There are thousands of varieties of steel pipes used in various industries. Each steel pipe has a different trade name due to different properties, chemical composition, or alloy type and content. Although the fracture toughness value greatly facilitates the selection of each steel, these parameters are difficult to apply to all steel.
The main reasons are:
First, because a certain amount of one or more alloying elements need to be added during the smelting of steel, different microstructures can be obtained after simple heat treatment after the material is formed, thus changing the original properties of the steel;
Second, because the defects generated during steelmaking and casting, especially concentrated defects (such as pores, inclusions, etc.) are extremely sensitive during rolling, and different changes occur between different furnaces of steel with the same chemical composition, and even in different parts of the same billet, thus affecting the quality of the steel.
The toughness of steel mainly depends on the dispersion of microstructure and defects (strictly prevent concentrated defects), rather than a chemical composition. Therefore, the toughness will change greatly after heat treatment. To deeply explore the properties of steel and the causes of its fracture, it is also necessary to master the relationship between physical metallurgy and microstructure and steel toughness.
1. Fracture of ferrite-pearlite steel
Ferrite-pearlite steel accounts for the vast majority of total steel production. They are usually iron-carbon alloys with carbon content between 0.05% and 0.20% and other small amounts of alloying elements added to improve yield strength and toughness. First, when the carbon content is below 0.03%, carbon exists in the form of pearlite nodules, which has little effect on the toughness of the steel; second, when the carbon content is higher, it directly affects the toughness and Charpy curve in the form of spherulites.
2. Effect of treatment process
It is known in practice that the impact properties of water-quenched steel are better than those of annealed or normalized steel because rapid cooling prevents cementite from forming at grain boundaries and promotes the refinement of ferrite grains.
Many steels are sold in the hot-rolled state, and rolling conditions have a great influence on the impact properties. A lower final rolling temperature will reduce the impact transition temperature, increase the cooling rate, and promote the refinement of ferrite grains, thereby improving the toughness of the steel. Because the cooling rate of thick plates is slower than that of thin plates, the ferrite grains are coarser than those of thin plates. Therefore, under the same heat treatment conditions, thick plates are more brittle than thin plates. Therefore, normalizing treatment is often used after hot rolling to improve the properties of steel plates.
Hot rolling can also produce anisotropic steels and various mixed structures, pearlite bands, and directional toughness steels with the inclusion of grain boundaries consistent with the rolling direction. Pearlite bands and elongated inclusions are coarse and dispersed into scales, which have a great influence on the notch toughness at low temperatures in the Charpy transformation temperature range.
3. The influence of ferrite-soluble alloying elements
Most alloying elements are added to low carbon steel to produce solid solution hardening steel at certain ambient temperatures and increase the lattice friction stress δi. However, it is not possible to predict the lower yield stress using only the formula unless the grain size is known. Although the determining factors of yield stress are normalizing temperature and cooling rate, this research method is still important because it can predict the range in which a single alloying element can reduce toughness by increasing δi. There is no report on the regression analysis of the non-ductile transition (NDT) temperature and the Charpy transition temperature of ferritic steel, and these are limited to the qualitative discussion of the effect of adding a single alloying element on toughness. The following is a brief introduction to the effects of several alloying elements on steel properties.
Post time: Oct-30-2024