Research on the hazards of cold brittleness of steel structures in low-temperature environments in cold areas. How much strength below zero is affected by steel structures?
While browsing the website today, someone was asking: “How much strength below zero is affected by the steel structure?” Now let’s study this issue. The obvious decrease in the strength of steel structure in low temperature environment is actually the occurrence of cold brittleness and brittle fracture. The two most important factors for the brittle fracture of steel structure are ambient temperature and steel thickness. As the temperature drops As the thickness increases, the steel becomes brittle. This article first analyzes the low-temperature brittleness and the influencing factors of the brittle-ductile transition of steel, and then uses three-point bending test and low-temperature impact toughness test to analyze the mechanism of steel brittle fracture from occurrence, development to fracture. The experiment verified the distribution law of steel brittle fracture and temperature and thickness, and proposed methods to prevent cold brittleness hazards.
Steel structure is one of the types of building structure. It is a structure composed of columns, beams, trusses and other components made of steel plates and various section steels (or mainly composed of steel materials), which are usually made into parts or components in the factory, and then transported to the site for welding, Bolt (or high-strength bolt) or rivet installation. Compared with the structure of other materials, the steel structure has lighter weight, high strength, superior seismic performance, good toughness and plasticity, good water tightness and air tightness, good energy saving effect, and can span a larger span. Factory assembly It is widely used in industrial and civil construction fields such as various factories, stadiums, high-rise buildings and so on.
China has a vast territory, and the regions for the cold climate are divided into severe cold regions (including northwest Heilongjiang Province, northeastern Inner Mongolia Autonomous Region, northern Xinjiang, northern Tibet, and Qinghai), cold regions (including most of North China, southern Liaoning, and most of Shaanxi) , Central and eastern Gansu, etc.), hot summer and cold winter areas (including northern South China, eastern Sichuan Basin, Guizhou and other areas), and winter low temperature and severe cold areas are widely distributed. With the development of society, steel structures are in negative or even extremely low temperature environments. The low temperature is becoming more and more common, and the influence of low temperature on the steel structure is gradually showing up. Since the 1930s, there have been many accidents of brittle fracture of steel components due to low temperature in the world. People have gradually realized that low temperature has an impact on the mechanical properties of steel structures.
Table of Contents
- 1 Low temperature cold brittle mechanism of steel
- 2 Characteristics of brittle fracture
- 3 Experiment 2: Low-temperature impact toughness test
- 4 Measures to avoid cold brittleness of steel structures
- 5 Conclusion
Low-temperature cold brittleness of steel refers to the phenomenon that the steel evolves from toughness to brittleness at low temperature until sudden failure occurs.
Many mechanical properties of steel are directly related to changes in temperature. When the steel structure undergoes brittle failure, the nominal stress decreases with the decrease of temperature, the plasticity of the steel decreases, the brittleness increases, and the performance of the steel structure also changes accordingly.When the temperature drops below a certain critical value, the impact toughness of the steel decreases very much. Fast, leading to brittle fracture .
Studies have shown that it is austenite with a face-centered cubic lattice structure that does not produce low-temperature brittleness. The transformation of austenite to ferrite occurs with a decrease in temperature, and the further formation of ferrite and cementite is distributed in layers. The low-temperature brittleness of pearlite often occurs in the ferrite of the body-centered cubic lattice.
The low-temperature brittleness not only depends on the structure and composition of the material, but also the type of crystal lattice. The specific explanation is:
- (1) From a microscopic point of view, the resistance of dislocations when moving in the crystal lattice affects the low-temperature brittleness. The yield strength of steel is positively correlated with the increase of resistance. Dislocation movement is the main cause of plastic deformation of steel. For metals with low symmetry, as the temperature decreases, the lattice resistance of the dislocation movement increases, thereby reducing the thermal activation ability of the atoms and increasing the yield strength of the material.
- (2) From a macro point of view, the yield and fracture of steel are related to temperature, especially for metals with low symmetry. Generally, the fracture strength of steel is negatively correlated with temperature, and the yield strength is positively correlated with temperature. Below the brittle-ductile transition temperature, the yield strength of the steel is greater than the fracture strength, and the steel is brittle and fractured before it yields under stress .
Influencing factors of brittle-to-ductile transition of steel:
- (1) The influence of microstructure: The size of the grains has a certain correlation with the occurrence of cracks. The toughness of the material is improved due to the refinement of the grains to make the matrix deformation more uniform, and the expansion of the cracks is effective due to the increased grain boundaries. To prevent it, the grain boundary area is large so that the packing of dislocations caused by plastic deformation is not too large, which can prevent the formation of cracks, and the strength, plasticity and toughness of steel can be improved by refining the grain;
- (2) The influence of chemical composition: The alloying elements or impurities used to improve the strength and hardness of the steel will enhance the brittleness of the steel and make the toughness and plasticity worse. For example, the cold brittleness of the steel will increase significantly with the increase of manganese and phosphorus content. In addition, the aging sensitivity and cold brittleness of steel will increase with the increase of carbon content, thereby reducing the plasticity and impact resistance of steel;
- (3) The influence of crystal structure: body-centered cubic and close-packed hexagonal steels with low symmetry have a higher transformation temperature, poor plasticity, and a tendency to brittle fracture;
- (4) Influence of temperature: Temperature will affect the thermally activated diffusion process of impurity atoms in the crystal, and pinning dislocation atom gas clusters reduces the plasticity of steel;
- (5) The effect of loading speed: The effect of increasing the loading speed is equivalent to reducing the temperature of the material, which increases the embrittlement temperature of the steel and reduces the plasticity;
- (6) The influence of the shape and size of the steel: the strength of the steel will increase with the decrease in temperature, and the toughness will decrease, showing low temperature and cold brittleness (see Figure 1). The ductile-brittle transition temperature is the upper limit temperature for the evolution of steel from ductile failure to brittle failure. In practice, measures will be taken to make the minimum allowable working temperature of the steel higher than the upper limit of the ductile-brittle transition temperature to avoid low-temperature brittle failure.
Characteristics of brittle fracture
In order to ensure the safety of the structure, the changes in the mechanical properties of the structure at low temperatures should be considered during the design. The brittle fracture of steel has the following characteristics:
- (1) The stress generated during brittle fracture will be much lower than the yield limit of the material, which is usually attributed to the category of low stress failure;
- (2) The brittle fracture temperature of the material is often close to the ductile brittle transition temperature of the material;
- (3) There is no sign of brittle fracture and rapid cracking;
- (4) The stress concentration position of the component is the source of cracks that occur in brittle fracture.
We use laboratory tests to explore the mechanism of steel brittle fracture from occurrence, development to fracture.
Experiment 1: Three-point bending test
(1) The steel sample selection test selects the Q235 series steel plates produced by Anshan Iron and Steel, which are currently widely used in the construction industry, and we select 3 sets of specimens 1 with thicknesses of 12, 24, and 36 mm (see Figure 2) for testing.
(2) Test method
Use absolute ethanol as the cooling medium, and the coolant is liquid nitrogen. Use a low-temperature alcohol thermometer to measure the temperature. The test piece is kept in the cooling medium for 15 minutes. The test temperature points are 20C, 0C, -20C, -40C and -60C. In accordance with “Test Method for Crack Tip Opening Displacement of Metallic Materials” GB/T2358-94, the test is carried out with a straight three-point bending specimen.
The width of the experimental specimen is: H=2B, B—the thickness of the specimen, and the span moment is: L=8B. The schematic diagram of specimen loading is shown in Figure (Figure 3).
(3) Test results
Starting from room temperature, five test temperatures were selected. The steel has good toughness at room temperature without brittle failure. As the temperature decreases, the probability of brittle fracture of the specimen increases (see Figure 4).
Figure 4 reveals the distribution of steel brittle fracture, the proportion of specimens with brittle fracture is represented by fractions (the denominator represents the total number of specimens, and the numerator represents the number of specimens with brittle fracture). Having said that, we can see from the picture that the answer to the question at the beginning of this article is that the strength of the steel structure is affected at about minus 30 degrees.
Experiment 2: Low-temperature impact toughness test
(1) Selection of steel samples
The Q235 series steel plate, which is produced by Anshan Iron and Steel and is currently widely used in the construction industry, is selected for the test, with a thickness of 60-150 mm. The test shall measure and record the longitudinal impact energy, and the direction of the V-shaped notch of specimen 2 (see Figure 5) is consistent with the rolling direction of the steel.
(2) Test method
The experimental equipment adopts the ZBC3000 pendulum impact tester. Use a mixture of alcohol and liquid nitrogen as the cooling medium. After the sample is cooled to the specified temperature and kept in an incubator for a period of time, the impact test is carried out. The measuring tool is a thermometer (range: -80~50℃, minimum graduation Value: 1℃).
(3) Experimental results
The value of impact energy is positively correlated with temperature, and steel is obviously brittle at low temperature; impact toughness is negatively correlated with plate thickness. The lower the plate thickness, the more obvious the low temperature brittleness (see Figure 6).
The impact energy-temperature curve is generally S-shaped, and the graph is divided into three parts: the upper and lower platform and the transition temperature zone (see Figure 7). The ductile-brittle transition temperature is the arithmetic average of the maximum and minimum impact energy (upper and lower platform energy).
Analysis of test results
Experiments show that when only the thickness and temperature are considered, the brittle failure of the sample can be followed regularly.
- (1) Figure 4 shows that all the specimens with brittle fracture are located in the lower temperature area. Limited by the test conditions, the test points are discretely distributed, and the transition zone is not obvious. Steel brittle fracture occurs when the temperature is lower or the thickness is thicker;
- (2) The boundary of the brittle fracture area is relatively regular, which is a straight line with an approximate slope of negative;
- (3) As the temperature decreases, the yield strength and ultimate strength of the steel are increased, and the shrinkage and elongation of the section are reduced accordingly;
- (4) The shape of the fracture changes with the decrease of the test temperature. The area of the crystalline fracture surface with metallic luster (located in the center, flush) gradually increases, and the area of the fibrous fracture surface without metallic luster gradually decreases. The toughness of the steel structure decreases significantly with the decrease of temperature, and the brittleness of the steel structure increases ;
- (5) The impact energy value of steel decreases rapidly with the decrease of temperature, and the impact toughness decreases with the decrease;
- (6) At the same temperature, as the thickness of the steel plate increases and the distance from the surface to the center changes, the ductile-brittle transition temperature will increase and the impact toughness will decrease.
Factors affecting low temperature brittleness:
- (1) The properties of steel, the crystal structure, chemical composition and smelting method of steel determine the toughness and plasticity of steel, and it is also the main factor of steel brittle failure. Studies have shown that the cold brittle resistance of steel with low carbon content is lower than that of low-alloy steel;
- (2) The state of stress, which has a greater impact on the toughness and plasticity of steel components. The failure of the component under the two-way or three-way stress state indicates that the tensile steel component with high local stress concentration will appear in the two-way and three-way tensile stress state. This state will damage the steel component and increase the probability of brittle fracture of the steel component ( 7];
- (3) Structural form. The structural form of the steel component (considered as a comprehensive factor of brittle failure) determines the actual stress and working state of the component. The processing technology and initial defects of the component are also related to the structural form.
Measures to avoid cold brittleness of steel structures
Factors to be considered when selecting steel and steel components
The thickness of the steel, the temperature and process conditions of the processing and installation of the steel component, the structural type of the steel component, and the importance of the building or component. In order to improve the reliability of steel components, in addition to ensuring the strength of steel, it should also ensure that there are better work and process technical indicators (weldability, plasticity and resistance to crack growth, brittle fracture, fatigue, etc.).
Principles to be followed in selecting the structural type of steel components
Use thinner plates for steel; minimize stress concentration (caused by processing technology and structural type); minimize local plastic deformation in the stress concentration area (caused by welding heat); ensure a complete component combined section.
The stress along the thickness direction gradually increases due to the increase in thickness, which causes the position to be tensioned in three directions and gradually evolve to a plane strain state. The possibility of brittle fracture of steel components is increased, and the stress concentration of steel components (low carbon steel and Low alloy), its thickness should not be greater than 40mm.
Production, processing and installation should consider the following factors
When welding steel structures at sub-zero temperature, temporary thermal protection measures should be provided. When welding, prevent rain and snow from falling on the weld. Clean up the ice and snow on the site and steel components at any time, pay attention to anti-skid protection measures; take into account the shrinkage of steel when lofting under negative temperature, and the size of the cutting and planing of steel structure should be preset to a shrinkage gap of not less than 2mm.
Punching and shearing operations are not allowed when the temperature of the working place is lower than -15° (low alloy structural steel) or -20° (ordinary carbon structural steel), and the temperature of the working place is lower than -20° (low alloy structural steel) or- 16° (ordinary carbon structural steel) cold bending and correction are not allowed.
The grouping of components is carried out from the inside out according to the craft. The shrinkage value of the weld must be considered when the temperature is below zero. When assembled at room temperature, the spot weld is 50mm, and the weld is doubled when the temperature is below zero. Steel plates above 9mm (thickness) should be surfacing layer by layer from top to bottom. One weld should be welded at one time to prevent the temperature from falling too low. Re-welding should be heat treated to eliminate weld defects before continuing welding. Thick plates (tubes) should be preheated when welding at zero temperature. For common structural steels welded with medium heat input, the preheating temperature should meet the specification requirements .
Alkaline electrodes must be baked before use according to the process requirements; after drying, they should be placed in an incubator (80-100°C) and taken as needed. The exposed electrode is not allowed to exceed 2h (otherwise it needs to be re-baked), and the baking of the electrode is not allowed to exceed three times. Try to arrange welding during the day, and the second level welds are best arranged between 9 am and 4 pm.
Carbon dioxide (for gas shielded welding), the moisture content is not allowed to exceed 0.005% (weight ratio), and the purity must not be less than 99.5% (volume ratio). The internal pressure of bottled gas must not be lower than 1N/mm2. Use at zero temperature to check whether the bottle mouth is blocked due to freezing. When working below minus five degrees, use asbestos cloth to keep the cylinder warm. Electroslag welding and gas-electric vertical welding above 0°C need not be preheated; when the plate thickness is greater than 60mm, the base metal in the arc ignition zone should be preheated and not less than 50°.
The preheating method and the welding seam temperature control should meet the following requirements: use flame, electric or infrared heating methods to preheat before welding and maintain the temperature between passes, and simultaneously measure the temperature with a special thermometer; implement on both sides of the weld groove Preheating (the width of the preheating area is one and a half times the thickness of the weldment plate and not less than 100mm); the preheating temperature is measured on the opposite side of the weldment heating surface, and the measuring point should not be less than 75mm (at the distance All directions of the welding point before the arc passes); the front temperature measurement should be carried out after preheating and stopping heating.
The steel shall not be over-hardened and produce scratches, cracks and other defects during the operation to avoid cold deformation caused by cold working of the steel.
When welding components, the weld defects such as unwelded solids should be eliminated; the large thermoplastic deformation and welding internal stress left in the weldment should be eliminated; when the plate thickness of the welded structure is greater than 25mm, if the cooling is too fast, it may be after welding. Cracks occur and brittle fracture occurs. In view of this, preheating measures should be taken during welding to slowly cool the weld, thereby solving the problem of fracture.
Due to the constraints of shrinkage, cracks may appear in the weld during cooling. Therefore, a sufficient gap is left between the soft steel wire pad between the two steel plates to allow the weld to shrink calmly and avoid cracks. The fillet weld is made concave to reduce stress concentration. The surface of the finished concave seam has a large shrinkage tensile stress, and its 45° angle section weld has the smallest thickness, which is easy to cause cracking. The surface shrinkage and tensile stress of the convex seam is not large, and the 45° angle can strengthen the section, and it is not easy to crack after welding. By changing the concave weld seam to the convex weld seam, cracking can be effectively avoided.
Stress concentration is often caused by the increase of local stress caused by sudden changes in the outer dimensions of steel components, which is easy to form the most dangerous brittle failure. The welding process is also easy to form residual tensile stress that is unfavorable to the components. Therefore, avoiding excessive welding seams and sudden changes in cross-sections can help prevent brittle fracture.
The use of steel with good toughness can prevent brittle fracture from occurring. The energy absorbed by material fracture is closely related to temperature. The absorbed energy is divided into three regions according to elasticity, plasticity and elastoplasticity. In order to avoid a sudden fracture that is completely brittle, the toughness of the steel is required to be greater than the elasticity.
The structural gap or incomplete weld that intersects the weld is an inducement for brittle fracture in the structural details. The structural weld can be compared to a slender crack. The weld causes a higher residual tensile stress and causes nearby metals to deform due to thermoplastic When age hardening occurs, the brittleness of steel increases. For safety reasons, the construction environment of steel structures in low temperature areas should be considered when designing to ensure easy welding and penetration of structural details.
Stress concentration reduction method
Adjust the stress state of the component to reduce the stress concentration; change the structure type to reduce the ductile brittle transition temperature of the component and avoid brittle cracks in the component.
Influence of grain size
The toughness of steel increases as the grains become finer, and the ductile-brittle transition temperature also decreases; the smaller the grains in the steel, the shorter the slip line, the smaller the cracks generated on the slip surface, and the smaller the stress concentration. Cracks are less likely to grow, thereby improving the toughness of steel.
The three-point bending test of steel shows that the brittleness of steel increases with the decrease of temperature and the increase of thickness. Under cold conditions, the properties of steel at low temperatures change greatly, and the increase in brittleness leads to sudden brittle fracture of steel. The actual engineering application brings a lot of trouble. Experiments and research results show that brittleness
Fracture is most likely to occur in the ductile-brittle transition temperature range. In this interval, some toughness indicators of steel will change suddenly with temperature. In actual production operations, the influence of temperature should be judged in advance and effective preventive measures should be taken.
Author: Liu Zhu
Source: China Stainless Steel Structures 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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