Factors affecting brittle fracture of steel structural bolts

Factors affecting brittle fracture of steel structural bolts

When using steel structural bolts, we often avoid brittle fracture of steel structural bolts. To avoid this situation, we must first identify the factors that cause it. The following are the factors that contribute to brittle fracture of steel structural bolts.

1. The influence of loading rate on steel structure bolts

Numerous experiments have shown that high loading rates increase the risk of brittle fracture in materials, and it is generally believed that their impact is equivalent to reducing temperature. As the deformation rate increases, the yield strength of the material will increase.

The reason is that the material does not have enough time to undergo plastic deformation and slip, so the thermal activation time required for dislocations to break free and slip is reduced, resulting in an increase in the brittle transition temperature and susceptibility to 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 severe stress concentration at the end of the crack. This sudden increase in stress is equivalent to a high loading rate load, causing the crack to rapidly propagate and cause brittle failure of the entire structure.

2. Service environment for steel structural bolts

When steel structural bolts are subjected to significant dynamic loads or operate at lower ambient temperatures, the likelihood of brittle failure of the bolts increases.

Above 0 ℃, as the temperature increases, both the strength and elastic modulus of the steel change, generally with a decrease in strength and an increase in plasticity. When the temperature is within 200 ℃, the performance of the steel does not change much.

However, when the tensile strength of steel rebounds at about 250 ℃, fy is greatly improved, and the so-called "blue brittleness" occurs when the plasticity and impact toughness decline. At this time, cracks are prone to occur in Hot working steel.

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 slightly increases, while the plasticity toughness decreases and the brittleness increases. Especially when the temperature drops to a certain temperature range, the impact toughness value of the steel sharply decreases, leading to low-temperature brittle fracture. The brittle failure of steel structures at low temperatures is commonly referred to as the "low-temperature cold brittleness phenomenon", and the cracks generated are referred to as "cold cracks"

3. Stress concentration of steel structure bolts

When stress concentration occurs in a certain part of the steel, a two-dimensional or three-dimensional stress field of the same size appears, making it difficult for the material to enter a plastic state, leading to brittle failure. The more severe the stress concentration, the more plastic the steel decreases, and the greater the risk of brittle fracture. The stress concentration of steel structures or components is mainly related to structural details:

4. Material defects of steel structural bolts

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

An increase in carbon content in steel will increase the brittle transition temperature of the steel. As the carbon content increases, the larger Charpy impact value of the steel significantly decreases. The gradient between the Charpy impact value and the experimental temperature curve tends to slow down.

The brittle transition temperature significantly increases, and the increase in phosphorus content in the steel reduces the grain boundary fracture stress. As the brittle transition temperature increases, containing more than 0.1% phosphorus in the steel will cause a decrease in grain boundary fracture stress.

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. As the content of sulfur and phosphorus increases, the K1C value of steel decreases. The increase in sulfur and phosphorus content reduces the K1C of the steel, resulting in greater harm from sulfur.

The presence of manganese in steel helps 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 significantly decreases.

There are two main reasons for sulfur and phosphorus to reduce the Fracture toughness of steel: ① segregating at the original austenite grain boundary to promote the embrittlement of the grain boundary; ② The sulfur chemical reaction generates MnS, which forms the brittle microcrack origin core in the matrix, increasing the nucleation source of microcracks and making brittle fracture prone to occur.

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. Choosing appropriate smelting methods is a direct and easy way to improve the purity of steel.

Compared with the ordinary electric furnace steelmaking method, vacuum smelting can improve the purity of steel. Ultra high strength steel is generally remelted in a vacuum consumable furnace (or vacuum electric arc furnace) to reduce impurities and segregation in steel and improve the Fracture toughness of steel.

Advanced industrial countries have set lower regulations for sulfur and phosphorus content, generally limited to 0.06% or less, but the segregation of steel produced by major steel mills in China is still relatively heavy. The unstable quality and factors affecting segregation (such as iron ore elements, steelmaking methods, ingot size, smelting technology, etc.) are mainly caused by steelmaking methods and smelting technology. Large segregation will cause a series of problems such as hot brittleness, cold brittleness, cracks, fatigue, etc.

The main factors affecting brittle fracture of steel structural bolts include loading rate, usage environment, stress concentration, and material defects. We need to avoid and also avoid from these four situations.

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