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.

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In order to present some information for design criteria about high-temperature properties of austenitic stainless steels, some results of high-temperature mechanical test which had been previously reported through our research were summurized and discussed.
Six standard type austenitic stainless steel rolled bars of AISI 304, 316, 321, 347 and 318 were used for high-temperature mechanical testing, such as, short-time tensile, creep-rupture, fatigue tests and testing for embrittlement associated with the structual change in these alloys.
Of course, it depends on the service condition of the materials which property should be taken for the allowable stress. In the case of stainless steels, however, the allowable stress in lower temperature range can be usually obtained from yield strength, and higher temperature range, from limiting-creep stress. As it has been shown that the temperature dependance of fatigue strength of these alloys is not so remarkable as that of limiting-creep stress, fatigue strength should be first considered as allowable stress in lower temperature range. As the caracteristics of austenitic stainless steels, it was recognized that the ratio of the tensile, fatigue and rupture strength to the yield strength or limiting-creep stress was relatively high.
Consideration for embrittlement at grain boundary which may be predominant in some austenitic alloys must be taken, while setting up the criterion on load-carring-ability as described in the above. Type 316, 318 and 310 stainless steels are liable to show intergranular brittlement due to massive precipitates of chromium carbide, sigme phase or other intermetallic compounds at grain boundaries. Grain boundary precipitation affects the alloys, making them more brittle and promoting the fructure at grain boundary in rupture testing.

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Structure members which are subjected to load at high temperature are, in general, not in a steady condition of applied stress and temperature. Therefore, the studies on creep of structures under cyclic temperatures or cyclic stressing are important problem. In this paper, the influence of temperature history on creep is discussed. Experimental and analytical relationships of the creep and the rupture life under temperature cycling or under combined variation of stress and temperature to the static creep and the rupture life are presented.
(1) The effect of temperature history on creep is composed of transitional change in internal yield stress, that is, recovery or aging, where the internal yield stress is defined as the local stress at which deformation begins without the aid of thermal fluctuation. The influence of the temperature history results in the existence of “induction period” or transitional increase in creep rate immediately after the change of temperature. However, these effects are negligible small in most metallic materials and the concept of “mechanical equation of state in solid”or the Robinson's hypothesis is applicable for preduction of creep or rupture life under cyclic temperatures.
(2) By adopting the assumption the creep strain in the stage of transient or steady state creep under temperature cycling can be predicted analytically from the data of static creep test in transient stage, by introducing the equivalent steady temperature. The equivalent steady temperature means the temperature at which the transient creep strain under cyclic temperatures is equivalent to the strain in the static creep test. In like manner, creep rupture life under temperature cycling can also be estimated analytically from the data of static creep rupture test by introducing the equivalent steady temperature for creep rupture life. These analysises are usefull if severe metallurgical change such as microstructural phase change or corrosion does not occur. The assumption is also applicable for prediction of creep or the rupture life under combined variation of stress and temperature. However, under the condition that the stress amplitude which is synchronized with temperature cycle is large, the influence of recovery must be considered.

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Mechanical properties of some austenitic stainless steels at elevated temperatures were investigated and discussed from the viewpoint of carbide precipitation.
Though static strength of stainless steels decreases with increasing temperature, it is almost constant at temperatures about from 300°C to 600°C, and at this temperature range, a serrated discontinuous phenomenon appears in stress-strain curves during tensile test. A fatigue test was carried out at temperatures of 460°C, 630°C and 700°C. The fatigue limit is affected by carbon content of steel and is higher than the yield point, and a conspicuous coaxing effect is observed.
These observations may be related to the fact that chromium carbide or others precipite out at slip bands and grain boundaries during the high temperature tests and prevent dislocations from moving. This criterion was confirmed by the observation of microstructure of specimens.

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The machinery part which is to be exposed to a high temperature is designed on the basis of creep strength, creep rupture strength and high temperature fatigue strength. However, in the design of jet engine part and other machinery part as well as gas turbine for naval vessels subject to temperature fluctuation because of many start-and-stops, the above data are not sufficient, and data on cyclic thermal stress are required. This is an interim report on the tests we have been making on three kinds of heat resisting material by means of thermal fatigue test apparatuses manufactured to meet this requirement. The test apparatuses used are indirect heating thermal cycling test apparatuses, direct heating thermal cycling test apparatuses and a high temperature constant strain cycling test apparatus; the indirect heating thermal cycling test apparatus is mainly used at present. It has been found from the tests that thermal fatigue strength depends to a great extent on heat treatment condition of material, and has no relation with creep rupture strength.

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Various kinds of Cr-Mo steel tubes which are from 1% to 9% in Cr contents are widely used for superheater, reheater, main steam tubes and others. Cr is an effective element for increasing oxidation resistance, creep strength and corrosion resistance, and accordingly the Cr-Mo steel tubes containing Cr above 5% have been used mainly for petroleum or other industries which require both heat resisting and corrosion resisting properties. In this connection, especially, 1 Cr-1/2 Mo, or 21/4 Cr-1 Mo steel has creep strength superior to those of Cr-Mo steels such as 5 Cr-1/2 Mo, 7 Cr-1/2 Mo and 9 Cr-1 Mo at a temperature between 500°and 800°C in non-corrosive atmosphere. The creep strength of all Cr-Mo steels mentioned above are remarkably dependent on their heat treatment, and therefore creep tests have been carried out for test specimens heat-treated in several ways.
Five criterions are specified in ASME Boiler and Pressure Vessel Code as the basis for selecting design stress. From the viewpoint of design, among these criterions, the stress of 60% of the average or 80% of the minimum stress to produce rupture in 100000hrs have been considered as the most reasonable value for some tube materials such as Cr-Mo steels having good ductilities at higher temperatures.
As the results of our experiment, it has been found that, when 1 Cr-1/2 Mo and 21/4 Cr-1 Mo steel with C contents around the upper limit of their specifications are heat-treated by air cooling at a suitable temperature related to the C-C-T curve and are tempered at a lower side within a given temper temperature range, their creep strength is superior to those of specimens heat-treated by other ways. While their creep strength becomes remarkably lower by the tempering at higher temperatures in the cases mentioned in the above. In addition, the most suitable conditions for heat treatment are decided for other Cr-Mo steels such as 5 Cr-1/2 Mo, 7 Cr-1/2 Mo and 9 Cr-1 Mo.

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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.

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Dynamic creep and fatigue tests were carried out for a commercial pure titanium at a temperature of 300°C. Frequency of alternating stress was 1800cpm.
The followings are the facts clarified from the test results under repeated loading. Under the dynamic stress condition as well as under the static stress condition, there exists a critical stress above which the fracture occures in a short time (several minutes) after loading and below which the fracture does not take place. When a stress below the critical stress is applied, the major part of the strain is produced in the course of time as short as the fracture time in the case of applied stress above the critical value. As for the relation between dynamic creep strength and static creep strength, the peak of the critical dynamic stress is equal to the critical static stress, and the same strain is produced under the same maximum stress no matter whether the stress is static or dynamic. This agrees with the conclusion of the analysis made by the present authors based on the mechanical equation of state.
Under the reversed stress, the critical stress is not so clear as is the case under repeated stress. The fatigue limit in this case is appreciablly less than the critical static stress.

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Titanium and its alloy are widely used as structural materials because of their excellent corrosion-resistance and heat-resistance properties.
This report deals with the unnotched and notched fatigue strengths of 2 Al-2 Mn-Ti alloy and some commercially pure titaniums in a high temperature range from 20°C to 600°C, as well as the internal heating phenomenon at a room temperature of commercially pure titanium which affects their fatigue strength.
The high temperature fatigue tests were conducted on Ono's high temperature fatigue tester (3000rpm) and a Krouse high speed high temperature fatigue tester (5000rpm). The fatigue tests at a room temperature are conducted on Ono's fatigue tester and a Schenck's vibrating fatigue tester.
The high temperature mechanical properties of the specimens are shown in Fig. 3-7 and the notched and unnotched fatigue strengths in a high temperature at 10⁷ cycles are also shown in Fig. 14, respectively.
The unnotched and notched fatigue strengths of 2 Al-2 Mn-Ti alloy decreased remarkably from the values of room temperature up to 100°C, and in a range from 150° to 300°C, those rates steadily changed, and in the case of further temperature, those rates increased again.
The unnotched fatigue strength of commercially pure titaniums decreased remarkably with the increase of temperature up to 200°-300°C, and those rates changed smoothly in further temperatures. The high purity specimen shows lower fatigue strength than that of lower purity specimen. The reduction percentage of fatigue strengths and fatigue strength reduction factor (3.0 theoretical stress concentration) of each specimen are shown in Fig. 15 and 16.
In the high temperature fatigue tests, an unnotched specimen tested in high stress levels induced internal heating phenomenon and the specimen changed their colour due to generated heat. The authors presumed the temperature of generated heat from the degree of their discolouration of specimen and examined the modified S-N curves as shown in Fig. 17.
Internal heating phenomenon at a room temperature of commercially pure titanium is more remarkable as to the high purity material than in the case of lower purity, and the maximum temperature of generated heat is related to repeated stress levels as shown in Fig. 22, and also the size effect of specimen influences on their phenomenon as shown in Fig. 27.

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