Effect Of Physical Field On The Microstructure And Thermal Fatigue Behavior Of Cast Hot Work Die Steel

With the entrance to 21st century the resource and environment problems have become the principle problems in the sustainable development of human beings. Therefore in the long term, the trends of the industrial development all over the world can be reduced to the saving of energy and resource, and what is more, the pursuit of natural production. As a new technique, physical field that is good to the solidification can not only refine grain and improve solidification structure, but also affect actively the solid-state phase changes .So the physical field can enhance the all round property of the cast product. On the other hand, under the condition of saving energy and material physical field can reduce the pollution of environment and material, enhance the repeat usage of materials, and realize the sustainable development and clean production and procession of material, so the study of the physical field is hotspot in 21 century. Consequently the study of the procession with the physical field is significant to not only the development of the new materials but also the improvement of the traditional materials. Along with the proliferation of industrialization , the field of application of high temperature materials becomes wider and wider,the ceiling temperature needs much higher and the floor temperature needs much lower, so the break off phenomenon of thermal fatigue in workpieces and structures caused by dynamic heating increase badly. The thermal fatigue is one of mainly factors cause workpieces out of work such as Cast Hot Work Die Steel, heat exchanger tube, boiler tube, the wheel disk and blade of gas engine, high speed train break disk and heavy truck break hub. According to the Stat., 60~70% the hot working die which called "black gold"are disabled caused by thermal fatigue, and then the wastage is very huge. About 40~50 thousand of hot-work die steel are consumed every year in our country, which worth several billion Yuan. Therefore, there is great national economic significance to enhance the thermal fatigue property of hot die steel. This paper mainly studies the effect of the mechanical vibration with different vibration amplitude, different vibration time and direction on the microstructure and thermal fatigue behavior of new cast hot work die steel which is made in Ji Lin university .On the other side this paper also studies the effect of the current pulse with different current density and the operation time on the thermal fatigue behavior of new cast hot work die steel. The results of the experiment were analyzed by SEM ,TEM,OM,XRD. (1) The MV with different amplitudes, time and orientation can cause broken of the dendrites and formation of small grains structure during the solidification. Furthermore, the amount of shrinkage cavity and inclusion is decreased and the size is reduced. In evidence, after MV, shrinkage cavity and inclusion look smaller with a shape of ellipse or sphere and are distributed more uniformly. For all of the castings solidified treated with MV, the increase in compactness was between 0.55 and 2.26%. Simultaneously, It is easy to draw a conclusion that, the MV with increasing amplitude and time resulted in an increase in the degree of dendrites fragmentation, degasifying, deslagging and filling capacity. Under the different vibration orientation, the result with the Z direction of one dimension which is parallel the inertia is the best. The second is the X-Z direction of two dimention, then the last is the X-Y-Z direction of threedirection. (2) For all of the castings solidified treated with MV, the increase in hardness was between 1.46 and 14.75% and the increase in impact toughness was between 13.34 and 43.99% in the hardness and impact toughness tests. Simultaneously, It can be found that, the fracture of the samples treated with MV is predominantly ductile with evidence of plastic deformation and microscopic features comprising a great deal of deep dimples and ligament. It can be concluded from the fracture surface features that samples treated with MV represent nicer characteristic of ductile failure. (3) It is found that the MV is advantageous to enhancing the resistance of the TF crack nucleation and propagation in the TF test. The MV can delay the crack nucleation and reduce the crack propagation velocity. The increase in TF life was between 1.25 and 3.01 times in the steels treated with MV than in the untreated steel. Simultaneously, It is easy to draw a conclusion that the MV is advantageous to enhancing the anti-soften ability. (4) After a certain time of the hot circulation in the TF test, some tiny flaw are formed in the samples, however when it is not accumulated to form the TF crack, The Current pulse with different current density and operation time is exerted on the samples. It is found that the Current pulse is advantageous to enhancing the resistance of the TF crack nucleation and propagation in the TF test. The Current pulse can delay the crack nucleation and reduce the crack propagation velocity. The increase in TF life was between 1.52 and 4.85 times in the steels treated with Current pulse than in the untreated steel. Simultaneously, It is easy to draw a conclusion that the Current pulse with increasing density and operation time resulted in an increase in the TF life of Cast Hot Work Die Steel. (5) In the same current time (100 μs), if the current density exceed 37 MA/m2. It can be seen that the circular heat affected zone is formed at the notch root of the sample treated with current pulse. And the range of the circular heat affected zone is become larger alone with the increase of the current density. With the same current density (54 MA/m2) and a certain operation time, the range of the heat affected zone formed at the notch root of the sample become larger and the microstructures around notch root get more and more...