Research on cold brittleness of steel structure under low temperature environment in cold area
Mechanism of cold embrittlement of steel at low temperature
Low temperature cold embrittlement of steel refers to the phenomenon that steel evolves from toughness to brittleness at low temperature until sudden failure occurs.
Many mechanical properties of steel are directly related to the change of temperature. When the temperature drops below a critical value, the impact toughness of steel decreases rapidly, which leads to brittle fracture.
The results show that austenite with fcc lattice structure does not produce low temperature brittleness. The transformation of austenite to ferrite occurs with the decrease of temperature, and pearlite with lamellar distribution of ferrite and cementite is formed. Low temperature brittleness often occurs in ferrite with body centered cubic lattice.
Low temperature brittleness is not only determined by the structure and composition of materials, but also by the type of lattice. The specific explanations are as follows:
- From the microscopic point of view, the resistance of dislocations moving in 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, with the decrease of temperature, the lattice resistance of dislocation motion increases, which reduces the thermal activation ability of atoms and increases the yield strength of materials.
- From a macroscopic point of view, the yield and fracture of steel are related to temperature, especially for metals with low symmetry. Generally, there is a negative correlation between fracture strength and temperature, and a positive correlation between yield strength and temperature. Under the brittle ductile transition temperature, the yield strength of the steel is greater than the fracture strength, and the steel will brittle fracture before yielding.
The influencing factors of brittle ductile transition of steel are as follows
- (1) Influence of microstructure: there is a certain correlation between the grain size and crack generation. The toughness of the material is improved by refining the grains to make the deformation of the matrix more uniform. The crack propagation is effectively prevented by the increase of grain boundaries. The area of the grain boundary is very large, so the dislocation accumulation caused by plastic deformation is not very large, which can prevent the formation of cracks, The strength, plasticity and toughness of steel can be improved by grain refinement;
- (2) The influence of chemical composition: the alloy elements or impurities used to improve the strength and hardness of steel will enhance the brittleness of steel, and make the toughness and plasticity worse. For example, the cold brittleness of steel will increase 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, thus reducing the plasticity and impact resistance of steel;
- (3) The influence of crystal structure: BCC and HCC steel with low symmetry have higher transition temperature, poor plasticity and brittle fracture tendency;
- (4) The effect of temperature: temperature can affect the thermal activation and diffusion process of impurity atoms in the crystal, and pinning dislocation atom air mass reduces the plasticity of steel;
- (5) The effect of loading speed: the effect of increasing the loading speed is equal to reducing the temperature of the material, which makes the embrittlement temperature of steel increase and the plasticity decrease;
- (6) Influence of steel shape and size: the strength of steel will increase with the decrease of temperature, and the toughness will decrease, showing cold embrittlement at low temperature (see Fig. 1). The ductile brittle transition temperature is the upper limit temperature of steel from ductile failure to brittle failure. In practice, measures will be taken to make the minimum allowable working temperature of steel higher than the upper limit of ductile brittle transition temperature, so as to avoid low-temperature brittle failure.
Fig.1 Steel stucture mechanical index-temperature variation curve
Factors affecting low temperature brittleness
- (1) The properties of steel, crystal structure, chemical composition and smelting method of steel determine the toughness and plasticity of steel, and are also the main factors of brittle failure of steel. The results show that the cold brittleness resistance of steel with low carbon content is lower than that of low alloy steel;
- (2) Stress state: the stress state has a great influence on the toughness and plasticity of steel members. The failure of the component under two-way or three-dimensional stress state shows that the tensile steel member with local high stress concentration will have two-way and three-dimensional tensile stress state, which will damage the steel member and increase the probability of brittle fracture of steel member;
- (3) Structural form: the structural form of steel member (considered as the comprehensive factor of brittle failure) determines the actual stress and working state of the component, and the processing technology and initial imperfection of the component are also related to the structural form.
Measures to avoid cold brittleness of steel structure at low temperature
Factors to be considered in selection of steel and steel members
The thickness of steel, the temperature and process conditions of fabrication and installation of steel members, the structural type of steel members, and the importance of buildings or components. In order to improve the reliability of steel members, in addition to ensuring the strength of steel, it is also necessary to ensure that there are good working and technological indicators (weldability, plasticity and resistance to crack propagation, brittle fracture, fatigue and other properties).
Principles to be followed in selecting structural type of steel members
Thin steel plate is selected; stress concentration (caused by processing technology and structural type) is minimized; local plastic deformation in stress concentration area (caused by welding heat effect) is minimized; and complete component combination section is ensured. The stress along the thickness direction increases gradually with the increase of the thickness, which makes the three-dimensional tension and evolution to the plane strain state. The possibility of brittle fracture of steel members is increased. The thickness of steel members (low carbon steel and low alloy) with stress concentration should not be greater than 40mm.
The following factors shall be considered in fabrication, processing and installation
- When the temperature is below zero, temporary thermal protection measures should be taken. During welding, rain and snow should be prevented from falling on the weld. Clean up the ice and snow on the site and steel members at any time, and pay attention to anti-skid protection measures; when setting out under negative temperature, the shrinkage of steel should be considered, and the size of cutting and milling of steel structure should be preset with a shrinkage gap of no less than 2mm.
- Punching and shearing operations are not allowed when the temperature of the working site is lower than – 15 ° (low alloy structural steel) or 20 ° (ordinary carbon structural steel), and cold bending and correction are not allowed when the working temperature is lower than – 20 ° (low alloy structural steel) or – 16 ° (ordinary carbon structural steel).
- The components are assembled from the inside out according to the process. When the temperature is below zero, the shrinkage value of weld must be considered when assembling. When the temperature is below zero, the spot weld is 50 mm in length, and the weld is twice longer when the temperature is below zero. The steel plate with a thickness of more than 9mm shall be overlaid in layers from top to bottom. One weld shall be welded at one time to prevent the temperature from falling too low. Heat treatment shall be conducted before welding again to eliminate weld defects. The thick plate (pipe) should be preheated when welding at zero temperature, and the preheating temperature should meet the specification requirements for the commonly used structural steel with medium heat input welding.
- The alkaline welding rod shall be baked before use according to the process requirements; after drying, it shall be put into the heat preservation box (80-100 ℃) and taken as soon as it is used. Exposed welding rod is not allowed to be more than 2 hours (otherwise it needs to be baked again), and the welding rod is not allowed to be baked more than three times. Welding should be arranged in the daytime as far as possible, and the welding of secondary weld should be arranged between 9:00 a.m. and 4:00 p.m.
- Carbon dioxide (for gas shielded welding), the moisture content shall not exceed 0.005% (weight ratio), and the purity shall not be lower than 99.5% (volume ratio). The pressure in the bottle with bottled gas is not allowed to be lower than 1n / mm2. When using at zero temperature, check whether the bottle mouth is blocked due to freezing. When working below – 5 ℃, the cylinder shall be insulated with asbestos cloth.
- Preheating is not required for electroslag welding and gas electric vertical welding above 0 ℃; when the plate thickness is greater than 60mm, the base metal in the arc striking zone shall be preheated at least 50 ℃.
Preheating method and weld temperature control shall meet the following requirements:
- The method of flame, electric or infrared heating is used to preheat and maintain the inter pass temperature before welding, and the temperature is measured by a special thermometer simultaneously;
- Preheating shall be carried out on both sides of the weld groove (the width of preheating area shall be one and a half times of the thickness of the welding plate of the weldment and shall not be less than 100 mm);
- The preheating temperature shall be measured on the opposite side of the heating surface of the weldment, and the measuring point shall not be less than 75mm (in all directions from the welding point before the arc passes); the temperature measurement at the front side shall be conducted after the preheating stops heating.
- In order to avoid cold deformation caused by cold working of steel, it is not allowed to harden excessively and produce scratches, cracks and other defects in the process of operation.
- When welding components, weld defects such as incomplete welding shall be eliminated;
- Eliminate the large thermoplastic deformation and welding internal stress left in the weldment;
- When the thickness of welded plate is more than 25 mm, if the cooling is too fast, cracks may appear after welding and brittle fracture will occur. In view of this, preheating measures should be taken to make the weld cool down slowly, so as to solve the fracture problem.
Due to the restraint of shrinkage, cracks may appear in the weld during cooling. Therefore, the soft steel wire should be padded between the two steel plates to leave enough gap, so that the welding seam can shrink easily and avoid cracks. Make the fillet weld concave to reduce the stress concentration. The thickness of the weld is the smallest, and the surface of the weld is easy to crack due to the small thickness of the weld.
The surface shrinkage tensile stress of the convex joint is small, but the 45 ° angle can enhance the cross-section and is not easy to crack after welding. By changing concave weld to convex weld, cracking can be avoided effectively. The stress concentration is often caused by the sudden change of the external dimension of steel members, which leads to the most dangerous brittle failure. The welding process is also easy to form adverse residual tensile stress, so avoiding excessive concentration of weld and sudden change of section is helpful to prevent brittle fracture.
Brittle fracture can be prevented by selecting steel with good toughness. 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 brittle fracture, the toughness of steel is higher than that required.
The structural gap or incomplete penetration weld that intersects with the weld is the inducement of brittle fracture of the structural details. The structural weld can be compared to a slender crack. The weld causes high residual tensile stress, which makes the nearby metal age hardening due to thermoplastic deformation, and the steel brittleness increases. For the sake of safety, the construction environment of steel structure in low temperature area should be considered in the design to ensure easy welding and penetration of structural details.
Method of reducing stress concentration
The results show that the stress state of the component is adjusted to reduce the stress concentration, the structure type is changed to reduce the ductile brittle transition temperature of the component, and the brittle crack is avoided;
Influence of grain size
The smaller the grain size, the shorter the slip line, the smaller the crack generated on the slip surface, and the smaller the stress concentration, the more difficult the crack propagation, thus improving the toughness of the 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 temperature change greatly. The increase of brittleness leads to sudden brittle fracture of steel, which brings great trouble to practical engineering application.
The experimental and research results show that brittle fracture occurs most easily in the range of ductile brittle transition temperature. In this range, some toughness indexes of steel will change suddenly with the change of temperature. In actual production, the influence of temperature should be judged in advance and effective preventive measures should be taken.
Author: Liu Zhu (Science and Technology Bulletin)
Source: China Angle Bars Manufacturer – Yaang Pipe Industry Co., Limited (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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