Introduction to Incoloy800 alloy (including Incoloy 800, 800H, and 800AT alloys): Do you know everything about these special materials?

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Introduction 800 alloy is a Ni-Fe-Cr alloy used to resist oxidation and carburization. Its nickel content reaches 32%, which gives the 800 alloy good resistance to chloride-induced stress corrosion cracking as well as to the embrittlement caused by the precipitation of σ-phase. The 800 alloy also exhibits excellent resistance to uniform corrosion. In the solution-treated state, 800H and 800AT exhibit excellent creep and stress fracture properties. All three of these alloys based on 800 alloy have passed the approval under the ASME Code for Materials for Boilers and Pressure Vessels, Section I – Power Station Boilers ； Part Three—Nuclear Vessels ； Part 8: Application standards for 800 series alloys apply to pressure vessels that are not in direct contact with fire. The 800 series alloys are basically the same, except that 800H has a slightly higher carbon content (0.05–0.10%), and 800AT contains 1.00% Al+Ti. 800 alloy can generally be used up to 593°C. 800H and 800AT are generally used at temperatures above 593°C where high requirements exist for creep and stress corrosion cracking resistance. Chemical composition: The typical chemical composition of 800 series alloys is shown in the table below. Material standards: The table below lists the international standards for 800 series alloys. *Applicable only to 800 alloy: Corrosion resistance and applications. The nickel content in 800 series alloys is much higher than that in the 304 stainless steel family. In many types of media, 800 series alloys exhibit similar properties to 304 stainless steel; for example, in various industrial gases and chemical media such as nitric acid and organic acids, they have comparable performance. Neither 800 series alloys nor 304 stainless steel can be used in sulfuric acid environments, unless under conditions of low temperature and low concentration. Similar to austenitic stainless steels, if 800 series alloys are heated for too long at temperatures between 538–760°C, chromium carbides will precipitate at the grain boundaries of the alloy, leading to sensitization. This results in intergranular corrosion of the alloy in environments such as those involving pickling or 65% nitric acid. Although the 800 series alloys are not immune to chloride stress corrosion cracking, they still possess excellent corrosion resistance. It exhibits good corrosion resistance in industrial fields such as petroleum, chemicals, food, pulp and paper manufacturing. Clearly, the 800 series alloys can be used in moderate corrosive environments that cause stress corrosion cracking in conventional austenitic stainless steels. However, the 800 series alloys are not immune to stress corrosion cracking in MgCl2 solutions. The most typical application of 800 alloy is in high-temperature environments, such as components in incinerators, petrochemical reforming units, hydrocracking fittings, and superheated steam treatment equipment in conventional power plants and nuclear power stations. The high content of chromium and nickel gives 800 series alloys good resistance to oxidation and carburization. The 800 series can be characterized by the following features: 1. It possesses high design strength when used at 899°C in accordance with ASME standards ； 2. The tube can be used at temperatures below 982°C ； 3. It still exhibits antioxidant properties at 1038°C ； 4. Resistance to chloride corrosion. Mechanical properties The typical room-temperature mechanical properties of 800 series alloys are shown in the table below. Among them, the 800 alloy is annealed at 982°C, while the 800H and 800AT alloys are annealed at 1149°C; the difference in annealing temperatures is mainly due to the varying strengths of the materials. Processing and Heat Treatment 1. Cold Forming: 800 series alloys exhibit excellent formability. The high nickel content prevents the transformation of austenite into martensite, a transformation that can occur during the cold working of 301 and 304 stainless steels. Extensive forming operations can cause wavy “orange peel” patterns on the surface of the alloy, mainly due to its larger grain size; however, this phenomenon does not have any adverse effect on the material’s properties. 2. Heat treatment: The annealing of 800 alloy is generally carried out at temperatures between 982–1038°C, with the main purpose being to refine the grain structure. The heat treatment temperature for 800H and 800AT is generally between 1121–1177°C. In addition to softening the material, this process also promotes grain growth, thereby improving the material’s resistance to creep and stress fracture. 3. Welding: The 800 series alloys can be welded using methods such as GTAW or MIG. There are now a large number of electrodes and welding wires available for welding 800 alloy. A dense oxide layer forms near the welds of 800 alloy, which can only be removed by grinding. Additionally, it is best to use inert gas shielded welding during welding.