Microstructural characterization of API 5L X65 and X70 steels manufactured by TMCP process

This work shows the microstructural characterization of API-5L X65 and X70 steels manufactured by the TMCP process. Images are obtained by optical microscopy (OM), scanning electron microscopy (FEG-SEM) and atomic force microscopy (AFM). Besides, the EBSD technique pointed out that both samples are presenting a refined quasi-polygonal ferrite matrix with eutectoid aggregates in the contours and vertices of ferritic grains. Moreover, a grain size of 8.4 μm is estimated for X70 and 10.6 μm for X65 steel. In the AFM images, eutectoid aggregates displayed higher relief concerning the ferrite matrix, and these microconstituents’ higher hardness causes them. This behavior is in agreement with the results of the Vickers Hardness test. EBSD showed that the quantity of microphases in the X65 is slightly higher than in the X70. However, the X70 steel presented higher high and low angle lengths due to the greater refinement and stronger cooling rates applied during processing. The Vickers test showed that the ferrite hardness is similar for both steels. This same behavior is verified for the Vickers test in eutectoid aggregates. It indicates that the higher strength of X70 sample is mainly a consequence of the finer microstructure.