Causes of grain boundary fracture of seamless steel pipe

This is because of the formation of (Fe, Cr) 2B, the original chromium content will be lacking; and it is proved that by adding 2~3 titanium, corrosion resistance can be obtained. Since there was concern about stress corrosion cracking of austenitic stainless steel in liquid environments such as chemical plants, studies were conducted under environmental conditions to prevent cracking (1965~1970). The effect of oxygen in water, temperature, especially the effect of alloying elements (the stress corrosion cracking of seamless steel pipes in high-temperature water containing ions is described in Section 7.4). For the high-temperature water environment of nuclear reactors, because there was no data on the high-temperature water of 304 steel at the beginning, 347 steel with resistance to intergranular corrosion has been used as the constituent material. However, Leggett et al. The 304 steel was tested in high temperature water (316 ℃) containing oxygen with different pH. The results showed that grain boundary cracks would not occur except when the pH was relatively low 3 to 4. Therefore, it was concluded that in the nuclear reactor In high temperature pure water, 304 steel can be used instead of 347 steel. In addition, with regard to stress corrosion cracking, it was concluded before the early 1960s that it is difficult to generate at least primary pure water. However, in the United States of light water reactors, there have been many cases of stress corrosion cracking, so this prediction was rejected.

According to the results of the 1973 U.S. and others on fracture accidents, primary fractures mostly occur in piping, and secondary fractures mostly occur in heat exchangers and steam generators. The main material problems in the nuclear reactor environment demonstrated at Nagasaki 21 in 1975 are excerpted from the light water furnace and attached in Table 4.1, among which stress corrosion cracking and corrosion fatigue caused by high temperature water of stainless steel are the main problems. Regarding the fractures generated by the primary system, the fracture forms are mostly grain boundary fractures; due to or after the stress heat treatment, the generated parts are more sensitized; the compounds are not necessarily necessary. An example of grain boundary fracture of 304 steel produced by the BWR of Dhesden 1, and subsequent fractures were found mainly in the fuel coating, piping, heat exchangers, pressure vessels, and nozzles of  seamless steel pipes. 2). It was initially suspected to be the influence of ions and dissolved oxygen, but it was later found in the laboratory that ions may also fracture in pure water below 0.1 × 10-4 231. Especially since 1974.

Since the grain boundary fracture was discovered in the heat-affected part of the seamless steel pipe joint used in the piping system of Dresden 2 BWR, the same phenomenon has occurred many times not only in the United States but also in Japan, and a lot of research has been carried out to this end241 The reason for grain boundary fracture seamless steel pipe is the same as intergranular corrosion in metal, which is due to the sensitization of grain boundaries. Therefore, it is an element that affects stress corrosion cracking in high-temperature pure water of sensitized stainless steel. Discussion started. Xiaoruo et al. (1976) 2 passed the double U-bending test piece of 18Cr13Ni steel sensitized at 650℃ for 30h, and compared the C, Si, P, S which caused grain boundary fracture in pure water at 250℃ under normal temperature and oxygen saturation state. , Mn and N influences were discussed. As shown in the result 4125, carbon significantly improved the fracture property, even if 0.01C was contained, the properties were improved, and the properties of P and S were also improved, but the effects were small, and the effects of S, Mn, and N were not seen. Abe et al. (1976)2 also investigated the influence of impurity elements on 18CrNi steel in which the carbon content was kept below 0.02 and grain boundary.