Study On Fracture Toughness Of High Carbon Si-mn Steel Steel With High Temperature Tempering

With the extension of the application filed of high carbon steel,high strength and toughness matching has become a hot research topic.In this study,the high carbon Si-Mn steel with high temperature tempering was used as experimental materials,studied the effect of preparatory structure before quenching,heat treatment process and network carbide on fracture toughness.The relationship between microstructure and heat treatment process were obtained by micro-analysis methods such as OM,SEM,XRD and image software etc.The strength,hardness,fracture toughness and other mechanical parameters of specimens were measured by mechanical test equipment.And the KIC fracture characteristics were analyzed.The effect mechanism of preparatory structure before quenching,structure after high temperature temper and network carbide were discussed.The main results are follows:Different preparatory structure were obtained after forging with different cooling rates.lamellar pearlite were obtained after the slow cooling and air cooling,fine pearlite and martensite organization were obtained after water cooling.The quantity of carbides in fan cooling specimen was largest after pretreatment(isothermal spheroidizing annealing),and the quantity of carbides in water cooling specimen was smallest.There was no obvious difference between the three specimens after final heat treatment(quenching and tempering),and the strength and toughness of these three kinds of specimens were basically the same.In the final heat treatment,with the increase of the quenching temperature,the large undissolved carbide in the structure of specimen has been gradually dissolved,the grain size was increasing continuously,KIC was descending in general and decreased rapidly from 920 to 1000℃.The growth of fracture toughness was slow with the increasing of tempering temperature under 400℃,then it will grew rapidly when the tempering temperature exceeded 400℃,the fracture climbed up to 80 MPa·m1/2 at 650℃.Homogenization Annealing was carried out after forging,large amount of continuous network carbide existed within the specimen after furnace cooling treatment,it leads to the intergranular brittle fracture easily,which can reduce the fracture toughness of specimens seriously.There were less network carbide in salt bath treatment specimens,and the fracture toughness was significantly higher than that of furnace cooling treatment specimens.With the increase of quenching temperature in the final heat treatment process,the network carbide of furnace cooling treatment specimen was gradually dissolve.The network carbide were completely disappeared at 920℃,when quenching temperature was higher than 920℃,the fracture toughness was constantly decline with the increase of the quenching temperature.The fracture toughness of salt bath treatment specimen decreased gradually with the increase of quenching temperature.