Research On The Morphological Evolution Of Upper Bainite And Property In The Silicon-Containing Steel

The influence of austenitic temperature, isothermal temperature on the transformation products of bainite in 9SiCr steel was studied by metallographic analysis. The evolution of the morphology of upper bainite in 9SiCr steel with the isothermal holding time was studied by metallographic analysis,X-Ray diffraction and transmission electron microscopy. We also studied the mechanical properties of the samples obtained after different isothermal holding time.The experimental results showed that the upper bainite transformed after short isothermal holding time when the austenitic temperature was 900℃and the isothermal holding temperature was 380℃had rod-like micromorphology. With longer isothermal holding time, new rod-like upper bainite generated at the side and the tip of the primary rod. Hence the upper bainite presented thick rod-like micromorphology. The thick rod-like upper bainite is arranged by small rod-like upper bainite. With the isothermal holding time prolonged, the thick rod-like upper bainite associated and gradually developed into featherlike bainite. When the isothermal time is long enough,the carbide precipitated between the rod-like upper bainite, so the upper bainite presented normal featherlike micromorphology.The results showed that the changes of mechanical properties of the samples are in accord with the changes of upper bainitic microstructure. The rod-like upper bainite transformed after short isothermal holding time had higher hardness and strength of extension. When the isothermal time prolonged, the hardness number and the strength of extension become lower, but the notched bar impact strength become higher. The reason of mechanical properties changes is that rod-like upper bainite had higher hardness itself and it evenly distributed in the parent phase. When isothermally held for a sufficient time the rod-like upper bainite gradually transformed to normal featherlike upper bainite. Therefore the rod-like upper bainite played an important role in the sample's mechanical properties.The fine structure of upper bainite had been investigated. We found that rod-like upper bainite was composed ofαmonophase and it did not have carbide. It has character of lattice expansion. There are high stress–strain field in the parent phase around the rod-like bainite. Therefore new laths could continuously grow around original bainitic lath. As a result, the original lath is lengthened and thickened. Therefore the transformation mechanism of bainite is in the manner of martensitic shear.