What is low stress brittle fracture
What is low stress brittle fracture?
Table of Contents
- 1 What is low stress brittle fracture?
- 2 Low stress brittle fracture
- 3 Factors affecting low temperature toughness
- 4 Design principles for preventing low stress brittle fracture
- 5 Evaluation method of low temperature toughness of steel
Low stress brittle fracture refers to the sudden fracture of the structure when the stress level is low or even lower than the yield point stress of the material. The main macroscopic feature is that there is no obvious plastic deformation in the structure, which is similar to the fracture characteristics of brittle materials, but the material is not necessarily brittle, and the fracture does not necessarily show crystalline morphology. Low stress brittle fracture is mostly related to macro defects (mainly cracks) in structural parts, as well as the toughness of materials.
Low stress brittle fracture is caused by the propagation of macro cracks (process cracks or service cracks). Because the crack destroys the uniformity and continuity of the material, it changes the internal stress state and stress distribution of the material.
Low stress brittle fracture is easy to occur when the service temperature is low and the material has defects or improper material selection.
- 1. Control the service stress state of components;
- 2. Avoid or minimize crack size.
Low stress brittle fracture
Since the end of the 19th century, a series of low-temperature brittle fracture accidents have occurred in railway tracks, bridges and structural parts in severe cold areas. Due to the limitations of science and technology at that time, the research on the cold brittleness of steel has not made substantial progress. Since the 1940s, many ships, pressure vessels, pipelines, chemical equipment and large structures, especially some welded structures, Low stress brittle fracture occurred many times, resulting in huge losses. Therefore, low stress brittle fracture has become a subject of great concern. Through the investigation and analysis of a large number of accidents, it can be concluded that low stress brittle fracture has the following characteristics.
- ① At the time of fracture, the working pressure of the container is relatively low, its fracture nominal stress is lower than the yield strength of the material, and there is no or only local minimal plastic deformation before fracture.
- ② The crack growth rate is large.
- ③ Low stress brittle fracture mostly belongs to cleavage fracture or quasi cleavage fracture. The fracture has the characteristics of grain shape, bright and smooth.
- ④ Low stress brittle fracture often occurs on vessels with notches or cracks, and various process defects and impurities existing in the cylinder itself are used as the crack source.
- ⑤ The fracture usually occurs at a lower temperature, when the toughness of the material is very poor.
Based on the above characteristics of low stress brittle fracture and the principle of fracture mechanics, the metal fracture mechanism is analyzed. It is found that the low-temperature toughness of metal, that is, the micro plastic deformation ability of metal at the notch tip determines the ability of pressure vessel to resist stress brittle fracture failure.
Factors affecting low temperature toughness
Effect of crystal structure
The results show that ferritic steels with body centered cubic matrix (BCC) structure have higher brittle transition temperature and greater tendency to brittle fracture, followed by dense hexagonal structure (HCP), while metals with face centered cubic structure (FCC) such as copper, aluminum, nickel and austenitic steels basically have no such temperature effect, that is, there is no low stress brittle fracture.
In fact, unless there is a second phase or in the environment leading to stress corrosion cracking, face centered cubic metals generally do not undergo brittle fracture. The main reason is that when the temperature decreases, the yield strength of face centered cubic metals does not change significantly, and deformation twins are not easy to occur, dislocations are easy to move, local stress is easy to relax, and cracks are not easy to propagate, Generally, there is no brittle transition temperature.
However, the BCC metal is different. In the medium temperature region, its strength (especially yield strength) is obviously affected by impurities, loading speed and alloy elements. In the low temperature region below 0.2t0 (t0 is the melting point of the metal, unit: k), its yield strength increases rapidly with the decrease of temperature, and finally is almost equal to the tensile strength, Especially at low temperature, deformation twins are easy to occur, so it is easy to cause low stress brittle fracture.
Influence of chemical composition
For low-temperature pressure vessel steel, increasing the carbon content will increase the brittleness of the material and make the brittleness transition temperature rise sharply. Therefore, the carbon content of low-temperature steel does not exceed 0.2%. In recent years, there is an obvious trend of developing and applying low-carbon (< 0.15%) or micro carbon (< 0.06%) steel abroad.
Manganese is an element to expand the austenite zone. The increase of manganese content can make the steel obtain fine and ductile ferrite and pearlite grains, so it can improve the toughness of the steel at low temperature. When the carbon content is certain, increasing the manganese ratio can obtain a lower non ductile transition temperature, reduce the carbon content, increase the manganese carbon ratio, reduce the non ductile transition temperature and reduce the allowable service temperature of the steel plate.
Nickel is also an important element to improve the low-temperature toughness of steel, even better than manganese. When the nickel content is 3.5%, the steel can still maintain high toughness at – 100 ℃, while the steel containing 9% nickel can be used as liquid nitrogen container to withstand the low temperature of – 196 ℃.
In the ferritic low temperature steel containing manganese, a small amount of gold containing elements such as V, Ti, Nb and Al are added. Through rolling or subsequent heat treatment, carbides and nitrides are dispersed and precipitated for precipitation strengthening, so as to obtain high strength and good low temperature toughness.
Effect of grain size
Grain size is an important factor affecting the low stress brittle fracture of steel. Fine grains not only make the metal have higher fracture strength, but also reduce the brittle transition temperature. This is because there are impurities and brittle phases at the grain boundary, which are often the source of cracks.
Grain refinement, on the one hand, relatively reduces the brittle phase per unit area, improves the surface energy and reduces the probability of crack nucleation and propagation, so as to improve the low-temperature brittle fracture resistance of the steel. On the other hand, the properties of fine grain steel are relatively uniform and reduce the brittle transition temperature.
Effect of inclusions
Phosphorus is easy to produce grain boundary segregation, and oxygen in steel precipitates at the grain boundary in the form of various oxides. Both of them greatly improve the brittle transition temperature of steel, resulting in low stress brittle fracture. Therefore, low-temperature steel must be fully deoxidized. For example, the low temperature toughness of killed steel is better than that of rimmed steel; If Si + AI and AI + Ti (V, Nb) are used for comprehensive deoxidation, the grain can be further refined and its low-temperature toughness is better.
Full deoxidation can not only effectively reduce the content of low oxygen, sulfur, phosphorus and other gases, but also spheroidize inclusions and reduce the stacking of dislocations, so as to reduce the brittle transition temperature of steel.
The results show that the low temperature brittleness of very pure metals is independent of grain type. For example, pure iron without carbon, nitrogen, oxygen and boron is plastic even at a low temperature of 4K. Impurities (especially grain boundary brittle phase) have a great influence on low stress brittle fracture. For example, trace carbon, oxygen and nitrogen in Fe Cr alloy with 25% Cr is an important reason to promote low stress brittle fracture.
Effect of heat treatment and torsion on Microstructure
Heat treatment has a great influence on the low stress brittle fracture of steel. Quenching and tempering treatment is a common method to obtain ferrite and granular carbide structure, which can significantly improve the low-temperature toughness of steel. However, with the increase of tempering temperature during quenching and tempering treatment, the aggregation of granular carbides affects the low-temperature toughness, so the tempering temperature during quenching and tempering treatment should be strictly controlled not to be too high.
Normalizing is the most widely used heat treatment method for low temperature steel. If the alloying elements in steel increase, the normalizing temperature should be increased accordingly. The annealing structure of steel is coarser than normalizing structure, and its low-temperature toughness is much worse than that after normalizing or quenching and tempering treatment. Therefore, the steel for low-temperature pressure vessel does not undergo annealing treatment. For low-temperature pressure vessels and pressure components requiring post weld heat treatment, the temperature of post weld heat treatment shall not exceed the tempering temperature of steel under any circumstances.
Heat treatment can also inhibit the precipitation of brittle phase from grain boundary, change the morphology, size, quantity and distribution of precipitated phase, uniform structure, and improve the strength and low-temperature toughness of steel. A certain amount of retained austenite or ferrite in tempered structure (tempered martensite) can effectively prevent crack propagation. Both quenching aging and strain aging increase the brittle transition temperature of steel and increase the sensitivity of low stress brittle fracture. Therefore, the boiling steel sensitive to aging should not be used as low temperature steel.
Effect of cold deformation
Cold deformation reduces the toughness of steel, strain aging worsens the low-temperature toughness and increases the brittle transition temperature. Therefore, for large high-pressure vessels, notch toughness must be paid attention to in use. Because cold deformation, cold pressing and welding deformation will lead to embrittlement in the manufacturing process, low-temperature annealing shall be carried out after cold deformation and welding.
Effect of stress state
Low stress brittle fracture is closely related to stress state. When there are cracks or notches in the container, it is easy to produce low stress brittle fracture. The sharper the notch and the larger the pre crack size, the easier it is to cause low stress brittle fracture. When there are cracks and residual stress in the welded joint, the low stress brittle fracture is more obvious.
Design principles for preventing low stress brittle fracture
At present, the allowable stress determined by room temperature tensile strength or yield strength is used in the design of low-temperature pressure vessels in all pressure vessel codes. This method can effectively prevent the failure of large plastic deformation. In order to prevent the pressure vessel designed according to this design method from low stress brittle fracture at low temperature, the steel must have certain toughness, and some requirements for design and manufacture are also put forward. How to determine the required toughness level first depends on which principle to prevent brittle fracture is adopted.
- ① Certain defects are allowed, but cracking can be prevented. Generally speaking, there are many defects and poor toughness in the welding area. The fracture always starts from the place with poor defect and toughness. Therefore, when adopting this principle, it is not enough to simply measure the properties of base metal, but also the properties of heat affected zone and fusion line. It is required that the place with the worst toughness can bear the strain caused by external load.
- ② It is allowed to have defects and may crack in the welding area with poor self toughness. It mainly depends on the base metal to prevent crack propagation and avoid fracture accidents. However, because the toughness of weld metal, fusion line and heat affected zone is often worse than that of base metal, cracks often propagate along the welded joint zone. Therefore, this method is not reliable to prevent brittle fracture.
- ③ Cracking from defects is allowed, and all parts of the container can stop cracking. At first glance, this method seems to be the safest, but it has two disadvantages.
First, when using this method to prevent brittle fracture, materials with very good toughness should be selected, which means that the material cost is very high.
Second, its fatal disadvantage is that its effectiveness as an absolute safety criterion is related to the type of structure.
The blasting test results of the container with defects show that it is easy to stop the crack under the condition of complete hydraulic pressure. Under the condition of air pressure or hydraulic pressure with part of gas, it is difficult to stop the crack due to the large energy stored in the system, or a special crack stop structure must be designed.
For low-temperature pressure vessels in petrochemical and refrigeration air separation industries, their internal medium is often gas-liquid two-phase or although it is liquid phase, but its working temperature is higher than its normal boiling point, so the crack arrest principle can not be used to prevent low stress brittle fracture.
Evaluation method of low temperature toughness of steel
Since the brittle fracture of steel structure attracted people’s attention in the 1940s, the evaluation methods and evaluation indexes of steel low temperature toughness have been widely studied and tested in various countries. The test methods closely related to pressure vessels are as follows: low temperature impact toughness test (V-notch, DVM sample); Drop weight test; Full thickness test plate test (wide plate test, double tensile test, Esso test); Fracture mechanics test (plane strain fracture toughness KIC and crack tip opening displacement cod method).
Among them, the low-temperature Charpy (V-notch) impact toughness test is the most widely used, and the low-temperature toughness of the material is evaluated by a certain absorption work Akv or a certain percentage of fracture fiber (i.e. brittle transition temperature) in the impact test.
In the early ASME Code of the United States, for vessels made of low carbon steel and some low alloy steels, when working below a certain temperature (which is related to the material thickness), the Charpy (V-notch) impact test impact energy of the material is required to be no less than 20j. This provision is based on a large number of damage accidents and material tests, and is applicable to a large number of Charpy of steel plates recommended in the code at that time In the impact test results of (V-notch), it is found that the maximum impact energy of crack initiation type steel plate is about 14J, the maximum impact energy of crack propagation type is no more than 18J, and those greater than 27J belong to crack arrest type.
Based on the research results at that time, people took the Charpy (V-notch) impact test impact energy Akv = 20J as the toughness assessment index of the material at its lowest service temperature. By 1953, due to the use of high-strength steel, the base point of its critical transition temperature was transferred to the higher position of Akv impact energy curve, and the Akv impact energy index of 20J could not avoid the occurrence of brittle fracture.
At present, in foreign pressure vessel specifications, 20J is used as the only criterion for Notch Toughness of low carbon steel at the lowest working temperature or design temperature: American ASMEVIII-1 and ASMEVIII-2, French specifications, etc. German ad specification W10 uses the impact energy of DVM sample as the criterion. The specific requirements are that the impact energy toughness of DVM sample at design temperature is 35J / cm in the transverse direction ²， This value is equivalent to using V-notch Charpy specimen to reach longitudinal 27J under the test temperature with the design temperature increased by 10 ℃. From the data of improving the steel standard, on the premise of using the same specimen type, the ratio of longitudinal and transverse impact energy is about 1:0.7.
China’s current steel pressure vessel standard GB150.1-150.4 refers to the relevant provisions of ASMEVIII-1, so it also takes 20J as the acceptance criterion of steel equivalent to the strength grade of low carbon steel. For steel plates, China’s standard requires horizontal sampling according to the domestic steel conditions. Compared with the foreign longitudinal sampling, the impact energy requirements are not lower than those in the foreign codes designed according to rules Toughness requirements for steel.
Source: Network Arrangement – China Pipe Fitting Manufacturer – Yaang Pipe Industry (www.epowermetals.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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