Microstructure Size Control And Phase Transition Conditions Of Skeleton Of Steel Wire

With the improvement of the production technology of steel wire, the strength level of the wire is gradually increasing. And the damage of the existence of material inner defects or microcrack on the service process and the subsequent processing is taken more seriously. Fracture toughness, which is viewed as the typical property index for the resistance capability to crack instability propagation, is also getting popular. Meanwhile the research for the fracture toughness is important to the study of crack growth path and the connection between crack growth capability and the microstructure. This thesis focused on pearlitic steel and different organizations were obtained by related treatment methods and then investigated the effect of the microstructure on the properties of the materials. The mechanical property, quasi-static fracture toughness(J0.2BL) and impact toughness test were measured. The microstructure, the path of crack propagation and the appearance of fracture were observed using SEM. Moreover using related theories, we built a relational model between microstructure and the fracture toughness of pearlitic steel. In addition, based on the relational noderl, we analyzed the fracture toughness of pearlitic steel with different microstructure. The result showed that this model is reasonable.According to the research results, we found that the temperature of salt bath is the major controlling factor to the size of pearlite lamellar spacing. The size of pearlite lamellar spacing gradually increased with the increase of the temperature of salty bath. Beside considering the other influencing factors, the fracture toughness of pearlitic steel decreased with the increase of the size of pearlite lamellar spacing. The austenitizing temperature mainly influenced the grain size of material. With the decrease of austenitizing temperature, the grain size turned smaller, the size of pearlite colony also turned smaller. The smaller pearlite colony made the crack growth path more circuitous, which increased the resistance of crack growth and improved the fracture toughness of material.The existence of small amount of proeutectoid ferrite could effectively reduce the crack-tip stress and the speed of crack growth. But amount of proeutectoid ferrite would obviously reduce the integral strength of material and had negative effect on the improvement of fracture toughness. Thus proper amount of proeutectoid ferrite is benefit to the improvement of fracture toughness.The relational model between the microstructure and fracture toughness of pearlitic steel is well consistent with the experimental results. By using this model, we can conveniently forecast the changing trend of fracture toughness of pearlitic steel with the changing of its microstrusure...