The fine structure of M/A islands in pipeline steel X70 has been studied by transmission electron microscope (TEM). It is shown that the M/A islands are about 1-2 μm in size and distribute at the grain boundary of ir...The fine structure of M/A islands in pipeline steel X70 has been studied by transmission electron microscope (TEM). It is shown that the M/A islands are about 1-2 μm in size and distribute at the grain boundary of irregular massive ferrite or acicular ferrite in the microstructure of the steel undergoing TMCP (thermo-mechanical controlled processing). The analysis of diffraction contrast shows that the M/A islands consist of retained austenite and some martensite lamellae different in size and orientation. The microt winning and midrib exist in the lamella of lenticular martensite, which exhibit the typical character of high-carbon martensite. The influences of TMCP parameters on M/A islands have been studied carefully. With the increase of the cooling speed, the amount of M/A islands decrease slightly and the morphology of M/A islands changes to thin dispersive short bars from thick irregular long strips.展开更多
The secondary hardening reaction is accompanied with precipitation of fine carbides in high CoNi ultrahigh strength steel. The crystal structure of the precipitating carbides is unambiguously determined by microbeam d...The secondary hardening reaction is accompanied with precipitation of fine carbides in high CoNi ultrahigh strength steel. The crystal structure of the precipitating carbides is unambiguously determined by microbeam diffraction in transmission electron microscopy. It is identified that the needle-shaped carbides are M2C with a hexagonal structure. The concentration of substitutional alloying elements in the carbides quantified by energy dispersive X-ray spectroscopy (EDS) also supports the result above. The spatial structure of M2C is identical with L'3 type. Metal atoms are in a close packed hexagonal structure, the carbon atoms partly distribute with random in the octahedral interstices and the filling probability is less than 1/2. Particular attention was paid to the relationship of needle-shaped carbides/ferrite matrix at secondary hardening peak tempered at 482癈 for 5 h. Observation by high resolution transmission electron microscopy (HRTEM) confirms that carbides with black-white contrast are fully coherent with ferrite and have individual crystal structure, and the coarsened carbides with moire' fringe are partially coherent with matrix. The orientation relationship between M2C and ferrite matrix is directly observed and identified with the well established P-S relationship [001]a //[111]c.展开更多
文摘The fine structure of M/A islands in pipeline steel X70 has been studied by transmission electron microscope (TEM). It is shown that the M/A islands are about 1-2 μm in size and distribute at the grain boundary of irregular massive ferrite or acicular ferrite in the microstructure of the steel undergoing TMCP (thermo-mechanical controlled processing). The analysis of diffraction contrast shows that the M/A islands consist of retained austenite and some martensite lamellae different in size and orientation. The microt winning and midrib exist in the lamella of lenticular martensite, which exhibit the typical character of high-carbon martensite. The influences of TMCP parameters on M/A islands have been studied carefully. With the increase of the cooling speed, the amount of M/A islands decrease slightly and the morphology of M/A islands changes to thin dispersive short bars from thick irregular long strips.
文摘The secondary hardening reaction is accompanied with precipitation of fine carbides in high CoNi ultrahigh strength steel. The crystal structure of the precipitating carbides is unambiguously determined by microbeam diffraction in transmission electron microscopy. It is identified that the needle-shaped carbides are M2C with a hexagonal structure. The concentration of substitutional alloying elements in the carbides quantified by energy dispersive X-ray spectroscopy (EDS) also supports the result above. The spatial structure of M2C is identical with L'3 type. Metal atoms are in a close packed hexagonal structure, the carbon atoms partly distribute with random in the octahedral interstices and the filling probability is less than 1/2. Particular attention was paid to the relationship of needle-shaped carbides/ferrite matrix at secondary hardening peak tempered at 482癈 for 5 h. Observation by high resolution transmission electron microscopy (HRTEM) confirms that carbides with black-white contrast are fully coherent with ferrite and have individual crystal structure, and the coarsened carbides with moire' fringe are partially coherent with matrix. The orientation relationship between M2C and ferrite matrix is directly observed and identified with the well established P-S relationship [001]a //[111]c.
