Maraging steels, having ultra-high performance, are widely used in both military and civil engineering industries. Fine grains are necessary to assure good ductility and toughness in the ultra-high strength level for this type of material. Cyclic phase transformation is an important grain refining method in maraging steels. The research on Co-bearing maraging steels showed that the alloying elements have a great influence on both the processing of cyclic phase transformation and the limit grain size after cyclic phase transformation. Furthermore, the processing of cyclic phase transformation could also have a great effect on the limit grain size after cyclic phase transformation.
But up to the present, there is less research work on refining grains through cyclic phase transformation for Co-free maraging steels and B-bearing maraging steels, especially there is no systemic research work on the effect of the processing of cyclic phase transformation on the limit grain size after cyclic phase transformation for various types of maraging steels. Therefore it is necessary and meaningful to carry out some work on the subject mentioned above, in order to provide reference for real applications of refining grains through cyclic phase transformation.
Therefore the present work systemically studied the effect of the processing of cyclic phase transformation on the limit grain size for three types of maraging steels, a Co-bearing maraging steel CM350, a Co-free maraging steel TM300 and a B-bearing Co-free maraging steel T250. The purpose of present work is to establish a rule of the parameters selection for cyclic phase transformation. Meanwhile, the behavior of the precipitated borides in T250, and the effect of un-recrystallized solution treatment on the microstructure and tensile properties of the Co-free maraging steel was also investigated in the present work. Conclusions of this thesis are as follow.
（1）The process of refining grains through cyclic phase transformation is: heating (α→γ transformation)-holding (solution and reverse austenite recrystallization)-cooling (γ→α transformation), and then repeated the process again for several times.
（2）The rule of parameters selection for cyclic phase transformation is as follow: the holding temperature and holding time are required to make sure the finish of the reverted austenite recrystallization and to avoid the severe growth of grains after recrystallization. The shorter of the holding time, the finer of the grains and the cycling times should be 4~5. The heating rate should be different for different maraging steels and be as fast as possible for TM300 and T250.
（3）The limit grain sizes for different materials under various processing of cyclic phase transformation were 9~12μm for CM350, 28~36μm for TM300 and 17~32μm for T250. The finest grain of 7μm for CM350 could be obtained in the condition of fast heating to the recrystallization temperature after pre-tempered at 650℃.
（4）The effect of microstructure revolution during heating on the limit grain size after cyclic phase transformation is as follows: microstructure change in the aging has no influence on the limit grain size after cyclic phase transformation; holding in the un-recrystallized temperature region could enlarge the limit grain size after cyclic phase transformation; holding in the phase transformation temperature region could reduce the limit grain size for CM350, enlarge the limit grain size for T250 and have no influence on TM300.
（5）After refining grains through cyclic phase transformation for the maraging steels, the tensile strength increased and the ductility became much better. The tensile fracture changed from brittle failure to ductile failure as the grain size became small, and the smaller of the grains, the smaller of the dimples.
（6）For the B-bearing Co-free maraging steel T250, coarse borides precipitated on the grain boundaries during the slow cooling after high temperature solution treatment, and small borides precipitated both on grain boundaries and within grains in the solution temperature range of 800~1000℃, and the dissolution of coarse borides needs holding at the temperature higher than 1050℃. The optimizing processing of cyclic phase transformation is step decrease temperature at 1050~950℃ short holding time.
（7）Un-recrystallized solution treatment could be successfully used for Co-free maraging steels of TM300 and T250. After un-recrystallized solution treatment, the morphology of martensites changed from blocky to stringer, the dimples on the tensile fracture surface became much finer, the hardness and tensile strength increased and the tensile ductility remained nearly constant.
修改评论