The austenitic grade 800 comprises of nickel-chromium-iron as its major constituent and is specifically used for applications at elevated temperatures. It can suitably resist oxidation and partially resist cyclic oxidation, carburization and nitride environments, all while keeping its strength at elevated temperatures. For achieving optimum properties at high temperatures, the grade 800 gets a slight modification in its chemical composition and is certified as grade 800H and grade 800HT.
The 800H & 800HT are heat treated for desired grade properties. They go through annealing to acquire fine grain structure. Both the grades have increased level of carbon which makes them conducive for applications at elevated temperature. The grade 800HT has extra levels of aluminium and titanium content. A close control on the addition of carbon, aluminium and titanium content is maintained.
The modification made gives the alloy its use in high-temperature structural applications. They remain ductile during long-term use due to their composition. The grade 800 is efficiently used below 590°C. For usage above 590°C, alloy 800H and alloy 800HT are used. These grades show exceptional creep resistance properties without failing even after long exposure to high temperature. These grades are also used in the sigma phase temperature range since they effectively resist embrittlement due to their composition and the titanium content stabilizes the carbides. The presence of aluminium in grade 800HT might be used to form intermetallic compounds during precipitation, this would subsequently increase the strength of the metal. Aluminium also helps in improving oxidation resistance.
The alloy grades can be conveniently hot worked as well as cold worked. Hot working needs to be done with precision. If the deformation is under 5%, the temperature range needs to be done around 850-1100°C, if the deformation is more, the temperature range needs to be around 950-1200°C. Less than the required temperature might make the metal lose its strength, with subsequent heat treatment required to retain its normal state. After this process, the alloys are gone through rapid air cooling or water quenching. Followed by annealing, to properly retain its strength and corrosion as well as creep resistance properties. The grade has a high work hardening rate and it should be considered before cold working. After cold working, if the deformation is still high, the metal needs to be annealed properly to remove brittleness.
