Basic analysis of steel fracture

Thread Content

The main reasons are as follows: First, since a certain amount of one or more alloying elements must be added during the steel manufacturing process, simple heat treatment after the steel is formed can result in different microstructures, thereby altering the original properties of the steel； Second, defects arising during steelmaking and casting, particularly 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 primarily on its microstructure and the degree of dispersion of defects (with a strict avoidance of concentrated defects), rather than on 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. 01 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 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 certain amounts of ferrite, which is known as pearlite; it also tends to exist as \"grains\" and spherical aggregates (grain boundary precipitates) dispersed 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. Typically, 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 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 ； Secondly, at higher carbon contents, it directly affects toughness and the Charpy curve in the form of spherulites. 02 Influence of processing techniques: 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 strength. 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 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 widely dispersed in a flaky shape, have a significant impact on the notch toughness at lower temperatures within the Charpy transition range. 03 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 of 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 individual 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 have a manganese content of about 0.5%. Added as a deoxidizer or sulfur fixer, it prevents hot cracking in steel; in low-carbon steel, it also has the following functions. 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 the austenite to transform into brittle upper bainite, rather than ferrite or pearlite. 2) Nickel. Its role in steel is similar to that of manganese, as it can improve 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 at this point 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 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 at the ferrite grain boundaries reduces the tensile strength Rm, leading to 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 steel, most of the silicon dissolves in ferrite, while δi is increased through solution hardening. This effect, combined with that of silicon addition on improving impact strength, results in the addition of silicon by weight percent to iron-carbon alloys with a stable grain size raising 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 percent, the addition of silicon to normalized steel increases the average energy conversion temperature by about 60°C. 5) Aluminum. The addition of alloys and deoxidizers to steel is based on two reasons: first, to form 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 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 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. 04 Effect of carbon content between 0.3% and 0.8%: In hypoeutectoid steel, where the carbon content is between 0.3% and 0.8%, pro-eutectoid ferrite acts as a continuous phase and forms first at the boundaries of the austenite grains. Pearlite forms within the austenite grains, accounting for 35% to 100% of the microstructure. Furthermore, various aggregates are formed 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 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 among the ferrite plates and within the ferrite grains in the adjacent aggregated structures. 05 Fracture of bainitic steel: Adding 0.05% molybdenum and boron to low-carbon steel with a carbon content of 0.10% can optimize the austenite-ferrite transformation, which typically occurs at 700–850°C, without affecting the kinetic conditions of the austenite-bainite transformation that takes place at 450°C and 675°C. Bainite formed between approximately 525–675°C is commonly referred to as \"upper bainite\"” ； The one formed between 450 and 525°C is called \"lower bainite\". Both tissues are composed of acicular ferrite and dispersed carbides. When the transformation temperature is reduced from 675°C to 450°C, the tensile strength of untempered bainite increases from 585 MPa to 1170 MPa. Because the transition temperature is determined by the content of alloying elements, which indirectly affects yield and tensile strength. The high strength achieved by these steels is the result of two effects: 1) As the transformation temperature decreases, the size of the bainite-ferrite lamellae continues to decrease. 2) Fine carbides continue to disperse within the lower bainite. The fracture characteristics of these steels depend to a large extent on tensile strength and transformation temperature. Two effects need to be noted: First, there is a certain level of tensile strength, and the Charpy impact performance of bainite after tempering is far superior to that of upper bainite without tempering. The reason is that in upper bainite, the cleavage planes within spherulites cut through several bainite grains, and the main dimension determining fracture is the size of the austenite grains. In lower bainite, the cleavage planes within the acicular ferrite are not aligned in a straight line; therefore, the main factor determining whether the quasi-cleavage fracture surface fractures is the grain size of the acicular ferrite. Because the grain size of the acicular ferrite here is only 1/2 of the grain size of the austenite in upper bainite. Therefore, at the same strength level, the lower bainite transformation temperature is much lower than that of upper bainite. In addition to the reasons mentioned above, there is the distribution of carbides. In upper bainite, carbides are located along the grain boundaries, increasing brittleness by reducing the tensile strength Rm. In the tempered lower bainite, carbonites are distributed very uniformly within the ferrite, and at the same time, cleavage cracks are restricted to improve tensile strength and promote the refinement of spheroidized pearlite. Second, attention should be paid to the changes in transformation temperature and tensile strength in untempered alloys. In upper bainite, a decrease in the transformation temperature leads to a reduction in the size of acicular ferrite while increasing the tensile strength Rp0.2. In lower bainite, to achieve a tensile strength of 830 MPa or higher, it is also possible to increase the strength by reducing the transformation temperature. However, since the fracture stress of upper bainite depends on the austenite grain size, and the size of the carbide particles is already large at this point, the effect of tempering in increasing tensile strength is minimal. 06 Fracture of martensitic steel: The addition of carbon or other elements to steel can delay the transformation of austenite into ferrite, pearlite, or bainite. Meanwhile, if cooling is fast enough after austenitization, austenite can be converted into martensite through a shear process, without the need for atomic diffusion. Ideal martensitic fracture should possess the following characteristics: because the transformation temperature is very low (200°C or lower), the tetrahedral ferrite or acicular martensite is extremely fine. Because the transformation occurs through shearing, the carbon atoms in austenite do not have time to diffuse out of the crystal, which saturates the carbon atoms in ferrite and causes the martensite grains to elongate, leading to lattice expansion. The martensitic transformation requires exceeding a certain temperature range, as the initially formed martensite plates increase the resistance to subsequent austenite transformation into martensite. Therefore, the transformed structure is a mixture of martensite and residual austenite. To ensure stable steel properties, tempering is necessary. High-carbon (above 0.3%) martensite, when tempered for about 1 hour within the following range, goes through the following three stages. 1) When the temperature reaches about 100°C, some of the supersaturated carbon in martensite precipitates to form very fine ε-carbide particles, which are dispersed within the martensite and reduce its carbon content. 2) At temperatures between 100 and 300°C, any residual austenite can transform into bainite and ε-carbides. 3) During the third stage of tempering, starting from around 200°C, it depends on the carbon content and alloy composition. When the tempering temperature rises to the eutectoid temperature, the carbide precipitates become coarser and Rp0.2 decreases. 07 Fracture of medium-strength steel Medium-strength steel (620MPa