Study On Heat Treatment And Mo Content On DT Steel Performance

In this thesis, on the basis of current research status and development of DT300steels, the content of alloying element Mo was adjusted to improve the high temperature mechanical properties of DT300steels, and to meet the performance requirement under high temperature condition. Variation of undissolved carbides and precipitate phases of DT tested steels in the process of quenching and tempering were calculated by Thermo-Calc software. The quenching temperature range was determined to be from890℃to970℃, and tempering temperature range is from200℃to500℃. The effect of different heat treatment processes on microstructure and mechanical properties of DT tested steels was studied. The effect of Mo content on microstructure and mechanical properties of tested steels at room temperature and high temperature were analyzed and discussed. The main results and conclusions of the research in this thesis are as follows:With increasing the quenching temperature, undissolved particles gradually dissolve, the size of martensite laths coarsened, and the volume fraction of retained austenite increase gradually. In the range of quenching temperature, the strength of DT steels is not sensitive to the increasing temperature, and the impact toughness increases significantly when temperature increased up to970℃. The optimum quenching temperature of DT tested steels is about970℃.After tempered in the temperature range of200-500℃, the tensile strength decreases firstly and then increases, and reaches minimum value at400℃; the yield strength and impact toughness increases firstly and then decreases, and reaches a peak at300～350℃. When the DT steels has been tempered below350℃, a large number of tiny ε-carbides are precipitated in the martensite body. The best mechanical properties of the DT steels can be obtained when the DT steels was tempered at300℃. When the tempering temperature is higher than350℃, numerous lamellar cementite are precipitated in the martensitic laths, which causes the decrease in impact toughness drasticly. The DT steels present the temper brittleness, and the cracking mechanism is quasi-cleavage fracture. The volume fraction of retained austenite decrease with the increasing of tempering temperature in the range from200℃to450℃. The amount of cementite of DT steels decreases, when the tempering temperature is higher than450℃, the reverted austenite forms, and the toughness increases. It can be concluded that the optimum heat treatment regime is970℃×1h OQ+300℃×3h AC.The solution temperature of DT steels were improved with increasing the Mo content, and the temper brittleness was eliminated. After heated under the best heat treatment regime (quenching at970℃and tempering at300℃), the microstructure refines with increasing of Mo content, but the precipitates in the martensite body of tested steels with various Mo content are both e-carbides. The yield strength, hardness, elongation, reduction of area and impact toughness increase with increasing of Mo content, though tensile strength remain constant.The mechanical properties of the DT steels at room temperature are improved and increased significantly. Under the high temperature, the tensile strength and yield strength decreases withthe increasing of temperature, and reaches minimum value at700℃; and reduction of area, elongation and impact toughness increases firstly and then decreases, reaches a peak at700℃. At the same temperature, the matrix stability of DT steels with high Mo content at high temperature was enhanced, the DT tested steels has high heat-resistance. In addition, the high temperature strength of tested steels has been improved significantly. In the temperature range of400～600℃, the tensile strength and yield strength are raised over100MPa. At600℃, large amount of Fe2Mo phases are found in the martensite of DT300steels, a large number of intergranular fracture appears among the fracture morphology because of the serious segregation of C and impurity element (N, S) which weak interfacial bounding strength and make grain boundary become to be brittle, at the same time the ductility and toughness of DT300steels are lower. The segregation of C, N and S has been inhibited obviously with increasing Mo content up to1.84%, there are large amount of larger and deeper dimples, the fracture morphology of DT steels shows typical ductile fracture, and the elongation, reduction of area and impact toughness of DT tested steels increases significantly. The DT steels with high Mo content has outstanding mechanical properties at room temperature and high temperature, and which can meet the performance requirements of materials.