本文应用电镜薄膜技术,观察30CrMnSiNi2A超高强度钢在淬火与回火过程中马氏体的精细结构,发现此钢淬火后存在位错型和孪晶型两类马氏体。低温回火的断裂韧性值,主要取决于马氏体的分解产物的性质和形态,而与挛晶马氏体是否存在无关。均匀分布的χ-Fe_2C的消失以及脆性的Fe_3C相的择优偏析,导致400℃退火钢的严重脆化。550℃回火后出现高温回火脆性,除了合金元素和合金碳化物在晶界富集外,晶内的微细沉淀相对滑动位错的钉扎,都对它的断裂韧性下降有贡献。回火温度进一步升高,沉淀相球化,基体回复,使断裂韧性再次上升。
Fine structure of tempered martensite of ultra-high strength steel 30CrMnSiNi2A was investigated under TEM. It was found that two kinds of martensite, the twinned type and the dislocated type were existed in the quenched state. Fracture toughness of this steel tempered below 400℃ depends primarily on the species and morphology of the products of martensite decomposition and has little concerned with the presence of twinned martensite. The severe embrittlement at 400℃ temper is attributed to the elimination of the homogeneously distributed χ-Fe_2C with the simultaneous preferred precipitation of the brittle Fe_3C phase. The high temperature temper brittleness at 550℃ is induced by the solute segregation(as alloyed carbide)along grain boundary as well as dislocation pinning by fine precipitates. Upon further increase in tempering temperature however, the precipitate phases are more or less spheroidized which, together with the matrix recovery, cause the fracture toughness to increase.
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