A brief discussion on the toughness of steel and the causes of fracture

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The main reasons are as follows: First, since a certain amount of one or more alloying elements must be added during the steel-making process, different microstructures can be obtained through simple heat treatment after the steel is formed, thereby altering its original properties； Second, defects arising during steelmaking and casting, particularly concentrated defects such as pores and inclusions, are extremely sensitive during rolling. Moreover, these defects can vary between different batches of steel with the same chemical composition, or even across different parts of the same steel billet, thereby affecting the quality of the steel. Since the toughness of steel mainly depends on its microstructure and the dispersion of defects (with a strict prevention of concentrated defects), rather than its chemical composition. Therefore, toughness changes significantly after heat treatment. To thoroughly understand the properties of steel and the reasons for its fracture, it is also necessary to master physical metallurgy as well as the relationship between microstructure and the toughness of steel. 1. Fracture of ferrite-pearlite steel: Ferrite-pearlite steel accounts for the vast majority of total steel production. They are typically alloys of iron and carbon with a carbon content ranging from 0.05% to 0.20%, along with small amounts of other alloying elements added to improve yield strength and toughness. The ferritic-pearlitic microstructure consists of BBC iron (ferrite), 0.01% C, soluble alloy, and Fe3C. In low-carbon steel with very low carbon content, cementite particles (carbides) remain at the boundaries and within the ferrite grains. However, when the carbon content is above 0.02%, the vast majority of Fe3C forms a flaky structure with some ferrite, known as pearlite, and tends to disperse as \"grains\" and spherules (grain boundary precipitates) within the ferritic matrix. In the microstructure of low-carbon steel with a carbon content of 0.10%–0.20%, the proportion of pearlite ranges from 10% to 25%. Although pearlite particles are very hard, they can be dispersed widely throughout the ferritic matrix and deform easily around the ferrite. Generally, the ferrite grain size decreases as the pearlite content increases. Because the formation and transformation of pearlite spheres hinder the growth of ferrite grains. Therefore, pearlite indirectly increases the tensile yield stress δy by raising d-1/2 (where d is the average grain diameter). From the perspective of fracture analysis, there are two carbon content ranges in low-carbon steel whose properties are of interest. First, the carbon content is below 0.03%, and the carbon exists in the form of pearlite spheres, having little impact on the toughness of the steel ； Second, when the carbon content is relatively high, it directly affects toughness and the Charpy curve in the form of pearlite. 2. Influence of the processing technique: Practice has shown that water-quenched steel exhibits better impact strength than annealed or normalized steel, as rapid cooling prevents the formation of cementite at the grain boundaries and promotes the refinement of ferrite grains. Many steel grades are sold in the hot-rolled state, and the rolling conditions have a significant impact on impact properties. A lower final rolling temperature reduces the impact transformation temperature, increases the cooling rate, and promotes the refinement of ferrite grains, thereby improving the toughness of the steel. Due to the slower cooling rate compared to thin plates, the ferrite grains in thick plates are coarser than those in thin plates. Therefore, under the same heat treatment conditions, thick plates are more brittle than thin plates. Therefore, normalizing treatment is commonly used after hot rolling to improve the properties of steel plates. Hot rolling can also produce anisotropic steels, as well as steels with various microstructures, pearlite bands, and grain boundaries containing inclusions, resulting in orientationally tough steels with the grain direction aligned. The pearlite bands and the elongated inclusions, which are heavily dispersed in a flaky manner, have a significant impact on the notch toughness at lower temperatures within the Charpy transition range. 3. Influence of ferrite and soluble alloying elements: The vast majority of alloying elements are added to low-carbon steel in order to produce solution-hardened steels that exhibit increased lattice friction stress δi at certain ambient temperatures. However, at present it is not possible to predict the lower yield stress using formulas alone, unless the grain size is known. Although the determining factors for yield stress are normalizing temperature and cooling rate, this research method remains important because it allows the range by which a single alloying element can reduce toughness to be predicted by increasing δi. There have been no reports to date on the regression analysis of the nil ductility transition (NDT) temperature and Charpy transition temperature in ferritic steels; however, these studies are also limited to qualitative discussions regarding the effect of adding a single alloying element on toughness. The following provides a brief introduction to the effects of several alloying elements on the properties of steel. 1) Manganese: The manganese content in the vast majority of cases is around 0.5%. Adding it as a deoxidizer or sulfur fixer can prevent thermal cracking of steel. It also has the following functions in low-carbon steel. ◆Steel with a carbon content of 0.05% shows a tendency to reduce the formation of cementite films at the grain boundaries after air cooling or furnace cooling. ◆It can slightly reduce the ferrite grain size. ◆It can produce a large number of fine pearlite particles. The first two effects indicate that the NDT temperature decreases as the manganese content increases, while the latter two effects cause the peak of the Charpy curve to become sharper. When the carbon content in steel is high, manganese can significantly reduce the transformation temperature by about 50%. The reason may be the large amount of pearlite, rather than the distribution of cementite at the boundaries. It should be noted that if the carbon content of the steel is higher than 0.15%, the manganese content plays a decisive role in the impact strength of normalized steel. This is because the high hardenability of steel causes austenite to transform into brittle upper bainite, rather than ferrite or pearlite. 2) Nickel, like manganese, plays a role in steel by improving the toughness of iron-carbon alloys. Its effectiveness depends on the carbon content and heat treatment. In steel with a very low carbon content (about 0.02%), an addition of up to 2% can prevent the formation of cementite at grain boundaries in hot-rolled and normalized steel; it also significantly lowers the onset transformation temperature TS and raises the peak value of the Charpy impact curve. Further increasing the nickel content reduces the effect of improving impact toughness. If the carbon content is low enough that no carbides remain after normalizing, the effect of nickel on the transformation temperature becomes very limited. The greatest advantage of adding nickel to normalized steel with about 0.10% carbon is the refinement of grain structure and the reduction of free nitrogen content, although the mechanism behind this is not yet clear. It may be because nickel acts as a stabilizer for austenite, thereby lowering the temperature at which austenite decomposes. 3) Phosphorus: In pure iron-phosphorus alloys, phosphorus segregation occurs at the ferrite grain boundaries, which reduces the tensile strength Rm and causes embrittlement between the grains. Furthermore, phosphorus is also a stabilizer for ferrite. Therefore, adding steel will **increase the δi value and the ferrite grain size. The combination of these effects makes phosphorus an extremely harmful embrittlement agent, leading to transgranular fracture. 4) Silicon: Silicon is added to steel for deoxidation, and it also helps to improve impact strength. If both manganese and aluminum are present in the steel, most of the silicon dissolves in the ferrite, thereby increasing δi through solid solution hardening. This effect, combined with the improvement in impact strength resulting from the addition of silicon, means that adding silicon in weight percent to iron-carbon alloys with a stable grain size raises the 50% transformation temperature by approximately 44°C. Furthermore, similar to phosphorus, silicon is a stabilizer for ferritic iron and can promote the growth of ferrite grains. By weight percentage, the addition of silicon to normalized steel increases the average energy conversion temperature by about 60°C. 5) Aluminum is added to steel in the form of alloys and deoxidizers for two reasons: first, it forms AlN with nitrogen in the melt, thereby removing free nitrogen ； Second, the formation of AlN refined the ferrite grains. As a result of these two effects, an increase of 0.1% in aluminum lowers the transition temperature by about 40°C. However, when the amount of aluminum added exceeds what is necessary, the effect of \"fixing\" free nitrogen weakens. 6) Oxygen: Oxygen in steel causes segregation at grain boundaries, leading to intergranular fracture in ferroalloys. When the oxygen content in steel is as high as 0.01%, fracture occurs along the continuous pathways formed at the grain boundaries of the embrittled grains. Even at low oxygen levels in steel, it causes cracks to nucleate at grain boundaries and then propagate through the grains. To address the problem of oxygen embrittlement, deoxidizing elements such as carbon, manganese, silicon, aluminum, and zircon can be added; these elements combine with oxygen to form oxide particles, thereby removing oxygen from the grain boundaries. Oxide particles are also beneficial substances for delaying ferrite growth and increasing d-/2. 4. Influence of carbon content between 0.3% and 0.8%: In hypoeutectoid steels with a carbon content of 0.3% to 0.8%, pro-eutectoid ferrite is the continuous phase and forms first at the austenite grain boundaries. Pearlite forms within the austenite grains, accounting for 35% to 100% of the microstructure. Furthermore, various aggregates form within each austenite grain, making the pearlite a polycrystal. Since the strength of pearlite is higher than that of pro-eutectoid ferrite, it restricts the flow of ferrite, thereby causing the yield strength and strain hardening rate of the steel to increase as the carbon content in pearlite increases. The restraining effect increases with the number of hardened patches, and pearlite enhances the refinement of the pro-eutectoid grain size. When there is a large amount of pearlite in steel, micro-cleavage cracks form during deformation at low temperatures and/or high strain rates. Although there are also some internal aggregated tissue sections, the fracture path initially follows the cleavage planes. Therefore, there are certain preferred orientations among the ferrite plates and within the ferrite grains in the adjacent aggregated structures.