C-276 alloy and its properties

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

Hastelloy C series alloy is Ni-Cr-Mo alloy. Because nickel itself has a face-centered cubic structure, its crystallographic stability allows it to contain more alloying elements (Cr, Mo, etc.) than iron-based alloys, so it can form alloys with a wide range of compositions to resist various environments Ability. Ni-Cr-Mo corrosion-resistant alloys still contain a single-phase face-centered cubic structure, that is, austenite (r) structure, when they contain a large amount of elements such as Cr and Mo. And because nickel itself has a certain corrosion resistance, especially resistance to stress corrosion caused by chloride ions, it is unmatched by all stainless steels. Therefore, the Ni-Cr-Mo corrosion-resistant alloy exhibits excellent corrosion resistance. They not only have good corrosion resistance in oxidizing media, but also in reducing media, especially in oxidizing acids such as F￣, Cr￣ ions, and reducing acids in the presence of oxygen or oxidants. The mixed acid of oxidizing acid and reducing acid has corrosion resistance that is difficult to compare with other corrosion-resistant alloys in both wet chlorine and chlorine-containing aqueous solutions. In order to reduce costs, some brands 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.

Table 5-1 Typical values of chemical composition of C series alloy (mass fraction)

Hastelloy C alloy is compatible and optimized for Ni-Cr alloy and Ni-Mo alloy. It has good corrosion resistance in oxidizing and reducing media as well as resistance to local corrosion, chloride stress corrosion cracking and seawater pitting . But it also has some serious shortcomings. In harsh oxidizing media, the chromium content of this alloy is not enough to keep it in a passive state and show a high uniform corrosion rate; a greater application obstacle is that the welding heat affected zone is in many It is sensitive to intergranular corrosion in oxidizing, low pH, and halide environments. Many occasions require that containers made of Hastelloy C alloy must undergo solution treatment after welding to eliminate segregation in the heat-affected zone, which severely limits the application of the alloy. In addition, the solution treatment process will also significantly reduce the plasticity and impact toughness of Hastelloy C alloy. Now, Hastelloy C alloy has been basically eliminated except for its use in certain casting materials.
The emergence of Hastelloy C-276 alloy cleared the biggest obstacle hindering the development of C alloy—the need for post-weld solution treatment. For the C alloy, welding will cause the corrosion resistance of the weld and heat-affected zone to drop sharply, and welding is a necessary process for most equipment manufacturing. C-276 alloy provides a solution to this problem. The emergence of the argon-oxygen decarburization remelting refining process has enabled the alloy to achieve extremely low carbon and silicon content, ensuring that the welding area still has the same corrosion resistance as the substrate. Due to its easy processing and wide adaptability to corrosion, C-276 alloy quickly became one of Haynes’s leading products as soon as it was launched in 1965, and it was extremely widely used. C-276 alloy is mainly resistant to the corrosion of wet chlorine, various oxidizing chlorides, chloride salt solutions, sulfuric acid and oxidizing salts, and has good corrosion resistance in low and medium temperature hydrochloric acid. Therefore, in harsh corrosive environments, such as chemical manufacturing, power plant flue gas desulfurization, papermaking, marine development and other industrial fields have a very wide range of applications.
However, under certain process conditions, even the low-carbon and low-silicon alloy C-276 is sensitive to intergranular corrosion, and the C-276 alloy does not have sufficient thermal stability. After long-term aging in the temperature range of 650 ~ 1090 ﾟ C, carbides will be precipitated at the grain boundary or the intermetallic compound will be produced, which will reduce the resistance to intergranular corrosion. In order to overcome this sensitivity, the alloy C-4 with better high-temperature stability was developed in the 1970s.
The carbon reduction, iron reduction and tungsten removal of the alloy C-4 and the addition of the stabilizing element titanium have solved the problem of intergranular corrosion caused by welding in the alloy C-276. alloy C-4 has remarkable high temperature stability. When placed in 650 ~ 1040 ﾟ C for long-term aging, it exhibits good ductility and intergranular corrosion resistance. It can resist the formation of grain boundary deposits in the welding heat affected zone. In many corrosive environments, the general corrosion resistance of alloy C-276 and alloy C-4 is essentially the same. Alloy C-276 performs better in strong reducing media like hydrochloric acid, and C in high oxidizing media. -4 alloy has better corrosion resistance.
In a highly oxidizing environment, C-276 alloy and C-4 alloy containing only 16% chromium cannot effectively provide corrosion resistance. This shortcoming is overcome by the development of other alloys, such as C-22 alloy. After the development of C-4 alloy, it mainly meets the needs of a small number of European users, and is currently only seen in older equipment.

Alloy C-22 is designed according to the atomic percentage factor (APF), APF = 4x (Cr) / [2x (Mo) + x (W)] is between 2.5 ~ 3.3, the alloy is both oxidizing and reducing All kinds of media have good corrosion resistance. The APF of the C-22 alloy is just in between, and the thermal stability and the ability to resist intergranular corrosion are also improved compared to the C-276 alloy. In addition, the C-22 alloy has excellent pitting and crevice corrosion resistance, and its stress corrosion resistance also exceeds that of the C-276 alloy, which was once considered the best. However, it does not perform as well as C-276 alloy in a strong reducing environment and under severe crevice corrosion conditions, because 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.
Alloy C-2000 is a patented product of Haynes in 1995, which is made by adding 1.6% copper on the basis of alloy 59 formula. Ni-Cr-Mo alloy is a medium with high Cr oxidation resistance, and a medium with high Mo and W resistance to reduction. However, due to the limitations of metallurgy, it is impossible to increase the oxidation resistance and reduction resistance by increasing the content of Cr, Mo, W. C-2000 alloy is designed to solve this problem. The biggest difference between it and other Ni-Cr-Mo alloys is the addition of 1.6% Cu, which greatly improves the resistance of the alloy to corrosion by reducing media. However, the addition of copper results in a substantial decrease in local corrosion resistance and the thermal stability is inferior to alloy 59. The resistance of this alloy to pitting and crevice corrosion is better than that of C-276 alloy, and its forming, welding and machining characteristics are similar to those of C-276 alloy. As a new generation product, alloy C-2000 provides a larger range of safe use, and is very suitable for users who can extend the life of equipment and test new processes.
In summary, the advantages and disadvantages of the corrosion resistance of C series alloys are shown in Table 5-2. It can be seen from the table that although the C-276 alloy is not the most perfect alloy for corrosion resistance, the current industry understanding of the C-276 alloy’s high reduction corrosion medium is superior to the alloy C-22, which has caused the alloy C-276 to become popular again. . At present, alloy C-276 is still the most used Ni-Cr-Mo alloy.

Table 5-2 Advantages and disadvantages of corrosion resistance of C series alloys