The9Ni steel is treated by QT heat treatment.Metallographic microscope is used to observe and analyze the microstructure.And the touch screen controlled PLC impact test machine is used to carry out the low temperature impact test,what is more,the phase analysis is carried out by XRD diffractometer.The experimental results show that the impact low temperature toughness of9Ni steel is obvious improved after QT heat treatment,and the average of experimental group is about3times that of the original group.The main reason...