Abstract
The influence of heat treatment on the creep properties of 21/4 Cr-1 Mo steel were investigated. Comparative creep tests with the duration of 1000 or 2000 hours at 550, 600 and 650°C, were made in the following six different conditions of heat treatment; that were the normalizing at 900°C, the annealing at 900°C followed by four different cooling rates of 400°C/hr, 100°C/hr, 25°C/hr and 5°C/hr and the isothermal transformation at 725°C whose austenitizing temperature was 920°C.
Under the stress of 7.5kg/mm2 at 600°C, the specimens cooled slowly show larger creep rates. The annealing with the cooling rate of 5°C/hr revealed the largest creep and the normalizing or the annealing with cooling rate of 400°C/hr gave relatively small creep. However, the creep rates of the annealed specimens with slower cooling rates decrease rapidly with the lowering of stress. Under the stress of 3.5kg/mm2, the largest creep rate were shown by the normalized specimen. The creep strengths for the creep rate of 0.01per cent per 1000 hours of normalized condition and annealed condition with the cooling rate of 400°C/hr are lower than 2.0kg/mm2 in spite of their higher creep resistance under the higher stress, and the creep strength of the annealed conditions cooled by 100°C/hr or slower are in the range from 2.8 to 3.0kg/mm2.
Isothermal transformation gave relatively small creep rates in all testing conditions, and the creep strength is the same as in the slowly cooled specimen.
The creep test under the higher stress would not be suitable for the criteria of creep strength of the material which is used under the lower stress in service conditions. And the annealing with cooling rate of 100°C/hr or slower or isothermal transformation would be preferable for this steel when they are used as the superheater.
