抄録
Along with the recent tendency of thermal engines becoming higher in efficiencies, acting gas temperature has become higher year by year, and the indications are that the steam temperature of 566°C for steam turbine is about to attain the stage of practicability while steam temperature of 650-750°C for gas turbine and that of 800-900°C for jet aircraft engine have become in practical use.
As for the research tendency of heat resistance materials, super alloys are now being actively developed. However, ferritic steels are being used in large quantities as they are suited for the medium temperatures of 538-600°C, beside being important from the economic viewpoint. But their effective data as the base for designing are still lacking.
The author, therefore, has investigated the trend of the developments of plants for marine and land uses, studied the recent cases of the failure of large forged rotors and thus has made some observations concerning the problems about the evaluation of high temperature properties of ferritic steels suitable for medium temperatures.
The salient points of the author's studies may be summed up as follows:
1) The steam temperature of the plant for marine or land use rises at an average of 6.5°C/Yr. and at present the temperature of 538-566°C for land plants is now being used in practise but so far as the plant for marine use is concerned, the temperature for practical use is no higher than 538-566°C. In order to compete with the diesel engines which are becoming larger and more powerful, the steam temperature of the plants for marine use has to be increased to 538-566°C degree within a few years, viewed from the stand point of fuel economy.
2) Examining the examples of large type forged rotors, the author has found many cases resulting from segregation and hydrogen embrittlement. Concerning the problem of the hydrogen embrittlement of large type forged rotors, it seems that it might be solved with the introduction of the method of stream degassing in vacuum.
In comparison with the steels melted in basic open hearth furnace, the steels melted in the basic electric furnace has generally higher reliability in point of quality and uniformity. However, so far as the amount of hydrogen remaining after the stream degassing in vacuum is concerned, the former becomes higher in quantity of hydrogen, reaching an average of 1.8ml/100gr which is still higher than 1.5ml/100gr that is regarded to be safety amount, judging from experience. Therefore, it is necessary to be treated by heat deliberately in order to drive out hydrogen.
3) At 538-600°C stage, the temperature inclination of the creep rupture strength of ferritic steels is extremely steep and accordingly the safety factor against excessive temperature is regarded especially important. Futhermore, there is the probability of the ferritic steels recovering softness after having become embrittled owing to heating for many hours. Naturally, there is the need for carrying out testing to the long hours side in connection with creep rupture deformity. So far as ferritic steels are concerned, there is, at present, no sure method of estimating the long time creep properties by conducting short time creep testing.
4) In order to reduce the mass effect of large type forgings, liquid cooling process is carried out, and in consideration of hardenability of the steels for such occasion, various special elements are added to the ferritic steels. At that time it is necessary to consider also the embrittlement to be caused by long time creeping and to select the steels having stable structure at temperatures in use.
