Factors of brittle fracture of steel structure bolts

  When we use steel structure bolt, we often avoid the situation of steel structure bolt brittle fracture, and to avoid this situation, we must find out the factors of this situation first, the following points are the factors of steel structure bolt brittle fracture.
             1. Influence of loading rate of steel structure bolts
         A large number of experiments show that the high loading rate will increase the risk of brittle fracture of the material, which is generally considered to be equivalent to lowering the temperature. With the increase of deformation rate, the yield strength of the material will increase.
              The reason is that the material has no time for plastic deformation and slippage, so the thermal activation time for dislocations to get rid of the bondage is reduced and the brittle transition temperature is increased, so brittle fracture is easy to occur.
          When there is a notch in the specimen, the effect of strain rate is more significant. Once a brittle crack is generated, there will be a very serious stress concentration at the crack tip. This sudden increase of stress is equivalent to a load with a high loading rate, which makes the crack rapidly unstable and propagates and causes brittle failure of the whole structure.
           2. Service environment of steel structure bolts
When the steel structure bolt is subjected to large dynamic load or working at low ambient temperature, the possibility of bolt brittle failure increases.
           When the temperature is higher than 0 ℃, the strength and elastic modulus of the steel are changed. Generally, the strength decreases and the plasticity increases. When the temperature is less than 200 ℃, the properties of the steel do not change much.
            At this time, the so-called "brittle" phenomenon of steel occurs at about 250 ℃, but the so-called "brittle" phenomenon occurs at about 250 ℃.
            When the temperature reaches 600 ~ C and E is close to zero, the steel structure almost loses its bearing capacity.
When the temperature is below 0 ℃, with the decrease of temperature, the strength of steel increases slightly, while the plasticity toughness decreases and the brittleness increases. Especially when the temperature drops to a certain temperature range, the impact toughness of the steel decreases sharply and brittle fracture occurs at low temperature. Generally, the brittle failure of steel structure at low temperature is called "cold brittle phenomenon" and the cracks produced are called "cold cracks"
           3. Stress concentration of bolt in steel structure
          When the stress concentration occurs in a certain part of the steel, the two-dimensional or three-dimensional stress field of the same sign appears, which makes it difficult for the material to enter the plastic state, thus leading to brittle failure. The more serious the stress concentration is, the more the plasticity of steel is reduced, and the risk of brittle fracture is also greater. The stress concentration of steel structure or component is mainly related to the structural details
         4. Material defects of steel structure bolts
When the content of carbon, sulfur, phosphorus, oxygen, nitrogen and hydrogen in steel is too high, the plasticity and toughness will be seriously reduced, and the brittleness will be increased accordingly.
The brittle transition temperature of steel increases with the increase of carbon content. With the increase of carbon content, the larger Charpy impact value of steel decreases significantly. The gradient between Charpy impact value and test temperature tends to be slow.

  With the increase of the content of phosphorus in the steel, the grain boundary fracture stress decreases. With the increase of  the brittle transition temperature, more than 0.1% phosphorus in the steel will cause the grain boundary fracture stress to decrease.
          The effect of phosphorus on the brittle transition temperature of steel increases with the increase of phosphorus content. The existence of sulfur and phosphorus is harmful to the fracture toughness of steel. With the increase of sulfur and phosphorus content, the K1C value of steel decreases. With the increase of sulfur and phosphorus content, K1C of the steel is reduced, and sulfur is more harmful.
          With the increase of the ratio of manganese to carbon, the harmful effects of carbon and phosphorus decrease, and the brittle transition temperature of steel decreases significantly.
            There are two main reasons for sulfur and phosphorus to reduce the fracture toughness of steel: 1) segregation in the original austenite grain boundary, which promotes the product boundary embrittlement; 2) the sulfur chemical reaction generates MNS to form the origin core of brittle microcracks in the matrix, which increases the nucleation source of microcracks, leading to brittle fracture.
           Reducing the content of sulfur and phosphorus in steel is an important way to improve the fracture toughness of steel, especially for ultra-high strength steel. Choosing suitable smelting method is a direct and easy way to improve the purity of steel.
Compared with ordinary EAF steelmaking process, the purity of steel can be improved by vacuum smelting. Ultra high strength steel is remelted in vacuum consumable furnace (or vacuum arc furnace) to reduce impurities and segregation in steel and improve fracture toughness of steel.
        The contents of sulfur and phosphorus are generally limited to less than 0.06% in all advanced industrial countries. However, the segregation of steel produced by major steel mills in China is still heavy. Among the factors affecting segregation (iron ore elements, steelmaking method, ingot size, smelting technology, etc.), the main reason is steelmaking method and smelting technology. Large segregation will cause a series of problems such as hot brittleness, cold brittleness, crack and fatigue.
The main factors of brittle fracture of steel structure bolts are loading rate, service environment, stress concentration and material defects, which we should avoid and avoid from these four situations.
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