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本文着重研究了两种结构钢的显微组织与试样破断时裂纹萌生之间的关系。结果表明,两种钢经250℃回火,析出ε-碳化物,断口呈迭波,这时a_k都有个极大值。高于350℃回火,两种钢中的ε-碳化物都趋消失,代之析出Fe_3C,这时断口上出现准解理结构。随着Fe_3C长大,准解理的比例增加,经450℃回火后达极大。进一步提高回火温度,随着Fe_3C球化,准解理也趋消失。对450℃回火试样的研究发现,试样破断时裂纹主要沿孪晶马氏体的内孪界和板条马氏体的条界萌生和扩展。这就表明准解理的解理面是这两种界面,其晶体学指数是{112)_a和{110}_a。Fe_3C析出和残留奥氏体转变,是导致这种界面脆化的原因;这也是37SiMnCrNiMoV钢的450℃回火马氏体脆性的机制。而60Si2Mn钢的350℃回火马氏体脆性机制是原始奥氏体晶界脆化。
This paper focuses on the relationship between the microstructure of two structural steels and the crack initiation at the time of specimen failure. The results show that the two steels are tempered at 250 ℃, ε-carbides are precipitated and the fracture is overlapped. In this case, a_k has a maximum value. Higher than 350 ℃ tempering, ε-carbides in both steel tend to disappear, instead of precipitating Fe_3C, quasi-cleavage structure appears when the fracture. With the growth of Fe_3C, the proportion of quasi-cleavage increases, reaching a maximum after tempering at 450 ℃. To further improve the tempering temperature, with Fe 3 C spheroidization, quasi-cleavage also tends to disappear. The study of tempering at 450 ℃ shows that the crack mainly starts along the boundary of twinned martensite and the martensite of lath when the specimen is broken. This shows that the quasi-cleavage cleavage plane is the two interfaces, the crystallographic index is {112) _a and {110} _a. Fe_3C precipitation and retained austenite transformation, is the cause of this interface embrittlement; This is 37SiMnCrNiMoV steel 450 ℃ tempered martensite brittle mechanism. However, the brittleness mechanism of 60Si2Mn steel at 350 ℃ tempered martensite is the embrittlement of the original austenite grain boundaries.