马氏体/贝氏体复相灰铸铁组织及形核特点

Phase transformation and nucleation of martensite/bainite dual-phase grey cast iron

研究了Ni、Mo微合金化灰铸铁在快速冷却条件下凝固获得片状石墨细小、组织细密的珠光体基体,分别经860℃、880℃、900℃、920℃奥氏体化后油淬,获得一定比例的马氏体/贝氏体（M/B）复相基体以及部分残留奥氏体。经X射线衍射和扫描电镜分析贝氏体、马氏体针状组织间和共晶团边界处存在两种不同成分的残留奥氏体（后者称之为白亮区）。在860℃进行5-6s短时淬冷后,能够观察到在Ni-Mo灰铸铁金相组织中有贝氏体存在,且形核位置最先在片状石墨与奥氏体界面处。在150℃和250℃回火后,部分白亮区转变为马氏体并且体积缩小,但很难完全消除。

Under condition of cast chill and microalloying with Ni and Mo elements, as-cast structure of small flake graphites and fine pearlite matrix were gotten. Subsequently, the samples were respectively austenized at 860 ℃ ,880 ℃ ,900℃ ,920℃ and oil quenched, then some martensite/bainite （M/B） dual-phase with some retained austenite microstructnre was obtained. Through analysis of XRD and SEM, two kinds of retained austenite were found, which mainly exist in the gap of M/B acicular structure and at the boundaries of eutectic cells （ the latter kind of retained austenite is named for whitebright zone）. The bainite nucleation could be observed from the metallurgical structure of Ni-Mo grey cast iron austenized at 860 ℃ subsequent with short oil-quenching for 5 - 6 s. The results show that the first nucleation site of bainite is the interface between flake graphite and austenite. Tempered at 150 ℃ and 250 ℃ respectively, part of the whitebright zones are transformed into martensite, and the size of white-bright zone is smaller than before , but it is unable to remove the white-bright zones completely.