Study On The Evolution Of Microstructure And The Thermal Fatigue Behavior Of H13 Steel

At present, the mold is applied into many areas of the industry. The developing of mold is growing at a very fast growth rate at home and abroad. The mold has become indispensable to the various tools. The hot-working die steel is always damaging due to its complexity and difficulty of working conditions, which means that the performance of the die material must has the higher demand and challenges on the economy and the environment. In recent years, people always use the hot forging die steel and the cast die steel, for example the H13 and QRO80M ,and so on. Because the production of steels costs relatively high and they also can be invalidated because of the alternated hotting and colding, how to improve the thermal fatigue properties of hot-work die steel is of a great national significance.There are many factors to affect the thermal fatigue of the hot-wok die steel, so it has many measures for improving the properties of the steel. It is commonly that people try to adjust the chemical composition, choice the right heat treatment and give the steel surface treatment for prolonging the service life time. At present, there is not much study on the thermal fatigue of the steel by the electropulsing. At the bionic principle point of view, if the damaging steel given the reasonable elcropulsing, it can change the stress concentration and the strain energy and repair some damage in location, which is a economic and environmental way to improve the hot-work die steel.In this paper, we use the self-discharge equipment to carry on the experiment. After the samples of H13 being treated by electropulsing, we studied that the affect of high-energy impulse current stimulation on the microstructure, mechanical properties, thermal fatigue behavior, oxidation of high temperature and corrosion of the H13 steel. We could found:At the maximal density of electric pulse current(105MA/m2), when the treated time reach to 340ms, the HAZ will have the largest area, which it is melted at the front of the prefab cut. When the treated time reach to 320ms, the HAZ will have the right area. So the time parameter is the best choice which is the follow-up test pulse current treatment parameters. The organization of HAZ remain refine after continuing to do 1500 cycles thermal fatigue when the sample which is treated by pulse current after 200 cycles thermal fatigue. Through to the TEM, we found that microstructure of HAZ of treated sample by pulse current after 1000 cycles thermal fatigue is more finer than that which is not treated by pulse current after 1000 cycle thermal fatigue. Also has it the higher dislocation and the larger amount of nanosized carbide particles separation and occurred in solution than the untreated samples.If the softened samples in the thermal fatigue cycles were treated by electric pulse current, the hardness of them can be improved, which can make hardness of the samples fallen slowly during the thermal fatigue cycles. When the fagigued samples which had undergone 1000 thermal cycles were treated by pulsing current, the tensile strength can be increased from 1342MPa to 2021MPa, relatively enhanced of 50.1% and the tensile strength can be increased from 1428MPa to 2021MPa, compare with the original ones, relatively enhanced of 41.5%, however, they have little difference between the extension.When resistance is applied into the test of thermal fatigue damage, we found that there is a relation between the variation of resistance and the thermal fatigue damage. The treating on the samples can make the resistance of the samples rise rapidly. When the resistance of samples dropped to the lowest value, the micro-crack at the front of pre-cut begin to emerge. The samples which had undergone different thermal fatigue cycles were treated by pulse current, we found that the discharge treatment on the sample can give the best treatment effect when the samples of H13 steel had undergone 200 thermal fatigue cycles. From the analysis about the variation of resistance and the treating by pulsing current, we can found that there is a good relation between them. It is the time that when the resistance is lowest to give the damaged samples pulse current treatment for having a best affect.When the damaged or undamaged samples of H13 were treated by electric pulsing current, the resistance of high temperature oxidation and acid corrosion of the HAZ of the samples all were improved, which furtherly give a good complementarity to the method that the pulsing current can repair the thermal fatigue damage during the thermal fatigue cycles.In short, this paper is not only a further in-study on the fatigue behavior of hot die steel treated by current, but also it is also provide a theoretical basis and significance for the applyment of pulse current.