Acta Metall Sin  2019, Vol. 55 Issue (4): 496-510    DOI: 10.11900/0412.1961.2018.00191
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An Investigation of the Crystallography of Pearlites Nucleated on the Proeutectoid Cementite
Wensheng XU,Wenzheng ZHANG()
Key Laboratory of Advanced Materials MOE, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
Abstract

Pearlite is common microstructure in the carbon steel, which is widely applied in the railway steel and cold drawn steel where high wear resistance and strength are required. The pearlite colony is a circumscribed aggregate within which lamellae of cementite and ferrite phases have the same orientation. A cluster of wedge-shaped pearlite colonies will form the pearlite group nodules. The morphology of pearlite colonies will be influenced by the crystallography of pearlite. The common orientation relationship (OR) between pearlitic ferrite and pearlitic cementite is the Pitsch-Petch, Bagaryatsky, and Isaichev ORs. Combined with deep etching, SEM was used to investigate the morphology and crystallography of pearlite colonies and pearlite group nodules nucleated on the proeutectoid cementite in a Fe-1.29C-13.9Mn steel. The results showed that the initial morphology of the pearlite is irregular, but the pearlite possesses a parallel lamellar structure at the later stage of growth. Mutual ORs between phases of austenite, cementite, and ferrite in pearlite, proeutectoid grain boundary cementite, and Widmannstätten cementite were measured with the EBSD technique. Several reproducible ORs between cementite and ferrite lamellar have been observed, including the Pitsch-Petch, Bagaryatsky, and Isaichev ORs, without a particularly dominant OR. Since the two phases in the pearlite colonies have reproducible preferential OR, they are usually not independently nucleated, otherwise the independent nucleation of the cementite and ferrite inside the austenite has special crystallographic requirements for the mutual ORs between ferrite, cementite, and austenite. Thus, there will be a phase that nucleates first, which is called the "active nucleus". The active nucleus of pearlite has been carefully examined mainly according to the preferred OR between the pearlitic phases and existing phases. While the development of the pearlite crystallography is influenced by the active nucleus, no clear relationship was found between the ORs within the pearlite and active nucleus of the pearlite. The ORs between austenite and major pearlitic ferrite are near the K-S OR, but the ORs between austenite and the pearlitic cementite are various, depending on the preferred ORs between pearlitic ferrite and both austenite and pearlitic cementite. Widmannstätten cementite has never been seen to grow into pearlite. The measured data suggests that active nucleus of the pearlite colonies and pearlite group nodules nucleated on Widmannstätten cementite is ferrite. In some cases, grain boundary cementite was seen to grow as part of pearlite. Consequently, the grain boundary cementite is regarded as the active nucleus, though a preferred OR often coexists between pearlitic ferrite and either austenite or proeutectoid cementite. In other cases, the orientations of pearlitic cementite and grain-boundary cementite are discontinuous. For these cases, the ferrite is likely the active nucleus of pearlite. The orientation of pearlitic ferrite was seen to alter with the growth of pearlite, even causing the split of a single ferrite layer into two grain layers with a considerable misorientation. Significant distortion varying with the layers of pearlite was noticed in austenite near the pearlite growth front, indicating an evident strain field caused by the pearlite transformation. This requests a further investigation.

 ZTFLH: TG113
Fund: National Natural Science Foundation of China(No.51671111);National Key Research and development Program of China(No.2016YFB0701304)
Corresponding Authors:  Wenzheng ZHANG     E-mail:  zhangwz@tsinghua.edu.cn