Feasibility study for thixoforming nanostructured bainitic steels

Abstract

Super Bainite Steel (0.74% C, 1.64% Si, 1.82% Mn, 1% Ni, 0.36% Mo, 0. 21% Cr, 0.047% Al bal. Fe) after cold plastic deformation was used as the starting material for bainitic treatment preceded by controlled cooling from the thixoforming temperature range. Differential Scanning Calorimetry was used to determine the amount of liquid phase fraction vs temperature in the solidus–liquidus range. The steel was heated up to 1430°C, which corresponded to about 30% of liquid fraction. It was then rapidly cooled in oil at three different temperatures: 240°C, 270°C and 300°C. The samples were then held at those temperatures for 5h. The microstructure of the samples after treatment at 240°C showed globular grains (average size of 115µm), containing carbide-free bainite as a mixture of ferrite plates with average thickness of 63nm, and retained austenite plates of 40nm in thickness. Plates of martensite and blocks of austenite were visible in some grains. X-ray studies confirmed the presence of 20.1% of austenite and 79.9% of ferrite with martensite. The sample revealed the highest compression strength of 6651MPa, at yield strength of 1780MPa and compression strain ε=38%. With the increase of isothermal treatment temperature to 270°C and 300°C, yield strength decreased to 1370MPa, 1375MPa and compression strength was 5243MPa, 4138MPa, respectively, while plastic strain reached 38.5% and 25%, respectively. Higher temperature of bainitic treatment led to coarsening of super bainite plates and as a consequence, a decrease in mechanical properties. Initiation of cracks was observed inside grains. They propagate through the globules and eutectic, unlike in thixoformed tool steels, in which the crack path runs along the eutectic mixture between the globular grains. Preliminary studies of the corrosion resistance of Super Bainite steels containing a carbide-free bainite structure were carried out. Results indicate slightly a higher resistance in chlorides environment of the sample isothermally treated at 240°C/5h preceded by controlled cooling from thixoforming temperature range than the sample treated in the conventional way (1000°C/15min/240°C/20h).