Cause analysis of steel fracture
There are thousands of steel products used in various industries. Each steel has a different trade name because of its different properties, chemical composition or alloy type and content. Although the fracture toughness values greatly facilitate the selection of each steel, these parameters are difficult to apply to all steels.
Cause analysis of steel fracture
The main reasons are as follows:
- First, because a certain amount of some or more alloy elements are added to the steel during smelting, different microstructures can be obtained after simple heat treatment, thus changing the original properties of the steel;
- Second, because of the defects produced in the process of steelmaking and pouring, especially the concentrated defects (such as pores, inclusions, etc.), it is extremely sensitive during rolling, and different changes occur between different heats of the same chemical composition steel, or even in different parts of the same billet, thus affecting the quality of steel.
The toughness of steel mainly depends on the microstructure and the dispersion of defects, rather than the chemical composition. Therefore, the toughness will change greatly after heat treatment. In order to further explore the properties and fracture causes of steel, it is necessary to master the relationship between physical metallurgy, microstructure and toughness of steel.
Fracture of ferrite pearlite steel
Ferrite pearlite steel accounts for the vast majority of the total steel output. They are usually iron carbon alloys with carbon content between 0.05% and 0.20% and other small amount of alloying elements added to improve yield strength and toughness.
The microstructure of ferrite pearlite is composed of BBC iron (ferrite), 0.01% C, soluble alloy and Fe3C. In carbon steel with low carbon content, cementite particles (carbides) stay in ferrite grain boundary and grain. However, when the carbon content is higher than 0.02%, the majority of Fe3C forms a lamellar structure with some ferrite, which is called pearlite. At the same time, it tends to be a “grain” and ball junction (grain boundary precipitate) and dispersed in the ferrite matrix. In the microstructure of low carbon steel with carbon content of 0.10% ~ 0.20%, pearlite content accounts for 10% ~ 25%.
Although the pearlite particles are very hard, they can be widely dispersed on the ferrite matrix and easily deform around the ferrite. Generally, the grain size of ferrite decreases with the increase of pearlite content. Because the formation and transformation of pearlite nodule will hinder the growth of ferrite grains. Therefore, pearlite can indirectly increase the tensile yield stress by increasing D-1 / 2 (D is the average grain diameter) δ y.
From the point of view of fracture analysis, there are two kinds of steels with carbon content in low carbon steel, and their properties are of great concern. First, when the carbon content is less than 0.03%, the carbon exists in the form of pearlite spheroidization, which has little effect on the toughness of the steel; Second, when the carbon content is high, the toughness and Charpy curve are directly affected by the form of spheroidite.
Influence of treatment process
The practice shows that the impact property of water quenched steel is better than that of annealed or normalized steel. The reason is that rapid cooling prevents the formation of cementite at grain boundaries and makes the ferrite grains finer.
Many steel products are sold in hot rolling, and the rolling conditions have a great influence on the impact properties. Lower finishing temperature can reduce the impact transition temperature, increase the cooling rate and refine the ferrite grains, so as to improve the toughness of the steel. Because the cooling rate of thick plate is slower than that of thin plate, the ferrite grains are coarser than that of thin plate. Therefore, under the same heat treatment conditions, thick plate is more brittle than thin plate. Therefore, normalizing is often used to improve the properties of steel plate after hot rolling.
Hot rolling can also produce anisotropic steel and directional toughness steel with various mixed structures, pearlite strip, inclusion grain boundary and rolling direction. The pearlite band and elongated inclusions are coarse and dispersed into flakes, which have a great influence on the notch toughness at low temperature in the range of Charpy transition temperature.
Influence of ferrite soluble alloy elements
Most alloying elements are added to low carbon steel to produce solid solution hardened steel at some ambient temperatures and to increase lattice friction stress δ i。 However, it is not possible to predict the lower yield stress only by formula until the grain size is known. Although the determining factors of yield stress are normalizing temperature and cooling rate, this research method is still very important because it can be improved by increasing the temperature δ I predict the range of toughness that can be reduced by a single alloying element.
No plastic transformation (NDT) of ferritic steels. The regression analysis of temperature and Charpy transformation temperature has not been reported so far. However, these are only limited to the qualitative discussion of the effect of single alloying element on toughness. The following is a brief introduction of the influence of several alloy elements on the properties of steel.
1). manganese. Most of the manganese content is about 0.5%. As deoxidizer or sulfur fixing agent, hot cracking of steel can be prevented. It also has the following functions in low carbon steel.
After air cooling or furnace cooling, the formation of cementite film at grain boundary of steel with 0.05% carbon content tends to decrease.
The grain size of ferrite can be reduced slightly.
A large number of fine pearlite particles can be produced.
The first two effects indicate that the temperature of NDT decreases with the increase of Mn content, while the latter two effects lead to sharper peak value of Charpy curve.
When the carbon content of steel is high, manganese can significantly reduce the transformation temperature by about 50%. The reason may be due to the 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. Because of the high hardenability of the steel, austenite transforms into brittle upper bainite instead of ferrite or pearlite.
2). nickel. The effect of adding manganese into steel is similar to that of adding manganese, which can improve the toughness of iron carbon alloy. The effect depends on carbon content and heat treatment. In the steel with very low carbon content (about 0.02%), the addition of 2% can prevent the formation of grain boundary cementite in hot rolled and normalized steel, and substantially reduce the initial transformation temperature ts and increase the peak value of Charpy impact curve.
The effect of improving impact toughness decreases with the increase of nickel content. If the carbon content is so low that no carbide appears after normalizing, the effect of nickel on the transformation temperature will be very limited. Adding nickel to normalizing steel containing about 0.10% carbon has the greatest advantage of refining grain and reducing free nitrogen content, but its mechanism is still unclear. It is possible that the decomposition temperature of austenite is reduced by using nickel as the stabilizer of austenite.
3). phosphorus. In pure Fe-P alloy, due to phosphorus segregation at ferrite grain boundary, the tensile strength RM is reduced and the grain is brittle. In addition, phosphorus is the stabilizer of ferrite. Therefore, the addition of steel will increase greatly δ I value and ferrite grain size. The combination of these effects will make phosphorus become an extremely harmful embrittlement agent and lead to transgranular fracture.
4). silicon. Silicon is added to the steel for deoxidation and to improve the impact properties. If both Mn and Al are present in the steel, most of the Si is dissolved in the ferrite, which is enhanced by solid solution hardening δ i。 The combined result of this effect and the improvement of impact properties by adding silicon is that the 50% transformation temperature increases by about 44 ℃ when silicon is added to the Fe carbon alloy with stable grain size by weight. In addition, silicon, similar to phosphorus, is a stabilizer of ferritic iron and can promote the growth of ferrite grains. According to the weight percentage, the average energy conversion temperature will be increased by about 60 ℃ when silicon is added into normalizing steel.
5). aluminum. There are two reasons for adding alloy and deoxidizer into steel: first, AlN is formed with nitrogen in solution to remove free nitrogen; Secondly, the formation of AlN refines the ferrite grains. As a result of these two effects, the transformation temperature will decrease about 40 ℃ for every 0.1% increase of aluminum. However, when the amount of aluminum exceeds the requirement, the effect of “solidifying” free nitrogen will be weakened.
6). oxygen. The oxygen in the steel will segregate at the grain boundary and lead to intergranular fracture. When the oxygen content in the steel is as high as 0.01%, the fracture will occur along the continuous channel generated by the grain boundary of brittle grains. Even if the oxygen content in the steel is very low, the cracks will nucleate at the grain boundary and then diffuse through the grain boundary. The way to solve the problem of oxygen embrittlement is to add deoxidizers such as carbon, manganese, silicon, aluminum and zirconium to make them combine with oxygen to form oxide particles and remove oxygen from grain boundaries. Oxide particles are also favorable for delaying ferrite growth and increasing D – / 2.
The influence of carbon content between 0.3% and 0.8%
The carbon content of hypoeutectoid steel ranges from 0.3% to 0.8%. Proeutectoid ferrite is a continuous phase and first forms at austenite grain boundary. Pearlite is formed in austenite grains and accounts for 35% – 100% of the microstructure. In addition, a variety of aggregation structures are formed in each austenite grain, which makes pearlite become polycrystalline.
Because the strength of pearlite is higher than that of proeutectoid ferrite, the flow of ferrite is restricted, so the yield strength and strain hardening rate of steel increase with the increase of carbon content in pearlite. With the increase of the number of hardened blocks, the confinement is enhanced by the refinement of proeutectoid grain size by pearlite.
When there is a large amount of pearlite in steel, micro cleavage cracks will be formed at low temperature and / or high strain rate during deformation. Although there are some cross sections of internal accumulation structure, the fracture channel initially runs along the cleavage plane. Therefore, there are some preferred orientations in the ferrite grains between the ferrite sheets and in the adjacent aggregates.
Bainitic steel fracture
The addition of 0.05% Mo and B to the low carbon steel with 0.10% C can optimize the kinetic conditions of austenite ferrite transformation, which usually takes place at 700-850 ℃, without affecting the subsequent Austenite Bainite Transformation at 450 ℃ and 675 ℃.
Bainite formed at about 525-675 ℃ is usually called “upper bainite”; The lower bainite formed between 450 ℃ and 525 ℃ is called “lower bainite”. Both structures are composed of acicular ferrite and dispersed carbide. When the transition temperature is reduced from 675 ℃ to 450 ℃, the tensile strength of untrained bainite increases from 585mpa to 1170mpa.
Because the transformation temperature is determined by the content of alloy elements, and indirectly affects the yield and tensile strength. The high strength obtained by these steels is the result of two actions:
- 1). When the transformation temperature decreases, the size of bainitic ferrite sheet is refined continuously.
- 2). The fine carbides in the lower bainite are continuously dispersed. The fracture characteristics of these steels depend largely on the tensile strength and transformation temperature.
There are two effects to be noted: first, at a certain tensile strength level, the Charpy impact property of tempered bainite is much better than that of non tempered upper bainite. The reason is that in the upper bainite, the cleavage facet in the spheroid cuts a number of bainite grains, and the main fracture size is the austenite grain size.
In the lower bainite, the cleavage planes in acicular ferrite are not aligned, so the main characteristic of fracture of quasi cleavage fracture plane is the grain size of acicular ferrite. Because the grain size of acicular ferrite is only half of that of austenite in upper bainite. Therefore, the transformation temperature of lower bainite is much lower than that of upper bainite at the same strength level.
In addition to the above reasons, it is the distribution of carbides. In the upper bainite, the carbides are located along the grain boundary and increase the brittleness by reducing the tensile strength RM. In tempered lower bainite, carbides are evenly distributed in ferrite, and the tensile strength and spheroidized pearlite refinement are improved by limiting cleavage cracks.
Secondly, it is important to pay attention to the change of transition temperature and tensile strength in the untrained alloy. In the upper bainite, the decrease of transformation temperature will refine the acicular ferrite size and increase the elongation strength Rp0.2.
In order to obtain the tensile strength of 830mpa or higher in lower bainite, it can also be achieved by reducing the transformation temperature to increase the strength. However, because the fracture stress of upper bainite depends on the austenite grain size, and the carbide grain size is already large, the effect of Tempering on improving the tensile strength is very small.
Fracture of martensitic steel
The addition of carbon or other elements can delay the transformation of austenite into ferrite, pearlite or bainite. At the same time, if the cooling rate of austenite is fast enough, austenite will be transformed into martensite by shearing process without atom diffusion.
The ideal martensitic fracture should have the following characteristics:
Tetrahedral ferrite or acicular martensite is very fine because of the low transformation temperature (200 ℃ or lower).
Because of the shear transformation, the carbon atoms in austenite have no time to diffuse out of the crystal, so that the carbon atoms in ferrite are saturated, which makes the martensite grains elongate and leads to lattice expansion.
The martensitic transformation should take place over a certain temperature range, because the initial martensitic sheet increases the resistance for the subsequent transformation of austenite into martensite. Therefore, the structure after transformation is a mixture of martensite and retained austenite.
In order to ensure the stability of steel properties, tempering must be carried out. High carbon (above 0.3%). Martensite, tempered in the following range for about 1 h, goes through the following three stages.
- 1). When the temperature reaches about 100 ℃, some supersaturated carbon in martensite precipitates and forms very fine grains ε- Carbide particles disperse in martensite and reduce carbon content.
- 2). Any retained austenite may be transformed into bainite and bainite when the temperature is between 100 ℃ and 300 ℃ ε- Carbides.
- 3). In the third stage of tempering, it depends on carbon content and alloy composition from about 200 ℃. When the tempering temperature rises to eutectoid temperature, the carbide precipitation becomes coarser and Rp0.2 decreases.
Fracture of medium strength steel
In addition to relieving stress and improving impact toughness, tempering has the following two functions: first, transformation of retained austenite. Retained austenite will transform into ductile acicular lower bainite at about 30 ℃. At higher temperature such as 600 ℃, the residual austenite will be transformed into brittle pearlite. Therefore, the steel is tempered for the first time at 550-600 ℃ and for the second time at 300 ℃ to avoid the formation of brittle pearlite, which is called “secondary tempering”.
Secondly, increasing the content of dispersed carbides (tensile strength RM increases) reduces the yield strength. If the tempering temperature is increased, both of them will cause impact and decrease the tempering range. Because the microstructure becomes fine, the tensile plasticity will be improved at the same strength level.
Temper brittleness is reversible. If the tempering temperature is higher than the critical range and the transition temperature is reduced, the material can be reheated and treated in the critical range, and the tempering temperature can be increased again. If there are trace elements, the brittleness will be improved. The most important trace elements are antimony, phosphorus, tin, arsenic, plus manganese and silicon. If other alloying elements exist, Mo can also reduce temper brittleness, and Ni and Cr can also play a role.
Fracture of high strength steel
High strength steel (Rp0.2 > 1240mpa) can be produced by quenching and tempering; Austenite Deformation before quenching and tempering; Precipitation hardening steel is produced by annealing and aging. In addition, the strength of the steel can be further improved by strain and re tempering or tempering strain.
Stainless steel fracture
Stainless steel is mainly composed of Fe Cr, Fe Cr Ni alloy and other elements to improve mechanical properties and corrosion resistance. Stainless steel corrosion protection is due to the formation of chromium oxide impermeable layer on the metal surface, which can prevent further oxidation.
Therefore, stainless steel can prevent corrosion and strengthen chromium oxide layer in oxidizing atmosphere. But in the reducing atmosphere, the chromium oxide layer is damaged. The corrosion resistance increases with the increase of Cr and Ni content. Nickel can improve the passivation of iron.
The increase of carbon is to improve the mechanical properties and ensure the stability of the properties of austenitic stainless steel. In general, stainless steels are classified by microstructure.
Martensitic stainless steel. It belongs to Fe Cr alloy and 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% – 18% chromium and 0.12% carbon. Because chromium is the stabilizer of ferrite, the austenite phase is completely inhibited by more than 13% chromium, so it is a complete ferrite phase.
Austenitic stainless steel. Nickel is a strong stabilizer of austenite. Therefore, at room temperature, below room temperature or high temperature, the content of nickel is 8%, and the content of chromium is 18% (300 type). The austenite phase is very stable. Austenitic stainless steel is similar to ferritic type and cannot be hardened by martensitic transformation.
The characteristics of ferritic and martensitic stainless steels, such as grain size, are similar to those of other Ferritic and martensitic steels of the same grade.
Austenitic stainless steel is fcc structure, and cleavage fracture is impossible at freezing temperature. After 80% cold rolling, 310 type stainless steel has very high yield strength and notch sensitivity, and even has a notch sensitivity ratio of 1.0 at – 253 ℃. Therefore, it can be used in liquid hydrogen storage tank of missile system. Similar type 301 stainless steel can be used for liquid oxygen storage tanks with temperatures as low as 183 ℃. However, it is unstable below these temperatures. If any plastic deformation occurs, the unstable austenite will become brittle non tempered martensite. Most austenitic steels are used in anti-corrosion environment. When heated to 500-900 ℃, chromium carbide will precipitate at austenite grain boundary. As a result, the chromium layer near the grain boundary will be completely depleted. This part is very vulnerable to corrosion and local corrosion, if there is stress, it can also lead to brittle fracture.
In order to reduce the above hazards, a small amount of elements with better properties than chromium carbide, such as titanium or niobium, can be added to form alloy carbide with carbon to prevent chromium depletion and subsequent stress corrosion cracking. This treatment is often called “stabilization treatment”.
Austenitic stainless steel is also commonly used in high temperature, such as flanges, to prevent and meet corrosion and creep resistance. Some steels are very sensitive to the cracks in and near the heat affected zone (HAZ) due to the post weld heat treatment and high temperature environment. Therefore, when welding reheating, under the action of high temperature, niobium or titanium carbide will precipitate in grains and grain boundaries, resulting in cracks and affecting service life, which must be paid great attention to.
Source: China Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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