(I) Relationship between fracture toughness and conventional mechanical properties
Fracture toughness is a comprehensive representation of the strength and plasticity of a material. Whether it is in the occurrence of ductile fracture or in the occurrence of brittle fracture, it is closely related to the strength index and plasticity index of the material.
In the conventional mechanical properties, the impact toughness of the notched specimens is the toughness index of the steel, which characterizes the level of the comprehensive mechanical properties of the steel. Naturally, there must be a closer relationship between steel and its impact toughness.
(II) Effect of metallurgical factors on fracture toughness of steel
The gas in the steel has a bad influence on the fracture toughness of the steel. Therefore, the forgings produced by vacuum melting, vacuum casting, vacuum degassing and the like have high fracture toughness. In addition, the process measures that can reduce the sulfur and phosphorus content of harmful impurities in the steel, reduce the amount of inclusions in the steel, and make the particles smaller and rounder can also increase the fracture toughness of the steel. From this point of view, the addition of rare earth elements, the argon agitation of molten steel before pouring, especially the application of ultra-pure steel, will certainly increase the fracture toughness of large forgings.
(III) Effect of heat treatment process and microstructure on fracture toughness of steel
1. The impact of the heat treatment system. After the low-carbon steel is completely quenched and then tempered at 200-250 °C for low temperature, the value of steel is the highest; for medium carbon steel, after quenching and then tempering at -600 °C, the steel has the highest value. Therefore, for welded forgings made of low carbon alloy steel, the temperature after post-weld annealing and the tempering temperature at the final heat treatment should be as low as possible. On the contrary, for the medium carbon alloy steel forgings, the final heat treatment process of the final heat treatment should be high temperature tempering.
2. Effect of grain size on fracture toughness of steel. Refining the grain is an important method to improve the comprehensive mechanical properties of the steel, and it should make a great contribution to the improvement of the fracture toughness of steel. However, if the plasticity of the steel is very good and the fracture belongs to the dimple fracture, the contribution of grain refinement will be relatively small. Because of this, it is the amount of energy that is consumed by the plastic deformation work when the crack propagates. In this regard, grain refinement has little contribution, so the impact is minimal. If the plasticity of the steel is poor or not high and belongs to cleavage or quasi-cleavage fracture, grain refinement will have a great influence on the fracture toughness of the steel. Because, when a crack propagates from one grain to another, it must pass through the grain boundary with a complex dislocation structure, which consumes more energy; once it passes through the grain boundary, the crack will develop along the cleavage plane, and the resistance Smaller. Therefore, for most large forgings, refining the grain is an important way to improve the fracture toughness of the forging material.
3. Effect of carbide particles on the fracture toughness of steel. Carbide particles in steel often become a source of cracking when steel breaks.