The Divorced Eutectoid Transformation In GCr15 Bearing Steel

Hypereutectoid steels are usually subjected to spherpidization annealing before cold forming to obtain spherodized microstructure consisting of finely distributed globular cementite in ferrite matrix.The main purpose of this heat treatment is to reduce the hardness and thus provide a good workability.Austenite with cementite particles transforms to a mixture of ferrite and spherical cementite by a reaction known as divorced eutectoid transformation(DET).GCr15 steel is developed to be studied.The network carbide in hot rolled microstructures of GCr15 bearing steel wire,the dissolution of cementite during intercritical austenitization,and the new base for DET undercooling is described.The temperature range for DET was established and verified by experiments.The main contents are present as follows:(1)The hot rolled microstructures of GCr15 bearing steel wire were investigated by acid erosion.The development of acid holes defect for different acid erosion time have been investigated.The presence of carbide network were considered to be the main reason for the occurrence of the acid holes defect.There is a significant reduction in the amount of acid holes by the heat treatment for eliminating carbide network;(2)Laboratory experiments were carried out for the reduction of the spheroidization annealing time by spheroidization of different initial microstructures.The martensitic and bainitic microstructures spheroidize more rapidly than pearlite,because the thermodynamic driving force is greater.The pearlite is difficult to spheroidize because of the low energy interfaces of this microstructure.In addition,Cr and Mn have an effect on the spheroidization kinetics of cementite in pearlitic steel;(3)In DICTRA,the dissolution of one spherical cementite particle in a spherical austenite cell was simulated.It is believed that the partitioning and redistribution of substitutional alloying element plays an important role during the dissolution of cementite.Influence of alloying elements addition on coarsening of cementite particles was considered.The slow diffusion of substitutional elements can remarkably retard the coarsening of cementite particles;(4)We can contrast growth rate of lamellar pearlite with that of divorced pearlite to identify spacing as a function of undercooling.In this paper,we revised the undercooling base.The actually eutectoid temperature was calculated by Thermo-calc when the composition of austenite matrix at a cooling rate falls to the down of ?+?/?phase boundary.The temperature range for DET was established.The experimental observations suggested that this new concept is reasonable.