Brief introduction of reasons for brittle fracture of high-strength bolts

1 Material defects

When the content of carbon, sulfur, phosphorus, oxygen, nitrogen, hydrogen and other elements in the steel is too high, its plasticity and toughness will be seriously reduced, and the brittleness will increase accordingly.

The increase of carbon content in steel will increase the brittle transition temperature of steel. As the carbon content increases, the maximum Chapy impact value of steel decreases significantly. Chabe impact value and test temperature

The gradient of the degree curve tends to be slow, and the brittle transition temperature increases significantly. The increase of the phosphorus content in the steel reduces the grain boundary fracture stress, and the brittle transition temperature increases. The steel containing more than 0.1% phosphorus will cause The grain boundary fracture stress is reduced. The effect of phosphorus on the brittle transition temperature of steel increases with the increase of phosphorus content, and the brittle transition temperature of steel increases. The presence of sulfur and phosphorus has a detrimental effect on the fracture toughness of steel. As the content of sulfur and phosphorus increases, the K1C value of steel decreases. The increase of sulfur and phosphorus content reduces the steel K1C, and sulfur is more harmful.

The presence of manganese in steel is helpful to improve its brittleness. As the ratio of manganese to carbon increases, the harmful effects of carbon and phosphorus decrease, and the brittle transition temperature of steel is significantly reduced. (Guide: Brief introduction of various types of gaskets)

Sulfur and phosphorus reduce the fracture toughness of steel. There are two main reasons: ①It is concentrated in the original austenite grain boundary, which promotes the embrittlement of the product boundary; ②The sulfur chemical reaction generates MnS to form brittle microcracks in the matrix. The core increases the nucleation source of micro-cracks, causing brittle fracture to occur easily.

Reducing the content of sulfur and phosphorus in steel is an important way to improve the fracture toughness of steel, especially ultra-high strength steel. Selecting an appropriate smelting method is the most direct and easy way to improve the purity of steel. Compared with ordinary electric furnace steelmaking, vacuum smelting can improve the purity of steel. Ultra-high strength steel generally uses vacuum consumable furnace (or vacuum arc). Furnace) remelting to reduce impurities and segregation in the steel to improve the fracture toughness of the steel. All advanced industrial countries have made lower regulations on sulfur and phosphorus content, which are generally limited to less than 0.06%, but the segregation of steel produced by major steel plants in my country is still heavy. The quality is unstable. Among the factors that affect segregation (iron ore elements, steelmaking method, steel ingot size, smelting technology, etc.), the main reason is steelmaking method and smelting technology. Large segregation will cause hot embrittlement, cold embrittlement, cracks, fatigue, etc. A series of questions.

2 Stress concentration

When the steel has a stress concentration in a certain part, a two-dimensional or three-dimensional stress field of the same number appears to make the material difficult to enter the plastic state, which leads to brittle failure. The more serious the stress concentration, the more the plasticity of the steel decreases, and the greater the risk of brittle fracture. The stress concentration of steel structures or components is mainly related to the details of the structure:

3Use environment

When the bolt is subjected to a larger dynamic load or works at a lower ambient temperature, the possibility of brittle failure of the bolt increases.

Above 0℃, when the temperature rises, the strength and elastic modulus of the steel will change. Generally, the strength decreases and the plasticity increases. When the temperature is within 200°C, the performance of the steel does not change much. However, the tensile strength of the steel rebounds at about 250°C, fy is greatly improved, and the plasticity and impact toughness decrease, and the so-called blue brittleness occurs. At this time, the hot-worked steel is prone to cracks. When the temperature reaches 600~C, and E is close to zero, the steel structure almost completely loses its bearing capacity.

When the temperature is below 0℃, as the temperature decreases, the strength of the steel increases slightly, while the ductility decreases and the brittleness increases. Especially when the temperature drops to a certain temperature range, the impact toughness value of the steel drops sharply, and low-temperature brittle fracture occurs. The brittle failure of steel structure at low temperature is usually called low-temperature cold brittleness, and the cracks produced are called cold cracks.

4The influence of loading rate

A large number of experiments have shown that a high loading rate will increase the risk of brittle fracture of the material, and it is generally believed that its effect is equivalent to lowering the temperature. With the increase of the deformation rate, the yield strength of the material will increase. The reason is that the material is too late for plastic deformation and slippage, so the thermal activation time required for the dislocation to get rid of the restraint and slippage is reduced, and the brittle transition temperature is increased. So it is easy to produce brittle fracture. When there are notches on the specimen, the effect of strain rate is more significant. Once a brittle crack occurs, there will be a serious stress concentration at the crack tip. This sudden increase in stress is equivalent to a load with a very high loading rate, which causes the crack to rapidly destabilize and expand, and finally causes brittle failure of the entire structure.

In summary, material defects, stress concentration, use environment and loading rate are the main factors affecting brittle fracture, and stress concentration is particularly important. It is worth mentioning here that the stress concentration generally does not affect the static ultimate bearing capacity of the steel structure, and its influence is usually not considered in the design. But under the action of dynamic load, serious stress concentration plus material defects, residual stress, cooling hardening, low temperature environment, etc. are often the root causes of brittle fracture.

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