材料期刊网

低碳马氏体形成时,碳由马氏体扩散到周围奥氏体。使奥氏体由0.27％富碳至1.04％所需的时间,经计算仅为10~(-7)s数量级,证明碳原子的扩散跟得上条状马氏体的形成。由热力学计算,可以合理地认为碳由马氏体脱溶使奥氏体富碳。经过透射电镜观察,0.12C-低Ni-Cr钢的淬火组织主要为条状马氏体及条间残余奥氏体,也存在挛晶马氏体。后者进一步证明,在低碳马氏体形成时碳的扩散使奥氏体富碳,在有些富碳不太高的母相区域就形成这类组织。奥氏体和马氏体之间的界面为较平直的界面。同一钢的贝氏体组织具有正常上贝氏体(B_Ⅱ)、B_Ⅲ型贝氏体和无碳化物贝氏体(B_Ⅰ)。奥氏体和贝氏体铁素体之间的界面上存在巨型台阶,与奥氏体和马氏体之间的界面具有很大的差别。又从动力学观点考虑,低碳马氏体的长大速率与上贝氏体的伸长速率相差达3—4个数量级,因此认为低碳马氏体的形成和贝氏体具有不同的机制。

The time required for carbon diffusion from martensite to enrich surrounding austenite from 0.27 to 1.04% C during the formation of low-carbon matensite is 10~(-7)s in order of magnitude as calculated. It does prove that the diffusion of carbon atoms can keep pace with the formation of lath martensite. From thermodynamical calculation, it is reasonable to recognize that the precipitation of carbon from martensite results the enrichment of austenite. TEM observation revealed that the quenched structure in a 0.12C-low Ni-Cr steel mainly contains lath martensite and interlath retained austenite, and also twin martensite. The existence of the latter further confirms the occurrence of carbon diffusion to enrich austenite during the martensite formation, and twin martensite forms at the parent phase where carbon enrichment is not very high. The interface of austenite and martensite is somewhat straight. The typical upper bainite (B_Ⅱ), B_Ⅲ type bainite and carbide-free bainite (B_1) can aU appear in the ame steel and there exists superledges at the interface of austenite and bainitie ferrite which is quite different to that of austenite and martensite. In addition, from the kinetics point of view. the growth rate of low-carbon martensite is 3 to 4 order of magnitude greater than that of upper bainite. It is more likely to conclude that the mechanism of the formation of lath martensite is not identical with that of bainite.