Delayed fracture performance
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High-strength bolts are notched parts with high notch sensitivity and are prone to delayed fracture at the point where the notch is concentrated, such as the transition between the rod and the head or the root of the thread. Therefore, the delayed fracture of high-strength bolt steel is a typical example, and the resulting accidents occur frequently, and the economic loss is quite alarming. For example, in Japan in the early 1980s, a number of delayed fractures occurred on the Apollo Bridge on the Hanshin Expressway and the high-strength bolts on the Tokyo Expressway. It was decided to use ultrasonic to periodically inspect all the bolts. It is very huge, and the annual cost is as high as 100 million yen. Due to the frequent occurrence of delayed fracture accidents of high-strength bolts, bolts with tensile strength exceeding 1200 MPa should be used as much as possible. In the JIS B1186-1967 standard revision, the high-strength bolt grades are divided into F8T, F10T, and F11T, and the F13T-class bolts are explicitly abolished; after 1977, even the F11T-class bolts used in the normal strength range have many delays. In the 1979 revision, it was proposed to no longer use F11T bolts; in 1981, the railway bridge terminated the use of F11T bolts.
The concept of delayed fracture and characteristic delayed fracture, also known as delayed fracture, is a phenomenon of sudden brittle failure of a material under static stress after a certain period of time. The word change was first used in the 1940s to refer to the hysteresis fracture phenomenon of brittle materials such as glass. After the 1950s, it became widely used with the lag-breaking of cadmium-plated high-strength bolts for aviation.
Delayed fracture is an environmental embrittlement that occurs due to material-environment-stress interaction and is a form of hydrogen-induced deterioration of materials (hydrogen damage or hydrogen embrittlement). Other hydrogen-induced material deteriorations include hydrogen-induced plasticity loss, microcracks caused by hydrogen pressure (such as white spots in steel, cold cracks in welds, soaking cracks in H2S or acid), high-temperature and high-pressure hydrogen corrosion, hydride phase generation, and hydrogen. Lead to martensitic transformation and so on. Hydrogen-induced plastic loss and hydrogen-induced delayed fracture due to atomic hydrogen damage diffusion and enrichment are reversible hydrogen embrittlement. The so-called retardation refers to the accumulation of atomic hydrogen through stress-induced diffusion to criticality under constant load (or constant displacement) conditions. The value needs to pass a period of time, so the hydrogen induced crack will nucleate and expand after a certain period of time after loading. If the atomic hydrogen is removed, delayed fracture does not occur, so it is reversible.
It should be pointed out that the deterioration of the material of steel materials caused by hydrogen is various, and the names are often different due to different material strength levels and hydrogen-containing environments: for high-strength steels (tensile strength Rm ≥ 980 MPa) It is called delayed fracture; it is usually called sulfide crack and sulfide stress corrosion crack for medium strength steel (980MPa>Rm≥490MPa); it is called hydrogen induced crack, hydrogen induced crack, hydrogen for low strength steel (Rm<490MPa). Bubbling, etc.
Generalized delayed fractures include hydrogen embrittlement, stress corrosion, and liquid metal embrittlement. Since the stress corrosion of high-strength steel in aqueous media is essentially a hydrogen-induced cracking process, the delayed fractures referred to herein are mainly hydrogen-induced delayed fracture of high-strength steel and stress corrosion in aqueous media.
The delayed fracture phenomenon is a major factor that hinders the high strength of mechanical manufacturing. It is broadly divided into the following two categories:
1. It is mainly delayed fracture caused by hydrogen (external hydrogen) invaded by the external environment. Bolts used in bridges and the like are delayed in long-term exposure to humid air, rain, etc.
2. Delayed fracture caused by hydrogen (internal hydrogen) invading the steel during the manufacturing process such as pickling and electroplating. Delayed fracture occurs after a short time of several hours or days, such as plating bolts, after loading.
For the former, it is generally caused by corrosion during the long-term exposure process and the intrusion of hydrogen from the corrosion reaction at the corrosion pit; the latter is due to the stress intrusion of the steel in the steel during the manufacturing process such as pickling and electroplating. Caused by the concentration of stress concentration,
The main reason for the delayed fracture of steel in the natural environment is tempered martensitic steel, which generally has the following characteristics:
1. When the tensile strength is greater than 1200 MPa and the hardness is HRC ≥ 38, the sensitivity of delayed fracture increases remarkably;
2. Delayed fracture usually occurs near room temperature, but from room temperature to around 100 ° C, the sensitivity of delayed fracture increases with increasing temperature (distinction from low temperature brittle fracture);
3. Macroscopically, delayed fracture is not accompanied by large plastic deformation (distinction from creep fission);
4. Occurs under static load (strain rate is zero) (different from fatigue fracture);
5. Occurs at a stress that is much lower than the yield strength;
6. After tempering at a temperature of 350 ° C near the low temperature temper brittleness, the delayed fracture sensitivity is the highest;
7. Subject to the crack of the prior austenite crystal.