowed researchers to identify lower bainite with a plate thickness of about 500 nm (SAEDP from that area showed reflections from α-Fe and Fe3C carbides). X-ray diffraction confirmed the presence of 90% ferrite, 6% austenite and 4% carbide. The average hardness of samples annealed at 235 °C decreased to 627HV10, while the compression strength decreased to 3100 MPa. The plastic strain increased to 32%. The microstructure of the samples after cooling and treatment at 335 °C consisted of globular grains, inside which upper bainite plates were present. The average hardness was 519HV10, while the compression strength decreased to 1420 MPa. The obtained experimental results correspond to the assumed model, in which the range of the bainitic transition terminates at the temperature from 135 to 370 °C.