| 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.
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