Abstract: Homogenization treatment of AISI 4330 steel with banding defects was carried out through a step-by-step cyclic quenching and tempering heat treatment processes. The microstructure evolution and its effect on mechanical properties was studied during homogenization process. The results show that the original specimen with banded defects consists of ferrite and pearlite, with a banded structure level of 3. The element segregation of C, Cr, and Mo is significantly revealed by means of electron probe micro-analysis (EPMA). After undergoing multiple high-temperature quenching+quenching and tempering cycles, the microstructure transforms into a homogeneous structure consisting of ferrite, carbides containing Cr, and tempered sorbite. The element segregation is improved to a certain extent. Although the diffusion of C element is evident, segregation of Cr and Mo elements still exists. The step-by-step cyclic quenching process leads to lots of phase transformation, resulting in the refinement of microstructure of the steel. Furthermore, the mechanical properties of the steel are enhanced, resulting in tensile strength exceeding 1200 MPa, hardness of about 350 HBW, and impact absorbed energy at -40 ℃ of about 70 J. The refinement of microstructure contributes to the improvement in both strength and toughness of the specimen. Additionally, the large amount of precipitated carbonitride plays a beneficial role in increasing the strength of the steel. The enhanced strength and toughness characteristics fulfill the requirements of fracturing pump products and their service life.

Key words: AISI 4330 steel, band structure, homogenization, microstructure, properties