Brittle Analysis of Steel Structure in Extremely Cold Environment

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1 Brittle Analysis of Steel Structure in Extremely Cold Environment Liu Zhu Chongqing College of Electronic Engineering,Chongqing ,China Abstract The brittleness of steel has obvious correlation with temperature and thickness. The results indicate that the brittleness of steel has obvious regularity with the distribution of temperature and thickness. The lower the temperature of the steel, the higher the thickness of the steel. Key words: Extreme Cold Environment, Steel Structure, Brittleness, Analysis 1. INTRODUCTION Steel structure is one of the building structural types. It is made of steel or the material is mainly composed of steel, it is mainly made of steel plate and various kinds of steel column and beam, truss and other artifacts. Typically, it is produced in the factory and install on-site. Welding process is required among each components, bolts or high strength bolts, and rivet connection. Compared with other materials structure, steel structure is widely used in various types of industrial or residential buildings like factories, stadium and skyscrapers. Main reasons are: Lighter weight High intensity Superior seismic performance Excellent toughness and plasticity Good water and air tightness, Saving energy, Able to cross a larger span High degree of factory assembly, Excellent quality and performance Short construction period. The world's cold regions are mainly distributed in high latitudes in the northern hemisphere. (see table 1). With the development of the society, the steel structure has become more and more common under cold and even extremely cold environment. The influence of low temperature on steel structure has gradually appeared. Many brittle accidents of steel components have happened around the world due to low temperature since the 1930s. People have gradually realized the influence of low temperature on mechanical properties of steel structures. Temperate monsoon climate Temperate continental climate, subterminal coniferous forest climate Tundra climate Ice climate High mountain and plateau climate Table 1. distribution of cold regions worldwide Winter is cold and dry, mainly in northeast China, southeastern Russia, the Korean peninsula and nearby island areas. Cold winter, mainly distributed in south and north latitude 35 ~ 50, the Eurasian 50 ~ 65, the interior of the continent. Extremely cold winter, short summer, few precipitation, mainly distributed in the Eurasian continental arctic ocean along the coast of North America. Cold weather with few rainfall all year long, is mainly distributed in Greenland, arctic islands and Antarctica. It is mainly distributed in the Pamirs plateau of the Tibetan plateau in Asia, the east African plateau, the Rocky Mountains in North America, the Andes in South America, etc. 2. EXPERIMENTAL STUDY ON BRITTLENESS OF STEEL In order to ensure the safety of the structure, the changes of mechanical properties of the structure under low temperature should be considered. The characteristics of brittle fracture of steel are as follows: 1) The failure stress of brittle fracture is often far below the yield limit of the material, which belongs to low stress rupture; 90

2 2) The temperature of the material's brittle rupture is usually close to the material's ductile brittle transition temperature; 3) Before the brittle fracture occurs, there is no indication and the cracking process is fast; 4) The fracture source where the brittle fracture occurs is the stress concentration point in the component (I) we use laboratory tests to explore the mechanism of brittle fracture Experiment 1: three-point bending test Steel sample selection. In the experiment, three sets of specimens (12mm, 24mm and 36mm) of Q235 series steel plates, which are widely used in the construction industry, are tested respectively. Test method. We use anhydrous ethanol as the cooling medium, liquid nitrogen as the coolant and a low-temperature alcohol thermometer to measure temperature. The specimen is kept warm for 15min in the cooling medium. The test temperatures are 20C, 0C, -20c, -40c and -60c.The experiment is carried out with three-point bending method according to "metal material crack tip open displacement test method" GB / T , using straight 3 - point bending test specimens The width of the specimen is H=2B. B stands for the thickness of the specimen, span loading L=8B. The loading diagram of the specimen is shown in the following figure. Figure 1. Test piece loading diagram Test results. Starting at room temperature, five test temperatures are chosen and good toughness of steel is observed at room temperature. No brittle failure happens. The possibility of brittle fracture specimens has a negative correlation with the temperature (see chart 2). Figure 2. The Diagram of the Relationship Between the Temperature, Thickness of Steel and Brittleness Figure 2 shows the distribution of the brittle fracture of the steel, and the ratio of the brittle fracture parts is represented in fractions (the denominator represents the total number of specimens, and the numerator represents the number of the specimens with brittle fracture) Experiment 2: impact toughness test under low temperature Selection of steel sample. The Q235 series of steel plate with thickness of 60 ~ 150 mm is selected. It is widely used in the construction industry. In order to measure the longitudinal impact work, the gap direction of test piece should be in line with the rolling direction of the steel plate. Test method. ZBC3000 pendulum impact testing machine is used for testing. Using the mixture of alcohol and liquid nitrogen as cooling medium, the impact test should proceed after the sample reaches the required temperature and cools in the incubator for a period of time. The temperature is measured using the thermometer 91

3 (range: - 80 ~ 50, minimum degree value: 1 ) (Su Renquan And Wang Wanzhen, 2011). Experimental results. The impact work value is positively correlated with temperature, and the low temperature brittleness of steel is obvious. The impact toughness is negatively correlated with plate thickness, and the lower the thickness, the more obvious the low temperature brittleness (see figure 3). Figure 3. The rule of change in inpact toughness of steels with different thickness with respect to temperature The impact work - temperature curve is generally s-shaped, which is divided into three parts: the upper flat zone, the temperature transition zone and the lower flat zone (see figure 4).The arithmetic mean of the minimum impact work (lower flat zone energy) and the maximum impact work (upper flat zone energy). Figure 4. The classical curve between impact energy and temperature 2.3. Analysis of Test Results The experimental results show that the brittle failure of the sample has obvious patterns when only the thickness and temperature are considered. 1) as shown in FIG. 2, the test pieces having brittle fractures are all located in the lower temperature zone. Limited by experimental conditions, the distribution of test points is discrete, and the transition zone (between lower left and upper right) is not obvious. Brittle fracture occurs when the temperature is low or the thickness is large (Qing-Xiang and Yang Yao,1997). 2) the boundary of brittle fracture zone has obvious patterns as well. The boundary of brittle fracture zone has a negative slope. 3) With the decrease of temperature, the strength of the steel (yield strength and ultimate strength) has been improved, and the plasticity index (the section shrinkage rate and elongation rate) has been reduced; 4) Shape of fracture has changed with respect to temperature. Crystal fracture area with surface of the metal luster (located in the center, flush) is gradually increasing, Fiber with no metallic luster fibrous fracture area gradually decreases. The toughness of steel structure decreases significantly with temperature, and the brittleness of steel structure increases (Zhang YuLing and Pan Interritis, 2003). 5) the impact work of the steel decreases rapidly with the decrease of temperature, and the impact toughness becomes worse. 6) Under the same temperature, the impact toughness decreases with the thickness of the steel plate. 7) as the plate thickness increases and the change of distance from the surface to the center, ductile-brittle transition temperature will increase. Factors that influence low temperature brittleness. 1) properties of steel The toughness and plasticity of steel depend on the chemical composition of steel and the method of smelting. These are the main influencial factors for the brittle fracture of steel. The research shows that low carbon steel has lower resistance to coldness and is more brittle than low alloy steel (Ye Weijiang and ZhangYouyu, 1977). 92

4 2) stress state The stress state has great influence on the plasticity and toughness of steel components. When the component in the complex stress state (two-way/three-way) being destroyed, the tendency of brittle failure shows that the tensile steel components of the local high stress concentration zone will appear two-way and three-way tensile stress state, This state greatly reduces the plastic deformation of the steel component during destruction, greatly improve the possibility of brittle fracture of steel components (Magdalen Editor, 2010). 3) structural form Under the same conditions, a comprehensive influence factor of brittle fracture is the structural form of steel components. It determines the actual stress and working state of the components. Component processing technology and initial defects are also related to the structure form Mechanism of Low Temperature Brittleness in Steel Structure The low temperature brittleness of steel refers to the phenomenon that the steel is transformed from toughness into brittleness and then destroyed under low temperature. Many mechanical properties of steel are correlated with temperature. Nominal Stress in brittle failure of steel structures decrease with the decrease of the temperature. The plasticity of steel is reduced and the brittleness increases. The performance of the steel structure changes accordingly. when the temperature drops to a certain value, the impact toughness of steel suddenly dropped significantly and steel brittle fracture happens (Wang Ziyu, 1977). Studies have shown that a face-centered cubic lattice structure of austenite will not have low temperature brittleness. Temperature decrease makes austenite transform to ferrite, and thus generate pearlite with ferrite and cementite layered distribution. However, the ferrite body centered cubic lattice can have low temperature brittleness at low temperature. The influence factors of low temperature brittleness are many. It depends not only on factors such as the organization and composition of materials, but also the influence of lattice type, which can be explained in two aspects: From the microcosmic point of view, the resistance of dislocation movement in the crystal lattice affects the low temperature brittleness. Steel yield strength was positively correlated with the increase of resistance. Dislocation movement is the main reason for plastic deformation of the steel. The lattice resistance of the dislocation movement of the metal with low symmetry increases with the decrease of temperature. It reduces the thermal activation capacity of the atoms, thus increase the yield strength of the material. On the macro level, the yield and fracture of the steel are related to the temperature, especially for the metal with low symmetry. Usually, the fracture strength of steel is negatively correlated with temperature and yield strength is positively correlated with temperature. Below the ductile-brittle transition temperature, the fracture strength of the steel is less than the yield strength. Thus, brittle fracture occurs even when the steel does not yield (Pei Guming and Mr. Xie, 1977). Influence factors of steel's ductile-brittle transition: 1)the influence of the microstructure, grain size has a certain correlation with the occurrence of the crack. The toughness of the material is improved by refining the grain to make the deformation more uniform. The expansion of the crack is effectively prevented by increasing grain boundaries. Grain boundary area is so large that the pile-up of the dislocation caused by the plastic deformation will not be great. It can prevent the crack. Thus the improvement of the steel strength, plasticity and toughness can be done by refining the grain size; 2) the influence of chemical composition: the alloying elements and impurities used to increase strength and hardness of steel will strengthen steel brittleness and worsen the toughness and plasticity, such as the increase of manganese and phosphorus in steel can significant increase cold brittleness of steel. External carbon also can increase the aging sensitivity of steel and cold brittleness, reduce the plasticity and impact resistance of steel; 3) the influence of crystal structure: Body-centered cubic and dense hexagonal steel with low symmetry has a higher transition temperature and bad plasticity. It shows tendency of brittle fracture. 4) influence of temperature: temperature can influence the thermal activation and diffusion process of impurity atoms in the crystal. 5) influence of loading speed: the effect of increasing loading speed is equivalent to lowering the temperature of the material. It makes the brittle temperature of steel increase and the plasticity decreases. 6) influence of the shape and size of the steel: as the temperature decreases, the strength of the steel generally increases, the toughness decreases and the steel has cold brittleness (see figure 5). The ductile-brittle transition temperature is the upper limit for steel from ductile damage to brittle failure. To avoid brittle damage, the minimum allowable working temperature of the steel should be higher than the ductile brittle transition temperature. 93

5 Figure 5. The curve of main mechanical indexes of steel structure with respect to the temperature 3. PREVENTION MEASURES OF LOW TEMPERATURE BRITTLENESS OF STEEL STRUCTURE 3.1. Factors to be Considered when Selecting Steel and Steel Components: The thickness of steel, the temperature and process conditions of the fabrication and installation of steel components, the structural type of steel components, the importance of buildings or components. In order to improve the reliability of steel components, in addition to ensure the strength of the steel, the good work and technology indicators, (such as brittle resistance and fatigue resistance, plasticity limit, good welding performance and crack propagation resistance, etc.) should also be guaranteed Principles of Selection of Structural Type of Steel Components: Thin steel plates are generally better; Minimize stress concentration (caused by machining process and structure type); Minimize the local plastic deformation of the stress concentration area (caused by welding thermal influence); Ensure the integrity of the composite cross section. As the thickness increases, the stress increases in the thickness direction, the three-way tension happens and gradually transform to the plane strain state. The possibility of brittle fracture of steel components is increased. For stress-concentrated steel components (mild steel and low alloy), the thickness should not be greater than 40mm (Liu Qu and Liu Zhongming and Liu Xiang, 1977); 3.3. Factors to be Considered in Production, Processing and Installation: Under negative temperature, the shrinkage of steel should be considered. Not less than 2mm shrinkage gap should be preset when doing cutting of steel structure and considering the size of the planning. When workplace temperature is below -15 (low alloy structural steel) or -20 (ordinary carbon structural steel) punching and shearing process shall not be initiated. When work temperature is below - 12 (low alloy structural steel) or - 16 (ordinary carbon steel structural) correct and cold bending process shall not be initiated; The assembly of components is carried out from inside to outside. The shrinkage gap shall be preset when welding negative temperature. At normal temperature, the weld is 50mm, and the weld length is doubled when the temperature is below 0. More than 9 mm thick steel plate should be layered welding. Welds are surfaced from top to bottom. In principle, a one-time welding is needed to prevent the temperature drop too low. welds should be heat-treated again before re-welding and elimination of weld defect is needed before welding. When welding thick steel plate, thick steel tube and medium thickness plate under negative temperature, we need to pre-heat the material. The minimum preheating temperature should meet the standard requirements when the steel is used in the medium heat input welding(gb/t ). Electroslag welding and gas welding don t need to be preheated; However, when the thickness is greater than 60 mm, it is appropriate to preheat the arc area of the parent metal preheating to not less than 50 ; Preheating method and temperature control between weld shall meet the following requirements: preheat before welding and keep temperature between welds needs to use flame heating, electric heating and infrared heating. At the same time, use special temperature measuring instrument to measure the temperature; Preheat on both sides of weld groove (preheating width is 1.5 times and not less than 100mm); The preheating temperature should be measured at the back of the heating surface of the welding part, and the measuring point should be no less than 75mm (in the direction of the welding points before the arc); Temperature measurement at front side should be carried out after the preheat process with flame heater. In the processing, the steel is not allowed to be excessively hardened, cracked and scratched, so as to avoid the cold deformation caused by cold processing of steel. The weld defects should be eliminated when welding components; Eliminate the large thermoplastic deformation and residual stresses in the welding parts; When the plate thickness of the welded structure is 94

6 greater than 25mm, if it is cooled too quickly, it is possible to crack after welding. In view of this, preheat process in order to make the weld slowly cooled is necessary to solve the fracture problem. Due to the constraints of shrinkage, cracks may occur during cooling. Therefore, it is necessary to set enough gap between two plates to make the weld easy to contract and avoid cracks. The surface of the fillet weld is concave to reduce stress concentration. The tensile stress of the concave seam is large, on a 45 Angle section the weld thickness is minimum, making it easy to crack after welding. Convex seam surface shrinkage stress is not big, and 45 Angle can enhance cross section, making it not easy to crack after welding. In the condition of the crack of concave weld, use convex weld to avoid cracking. Stress concentration tends to be caused by sudden changes in the geometry and size of steel components, and the increase of the local stress makes the brittle damage which is the most dangerous. The welding process is also easy to form the residual tensile stress of the components. Thus, avoid void too concentrated weld and sudden change in the cross-section. It can help prevent brittle fracture. The steel structure material with some toughness can prevent brittle fracture. The energy absorbed in the fracture process is closely related to the temperature. The energy absorbed is divided into three zones according to plasticity, elastoplastic and elasticity. In order to avoid a complete brittle fracture, the toughness of the steel should not be lower than the elasticity. The tectonic gap or impermeable weld intersecting the weld is the cause of brittle fracture in the structure. Tectonic welds can be compared to narrow cracks. Weld causes high residual tensile stress and make metal nearby age hardening by thermoplastic deformation. Steel brittleness increases accordingly. Therefore, it is necessary to consider the construction conditions of steel structure in low temperature area to ensure that the construction details can be welded through Reduce Stress Concentration Adjust the stress state of the component and reduce the stress concentration. The change of structural type can reduce the risk of brittle fracture of components and reduce the risk of brittle cracks Control the Effect of Grain Size The finer the grain of steel, the better the toughness. It can reduce the ductile-brittle transition temperature; The smaller the grain in the steel, the shorter the slip line, the smaller the crack in the sliding surface, the smaller the stress concentration and the more difficult the crack is to expand, thus improving the toughness of the steel. 4.CONCLUSION Cold bending test shows that the steel brittleness increases with lower temperature and the increase of the thickness of the steel. In cold conditions, the nature of the steel has greatly changed. The increase of brittleness causes a brittle fracture of the steel and brings a lot of trouble to the actual engineering application. According to the changing trend of the mechanical properties of steel with respect to temperature, the most dangerous temperature range of brittle fracture is the ductile-brittle transition temperature range. In this interval, some of the ductile indicators of steel can change dramatically as the temperature changes. In the process of actual production, the influence of temperature should be fully considered and preventive measures should be taken. REFERENCES Su Renquan, Wang Wanzhen (2011) Fracture test of high strength steel gap plate at low temperature, Cryogenic engineering, 24(5), pp Qing-Xiang,Yang,Yao(1997). Kind of sustenance, Structural steel ductile-brittle cleavage characteristic stress theory - Ⅰ cleavage characteristic stress concept and structural steel cleavage fracture behavior analysis. Journal of iron and steel research,15(2), pp Zhang Yuling, Pan Interritis(2003). Review of the effects of low temperature on the properties of steel and its components, China railway science,24(2), pp Ye WeiJiang, ZhangYouYu(1997). A brief analysis of the factors affecting the low temperature toughness of metals,natural gas and petroleum,13(1), pp Magdalen Editor(1990). Metal mechanical properties, Beijing: mechanical industry press. Wang Ziyu(2010). Summary of low temperature and fracture mechanism of structural steel, Scientific and technological information,11(8), pp.166. Pei Guming, Mr. Xie(2011). Mechanical properties of steel cryogenic environment research, Proc. of the 2nd national conference on structure engineering (the first Ⅰ copies), pp Liu Qu, Liu Zhongming, Liu Xiang(1999). Steel structure brittle fracture for engineering hazards and prevention, Low temperature construction technology, 34(3), pp GB/T , Test of open displacement test of crack tip of metal materials. 95