Analysis of the toughness of steel and the causes of fracture

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There are thousands of different types of steel used in various industries. Each type of steel has a different trade name depending on its properties, chemical composition, or the type and amount of alloys it contains. Although fracture toughness values **facilitate the selection of each type of steel, these parameters are difficult to apply to all steel materials. The main reasons are as follows: First, a certain amount of one or more alloying elements must be added during the steel-making process; after the steel is formed, simple heat treatment can be applied to obtain different microstructures, thereby altering the original properties of the steel ； Second, defects arising during the steelmaking and casting processes, especially concentrated defects such as pores and inclusions, are highly 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. The toughness of steel depends mainly on its microstructure and the degree of dispersion of defects (with a strict avoidance 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 grasp 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-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 ferrite-pearlite microstructure consists of BBC iron (ferrite), 0.01%C, soluble alloys, and Fe3C. In carbon steels 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 quite widely throughout the ferritic matrix and deform easily around the ferrite. Generally, the grain size of ferrite 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 ranges of carbon content in low-carbon steels 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 ； Secondly, when the carbon content is high, it directly affects toughness and the Charpy curve in the form of spherulites. 2. Influence of the processing technique: Practical experience shows that water-quenched steel has 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 enhancing 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 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 orientation aligned with the rolling direction. The pearlite bands and the elongated inclusions, which are widely dispersed in a flaky shape, 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 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 are no reports to date on the regression analysis of the non-plastic transformation (NDT) temperature and Charpy transformation temperature in ferritic steels; moreover, such discussions have been limited to a qualitative analysis of the effect of adding single alloying elements on toughness. The following provides a brief introduction to the effects of several alloying elements on the properties of steel. 1) Manganese: The vast majority has a manganese content of about 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 small 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 level of 2% is sufficient to prevent the formation of cementite at the grain boundaries in the hot-rolled and normalized states; simultaneously, it 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 remains unclear. 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 at the ferrite grain boundaries reduces the tensile strength Rm and causes brittleness 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 embrittling agent, leading to intergranular 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 steel, most of the silicon dissolves in ferrite, while δi is increased through solution hardening. This effect, combined with the improvement in impact strength brought about by the addition of silicon, results in an increase of approximately 44°C in the 50% transformation temperature when silicon is added to iron-carbon alloys with a stable grain size, on a weight percentage basis. 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 refines the ferrite grains. As a result of these two effects, an increase of 0.1% in aluminum reduces 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 reaches 0.01%, fracture occurs along continuous pathways formed at the grain boundaries of 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. Effects of a carbon content ranging from 0.3% to 0.8%: In hypoeutectoid steels, the carbon content is between 0.3% and 0.8%. Proeutectoid ferrite serves as the continuous phase and forms first at the boundaries of austenite grains. Pearlite forms within austenitic grains and constitutes 35% to 100% of the microstructure. Furthermore, various aggregates form within each austenite grain, turning the pearlite into a polycrystal. Since the strength of pearlite is higher than that of proeutectoid ferrite, it restricts the flow of ferrite; consequently, the yield strength and strain hardening rate of steel increase with the increase in the carbon content of pearlite. The restraining effect increases with the number of hardened masses, 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 aggregate tissue sections, the fracture path initially follows the cleavage planes. Therefore, there are certain preferred orientations within the ferrite grains in the spaces between the ferrite lamellae and in the adjacent aggregated structures.