Effect Of Heat Treatment On The Microstructure And Mechanical Properties Of Two Quenching And Tempering High Strength And Low Alloying Steels

Two kinds of offshore platform steels are studied in this dissertation. Their chemi cal compositions (wt.%) are Fe-0.089C-0.175Cr-0.53Ni-0.38Si-1.63Mn-0.415Mo-0.028Nb-0.052V and Fe-0.123C-0.531Cr-1.79Ni-0.219Si-1.08Mn-0.565Mo-0.0017Nb-0.0375V. T he relationship between the heat treatment-microstructure-mechanical property has been studied by optical microscope, scanning electron microscopy, transmission electron mi croscopy, tensile test and impact test.The results show that with increasing quenching temperature, the grain size of steels increase gradually, and the grain coarsening temperature was 950℃～980℃. After the end quenching test, the hardness of the steel falls down slowly with the distance increasing from the water-quenching end, which reflects a good hardenability; the critical quenching diameters are 140 mm and 240 mm, respectively. The experimental steels show a strong ability to transform to bainite through the microstructure observation in different site from the water-quenching end. The strength of experimental steel increases, and the plastic and toughness decreases, as the isothermal quenching temperature increases. This is due to increased carbide particles of granular bainite at higher temperature; the observation using transmission electron microscopy shows that the carbide precipitate mainly inside the grains at a lower temperature, and at higher temperature, carbide precipitated more along the grain boundary. The experimental steels possesse good toughness, and their ductile brittle transition temperatures are lower than -60℃.