Teishiro Oda

Creep Rupture of 18-8 Ti Stainless Steel

Recently, AISI 321 type 18-8 Ti stainless steel has widely been used as the material of superheater or reheater tubes for large power plants, and it is generally recognized that the creep rupture strength of this type of steel varies greatly with the grain size in connection with heat treatment. However, in view of the fact that both of the grain size and condition of precipitation are affected by the heat treatment, we consider that the grain size is not a sole factor to have an effect on the creep rupture strength, but also the condition of carbides or σ phase precipitation has a great effect on it. To confirm this we carried out 650°C creep rupture tests of the material treated at various heat treatments after working. As a result of the tests, we have reached the following conclusions:It makes a great difference in the creep rupture strength whether the heat treatment after working is carried out at a comparatively low temperature in the precipitation range, (less than about 1000°C), or at a high temperature in the solution range.During recrystallization, precipitation and coagulation develop very quickly, and the precipitation hardening effect that prevents creep deformation is lost due to over-aging. Therefore, the strength of the material subjected only to working or treatment at a low temperature is small at the long period side. On the other hand, the strength of the material treated at a high temperature after working is not reduced, because the precipitation never occurs, and precipitated carbides or σ phase are dissolved in matrix during heat treatment.There will be some relation between the creep rupture strength and the grain size. However, when the material is treated at a comparatively low temperature and has fine grains, it will also have an undue condition of precipitation. This coincidence might often cause a misconception that the creep rupture strength was affected only by the grain size, but we believe that the greater part of the reduction of the rupture strength is due to undue condition of precipitation. We shall make further studies on this point.