40CR Seamless steel tube work is characterized by strong impact load and alternating load

40CR Seamless steel tube work is characterized by strong impact load and alternating load
 

40CR Seamless steel tube is characterized by strong impact load and alternating load, its working environment is poor, so the requirements of bearing steel with high and uniform hardness, high contact fatigue strength, good toughness and a certain degree of hardenability.

 

Recently, a steel mill in the daily inspection found that individual GCr15 bearing steel specimen cracks, in order to find out the cause of the crack, the researchers carried out a detailed analysis and discussion, to provide a theoretical basis for practical production.

40CR Seamless steel tube chemical composition, hot-rolled state microstructure for fine flake pearlite + carbide (FE,CR) 3C, generally need to be annealed by the ball, quenching + low-temperature tempering heat treatment, heat treatment after the microstructure of the fine crystal martensite + cementite particles. The 40CR seamless steel tube crack flaw specimen is cut sampling, polished by the standard music grinding and polishing machine, using the Japanese electron JSM-6490 scanning electron microscope to observe the crack defect in plane and cross section, and using the Oxford instrument Oxford to analyze the spectrum component of the crack,

 

The micro-element segregation analysis of the crack and its vicinity was carried out by using the electronic probe of the EPMA-1720 of Japan Island, and then the microstructure of the crack was further observed with 4% nitric acid alcohol corrosion. The theory of metallization usually holds that the grain boundary is a reinforcing element, that is, the bond force of the grain boundary is higher than that of the crystal, and only when the grain boundary is weakened will the brittle fracture occur along the crystal boundaries. In general, there are several reasons for the weakening of grain boundaries: first, the grain boundary precipitation facies caused by the intergranular rupture, is caused by the inclusions in the grain boundary and the second phase precipitation, the precipitation phase on the grain boundary is usually discontinuous, it is spherical, rod or dendritic, sometimes it can cover more than 50% grain boundary area, the more the grain boundary precipitates, the lower the fracture stress. Second, impurity elements in the grain boundary segregation caused along the crystal brittle fracture, the main elements of the periodic table Ⅳ, Ⅴ, Ⅵ family of the main elements, such as GE, P, as, Sb, Bi, S, Se, TE and so on. The third is the intergranular rupture caused by the environmental medium leaching, such as copper embrittlement and cadmium embrittlement. Four is the intergranular fracture at high temperature, including weld thermal crack, grinding crack, creep fracture and so on.