将V-Nb-Mo微合金钢在1200℃固溶处理0.5 h后淬火, 然后在不同的温度(500-650℃)回火4 h, 或在650℃回火不同时间, 用显微硬度判定等温回火和等时回火时析出碳化物强化机制的转变点, 用三维原子探针(3DAP)表征碳化物的析出特征, 并测量其等效半径r_p.根据第二相与位错交互作用的强化理论, 共格应变强化主导切过机制,计算了位错切过向绕过机制转变时第二相的尺寸. 结果表明, 3DAP测量和强化理论计算得到的第二相的临界转变半径十分吻合, 约为1 nm, 表明用共格应变强化切过机制可以很好地预测析出强化作用最显著时第二相的尺寸.

Precipitation strengthening can occur by means of two mechanisms, depending on the size. First, the shearing mechanism is active for small precipitates and the strengthening increases with the increase in the precipitate size. Second, the Orowan bypass process, where dislocation loops around precipitates is active for larger precipitates and the strengthening decreases with increase in the precipitate size. Therefore, there must exists a critical transition size of precipitates, in which the strengthening reaches the maximum. V–Nb–Mo micro–alloyed steels were isochronally tempered for 4 h at different temperatures and isothermally tempered for different times at 650 ℃ after solution treatment at 1200 ℃ for 0.5 h. The tempering conditions where the transition of precipitate strengthening mechanism occurred could be ascertained by the isochronal and isothermal hardness curves. The critical transition size of precipitates was determined to be about 1.0 nm by three dimensional atom probe (3DAP). Based on the precipitation strengthening theory and shearing mechanism dominated by coherency strengthening, the critical transition size of precipitates was calculated to be about 1.0 nm, which was in good agreement with the value measured by 3DAP. It indicates that the size of second phase can be well predicted by the precipitation strengthening theory when the micro–alloy steel reaches the maximum precipitation strengthening effect.