At present, the nuclear reactors used in nuclear power plants and nuclear-powered ships at home and abroad are mainly pressurized water reactors and boiling water reactors with high temperature (280~350℃) and high pressure (80~185atm, 1atm=101325Pa) water as the working medium. Low induced radioactivity; The neutron absorption cross section is smaller. Therefore, chromium-nickel austenitic stainless steel and a small amount of high nickel corrosion resistant alloy with austenitic structure are used for equipment, components and pipe fittings contacting primary olefin in the working medium of nuclear reactor. According to statistics, a 1 million mw large pressurized water reactor nuclear power plant, nuclear reactor ontology, in pile component, main line and evaporator will need more than 2000 tons of stainless steel sheet, rod, tube and forgings, but does not include to bear the high pressure of nuclear reactor, and use of high strength low alloy steel by pressure inside a lot of the overlaying welding used in high temperature water corrosion cr-ni austenitic stainless steel. It is often said that nuclear reactors are "made" of stainless steel, and it is no exaggeration.
Because Cr-Ni austenitic stainless steels have face centered cubic austenite structure, they generally do not have the danger of embrittlement even under the action of high neutron flux in the reactor, so they all have high nuclear stability. Because Cr-Ni austenitic stainless steel has excellent corrosion resistance, strict control of its chemical composition, impurities contained and high surface finish, the corrosion of these stainless steel in the process of long-term operation of nuclear reactors, the release rate is very low, the induced emission line is less; Because of the strict control of the cobalt, boron and other elements in the stainless steel used in nuclear reactors, which have a large neutron absorption cross section, the stainless steel used in nuclear reactors also has a small neutron absorption cross section.
Therefore, the nuclear grade stainless steel can meet the three special requirements of nuclear reactor structural materials of stainless steel. Due to the organization structure of the cr-ni austenitic stainless steel and corrosion resistance can meet the first two requirements, therefore, people's attention to the stainless steel used for nuclear reactors is focused on the amount of cobalt, boron and other elements in steel, these are nuclear grade cr-ni austenitic stainless steel with the nuclear grade cr-ni austenitic stainless steel the primary and most important distinction.
Basis for the development of 304NG (nitrogen control 0Cr18Ni10) and 316NG (nitrogen control 00Cr17Ni12Mo2) :
The intergranular corrosion and fracture accidents of 304 and 316 stainless steel members used in light water reactors (including pressurized water reactors and boiling water reactors) have occurred in foreign countries. In order to improve the resistance of steel to intergranular corrosion and intergranular stress corrosion, it is necessary to reduce the content of C in steel ≤0.03% (to 0.035% in France); To compensate for the reduced carbon and led to a decline in the 304 and 316 the strength of the steel, but by adding nitrogen, offset by the solid solution strengthening, but in order to prevent excessive nitrogen, but also as a new brand to apply and get approval to enter the trouble of actual engineering application, choose the nitrogen control at present, 304 and 316, permitted scope of nitrogen (0.10%), and development
304NG (controlled nitrogen 0Cr18Ni10) and 316NG (controlled nitrogen 00Cr17Ni12Mo2).
The corrosion rate and corrosion product release rate of 304NG (controlled nitrogen 0Cr18Ni10) are lower than that of 0Cr18Ni10Ti (321), indicating that 304NG (controlled nitrogen 0Cr18Ni10) has better corrosion resistance than that of 0Cr18Ni10Ti (321).
Some tests also indicate that 304NG (nitrogen controlled 0Cr18Ni10) has good intergranular corrosion resistance without intergranular corrosion tendency, and has better resistance to pitting corrosion and chloride stress corrosion than 0Cr18Ni10Ti. The pitting resistance of 304NG (nitrogen controlled 0Cr18Ni10) is much better than that of 0Cr18Ni10Ti, which is related to the formation of TiN and other non-metallic inclusions in the titanium of 0Cr18Ni10Ti steel, which leads to the deterioration of the corrosion resistance of the steel.
It can also be seen that the release amount of 316NG (nitrogen-controlled 00Cr17Ni12Mo2) and corrosion products are lower than that of 0Cr18Ni10Ti.
Some tests also show that 316NG (nitrogen controlled 00Cr17Ni12Mo2) has better intergranular corrosion resistance, pitting corrosion resistance and stress corrosion resistance than 0Cr18Ni10Ti due to the addition of a small amount of nitrogen.
