Two carbide-free low alloy bainitic steels containing 0.60％and 1.17％Al were austempered with different processes and different microstructure and mechanical properties were obtained,and then the effect of austempering on the microstructure and properties of the tested steels was studied by using optical microscope,scanning electron microscope,transmission electron microscope,X-ray diffraction,as well as impact and tensile tests.The results show that when the bainitic transformation is incomplete,the microstructure of the carbide-free bainitic steels is composed of bainitic ferrite,retained austenite and a large amount of martensite,while the microstructure of the tested steels is composed of bainitic ferrite,retained austenite and a very small amount of martensite when the bainitic transformation is complete.And when the bainitic transformation is complete,the size distribution range and volume fraction of retained austenite in 0.60％Al steel are higher than those in 1.17％Al steel,while the bainitic ferrite lath of 1.17％Al steel is finer than that of 0.60％Al steel.The yield strength of 0.60％Al steel is slightly lower than that of 1.17％Al steel,but its impact property,tensile strength and total elongation after fracture are significantly higher than that of 1.17％Al steel,demonstrating much excellent comprehensive mechanical properties.
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