20CrMo 活塞缸体的淬火裂纹分析

The causes of cracks in the 20CrMo piston cylinder after carburizing and quenching treatment were analyzed using chemical composition testing, microstructure and crack morphology observation, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) . The results show that the crack nature of the 20CrMo piston cylinder is quenching crack, and two types of quenching cracks appear simultaneously in the cylinder, namely large transgranular cracks and micro intergranular cracks. The large transgranular cracks are mainly distributed at the step where the diameter of the borehole suddenly changes, and the crack fracture surface shows a transgranular cleavage morphology. The micro cracks are mainly distributed on the inner wall surface of the cylinder bore, characterized by propagation along austenite grain boundaries, sharp and fine tail ends, accompanied by secondary cracks along the grain, and there is no oxidation or decarburization phenomenon in the microstructure on both sides of the crack. The comprehensive analysis suggests that the excessive carbon content and the surface carburizing process increase the quenching tendency of the cylinder. The sudden change in the inner diameter of the cylinder hole is prone to stress concentration, and the occurrence of large quenching cracks is related to the rapid quenching cooling rate. Excessive austenitization temperature can easily lead to the weakening of grain boundaries and induce quenching intergranular microcracks on the inner wall of the cylinder under the action of microstructural stress.