新型有机物切削刀具材料的低温二次硬化机理研究及渗碳体超细化的析出控制

【作者】 陈明昕；

【导师】 史庆南； 雍岐龙；

【作者基本信息】 昆明理工大学 ， 材料学， 2009， 博士

【摘要】 通常只有高钴含量的高速高合金工具钢经过特殊热处理才可实现硬度达到HRC68-72,低合金工具钢中达到这样高的硬度未见报导。且钢的淬火与回火硬度几乎一致。另外,由于渗碳体或铁的碳化物析出后容易聚集长大而粗化,因此很难得到细小的尺寸(100nm以下)而达到显著的强化效果。因此,有机物切割刀具用低合金工具钢存在硬度偏低、刀刃锋利度不足等缺点,很难满足使用要求。本文依据钢铁材料的微合金化及强韧化原理,通过合金设计与优化,运用低温二次硬化原理成功开发了一种具有超高硬度(HRC68-72)的高碳低合金钢9CrV及新型的低温回火工艺,该钢以其良好的综合性能,满足了有机物切割刀具的高质量的切削性能需求。本文对该钢低温二次硬化机理及渗碳体的超细化析出控制进行了系统地研究。采用扫描电子显微镜、X射线衍射,相分析等手段研究和分析了该钢经不同的低温回火工艺处理后析出相的种类、数量及尺寸和低温二次硬化机理,并定量计算了合金渗碳体M3C相的Orowan机制对钢的强度贡献；利用钢铁材料中的第二相的相关理论计算了合金渗碳体M3C与ε-碳化物自铁素体及马氏体基体析出过程的相变自由能、比界面能、弹性应变能的变化,以及合金渗碳体M3C析出时的临界形核尺寸及临界形核功。研究结果表明,经过淬火880℃/5min油冷后基体为高碳马氏体组织,组织中有少量的、平均尺寸在1.2μm左右的合金渗碳体钉扎在晶界上,阻止了奥氏体晶粒长大,得到了11.5级细小晶粒,这对提高钢的硬度及韧性起到了重要的作用。再经180℃/10h低温回火处理,硬度达到HRC68-72,在马氏体组织基体上析出主要的强化相为合金渗碳体M3C相(非ε碳化物),并有少量的M(CN)相,尺寸为1-5nm的M3C相占总质量分数的14.2%,5-10nm占5.6%,细小、弥散的合金渗碳体与基体保持着共格或半共格关系,可使钢的强度提高640MPa,这是实现超硬化的重要原因。运用实验结果并依据相关理论计算得到,在200℃时M3C自马氏体中沉淀析出产生平均比界面能σ为0.58685 J/m2,ε碳化物自马氏体中沉淀析出产生平均比界面能厅为0.2J/m2,可见M3C自马氏体基体析出时需要克服的系统能量高于ε碳化物,这一结果证实了ε碳化物优先析出；根据第二相位错形核理论计算得到,M3C在马氏体基体析出时的核胚临界最小核心尺寸为Hed*=0.81nm、Led*=1.2nm、Wed*=1.08nm,最小临界形核功为0.694×10-18J,计算结果为控制析出相种类及尺寸提供了可靠的理论数据。析出相的溶解与析出伴随着系统能量的变化,能量的变化大小往往标志着该析出相沉淀析出的难易程度,本文通过计算得到合金渗碳体与ε碳化物溶入铁素体中标准反应的吉布斯自由能公式分别为：△GFe3C→α=77344-45.564T和△Gε→α=25558-1.149T,并计算得到含碳量为0.887%(除去M(CN)相中含碳量)的钢中M3C在铁素体中的析出自由能公式为△GM=-77344.184+46.56T,以及ε碳化物在铁素体中沉淀析出相变的自由能公式为：△GM=-25558+2.1456T,公式适用范围473-1000K,计算结果可对控制析出相的溶解与析出起到重要作用。同时本文还估算得到在200℃时M3C和ε-碳化物在铁素体中沉淀析出的粗化速率分别为0.08017 nm/s1/3、0.542064nm/s1/3,说明ε-碳化物的粗化速率快于合金渗碳体M3C,因此ε-碳化物沉淀析出后将会迅速长大。而本文所采用特殊的低温回火工艺抑制了ε碳化物的析出或促进ε碳化物转变为合金渗碳体,并在较低温度得到了粗化速率较慢的合金渗碳体,最终得到超细的颗粒尺寸,实现了渗碳体的超细化控制。更多还原

【Abstract】 Unless high speed steel of high cobalt content might be realized in special conditions, hardness of low alloy tool steels did not reach up to HRC68～72 in general case. Meanwhile, hardness of tool steels after quenching and tempering was almost coincide. It is very difficulty to obtain cementite and carbide precipitates with fine size by aggregation growth of cementite and carbide of iron in tempering. Therefore, obviours strengthen action of cementite or carbide precipitates can not be obtained for years.In this paper, a new type of process for low temperature temper and a new type of alloy tool steel 9CrV with ultra-high hardness, HRC68-72, were sucessly obtained,which has good propertities and can meet to the requirement of tool cut organism. Meanwhile, Secondary hardening mechanics at low temperature and ultra-refinement cementite of this new type of tool material used to cut organism, high carbon low alloy 9CrV steel, were systematically investigated. Using SEM, XRD and phase analysis et al, type, amounts, size of precipitates and secondary hardening mechanics at low temperature in this steel tempered during deferent process were investigated and analyzed. Meanwhile, the strength increments by M3C phases were quantitatively calculated. Variation in transformation free energy, special interface energy, elastic strain energy for the process of alloy cementite M3C andεcarbide precipitating from ferrite and martensite matrix, and critical nucleating size and work were calculated by the theoretic for the secondary phase in iron and steel.The results show that high carbon martensitic microstructure can be obtained in the tested steel quenched at 880℃, held for 5 minutes and then cooled in oil. Partial alloy cementites dissolve and small alloy cementites with average size of 1.2μm at grain interface can inhibit growing of austenite grains. Austenite grains size is 11.5 grade,which has action to increase hardness and ductility of the steel. Hardness of the steel tempered at 180℃and held for 10h with pressure is HRC68～72, in whose martensite matrix, alloy cementite M3C phases is mainly strengthening phase and there are small amount of M (CN) phase. Size of alloy cementite M3C phases distribute mainly 1～5nm, which is 14.2% mass fraction of total M3C phases and the phases with 5～10nm is 5.6%. That interface between refine and dispersed alloy cementite and matrix is semicoherent can make to increase the strength of the steel to 640MPa. This may be extremely important for ultra- hardening.It has be known that average special interface energy (?) and elastic strain energyΔGEV in the process of M3C phases precipitating from martensite at 200℃are respectively 0.58685 J/m2 and average special interface energy for s carbides is 0.2 J/m2, which implicated the energy barrier for M3C phases precipitating from martensite is lower than that for s carbides. Therefore, alloy cementite M3C phases prior precipitate. The critical nuclear size for M3C phase precipitating from martensite is Hed*=0.81nm, Led*=1.2nm, Wed*=1.08nm, and the critical nucleation work is 0.694×10-18J。The formula of variation in Gibbs free energy of the standard reaction for alloy cementite and s carbide dissolved in ferrite can be respectively expressed asΔGFe3C→α=77344-45.564T andΔGε→α=25558-1.149T.The formula of variation in Gibbs free energy of for M3C phases precipitating from ferrite in the steel containing carbon of 0.887% can be expressed asΔGM=-25558+2.1456T,where the range of temperature is 473-1000K.In this paper, it has been estimated that ripening rate of M3C phase andεcarbide precipitating from martensite at 200℃are respectively 0.08017 nm/s1/3 and 0.542064 nm/s1/3.The result shows that the ripening rate ofεcarbide is higher than that of M3C phase, and so it would grown quickly whenεcarbide precipitate from ferrite. However, martensitic microstructure can be obtained by quenching, and then precipitation ofεcarbides can be inhibited or can be transformed alloy cementites in low temperature(at 180℃, Pressure) temper, which can make to refine carbide particles and obtain ultra-refined alloy cementite particles in the rage of composition of the tested steel.更多还原