Study On High Temperature Wear Property Of New Type Die Casting Magnesium Alloys Die Steel

Nowadays, the dies are called as"black gold"because of their high cost and complicated process, and mould technology has been an important symbol that can estimates the manufacturing ability of one country.Recently, the die casting magnesium alloy has attracted much widespread attention due to its high specific strength and stiffness, as well as good anti-electromagnetic radiation ability and so on, and it has became a kind of more potential engineering material which can substitute die casting aluminum alloy and other die casting material. The squeeze casting magnesium alloy has been reputed for"the 21th century green material"and more and more widely applied in the automobile, electronic, electrical appliances, communication, measuring appliance and astronautics aviation applications increases day by day. Unfortunately, there are no especially suitable materials for the selection of the magnesium alloy die casting mold in the domestic die steel. Currently, only the aluminum alloy die casting molding steel has to been adopted widely. However, compared with the traditional thermal fatigue failure of aluminum alloy die casting mold, the failure forms are abrasion and erosion in the current use, so the life of mold is also low. Moreover, due to these differences in the thermal capacity, the melting point and the fluid of the magnesium alloy from those of aluminum alloy, therefore it is necessary to conduct the thorough research in the magnesium alloy die casting mould and the high temperature wear performance. So far, the researches on the high temperature wear resistance of die steels are relatively limited. Most of researches have been focused on the high temperature oxidation wear. The function of the formation, growth and desquamation of oxide during the wear process has been known. But the high temperature oxidation wear mechanism is still incomplete.The new type hot work die steel, used as magnesium alloy die casting mold, is used as the raw material. Optical microscope (OM), scanning electron microscopy (SEM), chemical composition analysis, transmission electron microscopy (TEM), impact toughness tests, hardness tests and high temperature wear tests have been carried out to study the factors and mechanism of the high temperature wear resistance and the delaminating mechanism of the oxide film.In the present study, the effects of the chemical composition and heat treatment on the mechanical properties and the high temperature wear resistance of the new type hot work die steel were investigated. Optimized chemical composition, temper temperature and parameters were carried out to be ready for practice products. Optimized chemical compositions are as follows (wt. %): C: 0.28~0.32, Cr: 9.5~10.5, Mo: 1.9~2.2, Si: 0.1~0.3, V: 0.5~0.9, Co: 5.0~7.5, W: 5.0~6.0, Mn: 0.4~0.6. Optimized heat treatment parameters are as follows: austenizing heating to 1130℃for 1 hour and oil quenching; tempering for 3 hour at 550℃and air cooling. The tempering microstructures of the new type hot work die steel are mainly the lath martensite, followed the bainite with a trace of remained austenites. The impact toughness is 9~24 J/cm2, and the hardness is 45~53(HRC).The experimental results indicate that the thermal stability of the new type hot work die steel is better than those of the steel of H13 steel and ASSAB8407 steel. Under the 650℃for 21 hours, the hardness of the new type hot work die steel is still larger than HRC30, while the hardness of the steels of H13 and ASSAB8407 decreased to HRC27~28. The oxidation resistance of the new type hot work die steel is much better than those of H13 and ASSAB8407 steels. With the tempering at 650℃for 50 hours, the oxidation adding weight of the H13 and ASSAB8407 steels are much larger than MGD die steel, respectively. All the three steels were investigated under the same experimental condition.In the process of high temperature wear, metals must be oxidated, so the high temperature wear mechanism of the new type hot work die steel is influenced greatly by oxidation. The formation and the desquamation of oxide film occur in turns during the high temperature wear process. The formation of oxide film improve the wear resistance, whenas the desquamation of oxide film can deteriorate the wear resistance.This article has studied the effect of the oxidation resistance on high temperature wear of the new type hot work steel. Abraded against the counter disk with HRC50, the better the oxidation resistance is, the better the high temperature wear resistance is; abraded against the counter disk with HRC65, the trend of the effect of oxidation resistance on high temperature wear is consistent with that with HRC50. However, due to the effect of the strength, toughness and thermal stability of the die steel on the support of oxidation film, the oxidation resistance is not main reason which affects the high temperature wear resistance. During the wear process, the sample with bad oxidation resistance is easy to oxide to form oxide layer. When the thickness of oxide film is beyond the critical thickness which can make the oxidation film desquamate, the probability of adherence abrasion increases, and thus the wear rate increases subsequently; In contrary, the sample with good oxidation resistance is difficult to oxide. Under the circular mechanical power, the oxidation is difficult to desquamate, which is in favor of the improvement of high temperature wear resistance. The formation of oxidation film can protect the matrix from abrading against the counter face directly. Therefore, at high temperature, oxidation film decreases the probability of adherence abrasion and support the load to prevent the wear of matrix, which improve the wear rate of materials.The results indicated that the high temperature wear mechanism varied with the change in the hardness of counter disk.The oxide film was formed on the worn surface during the wear process at high temperature. It has several desquamation forms of the oxide film, such as layer delamination,holes after particles brushing off, particles ploughing, groove and brittle cracking and so on. During the wear process, the oxide film may be formed again by the delamination oxide compacting.The new type hot work die steel, which has many superior aspect such as high life expectancy,low cost, can be applied to die casting production of magnesium alloy. This paper researched the high temperature wear theory, which provides a theoretical basis for practical application.