Abstract
High Mn austenitic stainless steel has been developed for high temperature applications especially for boiler tubes. The most suitable chemical composition has been found to be 18%Cr-6%Ni-8%Mn, the structure this steel is almost wholly austenitic and its properties of creep-rupture strength, resistance to oxidation at elevated temperature and hot workability are comparable to those of type 304 steel.
Small amounts of Ti and Nb have been added simultaneously to this type of steel (being called 18-6-8Ti Nb steel) to improve the elevated temperature strength. The carbon content being about 0.2%, the optimum amounts of Ti and Nb have been found to be the atomic ratio of (Ti+Nb)/C being in the range of 0.1 to 0.5. The resultant steel has considerably high creep-rupture strength in the temperature 600 to 700°C, and is believed to be very economical steel for high temperature applications.
The mechanism of raising the creep-rupture strength due to the additions of Ti and Nb is considered to be to depress the coalescence of M23C6 carbides in the grains and on the grain boundaries and to make the carbides dispersing very finely within the grains during its use at high temperature. This mechanism is seemed to be the same as in the case of 18%Cr-10%Ni austenitic steel.
