过共析钢在奥氏体化温度下加热后空冷或炉冷将会产生片状珠光体组织和网状渗碳体,在超细晶条件下得到了不遵循这一规律的固态相变新现象。本实验采用高能球磨快速烧结方法制备了超细晶过共析钢块体试样,通过控制球磨时间控制烧成试样晶粒度。结果表明：当球磨时间超过40 h,烧结后试样晶粒细化到2～4μm的量级,试样空冷无法得到片状珠光体,经共析转变得到纳米级球状渗碳体和铁素体组织,渗碳体球的尺寸在10～100 nm范围。热力学分析表明,晶粒细化导致原奥氏体中碳化物形核率增加,尺寸较小的粒状碳化物相对于片状碳化物具有更大的形核动力;动力学分析表明,晶粒细化导致钢中碳扩散速度提高,层片状珠光体的长大受到抑制,共析转变中珠光体更易长成为粒状。
Common steels of hypereutectoid composition form lamellar pearlite with network carbide when cooled from austenite temperature in air or in furnace.Here,we provide evidence that this is not the case in an ultrafine-grained hypereutectoid steel obtained by ball milling and plasma activated sintering.Different grain sizes were obtained by changing the time of ball milling.When the grain size was reduced to the scale of 2-4 μm,normal pearlite can not be obtained when the steel was cooled in air,instead ferrite and ultrafine granular cementite which were 10-100 nm in diameter were formed by eutectoid transformation.The thermal dynamic analysis indicates that the nucleation rate is accelerated by the fine grain effect,and the nucleation driving force for ultrafine granular pearlite is greater than that of lamellar pearlite.Dynamics analysis indicates that the carbon diffusion rate increases with decreasing grain size,the growth rate of lamellar pearlite is reduced and the granular carbide is finally precipitated by eutectoid reaction.
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