Microstructure and properties of Ni based corrosion resistant alloy hastelloy C-276 seamless pipe
The nickel-based corrosion-resistant alloy: Hastelloy C-276 blank tube is extruded by hot extrusion process. A 25 mmx2.5 mm seamless tube was developed through multi-pass cold rolling deformation. The results show that the average grain size of Hastelloy C-276 alloy pipe is about 7, and it has good mechanical and intergranular corrosion resistance. The yield strength of high temperature tensile test at 400 and 700 ℃ is more than 300N a. The chemical composition, structure and performance of the developed Hastelloy C-276 seamless pipe meet the requirements.
Hastelloy C-276 is a Ni-Cr-Mo nickel-based corrosion-resistant alloy. Its chemical composition is shown in Table 1. Hastelloy C-276 is based on Ni, and contains a large amount of elements such as Cr and MO. It has extremely low C and Si content, so it shows excellent corrosion resistance. It has good corrosion resistance not only in oxidizing media, but also in reducing media, especially in oxidizing acids containing fluoride and chloride ions, in reducing acids in the presence of oxygen or oxidants, and in The mixed acid of oxidizing acid and reducing acid, in wet chlorine and chlorine-containing aqueous solution, has corrosion resistance that is difficult to compare with other corrosion resistant alloys. Therefore, Hastelloy C-276 is widely used in chemical industry, such as low-pressure oxo acetic acid (acetic anhydride), chlorination method titanium dioxide, high-pressure method melamine, etc. The alloy is also increasingly used in nuclear industry, oil and natural gas, power plant flue gas desulfurization and other fields. The main product forms are pipes and plates.
Tab.l Chemical composition of Hastelloy C-276 (wt%)
|Remainder||15.0-17.0||14.5-16.5||4.0-7.0||3.0-4.5||2.5 max||1.0 max||.01 max||.35 max||.04 max||.03 max||.08 max|
At present, most domestic companies import C-276 plates from abroad and use them as pipes and various containers through welding. As we all know, nickel-based alloys are prone to sensitization behavior during the welding process, resulting in a decrease in the corrosion resistance of the device. Therefore, for a long time, most researchers have conducted a lot of research on the welding performance of C·276 alloy, post-weld heat treatment and its influence on corrosion resistance. There are few reports on the manufacture, processing, and performance of C-276 alloy seamless pipes.
C-276 alloy has the characteristics of high degree of alloying (especially high content of molybdenum), difficult smelting, and easy to produce defects. At the same time, due to the large deformation resistance of the alloy, its high temperature deformation resistance is as high as 300MPa or more, the hot working process is very difficult, and processing cracks are prone to appear. It is basically impossible to manufacture seamless pipes using conventional perforation processes. For a long time, most of C-276 alloy seamless pipes were imported from abroad. However, with the improvement of the smelting level of domestic steel mills, qualified C-276 alloy billets can also be smelted in China. Therefore, the localization of C-276 alloy seamless pipe is imperative.
Microstructure characteristics of C-276 alloy
Table of Contents
Ni-Cr-Mo series alloys exhibit excellent corrosion resistance in many fields. The theoretical basis comes from the thermodynamic and kinetic basis of alloy phase transformation that controls the precipitation order of phases and affects the long-term stability of the structure and mechanical properties. Therefore, in order to obtain a good structure through the formulation of reasonable thermal processing and heat treatment processes, and thereby improve the performance of C-276 alloy, it is necessary to grasp the precipitation law of C-276 alloy phase. Figure 1 is the equilibrium phase diagram of C-276 alloy (15Cr-16Mo-35W). It can be found from the figure that as the temperature of the liquid metal decreases, the liquid phase gradually transforms into the solid phase. The solidification range is about 1350 and 1380 ℃. When the temperature continues to decrease to about 1160 ℃, the austenite structure begins to produce Two phases, such as M6C type carbide and P phase. The morphology of the precipitated phase is shown in Figure 2. When this second phase appears, it may affect the hot workability and serviceability of the alloy (such as inter-product corrosion). Therefore, no matter in the process of hot working or in use, the precipitation of harmful phases should be avoided as much as possible to improve the performance of the alloy, reduce the maintenance cost of the equipment, and increase the service life of the equipment and the economic benefits of the enterprise. Therefore, the 0276 alloy In the product manufacturing process, once the second phase is produced, it must be dissolved by corresponding heat treatment to ensure the stability of the alloy structure and the performance of the product. The test results show that when C-276 alloy is solid-solved above 1100 ℃, the second phase will gradually disappear, but a small amount of second phase remains. When the solid solution temperature is above 1170 ℃, the second phase will basically completely re-dissolve to In the austenite matrix (Figure 3).
Fig.1 Calculated equilibrmm phase graph of C-276 alloy and its partial magnification
Fig.2 The second phase in C-276 alloy
Fig.3 Microstructure of C 76 alloy under different solution temperatures
Process and performance of C-276 alloy seamless pipe
Forming process of seamless pipe
The entire development and manufacturing process of C276 alloy seamless steel pipe can be simplified as follows: ingot smelting, forging, billet, tube blank processing, hot extrusion forming, cold rolling forming, solution treatment, pickling, inspection, and finished alloy billet using VIM+ ESR duplex process smelting. After re-inspection by the factory, the chemical composition of the tube blank meets the technical requirements (Table 2). The content of various inclusions in the billet is 0, 5, and 1.0, which meets the technical requirements.
Tab.2 Chemical composition of C一276 steel billet (wt%
After the extruding billet is stripped by the stripping machine, the defects (oxide scale, cracks, etc.) on the surface of the forged and rolled steel are eliminated. Then, according to the actual requirements, the long billet is segmented, drilled, and end-faced, and the dimensional tolerance and surface quality are checked. The tube blank enters the continuous degreasing cleaning device for cleaning and degreasing before heating to remove grease and dirt on the inner and outer surfaces of the tube blank. Prevent the tube billet from being contaminated during the heating process of the preheating furnace, and ensure the quality of the tube billet. The cleaned tube billet: sent to the ring preheating furnace for preheating, then sent to the induction furnace for heating, and vertical expansion Reaming on the extruder. Finally, the induction heated reamed tube blank is sent to the extruder for hot extrusion. An extruded blank tube with compact structure and uniform performance is obtained, thereby avoiding quality defects such as easy cracking and folding on the inner and outer surfaces of the two-roll cross-rolling perforated blank tube. The extruded blank tube is cold-rolled and deformed on a periodic cold-rolling tube mill. After multiple passes of cold rolling, the final rolled product: Φ25mmx25mm finished steel tube with a length of more than 12m.
Seamlessly managed performance
Samples were taken from the finished pipes for metallographic, room temperature and high temperature tensile tests, intergranular corrosion performance evaluation tests, flattening tests and flaring tests to evaluate the service performance and process performance of the pipe.
Organization and mechanical properties
The microstructure of the finished tube was observed (Figure 4). The results showed that the tube was basically austenitic with a grain size of about 7. There are few precipitated phases in the grain boundary and in the grain. The room temperature tensile test is in accordance with the provisions of GB/T228-2002. After the tensile test is taken on the whole tube at room temperature, the yield strength of the tube is 440MPa and the tensile strength It is 844MPa and the elongation is 49%. At the same time, the high temperature tensile test is carried out in accordance with the provisions of GB/T4338-2006. The high temperature tensile results show that at 400 and 700 ℃, the yield strength of the tube is still as high as 300Mpa or more (Figure 5).
Fig.4 Metallurgy microstructure Of C-276 seamless pipe
Fig.5 Tensile properties Of C-276 seamless Pipe at high temperature
Corrosion performance between products
In order to test the anti-corrosion performance between products, the corrosion test is carried out in accordance with ASTM G28A method (Fe2(S04)3+50%H2S04 solution), and the test time is 24h. The results show that the average corrosion rate is 3.9mm/a, the microstructure after corrosion is shown in Figure 6, and the corrosion depth is 82.9μm. The domestic standard for pressure vessels made of nickel and nickel alloys (JB/T4756-2006) stipulates that C-276 alloy seamless steel pipes shall be tested with reference to G28A method, and the average corrosion rate shall be less than 12 mm/a. It can be seen that C-276 alloy finished products The intergranular corrosion performance of the tube fully meets the requirements of relevant standards and can replace imported products.
Fig.6 SEM microstructure Of C-276 alloy seamless pipe after Intergraln corrosion
Flattening and flaring test Take samples of the finished pipe for flattening and flaring tests.
The flattening test is carried out in accordance with the provisions of GB/T246. The flattened pipe section sample length is 65mm. The flaring test is carried out in accordance with ASME SB829. The cone tip with an apex angle of 60 is used for the flaring test. The expansion rate is the outer diameter of the pipe. The 30% result shows that the sample has no visible cracks and cracks after being flattened and expanded, which meets the technological requirements.
To sum up, it can be seen that the 25 mmx2.5mm nickel-based alloy C·276 seamless pipe trial-produced by hot extrusion and cold rolling processes can meet the requirements of the ASME SB622 standard and can completely replace imported products.
The ballast 25mm 5mm C-276 nickel-based corrosion-resistant alloy seamless pipe developed by hot extrusion and cold rolling has an average grain size of grade 7, with good mechanical properties and resistance to inter-product corrosion at 400, 700 ℃ The high temperature yield strength is greater than 300MPa, which meets the requirements of ASME SB622 and other related standards, and can completely replace imported products
Source: Network Arrangement – China Hastelloy C-276 Seamless Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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