A Comprehensive Guide to Nickel-Based Superalloys: Incoloy 800HT (UNS N08811/W.Nr. 1.4959)
What is Incoloy 800HT?
Incoloy 800HT, also known as "Alloy 800HT", is designated UNS N08811 or DIN W.Nr. 1.4959. It is an iron-nickel-chromium alloy with the same basic composition as Incoloy 800 and Incoloy 800H but with significantly higher creep-rupture strength. The higher strength results from deliberate control of the carbon, aluminum, and titanium contents and from high-strength-temperature annealing. Used in chemical and petroleum processing, superheater and reheater piping in power plants, industrial furnaces, and heat-treating equipment.
Incoloy 800HT (N08811) is a fully Austrian homologous series of Incoloy 800, a low-carbon nickel-iron-chromium alloy whose cobalt content can be tightly controlled to less than 0.01%. Incoloy 800HT (N08811) resists corrosion from a wide range of corrosive media. Its high nickel content gives it good resistance to stress corrosion cracking in water-based corrosion conditions. The high chromium content gives it better resistance to pitting and crevice corrosion cracking. The alloy has good corrosion resistance to nitric acid and organic acids, but limited corrosion resistance in sulfuric and hydrochloric acids.
It also has good corrosion resistance in oxidizing and non-oxidizing salts, except for the possibility of pitting in halides. It also has good corrosion resistance in water, steam, and mixtures of steam, air and carbon dioxide. Used in nitric acid condensers, resistant to nitric acid corrosion, steam heating, good mechanical properties, heating element tubes, good mechanical properties, etc. For applications up to 500°C, the alloy is supplied in the annealed state.
Incoloy 800, 800H & 800HT have almost the same chemical composition and physical properties. The main difference between these nickel-based high-temperature alloys and their mechanical properties derives from the restricted composition of alloys 800H and 800HT and the high-temperature annealing used for these alloys. In general, alloy 800 has higher mechanical properties at room temperature and when exposed to high temperatures for short periods of time, while alloys 800H and 800HT have excellent creep and rupture strength when exposed to high temperatures for long periods of time.
Characteristics of nickel-based alloy Incoloy 800HT (UNS N08811)
(1) Excellent corrosion resistance in extremely high temperature aqueous media up to 500°C.
(2) Good resistance to stress corrosion.
(3) Good processing properties.
Corrosion resistance of Incoloy 800HT (N08811) alloy
Incoloy 800HT is a general-purpose engineering alloy that is resistant to acid and alkali metal corrosion in both oxidizing and reducing environments. The high nickel content gives the alloy effective resistance to stress corrosion cracking. It has good corrosion resistance in various media such as sulfuric acid, phosphoric acid, nitric acid and organic acids, as well as alkali metal solutions such as sodium hydroxide, potassium hydroxide and hydrochloric acid. The high comprehensive performance of Incoloy 800HT is demonstrated in nuclear combustion dissolvers with various corrosive media, such as sulfuric acid, nitric acid and sodium hydroxide, which are all treated in the same equipment.
Processing and heat treatment Incoloy 800HT (UNS N08811)
Incoloy 800HT (N08811) alloy heating
(1). Keep the workpiece clean before and during heat treatment.
(2). Do not contact low melting point metals such as sulfur, phosphorus and lead during heat treatment, otherwise the performance of the material will be damaged. Care should be taken to remove dirt such as marking paint, temperature indicator paint, colored crayons, lubricating oil, fuel oil, etc.
(3). The lower the sulfur content in the fuel, the better. The sulfur content in natural gas should be less than 0.1%, and the sulfur content in heavy oil should be less than 0.5%.
(4). Considering the need for temperature control and cleanliness, it is best to perform heat treatment in a vacuum furnace or a gas-shielded furnace.
(5). It can also be heated in a box furnace or a gas furnace, but the furnace gas must be clean, neutral to slightly oxidizing. The furnace gas should avoid fluctuations between oxidizing and reducing properties. The heating flame cannot be directly heated. Burn the workpiece.
Incoloy 800HT (N08811) alloy hot processing
(1). The heat treatment temperature range of Incoloy 800HT is 1200℃ to 900℃, and the cooling method is water quenching or cooling as quickly as possible between 760℃ and 540℃. Hot bending should be performed between 1150℃-1000℃.
(2). In order to obtain the best corrosion resistance and creep resistance, annealing should be performed after hot processing.
(3). The material can be directly fed into a furnace heated to 1200℃, and the material holding time is 100 minutes per 60mm thickness. After sufficient heat preservation time, the material is quickly taken out of the furnace and heat treated within the specified temperature range. When the material temperature drops below the heat treatment temperature, it needs to be reheated.
