| In order to study the influence of remanufacturing techniques on the performance and life of carburized gears and other components in automobiles, a 20MnCr5 gear and a 20CrMnTi carburized cylinder were cut into small samples and kept at 350℃for 20 min for once, three or five times, or kept at 500℃for 20min for once, three or five times. Optical microscope, scanning electron microscope, transmission electron microscope (TEM) and microhardness tester were used to characterize their microstructure and properties. Electron backscattered diffraction techniques (EBSD) were applied to study the crystallography of martensite of the 20MnCr5 gear and 20CrMnTi carburized cylinder samples. The results are as follows:(1) The microstructures in the surface layers of the 20MnCr5 gear and the 20CrMnTi carburized cylinder were composed of retained austenite and plate martensite. With the increase of distance from the surface, the concerntration of retained austenite decreased and the shape of martensite became both plate and lath. The microstructures in the center were mainly lath martensite, with carbides dispersed. In the 20MnCr5 gear, the peak hardness (850 HV) lay in the area 0.3-0.4 mm away from the surface and the thickness of effective carburized layer was 1.4 mm. In the 20CrMnTi carburized cylinder, the peak hardness (720 HV) lay in the area 0.2～0.3 mm away from the surface and the thickness of effective carburized layer was 1.2 mm.(2) The microstructure of the samples heat-treated at 350℃/20min and 500℃/20min changed obviously:Retained austenite decomposed, carbon supersaturated in martensite precipitated and carbide formed, martensite were partitially transformed to polygonal ferrite; the hardness of both the carburized layer and the center decreased.These changes are harmfull for the performances of the gears and carburized components.(3) The TEM results of 20MnCr5 gear samples showed that there wereχ-Fe5C2 carbides with relatively large dimensions dispersed in the microstructure of the original sample. Acicular carbides with small dimension were formed in the sample heated at 350℃/20min for three times. In the sample heated at 500℃/20min for three times, orthrombic 0-Fe3C carbides that kept certain orientation relationship (OR) with a-Fe formed. Though with small dimension, the 0-Fe3C carbides started to spheroidize.(4) A method for fitting{011}αpole figures of martensite from EBSD results was designed. By fitting the {011}αpole figures of martensite in 20MnCr5 and 20CrMnTi steel, the OR of martensite with its prior austenite grain was proved to follow both K-S and N-W ORs, and predominantly N-W OR. This was further proved by the discrepancy between practical and theoretical misorientation distribution. The discrepancy was explained by strain accommodation mechanism. A kind of (011)/54.7°lath boundary, which can only be formed by adjacent K-S OR lath and N-W OR lath, was proved to exist in 20MnCr5 steel martensite. This result also proved the coexistence of K-S OR and N-W OR in the martensite.(5) The crystallography of tempered martensite in 20MnCr5 steel heated at 350℃/20min and 500℃/20min were examined by EBSD. The results proved that the gray scale of the image quality map decreased, which indicated that the density of defects and the extent of distortion decreased. The length of grain boundaries with different misorientation in the scanned area of the samples decreased with the increase of time and temperature. In the scanned area of the heated samples both of 20MnCr5 and 20CrMnTi steel, the orientation of martensite distributed randomly. By selecting one prior austenite grain in each of the orientation image maps, the {011}αpole figures were drawn and fitted. The results showed that the ORs of martensite in the selected areas all followed the coexistence of K-S and N-W ORs, but with some variants absent. The heat treatment with higher temperature caused the orientation of martensite grains rotated to some extent, which caused most areas in the heated samples do not follow K-S and N-W OR accuratly. |
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