Study On Heat Treatment Technology Of Seamless Tube Of New High-strength Bainitic Steel

Seamless steel pipe is an important raw material for national infrastructure construction and is widely used in petroleum engineering,electric power construction,military manufacturing,aerospace and other fields.A new type of high-strength bainitic steel seamless tube was studied in this paper.The hardness-temperature method was used to determine the phase transition temperature of the experimental materials.The effect of heat treatment process on the micro structure and properties of the new type of high-strength bainitic steel seamless tube was studied by mechanical properties test,X-ray diffraction,optic microscope observation and observation of impact fracture morphology and microstructure by scanning electron microscope.The results show that the phase transition temperature of seamless tube of new high strength bainitic steel was determined by hardness-temperature method.The Ac1 point of the material is 700℃ and the Ac3 point is 880℃.When the tempered temperature of hot rolled new high strength bainitic steel seamless tube is below 550℃,as the tempering temperature increases,the tensile strength tends to decrease.The tensile strength varies from 983 to 1023MPa,and the decreased is not large.The minimum tensile strength after tempering at 650℃ is 794.1 MPa.250～350℃ tempered seamless pipe has high strength and plasticity.Tempering at 400℃ appears bainite temper brittleness.Tempering at temperatures above 500℃ can increase the impact value of hot-rolled bainite steel pipes.Tempering at temperatures below 400℃,the tempered microstructure of the bainite steel seamless pipe is lath bainite,granular bainite,a small amount of massive ferrite and retained austenite.Tempered over 500℃,the tempered microstructure is ferrite and granular bainite.When the austenitizing heating temperature of the new high-strength bainite steel seamless pipe is 930℃,the seamless pipe has a higher strength and toughness combination,which is the best austenite heating temperature.Normalizing by heating at 930℃,tempering at different temperatures,the impact value and plasticity index of the experimental material is the highest when tempering at 300℃,which is the best tempering temperature.Bainitic steel temper brittleness appears at 400℃.Normalizing at 930℃ and tempering at 300℃,when the tempering holding time is 2h,it is the best tempering holding time.Normalizing at 930℃ and tempering at 300℃ for two times can increase the impact value of the new type of bainite steel seamless pipe by 50%.The reason for the increase in impact value is related to the change in the microstructure of the experimental material during tempering.The best normalizing heat treatment process for the new type of bainite steel seamless pipe is heating at 930℃ and tempering twice at 300℃ for 2h tempering holding time.The mechanical properties obtained are tensile strength of 960MPa,elongation of 16.3%,reduction of area of 69.8%,and impact energy is 114.6J.The mechanical properties meet the requirements of practical pipe engineering applications(Rm≥930MPa,A≥14%,Z≥50%,AKV≥70J).The microstructure of normalizing at 930℃ and tempering at 300℃is mainly carbide free bainite ferrite and massive ferrite.The experimental material was quenched at 930℃ and tempered at different temperatures,and the impact value appeared peak when tempered at 200℃ and 700℃.The mechanical properties obtained by tempering at 200℃ are 1375MPa for Rm,13.8%for A,60.9%for Z,and 63.4J for AKV.The mechanical properties obtained by tempering at 700℃ are 981 MPa for Rm,18.8%for A,51%for Z,and 68.7 J for AKV.The tempered microstructure at 200℃ is tempered martensite and a small amount of retained austenite,and the tempered microstructure at 700℃ is sorbate.When quenching at 930℃ and tempering at 200℃,the tempering holding time is 6h,which is the best tempering holding time.The second tempering at 200℃ can increase the impact value of the experimental material by 32%,and the obtained mechanical properties Rm is 1340MPa,A is 15.5%,Z is 65.5%,and AKV is 83.9J.After quenching at 930℃ and tempering at 200℃,the microstructure of primary and secondary tempering is tempered martensite and a small amount of austenite.