40cr seamless steel pipe characteristics

40cr seamless steel pipe works to withstand strong impact loads and alternating loads
Because the working characteristic of 40cr seamless steel pipe is to withstand strong impact load and alternating load, its working environment is harsh. Therefore, bearing steel is required to have high and uniform hardness, high contact fatigue strength, good toughness and certain hardenability. Recently, a steel mill found cracks in individual GCr15 bearing steel samples in daily inspection. In order to find out the causes of cracks, the researchers conducted detailed analysis and discussion, which provided a theoretical basis for actual production.
The chemical composition of 40cr seamless steel pipe, the hot-rolled structure is fine-grained pearlite + carbide (Fe, Cr) 3C, generally requires spheroidizing annealing, quenching + low temperature tempering heat treatment, and the microstructure is fine cryptocrystalline after heat treatment. Martensite + cementite particles.
The 40cr seamless steel pipe crack defect samples were cut and sampled, polished by Bühler grinding and polishing machine, and the plane defects and cross-sections were observed by Japanese electronic JSM-6490 scanning electron microscope. The Oxford Instruments OXFORD was used to conduct cracks. Spectral composition analysis, using the Shimadzu EPMA-1720 electron probe to analyze the segregation of the micro-components in the crack and its vicinity, and then observe the change of the microstructure near the crack after corrosion with 4% nitric acid.
The metallurgical theory generally considers that the grain boundary is a strengthening element, that is, the bonding force of the grain boundary is higher than that of the crystal, and the edge brittle fracture occurs only when the grain boundary is weakened. Under normal circumstances, there are several reasons for the weakening of the grain boundary: First, the intergranular fracture caused by the grain boundary precipitation phase is caused by the inclusion of the grain boundary and the precipitation of the second phase. The precipitation phase on the grain boundary is usually not Continuous, spherical, rod-like or dendritic, sometimes covering more than 50% of the grain boundary area, the more precipitation phase at the grain boundary, the lower the fracture stress. Second, the impurity elements are segregated at the grain boundary to cause brittle fracture along the crystal. There are mainly the main group elements of the IV, V, and VI groups in the periodic table, such as Ge, P, As, Sb, Bi, S, Se, Te, etc. . The third is the intergranular fracture caused by the etching of the environmental medium, such as copper brittleness, cadmium brittleness and the like. The fourth is the intergranular fracture at high temperature, mainly including welding hot crack, grinding crack, creep fracture and the like.

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