| With the rapid the development of electronics,automotive and other industries and the improvement of quality in the life,the demand for high-grade plastic mold steels,such as pre-hardened NAK80 plastic mold steel,is also increasing.Though we can produce NAK80 steel,but there are many problems such as poor mechanical properties,low injection times,short life and so on.In order to improve the performance of the domestic NAK80 steel and its competitiveness in the market,we study the influence of solution treatment and aging process on microstructure and properties of NAK80 steel by metallographic,scanning electronic microscope(SEM),hardness and impact toughness tests.The results show that:(1)when the quenching temperature is at 830??890?,the microstructure is small martensite,which has good mechanical properties.when the quenching temperature is at 870? the impact toughness is the best.The steel would get granular bainite after furnace cooling.Because of the effect of MA island,compared with the lath martensite,granular bainite has poor strength and toughness.(2)Because of the effect of precipitation strengthening,the strength of the steel increased sharply after aging.The aging strengthening effect of bainite NAK80 steel is much better than that of martensite NAK80 steel.Due to aging precipitation of intermetallic compounds,which hinder dislocation and weaken the grain boundaries,the impact toughness decreases significantly compared with that of the steel after solution.(3)The softening of matrix and the precipitation strengthening of second phase work together,which determine the performance variation of the steel in the aging process.When the aging temperature is low or the aging time is short,the steel is in underaged condition while the precipitation hardening is the leading role.When the steel is under overaged condition,the softening of matrix plays the main role.(4)The mechanical properties of martensite NAK80 steel is much better than that of bainite NAK80 steel after aging.Martensite NAK80 steel still has good strength and toughness even if at the aging peak,while bainite NAK80 steel has rather bad toughness which is unable to meet the requirements at the aging peak.The toughness of bainite NAK80 steel will be improved under overaged condition.(5)The aging temperature and aging time play an important role in Cu and Ni3A precipitation.In the condition of under aging,Cu and Ni3Al precipitation are very small and increase with the rising of aging temperature and aging time.There are the most of Cu and Ni3Al particles at the peak strength,which greatly increasing the strength.In the condition of over aging,the size of Cu and Ni3Al particles happen to coarsen obviously,while the number are decreased. |
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