C-276 alloy and hastelloy C alloy

C-276 alloy is an important nickel-based corrosion-resistant material. Because it contains about 16% (mass fraction) of Mo and Cr, it has strong resistance in both oxidizing and reducing media. It is called a general-purpose corrosion-resistant alloy. Compared with general stainless steel and other corrosion resistant materials, it has the ability to withstand various forms of corrosion damage (including uniform corrosion, local corrosion and stress corrosion) in various corrosive environments, including electrochemical and chemical corrosion. It has good mechanical properties and processing properties, especially suitable for harsh media environments in many fields such as chemical manufacturing, power plant flue gas desulfurization, paper making, marine development, etc. It is also considered to be one of the most widely used nickel-based corrosion-resistant alloys in the world.

The Hastelloy C alloy is a Ni-Cr-Mo alloy. Since nickel itself is a face-centered cubic structure, the crystallographic stability makes it possible to accommodate more alloying elements (Cr, Mo, etc.) than iron-based alloys, and thus can form a wide range of alloys to resist various environments. Ability. Ni-Cr-Mo corrosion-resistant alloys still have a single-phase face-centered cubic structure, that is, austenite (7) structure, when they contain a large amount of elements such as Cr and Mo. And because nickel itself has a certain degree of corrosion resistance, especially resistance to stress corrosion caused by chloride ions, is unmatched by all stainless steel. Therefore, the Ni-Cr-Mo corrosion resistant alloy exhibits excellent corrosion resistance. They not only have good corrosion resistance in oxidizing medium but also in reducing medium, especially in the oxidizing acid of F ̄, Cl ̄ plasma, in the reducing acid in the presence of aerobic or oxidizing agent, In the mixed acid of an oxidizing acid plus a reducing acid, it is difficult to compare corrosion resistance with other corrosion resistant alloys in both wet chlorine and chlorine-containing aqueous solutions. In order to reduce the cost, some grades of Ni-Cr-Mo corrosion resistant alloys will also have a small amount of Fe; in order to improve the corrosion resistance and mechanical properties of the alloy, some Ni-Cr-Mo corrosion resistant alloys also contain a small amount of W.
The typical chemical composition of the C-based alloy is shown in Table 5-1.

Hastelloy C alloy is compatible and optimized for Ni-Cr alloy and Ni-Mo alloy. It has good corrosion resistance and resistance to local corrosion, chloride stress corrosion cracking and seawater pitting corrosion in both oxidizing and reducing media. . However, it also has some serious drawbacks. In the harsh oxidizing medium, the chromium content of this alloy is not enough to keep it passivated and shows a high uniform corrosion rate; the greater application obstacle is the welding heat affected zone. Many oxidizing, low value, halide environments are sensitive to intergranular corrosion. In many cases, containers made of Hastelloy C alloy must be solution treated after welding to eliminate segregation in the heat affected zone, which severely limits the application of the alloy. In addition, the solution treatment process also significantly reduces the plasticity and impact toughness of the alloy only 3816110X0. Today, Hastelloy C alloys have been largely eliminated except for use in certain foundry materials.
The emergence of Hastelloy C-276 alloy has cleared the biggest obstacle to the development of C alloys – the need for post-weld solution treatment. For C alloys, the weld will cause a sharp drop in the corrosion resistance of the weld and heat affected zone, and welding is a necessary process for most equipment manufacturing. C-276 alloy provides a solution to this problem. The appearance of the argon-oxygen decarburization remelting refining process enables the alloy to achieve an extremely low carbon-silicon content, ensuring the same corrosion resistance as the substrate in the welded area. Because of its easy processing and corrosion resistance, C-276 alloy quickly became one of Haynes’s fist products in 1965, and it has been widely used. C-276 alloy is mainly resistant to wet chlorine, various oxidizing chlorides, chloride solutions, sulfuric acid and oxidizing salts. It has good corrosion resistance in low temperature and medium temperature hydrochloric acid. Therefore, in the harsh corrosive environment, such as chemical manufacturing, power plant flue gas desulfurization, papermaking, marine development and other industrial fields have a wide range of applications.

However, under certain conditions, even if the low-carbon low-silicon alloy C-276 are more susceptible to intergranular corrosion, alloy C-276 does not have sufficient thermal stability. After the long-term aging 650~10901 temperature range, the precipitation of carbides at the grain boundaries or accompanied by u-phase intermetallic compound, it can decrease resistance to intergranular corrosion. To overcome this sensitivity, C-4 alloys with better high temperature stability were developed in the 1970s.
The C-4 alloy reduces carbon, iron, and crane, and adds stabilizing element titanium to solve the problem of intergranular corrosion caused by welding of C-276 alloy. C-4 alloy has remarkable high temperature stability and exhibits good ductility and resistance to intergranular corrosion when placed at 650 ~ 1040oC for long-term aging. The formation of grain boundary deposits can be resisted in the heat affected zone of the weld. In many corrosive environments in general corrosion resistance alloy C-276 and C-4 alloys are substantially the same, the strongly reducing media like hydrochloric acid in C-276 alloy behave better, C in oxidizing medium high -4 superior corrosion resistance of the alloy.
In the highly oxidizing environment, with only 16% chromium alloy C-276 and C-4 alloy corrosion resistance can not be provided efficiently, this disadvantage is overcome by the development of other alloys, such as alloy C-22. C-4 alloy mainly to meet the needs of European users a small part of the development, currently only seen in older devices.
The C-22 alloy is designed according to the atomic percent factor (APF).

C-22 alloy has good corrosion resistance in both oxidizing and reducing media. The APF of the C-22 alloy is just in the meantime, and the thermal stability and intergranular corrosion resistance are also improved compared to the C-276 alloy. In addition, C-22 alloy has excellent pitting and crevice corrosion resistance and resistance to stress corrosion than the C-276 alloy which was once considered the best. However, it does not perform as well as C-276 alloy in a highly reducing environment and under severe crevice corrosion conditions because the C-276 alloy contains 16% molybdenum. At present, C-22 alloy is often used in the corrosive environment of flue gas desulfurization systems and complex pharmaceutical reactors.
C-2000 alloy is a patented product of Haynes Company in 1995. It is made by adding 1.6% copper to the alloy 59 formulation. Ni-Cr-Mo alloy is a high Cr oxidation resistant medium with high Mo and W anti-reduction medium. However, due to the limitations of metallurgy, it is impossible to increase the content of Cr, Mo, and W to improve oxidation resistance and resistance to reduction. C-2000 alloy is designed to solve this problem. The biggest difference from other Ni-Cr-Mo alloys is the addition of 1.6% Cu, which greatly improves the corrosion resistance of the alloy against reducing media. However, the addition of copper results in a substantial decrease in local corrosion resistance and is also inferior to alloy 59 in thermal stability. The alloy has better pitting and crevice corrosion resistance than C-276 alloy, and its forming, welding and machining characteristics are similar to those of C-276 alloy. As a new generation, C-2000 alloy offers a greater range of safe use, making it ideal for users who can extend equipment life and test new processes.
In summary, the advantages and disadvantages of corrosion resistance of C-based alloys are shown in Table 5-2. It can be seen from the table that although C-276 alloy is not the most perfect alloy for corrosion resistance, the current industry’s understanding of C-276 alloy is superior to C-22 alloy in high reducing corrosive medium, resulting in C-276 alloy again. Popular. Now, C-276 alloy is still the most widely used Ni-Cr-Mo alloy.

Since the c-276 alloy does not contain active elements such as aluminum and titanium, the preferred primary melting process is air melting. The raw materials required for this process are cheap and have high economic value. The raw material is first melted in the EAF of the electric arc furnace, and then the molten steel is transferred to the AOD of the argon-oxygen decarburization refining furnace, and the oxygen is blown at the bottom of the AOD.Keep the melt in a liquid state. During the “carbon boiling period”, oxygen reacts with carbon dissolved in the molten steel to form CO2, which releases a large amount of heat and plays an important role in controlling the final carbon content of the solution below 0.01%. In the later stage of the flame refining, argon is blown with less oxygen, and the melt is reductive, so that elements such as chromium which are oxidized by the oxygen stream are reduced from the slag into the molten metal. At the end of the refining, Al is added to maintain the melt temperature and deoxidize. The chemical composition is periodically sampled throughout the smelting process.
After the refining is completed, the melt is transferred to an electroslag furnace, and electroslag remelting ESR is performed with a consumable electrode to obtain a final ingot. Electroslag remelting is carried out in a copper water-cooled crystallizer containing a high temperature, high alkalinity slag, typically a mixture of CaF2, AL203 and CaO. The consumable electrode, the slag pool, the metal molten pool, the steel ingot, and the bottom water tank form a loop through the short wire and the transformer. When a large current flows through the circuit, the slag pool is heated to a high temperature by its own resistance. The top of the consumable electrode is gradually heated and melted by the slag pool to form metal droplets. The metal droplets then fall off the top of the electrode and pass through the slag pool into the metal bath. Due to the cooling action of the water-cooled crystallizer, the liquid metal gradually solidifies to form an ingot. During this period, the computer is used to control the current, voltage and the speed of the self-consumption electrode.

The ingot gradually solidifies from bottom to top, causing the metal bath and the slag pool to move upwards. The ascending slag pool first forms a slag shell on the inner wall of the water-cooled crystallizer.
This slag shell not only makes the surface of the ingot smooth and smooth, but also acts as a thermal insulation function, so that more heat is transferred from the ingot to the bottom cooling water, which is beneficial to the crystallization of the ingot from bottom to top. . The bottom-up sequential solidification process ensures that the crystal structure of the remelted ingot is uniform and dense, and is advantageous for suppressing segregation.
The crystallographic direction is controlled to obtain a crystal structure which tends to be axial. The electroslag remelting process can significantly improve the hot workability and plasticity of the alloy, making the inclusions fine and uniform.And because of its slower solidification rate, the ingot type segregation can be reduced.
However, the main difficulty of the alloy smelting is the control of carbon, silicon and nitrogen, hydrogen and oxygen gases, and it is sometimes difficult to meet the requirements only in the arc furnace. A more common approach is to use vacuum induction smelting to control gases more efficiently, with the added disadvantage of increased cost. At present, some foreign manufacturers have realized non-vacuum mass production of C-276 alloy, and domestic research and production of C-276 alloy is not much.

C-276 ALLOY Physical properties
The density of C-276 alloy is 8.89g/cm3, the specific heat capacity is 425J/(kg*K), and the melting temperature range is 1325 ~ 1370 °C.