Manufacturing performance of Hastelloy C-276

Manufacturing performance of Hastelloy C-276

In many modern industrial fields (such as: electric power, petrochemical, metallurgy, steel, medicine, food hygiene, paper making, light textile, environmental protection, etc.), the daily production of industrial instruments are required to measure their process parameters. But in some special industrial fields, many of the measured media are corrosive, and some of the measuring instruments to be measured directly in contact with the media, which requires our measuring instruments in contact with the media part should have good corrosion resistance. So in the design process, for different media we have to choose a different contact fluid material. To meet the requirements of use. Hastelloy is also considered to be a universal corrosion-resistant alloy because of its excellent corrosion resistance. Therefore, it has been widely used in chemical, petrochemical, flue gas desulfurization, pulp and paper, environmental protection and other industrial fields in the past 30 years. At the same time, Hastelloy C-276 has good corrosion resistance to mixed acid and salt solutions with chloride ions, so it is also widely used in marine, salt water and high chloride environments.

The following is an example of Hastelloy C-276 used in the production process of industrial instrument transmitters, introducing its physical properties, welding properties, casting properties, heat treatment properties, machining properties.

Chemical Composition of Hastelloy C-276

Chemical Requirements
Ni	Mo	Cr	Fe	W	Co	Mn	C	V	P	S	Si
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

Physical properties of Hastelloy C-276

Hastelloy C-276 is a tungsten-containing nickel-chromium-molybdenum alloy developed and produced by Haynes International, Inc. Its chemical composition is Ni:57, Cr:15.5-17.5, Mo:16-18, Fe:4.5-4.7, W:3.75-5.25, Mn:1Max, V:0.2-0.4, Si:1Max, C:0.12Max, P:0.04Max; S:0.03Max. From its chemical composition, it can be seen that it is a It is a nickel-chromium-molybdenum alloy containing tungsten. The nickel as the matrix itself has good corrosion resistance and is not easy to rust in the atmosphere, and can resist the corrosion of caustic alkali. In the air or oxygen, the NiO film is formed on the surface of nickel, which prevents further oxidation. At the same time, Hastelloy C-276 has a high content of chromium and molybdenum as well as tungsten, which determines its good strength at the same time. Since Hastelloy C-276 has good corrosion resistance in some strong acids and other environments, we usually choose it as the contact fluid material in these media tests.

The welding performance of Hastelloy C-276

In the industrial instrument transmitter as a key part of the transmission medium pressure – isolation diaphragm, it mainly plays a role in the transmission of medium pressure, to be measured in direct contact with the medium, then the need for the medium corrosive resisting effect. Otherwise, the diaphragm is corroded, the transmitter will not work. At the same time, in order to ensure that the pressure is evenly transmitted to the transmitter’s pressure measuring element, and can return to the original state after the pressure relief, the isolation diaphragm material requires a good modulus of elasticity, to prevent its deformation can not accurately transmit the test pressure. In accordance with this requirement, we usually choose Hastelloy C-276, which has a good coefficient of elasticity, in the environment of testing strong acid media.
However, because of the high price of Hastelloy C-276, in order to reduce the cost, the material of the diaphragm seat connected to the isolation diaphragm is usually selected from stainless steel, and due to the special structure and use, the two are usually connected by welding. In this case, it is necessary to understand the welding properties of Hastelloy C-276. Since the welding performance of Hastelloy C-276 is similar to that of ordinary austenitic stainless steel, before using a welding method to weld C-276, measures must be taken to minimize the decrease in corrosion resistance of the weld and heat affected zone. At the same time in the welding to use the appropriate heat input rate, in order to prevent the generation of thermal cracking. Because of some characteristics of Hastelloy C-276, so in the welding need to choose the appropriate welding parameters. For some of the low pressure membrane welding can also be used in the way of roll welding, as roll welding does not require melting of the metal. So welding is easier. In addition, in the Hastelloy C-276 isolation diaphragm for argon arc welding should pay attention to the weld should be cleaned before welding, to avoid the formation of chromium carbide between the grain boundaries in the welding process, making the weld corrosion resistance is reduced, can not meet the requirements of strength. In addition, during the welding process, special attention should be paid to the transfer of heat and heat dissipation synchronization, because when welding Hastelloy C-276 and stainless steel welding, two different materials have different specific heat and coefficient of thermal expansion, so when welding a large amount of heat should have good heat dissipation fixture, so as to avoid cooling due to the different coefficients of thermal expansion between Hastelloy C-276 and stainless steel The different coefficients of thermal expansion during cooling cause significant welding thermal stresses between Hastelloy C-276 and stainless steel. In the use of the transmitter, these stresses in the release will affect the accuracy of the use of the transmitter, serious deformation of the welded components or make the weld cracking.

The castability of Hastelloy C-276

There are many parts in the industrial instrumentation, because it is also in contact with the test medium, so in some very harsh environment of strong acid environment, usually also use Hastelloy C-276 manufacturing. However, the complex structure of some parts usually cannot be processed directly by bar or plate, so they are processed by blank. In the casting process of Hastelloy C-276, the casting mold for Hastelloy C-276 should be designed according to the shrinkage rate of the material and the size of the blank. -Shell cleaning -Surface finishing, etc. The casting process is different from the general material casting process. Hastelloy C-276 in the melting process, in order to obtain a more pure steel, reduce the gas content and the content of harmful elements, at the same time, due to the presence of oxidation-prone elements in some alloys, non-vacuum melting is difficult to control, in order to get better thermoplastic material, in the melting process of Hastelloy C-276 is usually used vacuum induction furnace melting, or even vacuum induction smelting plus vacuum self-consumption The melting process of Hastelloy C-276 is usually carried out by vacuum induction furnace, or even by vacuum induction smelting with vacuum self-consumption furnace or electroslag furnace remelting. In addition, the melting point of Hastelloy C-276 is 1325-1370℃. And the material contains high melting point of tungsten elements, according to this physical characteristic, we generally use 1560-1600 ℃ range in the casting and melting process to melt the material. This can ensure that the material can be fully melted, so that the chemical composition of the castings can meet the standard requirements. In addition to the melting process of slagging is also very important, because if the slagging is not complete, the casting process is prone to slagging and other defects. Causes casting scrap.

The heat treatment performance of Hastelloy C-276

After the casting is cast, the grain is coarse due to the natural cooling process, and the grain needs to be refined. So it is necessary to carry out heat treatment. Usually we verify whether the heat treatment process is correct or not Thi C-276 alloy material solution heat treatment includes two processes:

• (1) Heating temperature control at 1175 ℃ min heating, and then according to the size of the part to choose a period of time for insulation;
• (2) Using circulating water or compressed air to cool the casting quickly to room temperature, so that the treated material has good corrosion resistance, as well as good strength.

Because a good heat treatment process can further improve the corrosion resistance of the material. Therefore, stress relief heat treatment of Hastelloy C-276 only is not effective. It is worth noting that the surface of the casting needs to be cleaned before the heat treatment is done, firstly because surface stains may produce some fouling of the carbon elements during the heat treatment process. The second is that if there are impurities on the surface, in the Hastelloy C-276 heat treatment will produce oxides, so that the Cr content of the alloy is reduced, through the corrosion resistance test, it will be formed between the grain boundaries “poor chromium” area, so that its strength is reduced, thus affecting the corrosion resistance, so the surface should be cleaned. You can use stainless steel wire brush or grinding wheel for surface treatment, and then dip into the appropriate proportion of nitric acid and hydrofluoric acid mixture of pickling, and finally rinsed with clean water. Through the above precautions, you can ensure that the subsequent production process and the use of the process to meet the requirements of qualified castings.

Machinability of Hastelloy C-276

Because the mechanical properties of Hastelloy C-276 is very prominent, it has high strength and high toughness, so there are certain difficulties in machining. Compared to general stainless steel machining, Hastelloy C-276 can usually be machined with YW3 tools due to the high heat generation during cutting, and in terms of cooling, we should choose emulsions with good cooling and lubricating properties. When we are processing Hastelloy parts, due to the different chemical element content of each batch, resulting in different processing performance of the parts. Previously, we have encountered a batch of Hastelloy parts in the production process when processing the threaded hole, can not be processed, after testing the hardness of the parts found that the hardness of more than 30HRC. The general tool is unable to process, after replacing the tool, can barely be processed, but the tool wears quickly, and the speed can not be too high, and the cutting volume than the conventional material selection to be lower. According to this situation, the raw material analysis was conducted, and it was found that the composition and mechanical properties of the elements met the requirements, except that the mechanical strength was 50% higher than the standard value, and the corresponding hardness was also higher than usual. In order to reduce the wear of the tool, after repeated experiments we determined the casting annealed again, and in the annealing process, so that the parts in a very short period of time down to room temperature, so that the hardness can be reduced to about 10HRC, machining performance is almost equivalent to the processing of 316. Therefore, it is recommended that before processing such special alloys, the hardness range of the casting can be checked, and if it is too high, the annealing process should be repeated to ensure the smooth production.

The above is the application of Hastelloy C-276 encountered in the process of industrial instrumentation production and the problems encountered in the process of processing and some proposed solutions to the problem. Due to the special characteristics of the alloy, the use of fewer places, so its performance is also in the continuous exploration, but is not very comprehensive. We believe that we will have more understanding of this alloy in the future as we continue to apply it.

Author: Hu Huifeng

Source: China Hastelloy C-276 Forgings Manufacturer – Yaang Pipe Industry (www.steeljrv.com)