In austenitic stainless steels much ferrite precipitates by the adjustment of the chemical elements as austenite former and ferrite former. In this report, AISI 321 type stainless steels containing much ferrite or no ferrite were prepared, and the influeuce of ferrite in austenite on creep rupture strengh were detected. Moreover, 321 type stainless steels melted by conventional melting process (Ti/C≅2) or vacuum melting process (Ti/C≅8) were tested for confirming the influence of grain size and TiC changes obtained as the results of various heat treatmeuts on creep rupture strength.
The results obtained by above mentioned experiments are as follows.
1) AISI 321 type stainless steel containing much ferrite shows lower creep rupture strength than no ferrite ones.
2) Creep rupture strength of 321 type stainless steels at 650°C changes remarkably by heat treatments.
3) Creep rupture strength of specimens prepared by vacuum melting or conventional melting process shows same tendency corresponding with same heat treatment.
4) Grain size and TiC changes by heat treatments were supposed as the important factor for creep rupture strength, but the detail of their influence is studing from the various point of view.
5) The structural change and precipitated various compounds were detected with electron microscope and electronic diffraction.

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Relations between heat treatments and creep rupture lives of 21/4 Cr-1Mo steel for boiler tube used at elevated temperature and under higher pressure were discussed for the purpose of determining the best heat treatment for getting both proper toughness at room temperature and good creep rupture strength at eleveated temperature.
Creep rupture strength at 550°C of each specimens was studied in connection with heat treatment, hardness and microstructure. Wihin the range of Vickers hardness 140-190, the specimens having higher hardness have better creep rupture strength than the specimens having lower hardness. From the stand point of microstructure the specimen having ferrite+bainitic structure shows good creep rupture strength.
In practical heat treatment process, we must select proper tempering temperature under consideration of the influence of heating for stress relief after welding on the structural change of mother material and also insuring proper toughness for various working abilities at room temperature at the sacrifice of creep rupture strength. We suppose from the base on the above mentioned testing results that conventional after heating temperature ranging from 720°C to 760°C for stress relief is too high for maintaining initial good creep rupture strength of mother material.

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Generally speaking, austenitic heat resisting steels, when they have the tendency of agehardening during creep test, rupture with remarkably small elongation after the lapse of a certain time in the test. This state of poor elongation is sensitive to notch and attention on it is required from design viewpoint.
Now we dealt with creep ruptures of super alloy LCN-155, as its test pieces were subjected to creep rupture test at the temperature of 550°C, 650°C and 700°C for about 5000 hours at the longest with respect to three sorts of heat treatment; i.e., solution treatment, solution treatment plus 700°C aging and solution treatment plus 800°C aging.
Poor elongation ruptures were of intercrystalline rupture even when they occured after a short period. This fact is explainable if we assume that such ruptures are mainly caused by the united action of both the lattice distortion due to early stage of boundary precipitation and the voids developed by vacancy aggregation. We obtained the electron microscopic examination results which support the above assumption. Some other austenitic heat resisting steels which we examined have the same tendoncy with respect to rupture elongation.
Ferritic heat resisting steels sometimes show a brittle creep rupture, and such a nature becomes specially strong when tempering of them is not sufficient. We ascertained such a tendency on two ferritic steels H40 and H46.

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In recently, as it should be considered the high temperature strength of some copper alloys and commercially pure titanium for practical purpose, the author has been begining the study of high temperature tests, such as tensile, creep and fatigue tests of those materials. Following results were obtained.
(1) The short time tensile strength and creep strength of Albrac and cupro nickel alloys tend to show the different behaviour in the high temperature.
(2) High temperature fatigue strength of cupro nickel alloy shows the maximum value in the temperature range from 200 to 300°C.
(3) The relations between the speed of cyclic stressing and the fatigue strength of the aluminium alloys and commercially pure titanium tend to show quite contrary each other.
(4) In the case of tough and harder materials, such as stainless steel, commercially pure titanium and cupro nickel alloys, they generate heat by large repeated stress cycles and decrease their fatigue strength.
(5) When the high temperature strength of the alloys are studied, it is necessary to consider the effects of speed of cyclic stressing and internal heating phenomenon above mentioned.

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Machine parts often are subjected to repeated stresses at elevated temperatures and the fatigue deformation of the machine parts caused by repeated stresses may have undesirable effect upon the faculty and the accuracy of these machine parts.
In this paper, the effect of elevated temperatures (100°C, 250°C, 350°C) upon the fatigue deformations of cold worked and torsional stress-aged 0.37%C steels caused by slow cyclic torsional stress was investigated.
The results obtained are as follows:
(1) In cold worked specimen, the more temperature was elevated, the more fatigue yield strength increased, at the range of temperature from room temperature to 250°C, but at 350°C it decreased.
(2) In torsional stress-aged specimen, the more temperature was elevated, the more fatigue yield strength decreased, at the range of temperature up to 250°C in τ_a/τ_m≤1, but increased in τ_a/τ_m>1. And at 350°C it decreased in the all values of τ_a/τ_m.
(3) It seems that the above behavior of the fatigue yield strength is related to a structural change caused by elevated temperatures and repeated stresses, and especially the remarkable change of the fatigue strength in τ_a/τ_m>1 is related to the diminution of the Bauschinger effect caused by these factors.
(4) Torsional stress-aged steel is more useful in preventing the fatigue deformation than cold-worked one at the range of elevated temperature up to 350°C.

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Torsional creep tests of a mild steel were carried out under varying stress levels at 500°C by using thin walled tubular test pieces. In the present test conditions under constant stress, buckling occurs within the range of primary creep, where the relation between strain γ and time t is expressed by Andrade's equation, γ=γ₀+βtⁿ, like as in tensile creep tests. For varying stress tests, stress level is varied to the lower stress of τ_u=-12.8, -6.4, 0kg/mm² and is maintained for 60min after the upper stress τ₀=12.8kg/mm² for 60min, and the cycle is repeated. The instantaneous strain, creep rate, and total creep are greater when the previous stresses are higher in the reverse direction. Accordingly, the creep resistance is increasing with test time, and yet the increase is smaller when the stress returns to the constant τ₀ from higher τ_u. The instantaneous recovery of strain in releasing of stresses is constant if the stresses were in the same height, and is not varied by the number of cycles. The rate of recovery strain under τ_u=0, i.e. during the rest of load, is in linear relation with time when the both are shown by logarithmic coordinate, and is not affected by the number of cycles.

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