Defective Carburizing Layer Related To Internal Oxidation And Its Influence On Service Properties

In this paper,two kinds of defective carburized layer related to the internal oxidation in the carburizing process were found in the cracked big-gear and plunger-sleeves carburized,which are different from the traditional internal oxidation texture appearing in the carbonitrided or carburized layer.The microstructure features and formation mechanism of the special internal oxidation textures were investigated by SEM,OPM and EDAX The main results were concluded as follows:1.Internal oxidation texture in the cracked big-gear(1) These black mesh is not identical to the traditional grain-boundary internal oxidation. Accompanied by emergence of a gray block or strip oxide organizations,in addition to the formation of non-martensite,the most important thing is that most of the grain boundary region cracking,there has been continuous or intermittent along the distribution of crack crystal mesh,from the apparent morphology point view,this shortcoming organizations impact on performance even more serious than black organizations caused by intemal oxidation in carbonitriding.(2) Elastic modulus of oxides along the grain is different from the elastic modulus of the matrix,in the carburized layer under tensile stress,lack of coordination between the two deformed mesh will result in cracks along the grain.In addition,divorced oxides take a large number of cracks along the grain.(3) The defect organizations changes before and after shot analysis shows that shot peening treatment as an effective way to eliminate or reduce oxidation defects in carburized layer,make no significant effect on net cracks caused by internal oxidation.2.Internal oxidation texture in the cracked plunger-sleeve(1) The edge of the plunger-sleeve hole,and the plunger-sleeve shaft at the junction of both axial appear "spear-like" pits or "spear-like" blunt crack.(2) Main crack of cracking plunger-sleeve hole are the top and bottom of the "spear-like" blunt crack tip for the fatigue crack source and along the opposite axial expansion,which eventually led to the plunger-sleeve failure.(3) "spear-like" blunt cracks in the surface layer of the formation of cementite,and along the elongated extension of MnS inclusions,the formation of "Spear-like" blunt cracks is relate to gas carburizing process. (4) Mn and oxygen have a great affinity,in gas carburizing process,the oxygen and plunger-sleeve of axial surface MnS inclusions oxidation reaction occurs,so that MnS inclusions separated from the matrix→divorce→forming holes(spear-shaped crack)(5) Less impact on the performance of not exceeding the MnS inclusions in the aerobic environment carburizing(gas carburizing) may occur "special" internal oxidation, making non-metallic inclusions in an effective increase in size,resulting in abnormal failure workpiece...