Effect of Pre-tempering and Double Normalizing on Microstructure and Mechanical Properties of Low Activation Steel

The effects of heat treatment conditions on the microstructures and mechanical properties of low activation steel with Y, Zr and Ti were investigated. Two heat treatment processes are designed to control the microstructure and carbides in the steels, by pre-tempering and repetition of normalizing. The higher the pre-tempering temperature is, the higher the yield strength is. With the increase of pre-tempering temperature, the residual stress in steel decreases. The average diameter of M23C6 phase in the steel reduced from 141 to 130 nm, and the microstructure is full martensite. When the pre-tempering temperature is 500 degrees C, the yield strength of the steel is 663 MPa, the ductile-brittle transition temperature (DBTT) of the steel is - 61 degrees C. Double normalizing has a significant effect on the microstructure and precipitated phase of steel. The microstructure of the steel is sorbite, and the size of M23C6 in the steel is reduced to 93-102 nm. The size of MX increases to 23-26 nm. With the increase of the second normalizing temperature, the size of MX increases and the size of M23C6 decreases. MX preferentially precipitates, reducing the carbon in the matrix, thereby inhibiting the precipitation of M23C6. The yield strength of 1050 degrees C x 30 min (normalizing) + 950 degrees C x 30 min (normalizing) + 425 degrees C x 30 min (pre-tempering) + 750 degrees C x 90 min (tempering) experimental steel is 693 MPa, and the DBTT is - 99 degrees C. The refinement of M23C6 and the increase of MX volume fraction strengthen the precipitation strengthening effect. Double normalizing has a significant effect on the ductile-brittle transition temperature (DBTT) of the experimental steel. The change trend of DBTT of different heat treatment samples is consistent with the change trend of fracture strength caused by M23C6 calculated by improved Griffith model. M23C6 is the main reason for the change of impact property of steel.