S390粉末高速钢经1180℃真空加热气冷（4×105Pa）、1230℃真空加热气冷（6×105Pa）后得到以下贝氏体为主的显微组织。结果表明,在片状下贝氏体中部析出成排碳化物是碳按下坡扩散机制和按dr/dt∝t-1/2方程长成260 nm×65 nm椭球状。在残留合金碳化物边缘析出纳米椭球状碳化物是按碳上坡扩散在离碳化物某处形成碳浓度峰,并处于最大过冷条件下进行析出的机制。
The main microstructure of lower bainite in a powder metallurgy high speed steel S390 treated with vacuum heating at 1180 ℃ and 1230 ℃,and then gas quenching with high pressure nitrogen at 4×105 Pa and 6×105 Pa,respectively,was obtained.It is found that ellipsoidal carbides with the size of 260 mn×65 nm are precipitated with in the lower bainite plates the mechanism of downhill diffusion of carbon and the formula of dr/dt ∝t-1/2.Moreover,the nano-size ellipsoidal carbides are observed on the edge of retained alloy carbide particles,for which the carbides are formed at the locations of carbon concentration peak and under maximum supercooling condition by uphill diffusion mechanism of carbon.
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