Effect Of Heat Treatment On Low Temperature Impact Toughness Of High Strength Low Carbon Bainitic Steel

With the rapid development of infrastructure in China,the demand for steel materials is increasing,especially application in low-cold areas.Higher requirements are put forward for their low-temperature performance to meet the safety and usability of equipment or components.In this thesis,the effects of different heat treatment processes on the microstructure,mechanical properties and low temperature impact properties of carbide free bainite steel with components of Fe-0.2%C-1.2%Si-x%?Mn+Cr+Mo+Ni?were studied through heat treatment process experiments,mechanical properties tests,X-ray diffraction phase analysis,optical metallographic and transmission electron microscope microscopic observation.The results are as follows:The phase transition temperature of the experimental materials are measured by the thermal expansion method?Ac1:740?,Ac3:880?,Bs:370?and Bf:225??.The experimental materials are heating and air cooling at different austenitizing temperatures.When the heating temperature is lower than 920?,the tissue is fine,and when the heating temperature is higher than 920?,the tissue coarseness is obvious.The hardness of the experimental material decreases with the increase of heating temperature,and the tensile strength increases first and then decreases with the increase of heating temperature,when heated temperature is 880?,the tensile strength have peak.When heating temperature is920?,the room temperature impact energy have peak,and when heating temperature exceeds920?,the impact energy decreases.The best comprehensive mechanical properties are obtained when the austenitizing heating temperature is 920?.When heating temperature is 920?and the holding time coefficient is less than 2.0min/mm,the size of tissue strip bundle is small,when the holding time coefficient is more than2.0 min/mm,the size of tissue strip significantly increases and the tissue becomes coarse.The hardness of experimental materials decreased with the extension of the insulation time,and the tensile strength increased first and then decreased with the extension of the insulation time,the maximum value of the tensile strength occurs when the holding time coefficient is 2.0 min/mm,and the room temperature impact energy have peak when the holding time coefficient is 1.5min/mm and 4.0 min/mm.Holding time coefficient of 1.5 min/mm has the best comprehensive mechanical properties.The optimum normalizing process parameter of the experimental material was 920?×1.5 min/mm heating.The mechanical properties of the experimental materials obtained at 920?×1.5 min/mm+350?tempering were Rm 1428 MPa,A 10.8%,Z63.8%and A_KV?room temperature?91.3 J,and the structure is carbide free bainite composed of bainite ferrite and retained austenite.The mechanical properties for the experimental material of solid structural parts after heating at 920?and tempering at 350?for one time are Rm 1288 MPa,A 14.0%,Z 59.3%,A_KV?-40??25 J,indicating-40?low temperature impact toughness do not meet the technical requirements.The low temperature properties of the materials can be improved by increasing the tempering times at 350?.The mechanical properties of the experimental materials are Rm1308 MPa,A 14.4%,Z 58.4%,A_KV?-40??31 J for normalizing at 920?and tempering at350?for two times.Alter three times of 350?tempering A_KV?-40??32 J,low temperature impact energy meets the technical requirements of A_KV?-40???27 J.The experimental material of solid structural parts are heated at 920?,alternating cooling of water cooling and air cooling+350?tempering,and heated at 920?,alternating cooling of water cooling and air cooling+350?tempering for two times,which can refine the structure and improve the low-temperature performance,so that the low-temperature impact toughness of the large section structure meets the requirements of A_KV?-40???27 J.The mechanical properties of the materials obtained by the two heat treatment processes are Rm1374?1393 MPa,A 13?15%,Z 57?60%,and A_KV?-40??37?39 J.The morphology of the tissue is mainly lath carbide-free bainite,and the residual austenite in the tissue is distributed in the middle or above bainite ferrite lath.In the actual production of large section die forgings,after heat treatment with heating at 920?,alternating cooling of water cooling and air cooling+350?tempering.Both transverse and longitudinal low temperature impact energies for the bar and head of the die forgings have been improved.The low temperature impact energies all meet the technical requirements of A_KV?-40???27 J.