材料期刊网

电镜观察发现4340加Si钢在320℃及420℃等温形成的贝氏体宽面上均存在巨型台阶,其高度a为320—850(?),宽度b为1500—1800(?),和根据贝氏体增长速率为0.006μm/s计算所得的b=0.5μm在同一数量级,这些台阶位于惯习面上,为贝氏体的长大台阶。 无论在320℃或420℃等温形成的贝氏体中均存在ε碳化物,ε和α之间的位向呈Jack关系。因此,Hehemann提出的在350℃附近存在γ→α+ε转变的看法自属疑问。等温较长时出现渗碳体,它位于γ和α之间。Fe_3C和γ之间具有Pitsch关系,Fe_3C和α之间为Isaichev关系,γ和α之间具有K-S关系,由此也可建立γ-α-ε或γ-α-Fe_3C之间的位向关系,但尚不能肯定碳化物系相间沉淀。 发现部分γ/α界面呈弯曲状伸展,表示非共格界面。鉴于存在长大台阶以及非共格界面,因此贝氏体的长大机制很难归之于共格切变,而很可能为台阶的伸长。

The existence of superledges on the broad faces of bainite formed isothermally at 320 and 420℃ has been observed with TEM on the specimens of a Si-modifled AISI 4340 steel. The height, α, and the width, b, of the superledges are about 320—850 and 1500—1800 A respectively. From the thickening rate of bainire, 0.006μm/s, the width of the superledge, b, is calculated to be approximately 0.5μm, this being of the same order of magnitude as width of the superledges observed. These ledges did exist at the habit plane and act as the growing superledges, ε-carbide and cementite were also found in bainite formed during 320 and 420℃ reaction. The orientation relationship between ε and α is the Jack relationship. Therefore, the existence of a eutectoid decomposition γ→α+ε at about 350℃, as suggested by Hehemann, is questionable. As the holding duration was prolonged there appeared cementite existing between γ and α phases. The orientation relationship between Fe_3C and γ is the Pitsch, Fe_3C and α-the Isaichev and γ/α is K-S relationship, from which the orientation relationship among γ-α- γ(or γ-α-Fe_3C) can also be established. However, it is uncertain whether or not the carbides are interphase boundary precipitates. The results have shown that part of the γ/α interface boundaries extended in a curve shape. The morphology of such interfaces and the existence of the growing superledges indicate that the growing interface is not a coherent one, so it is difficult to ascribe the growing mechanism of bainite to coherent shearing mechanism and it is more likely that bainite grows through the lengthening of the ledges.