There are thousands of varieties of steel products and steel pipes used in various industries. Each steel 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:
1. 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 steel is formed, thus changing the original properties of the steel;
2. 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.
Because the toughness of steel mainly depends on the microstructure and the dispersion of defects (strictly prevent concentrated defects), rather than the 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.
First, ferrite-pearlite steel fracture
Ferrite-pearlite steels account for the vast majority of total steel production. They are usually alloys of iron-carbon with a carbon content between 0.05% and 0.20% and other small amounts of alloying elements added to improve yield strength and toughness. The microstructure of ferrite-pearlite consists of BBC iron (ferrite), 0.01% C, soluble alloys and Fe3C. In carbon steels with very low carbon content, cementite particles (carbides) stay at the ferrite grain boundaries and grains. But when the carbon content is higher than 0.02%, most of the Fe3C forms a lamellar structure with some ferrite, called pearlite, and tends to be dispersed in the ferrite matrix as “grains” and nodules (grain boundary precipitates). In the microstructure of low-carbon steels with a carbon content of 0.10% to 0.20%, the pearlite content accounts for 10% to 25%. Although pearlite particles are very hard, they can be dispersed very widely in the ferrite matrix and deform easily around the ferrite. Generally, the grain size of ferrite decreases with the increase of pearlite content. This is because the formation and transformation of pearlite nodules will hinder the growth of ferrite grains. Therefore, pearlite will indirectly increase the tensile yield stress δy by increasing d-1/2 (d is the average diameter of the grain). From the perspective of fracture analysis, there are two carbon content ranges in low-carbon steel whose performance is of concern. 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.
Second, the influence of the treatment process
It is known in practice that the impact performance of water-quenched steel is better than that of annealed or normalized steel because rapid cooling prevents the formation of cementite at the grain boundary and promotes the refinement of ferrite grains. Many steels are sold in the hot-rolled state, and the rolling conditions have a great influence on the impact performance. 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 steel. Thick plates have coarser ferrite grains than thin plates because the cooling rate is slower than that 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.
Third, 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. Regression analysis of the non-ductile transition (NDT) temperature and Charpy transition temperature of ferritic steel has not been reported so far, but these are limited to qualitative discussions on 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.
1) Manganese: The vast majority of manganese content is about 0.5%. Adding it as a deoxidizer or sulfur fixer can prevent the hot cracking of steel. It also has the following effects on low-carbon steel.
- Steel with a carbon content of 0.05% has a tendency to reduce the formation of cementite film at the grain boundary after air cooling or furnace cooling.
- The ferrite grain size can be slightly reduced.
- A large number of fine pearlite particles can be produced.
The first two effects indicate that the NDT temperature decreases with the increase of manganese content, and the latter two effects will cause the peak of the Charpy curve to be sharper. When the carbon content of 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 boundary. It must be noted that if the carbon content of the steel is higher than 0.15%, the high manganese content plays a decisive role in the impact properties of normalized steel. This is because the high hardenability of the steel causes the austenite to transform into brittle upper bainite, rather than ferrite or pearlite.
2) Nickel: Added to steel, it acts like manganese and can improve the toughness of iron-carbon alloys. The magnitude of its effect depends on the carbon content and heat treatment. In steels with very low carbon content (about 0.02%), an addition of 2% can prevent the formation of grain boundary cementite in hot-rolled and normalized steels, while substantially reducing the start transformation temperature TS and increasing the peak of the Charpy impact curve. Further increases in nickel content have a reduced effect on improving impact toughness. If the carbon content is so low that no carbides appear after normalizing, the effect of nickel on the transformation temperature will become very limited. The biggest benefit of adding nickel to normalized steels with a carbon content of about 0.10% is grain refinement and reduction of free nitrogen content, but the mechanism is not yet clear. It may be because nickel acts as a stabilizer for austenite, thereby reducing 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 grains. In addition, phosphorus is also a stabilizer for ferrite. Therefore, adding it to steel will greatly increase the δi value and the ferrite grain size. The combination of these effects will make phosphorus an extremely harmful embrittlement agent, causing transgranular fracture.
4) Silicon: Silicon is added to steel for deoxidation and is also beneficial for improving impact properties. If manganese and aluminum are present in steel at the same time, most of the silicon dissolves in the ferrite and increases δi through solution hardening. The result of this effect combined with the addition of silicon to improve impact properties is that the addition of silicon by weight percentage to an iron-carbon alloy with a stable grain size increases the 50% transformation temperature by about 44°C. In addition, silicon, like phosphorus, is a stabilizer for ferrite iron and can promote the growth of ferrite grains. By weight percentage, the addition of silicon to normalized steel will increase the average energy conversion temperature by about 60°C.
5) Aluminum: It is added to steel as an alloy and deoxidizer for two reasons: first, it forms AlN with nitrogen in the solution and removes free nitrogen; second, the formation of AlN refines the ferrite grains. As a result of these two effects, each 0.1% increase in aluminum will reduce the transformation temperature by about 40°C. However, when the amount of aluminum added exceeds the requirement, the effect of “solidifying” free nitrogen will weaken.
6) Oxygen: Oxygen in steel will segregate at the grain boundaries and cause intergranular fracture of ferroalloys. When the oxygen content in steel is as high as 0.01%, the fracture will occur along the continuous channel generated by the grain boundaries of the embrittled grains. Even if the oxygen content in steel is very low, the cracks will be concentrated at the grain boundaries and then diffuse through the grain. The solution to the oxygen embrittlement problem is to add deoxidizers such as carbon, manganese, silicon, aluminum, and zirconium to combine with oxygen to form oxide particles, thereby removing oxygen from the grain boundaries. Oxide particles are also favorable substances for delaying ferrite growth and increasing d-/2.
Fourth, the influence of carbon content of 0.3% to 0.8%
The carbon content of hypoeutectoid steel is 0.3% to 0.8%. Proeutectoid ferrite is a continuous phase and is first formed at the austenite grain boundary. Pearlite is formed in the austenite grains and accounts for 35% to 100% of the microstructure. In addition, there are a variety of aggregate structures formed in each austenite grain, making pearlite polycrystalline. Since pearlite is stronger than eutectoid ferrite, it restricts the flow of ferrite, so that the yield strength and strain hardening rate of steel increase with the increase of pearlite carbon content. The restriction effect increases with the increase of the number of hardened blocks and the refinement of the eutectoid grain size by pearlite. When there is a large amount of pearlite in the steel, micro cleavage cracks will be formed at low temperatures and/or high strain rates during deformation. Although there are also some internal aggregate structure sections, the fracture channel initially runs along the cleavage plane. Therefore, there are some preferred orientations between ferrite sheets and ferrite grains in adjacent aggregate structures.
Fifth, bainitic steel fracture
Adding 0.05% molybdenum and boron to low-carbon steel with a carbon content of 0.10% can optimize the austenite-ferrite transformation that usually occurs at 700-850℃, and does not affect the kinetic conditions of the subsequent austenite-bainite transformation at 450℃ and 675℃. Bainite formed between about 525℃ and 675℃ is usually called “upper bainite”; that formed between 450℃ and 525℃ is called “lower bainite”. Both structures are composed of acicular ferrite and dispersed carbides. When the transformation temperature drops from 675℃ to 450℃, the tensile strength of untempered bainite increases from 585MPa to 1170MPa. Because the transformation temperature is determined by the alloying element content and indirectly affects the yield and tensile strength. The high strength obtained by these steels is the result of the following two effects:
1) When the transformation temperature decreases, the size of the bainitic ferrite sheet continues to refine.
2) Fine carbides are continuously dispersed in the lower bainite. The fracture characteristics of these steels depend largely on the tensile strength and transformation temperature.
There are two effects to note:
① At a certain tensile strength level, the Charpy impact performance of tempered lower bainite is much better than that of untempered upper bainite. The reason is that in the upper bainite, the cleavage planes in the spheroid cut several bainite grains, and the main size that determines the fracture is the austenite grain size. In the lower bainite, the cleavage planes in the acicular ferrite are not arranged in a straight line, so the main feature that determines whether the quasi-cleavage fracture surface breaks is the acicular ferrite grain size. Because the acicular ferrite grain size here is only 1/2 of the austenite grain size in the upper bainite. Therefore, at the same strength level, the transformation temperature of lower bainite is much lower than that of upper bainite. In addition to the above reasons is the distribution of carbides. In the upper bainite, carbides are located along the grain boundaries and increase brittleness by reducing the tensile strength Rm. In the tempered lower bainite, carbides are very evenly distributed in the ferrite, while limiting cleavage cracks to improve tensile strength and promote spheroidized pearlite refinement.
②It should be noted that the change in transformation temperature and tensile strength in the untempered alloy. In the upper bainite, the reduction of the transformation temperature will refine the size of the acicular ferrite and increase the elongation strength Rp0.2. In the lower bainite, in order to obtain a tensile strength of 830MPa or higher, it can also be achieved by lowering the transformation temperature to increase the strength. However, because the fracture stress of the upper bainite depends on the austenite grain size, and the carbide particle size is already large at this time, the effect of increasing the tensile strength by tempering is small.
Sixth, Martensitic Steel Fracture
Carbon or other elements added to the steel can delay the transformation of austenite into ferrite and pearlite or bainite. At the same time, if the cooling rate after austenitization is fast enough, the austenite will become martensite through the shear process without atomic diffusion.
The ideal martensitic fracture should have the following characteristics.
- Because the transformation temperature is very low (200℃ or lower), tetrahedral ferrite or needle-shaped martensite is very fine.
- Because the transformation occurs through shear, the carbon atoms in austenite do not have time to diffuse out of the crystal, making the carbon atoms in ferrite saturated, so that the martensite grains are elongated and the lattice expands.
- The martensitic transformation must exceed a certain temperature range because the initially generated martensite sheets increase the resistance to the subsequent transformation of austenite into martensite. Therefore, the structure after transformation is a mixed structure of martensite and residual austenite.
In order to ensure the stability of the performance of steel, tempering must be carried out. High carbon (more than 0.3%) martensite is tempered for about 1h in the following range and goes through the following three stages.
1) When the temperature reaches about 100℃, some supersaturated carbon in martensite precipitates and forms very fine ε-carbide particles, which are dispersed in martensite to reduce the carbon content.
2) At temperatures between 100 and 300 °C, any retained austenite may transform into bainite and ε-carbide.
3) In the third stage of tempering, starting from about 200 °C, it depends on the carbon content and alloy composition. When the tempering temperature rises to the eutectoid temperature, the carbide precipitation becomes coarser and Rp0.2 decreases.
Seventh, medium-strength steel fracture
In addition to relieving stress and improving impact toughness, tempering has the following two effects:
① Transformation of retained austenite. Retained austenite will transform into tough needle-shaped lower bainite at a low temperature of about 30 °C. At higher temperatures such as 600 °C, retained austenite will transform into brittle pearlite. Therefore, the steel is tempered for the first time at 550-600 °C and the second time at 300 °C to avoid the formation of brittle pearlite. This tempering system is called “secondary tempering”.
② Increase the content of dispersed carbides (increase in tensile strength Rm) and reduce yield strength. If the tempering temperature is increased, both will cause impact and the transformation tempering range will be reduced. Because the microstructure becomes finer, the tensile plasticity will be improved at the same strength level.
Temper brittleness is reversible. If the tempering temperature is high enough to exceed the critical range and reduce the transformation temperature, the material can be reheated and treated in the critical range before the tempering temperature can be increased again. If trace elements appear, it indicates that brittleness will be improved. The most important trace elements are antimony, phosphorus, tin, and arsenic, plus manganese and silicon have a brittle removal effect. If other alloying elements are present, molybdenum can also reduce tempering brittleness, and nickel and chromium also have a certain effect.
Eighth, high-strength steel (Rp0.2>1240MPa) fracture
High-strength steel can be produced by the following methods: quenching and tempering; austenite deformation before quenching and tempering; and annealing and aging to produce precipitation-hardening steel. In addition, the strength of the steel can be further improved by straining and re-tempering or straining during tempering.
Ninth, stainless steel fracture
Stainless steel is mainly composed of iron-chromium, iron-chromium-nickel alloys, and other elements that improve mechanical properties and corrosion resistance. Stainless steel is corrosion-resistant because an impermeable layer of chromium oxide is formed on the metal surface to prevent further oxidation. Therefore, stainless steel can prevent corrosion in an oxidizing atmosphere and strengthen the chromium oxide layer. However, in a reducing atmosphere, the chromium oxide layer is damaged. Corrosion resistance increases with the increase of chromium and nickel content. Nickel can comprehensively improve the passivation of iron.
- Martensitic stainless steel. It is an iron-chromium alloy that can be austenitized and subsequently heat-treated to form martensite. It usually contains 12% chromium and 0.15% carbon.
- Ferritic stainless steel. It contains about 14% to 18% chromium and 0.12% carbon. Because chromium is a stabilizer for ferrite, the austenite phase is completely suppressed by more than 13% chromium, so it is a complete ferrite phase.
- Austenitic stainless steel. Nickel is a strong stabilizer of austenite, so 8% nickel and 18% chromium (type 300) can make the austenite phase very stable at room temperature, below room temperature, or at elevated temperatures. Austenitic stainless steels are similar to ferritic types and cannot be hardened by martensitic transformation.
The characteristics of ferritic and martensitic stainless steels, such as grain size, are similar to other ferritic and martensitic steels of the same grade. Austenitic stainless steels have an FCC structure and are unlikely to cleave at freezing temperatures. After cold rolling 80% of large pieces, type 310 stainless steel has extremely high yield strength and notch sensitivity, even at temperatures as low as -253°C, it has a notch sensitivity ratio of 1.0. Therefore, it can be used in liquid hydrogen storage tanks for missile systems. Similar type 301 stainless steel can be used for liquid oxygen storage tanks at temperatures as low as 183°C. However, it is unstable below these temperatures, and if any plastic deformation occurs, the unstable austenite will transform into brittle non-tempered martensite. Most austenitic steels are used in corrosion-resistant environments and are heated to a temperature range of 500-900°C. Chromium carbides will precipitate at the austenite grain boundaries, resulting in the complete depletion of the chromium layer in the vicinity of the grain boundaries. This area is very susceptible to corrosion and localized corrosion, and if stress exists, it can also lead to brittle fracture.
In order to mitigate the above hazards, a small number of elements with stronger properties than chromium carbides, such as titanium or niobium, can be added to form alloy carbides with carbon to prevent chromium depletion and the resulting stress corrosion cracking. This treatment is often called “stabilization treatment”. Austenitic stainless steel is also commonly used in high temperatures, such as pressure vessels, to prevent and meet corrosion resistance and creep resistance. Some steel grades are very sensitive to cracks in and around the heat-affected zone due to post-weld heat treatment and high temperature environments. Therefore, when welding is reheated, niobium or titanium carbides will precipitate in the grains and grain boundaries under the action of high temperature, causing cracks to occur and affecting the service life, which must be given great attention.
Post time: Mar-12-2025