| The influence of heat treatment process upon the structure and mechanic properties of a new type carburized bainitic steel was studied by using heat treatment, mechanics testing, optical microscope and X ray diffraction. The new type bainite hollow steel was fabricated by hot-penetrating and hot-rolling method and the influence of carburize temperature and after heat treatment on the structure and the hardness of steel was analyzed also, and the friction welding experiment between bainitic steel and 22CrNi3Mo steel was done as well.The results indicated that the higher the normal temperature the lower strength, hardness and the higher toughness, the optimally normal temperature range is 900～920℃, the mechanics are as follows:σb1149～1189MPa,δ5%11～12,ψ%49～51%, AKV 50～52J. As the increasing of quenching temperature the strength and hardness shows a slow decreasing tendency, and at 900℃a heat toughness peak is achieved, the optimally quenching temperature is 900℃and the performance are as follows:σb1488MPa,δ510%,ψ58/%, AKV 37J.The structure varied with different normal and tempering temperature which is ferrolites, bainite and austenite respectively, the austenite distributed among the ferrolites and bainite lathes, and the different quenching and tempering temperature the structure is normalized martensite and austenite respectively and the austenite distributed among the martensite lathes.When normalized at 920℃and with the increasing of tempering temperature the strength and hardness shows a decreasing tendency. Tempering bellow 350℃the material shows higher impact toughness, when tempering above 350℃the impact toughness decreased. Tempering at 500℃the temper brittleness of bainite occurred, above 500℃the temper impact toughness increased. When normalized at 920℃and tempering at 350℃with the lengthened time of tempering the strength increased and tempering for 8-10h the bainitic steel achieved optimal strength-touhhness match.A good surface quality and hot working performance can be achieved by using hot-penetrating and hot-rolling method fabricated new type bainitic hollow steel.The test results of the samples of hollow steel which normalized at 920℃and tempering at 200℃show that the performance are as follows:σb(MPa)≥1496,δ5(%)≥12,ψ(%)≥47, AKU(J)≥76 which can satisfy the properties of FF710 steel (σb(MPa)≥1470,δ5(%)≥9,ψ(%)≥45, AKU(J)≥63) can be used as drilling steel.With the increase of carburizing temperature the surface hardness of new type bainitic hollow steel decreased when which cooled in still air or wind. With different carburizing temperature and cooled in still or wind and tempering at 200℃, the carburized case is high carbon tempered martensite and resuidual austenite, the transition zone is tempered martensite and resuidual austenite and bainite and the center is bainitic ferrite and resuidual austenite. When carburizing at 920℃and cooled in still air or wind a highest surface hardness of material achieved (HRC62.5) and so as a fine structure of carburized case. The optimal carburizing and the heat treatment processing is carburizing at 920℃+680℃cooling in air and normalizing at 860℃and cooling in air and tempering at 200℃.The hardness of as friction welded bainitic steel and 22CrNi3Mo steel shows a convexity distribution, the welded seam showing the highest hardness.The welded seam showing a similar hardness when which normalized and temped after friction welded and a good strength and toughness joints can be achieved. The as friction welded welded seam is composed by bainite lathe and little austenite, and the heat treatment treated joints which the heat affected zone and the martix are composed by bainite lathe and little austenite.The practice shows that using new type bainitic steel for the manufacturing of heavy type drilling rod is very well and will be a good prospect.,...
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