Inconel 690 alloy along with the rapid growth of the demand for nuclear power, the core component of the nuclear power plant of pressurized water reactor nuclear power plant steam generator heat transfer tube demand is also more and more exuberant. Steam generator heat transfer tube bears the energy exchange of one or two circuits and ensures the important function of the integrity of the pressure boundary of one circuit, and the reliability of the heat transfer tube directly affects the technical performance and safety of the nuclear power plant. Grain boundary stress corrosion cracking (IGSCC) has been the main concern of pressurized water reactor (PWR) nuclear power plant steam generator heat transfer tube material. 1980s, in order to solve the problem of stress corrosion of Inconel 600 alloy tubes in the steam generator first and second side of the medium, France, the United States, Japan and other countries jointly developed a high corrosion-resistant Inconel 690 (TT) alloy tubes. Tube, the early 1990s began to be formally applied to the project.
Inconel 690 alloy is based on Inconel 600 alloy, the chromium content increased to 30%, C content down to 0.04% below the development of a new type of evaporator with a nickel-based corrosion-resistant alloys, which not only has excellent resistance to stress corrosion cracking, but also has a high strength, good metallurgical stability and excellent processing performance and other characteristics. At present, the world’s only French Valinox, Sweden Sandvik, Japan Sumitomo, Anton Metal and a few other companies to produce Inconel690 (TT) alloy tubes, almost monopolizing the steam generator high-performance heat transfer tubes supply market. Domestic from the 1970s began to develop steam generator heat transfer tubes, initially 18-8 stainless steel tubes, and then developed a new 13 alloy tubes. Since China’s development of nuclear power, nuclear steam generator heat transfer tubes are basically the introduction of foreign tubes. It is worth noting that the introduction of varieties of tubes is also with the object of cooperation and the state of technology and change, China’s self-designed Qinshan nuclear power plant with more contact with Europe, the choice of the Germans at the time favored Incoloy 800 alloy tubes; French-designed Dayawan nuclear power plant selected the development of the successful Inconel 690 (TT) alloy tubes at the time of the near future; Qinshan II and Lingao Qinshan II and Ling’ao nuclear power plant have chosen the most popular Inconel 690 (TT) alloy tubes. At present, almost all new designs and under construction of nuclear power plant steam generators are selected Inconel 690 (TT) alloy tubes for heat transfer, and many of the original use of 18-8 stainless steel tubes or Inconel 600 alloy tubes of the steam generator units have been replaced with Inconel 690 (TT) heat transfer tubes. TT stands for Thermal Treatment. According to the third generation of nuclear technology on the demand for Inconel 690 alloy tube, at least 350t of GH3690 (Inconel 690) tubes are needed every year to meet the construction of 2 million kW nuclear power units. Therefore, China’s special steel enterprises state machine metal need to have in a relatively short period of time in accordance with international standards, mass production of high-performance nickel-based GH3690 alloy tube capacity, in order to meet the construction of China’s nuclear power plants and nuclear power equipment upgrading needs.
Chemical composition of the alloy Inconel 690:
The most important property of Inconel 690 alloy for tubes for nuclear power generation is the resistance to intergranular stress corrosion, while the depletion of chromium at the grain boundaries, the segregation of impurities to the grain boundaries, the intergranular carbides and their mechanical effect on stress concentration, as well as the grain boundary orientation errors, are essentially the causes of corrosion of the material. As a result, there is an increasing demand for compositional control of Inconel 690 alloys used in heat transfer tubes for steam generators in nuclear power plants. Table 72-5 lists the average compositions of Inconel 690 alloys.
In the last two decades, many studies have been conducted on the role of both major and trace elements in Inconel 690 alloys. The main research results are summarized as follows.
1. The role of major elements
Ni: It is found that increasing the amount of Ni can reduce the stress corrosion cracking sensitivity of Inconel 690 alloy in alkaline solution.
Cr:Increasing the Cr content reduces the IGSCC sensitivity of Inconel 690 alloy in high temperature water containing chloride ions and oxygen. It has been found that a Cr control of about 30% results in an alloy with high corrosion resistance. The addition of large amounts of chromium, significantly improve the creep resistance of the alloy, Cr can reduce the stacking layer error energy in nickel-based alloys, thereby reducing the creep rate by impeding dislocation climb and cross-slip.
Fe: people early research on Inconel 600 alloy found that Fe in deoxidized pure water or ammonia water with stress corrosion cracking sensitivity, and increase Cr content can reduce stress corrosion sensitivity.
C: In the rolled or annealed condition, C content has only a small or slight effect on IGSCC. However, after low temperature TT treatment, higher carbon content (0.02%~0.03%) contains more intragranular carbide precipitation than lower carbon content (0.01%) alloys, so lower carbon content alloys have a greater tendency to grain growth than higher carbon content alloys. the optimum content of C is 0.1%~0.025%, in order to tightly control the performance of the material, the content of C is recommended to be 0.01%~0.02%, and C content is recommended to be 0.01%~0.02%, and C content is recommended to be 0.01%~0.02%, and C content is recommended to be 0.01%~0.01%. ~The optimum C content is 0.1% to 0.025%, and for tight control of material properties, a C content of 0.01% to 0.02%, or 0.015% to 0.025% is recommended. When the carbon content rises from 0.006% to 0.015%, the corresponding dissolution temperature or grain roughening temperature rises from 960°C to 1030°C. The solubility of C in the Inconel 690 alloy as a function of temperature [211:log(%C)=4.771×10-3T(°C)-6.819.
ANTON METAL researchers in the study of GH3690 alloy found that the carbon content from 0.05% down to 0.015%, its resistance to intergranular corrosion significantly improved, but on 290 ℃, 50% NaOH solution in the resistance to stress corrosion performance does not have a significant impact; alloy added aluminum, titanium so that the resistance to intergranular corrosion performance significantly improved, but on 290 ℃, 50% NaOH solution in the Stress corrosion sensitivity has no significant effect; solid solution temperature of GH3690 alloy intergranular corrosion performance has a slight effect, 1000 ℃ solid solution of the specimen than 1070 ℃ solid solution of the specimen stress corrosion cracking to be shorter.
2. Role of trace elements.
Nb and B: The addition of Nb, B, Mo, adversely affects the resistance of Inconel 690 alloy to IGSCC;
Al: Increasing Al (<0.05%~0.14%) increases the depth of intergranular stress corrosion cracking; Ti: Inconel 690 alloy contains Ti, which is able to combine C and Cr, and play a role in pinning and rolling the grain boundaries;
S, P: deadly harmful elements, especially S, if you can not get a very low S content, it will seriously affect the alloy’s hot workability, and even make the alloy difficult to process;
N: It is believed that the alloy contains a moderate amount of N (0.2%) can refine the grain, without endangering the grain boundaries poor Al, poor Cr. Because the formation of Cr23C6 cell, each C atom needs to be combined with 4 chromium atoms, and the formation of a Cr2N cell, each nitrogen atom only needs to be combined with 2 chromium atoms. As can be seen, Inconel690 alloy with nitrogen microalloying, it seems to obtain a better overall performance.
Alloy preparation process
1. Smelting process
The French standard stipulates that “the alloy is smelted in an electric furnace or in other technically equivalent smelting processes”, and then remelted in an electroslag or vacuum furnace. 1998 published literature pointed out that Inconel690 alloy is usually smelted in a vacuum induction furnace or by the AOD method, followed by electroslag remelting. Most domestic alloys are produced by vacuum induction furnace melting and electroslag remelting.
2. Processing technology
Inconel690 alloy depth hot forming temperature range of 1040 ~ 1230 ℃, light molding can be carried out at temperatures below 870 ℃. Domestic not only mastered the Inconel690 alloy smelting process, but also has mastered the forging, extrusion, multi-channel cold rolling and heat treatment technology.
3. Heat treatment process
(1) solution treatment
(1) the solid solution treatment temperature on the hardness and grain size effects
According to the Inconel 690 alloy solid solution heat treatment temperature on the hardness value of the impact, it can be seen that the solid solution temperature increases, the hardness value of the specimen is a decreasing trend. Inconel 690 alloy in the deformation and solid solution treatment under the action of the grain boundaries through the return and part of the re-crystallization or re-crystallization caused by grain boundary migration, the grain boundary migration caused by the inter-grain juxtaposition of the grains, the grain grows up, the higher the solid solution temperature, the alloy in a variety of The higher the solid solution temperature, the easier the various defects in the alloy to rearrange or even partially eliminate, so that the more likely to occur in the crystal back and recrystallization, the faster the speed of grain boundary migration, the larger the grain size.
3. Corrosion resistance
1) solution corrosion resistance due to the high Cr content of Inconel690 alloy, so that in a wide range of aqueous solutions and gaseous environments show a very high corrosion resistance, in part of the acid also has good performance. Particularly suitable for handling nitric and nitric/hydrofluoric acids, Inconel690 alloy shows excellent corrosion resistance in nitric and hydrofluoric acid mixtures used for stainless steel pickling and recycling of nuclear fuel elements, as shown in Table 72-11 In hydrochloric acid, Inconel690 alloy is resistant to corrosion at low concentrations (about 15% or less) at room temperature, but is very resistant to corrosion in hot solutions and at high concentrations. Inconel 690 alloy has high corrosion resistance to phosphoric acid at room temperature and moderate temperatures. However, in boiling solutions, the alloy is only resistant to low concentrations of acid. At room temperature, Inconel 690 alloy has good corrosion resistance to all concentrations of sulfuric acid. In hot sulfuric acid solutions, the alloy typically shows high corrosion rates
2) Stress corrosion cracking resistance
Inconel690 alloys have good stress corrosion cracking resistance in many environments including chloride-containing solutions, high-temperature water, even more SO4, and moderate concentrations of NaOH. Inconel690 alloy stress U-shaped bending specimens were exposed to boiling 45% MgCl solution for 30d without cracking.
Inconel 690 alloy has been subjected to a wide range of stress corrosion cracking tests in the high temperature water encountered in nuclear steam generating stations, and the results show that the alloy is highly resistant to cracking in chloride-containing water, oxygenated water under crevice conditions, and deoxygenated (oxygen content of 0.02 ppm or less) water.Stress corrosion cracking tests of U-shaped bent specimens in NaOH solution The results show that Inconel 690 alloy is resistant to cracking in boiling NaOH solutions at concentrations up to 50%. A small amount of general corrosion occurred at 260°C and 320°C at concentrations of 30% or higher. In a 1% NaOH solution deoxidized at 316°C, U-shaped bending specimens did not crack after 9400h. Resistance to oxidation and sulfidation Inconel690 alloy has good resistance to oxidation and sulfidation in high temperature gas environments.
Summary.
1. In order to meet the increasingly high quality requirements of pressurized water reactor heat transfer tubes, Inconel690 alloy has been commonly used to control the purity, especially the control of ultra-low sulfur and phosphorus smelting process. This is the key to obtain excellent processing performance, excellent performance alloy.
2. From the organizational characteristics, the formation of chain-like distribution of carbides on the grain boundaries, smaller grain size or increase the special grain boundaries is the way to obtain excellent performance alloy.
3. Heat treatment plays a vital role in the performance of the alloy, for a specific composition of Inconel690 alloy, to explore the optimization of the heat treatment system is an important part of industrial production.
4. The smelting process and processing process are rarely reported in the literature, but computer-aided design and manufacturing is the current development trend.
Conclusion.
Nuclear power plant is the most important and successful example of peaceful utilization of nuclear energy. Nuclear power is a safe, clean, economic and reliable energy source. Replacing some fossil fuels with nuclear power to generate electricity not only preserves the fossil fuels for long-term use, but also contributes to environmental protection and reduces the transportation of large amounts of fuels, and promotes the sustainable development of the national economy. The core of a nuclear power plant is the reactor. High-temperature alloys are mainly used in the manufacture of water reactor evaporator heat transfer tubes, element grids and compression springs, as well as high-temperature gas-cooled reactors and some fast reactors, such as superheaters and reheaters and other parts. This paper introduces the structural characteristics of high-temperature alloy parts for reactors, working conditions and requirements for materials, chemical composition, organization and mechanical properties of high-temperature alloys for reactors and corrosion resistance, and focuses on the third generation of high-temperature alloys for reactors with heat transfer tubes GH3690 alloy.
Post time: Aug-05-2023
