Hereunder is presented the report of experimental studies that have been made in our laboratory extending for a considerably long period of time, of the creep-rupture strength of boiler tube materials, i. e. of steels of 11 grades of carbon steel, molybdenum steel and Cr-Mo steels including even 9% Cr-1% Mo. The test samples were taken from at least 3 heats of each grade up to as many as 21 heats of 2.25% Cr-1% Mo steel. The main results obtained from these experiments are as follows. (1) Very small effect of conbon on carbon steel has been observed in the long time test at above 400°C for creep-rupture strength. (2) Similarly there has been but little difference in the creep-rupture strength of steels of 2 different 0.5% Mo grades with slightly different amount of carbon ingredient, in their long time test at above 450°C. These steels of variant molybenum grades occasionally showed very low values of rupture elongation, and had to be improved by addition of 0.5% Cr. (3) The steels of 2.25% Cr-1% Mo and of 5% Cr-0.5% Mo showed low values of rupture strength at comparatively low temperature in 105hr when fully annealed
In order to develop high strength austenitic heat resisting steel, the creep-rupture and metallo graphia data are presented on 25% Cr-28% Ni steels. The individual and combined effects of C, N, Mo, W, V, and B have been studied in detail. Some of the steels tested with high N content are prepared using a melting and casting technique under high-pressure nitrogen atmosphere. Of the 25% Cr-28% Ni steels tested two compositions 2 Mo-5.7 W-0.6 N-0.05 B and 1.6 Mo-2.2 W-0.3N are shown to give good combination of properties. The former possesses very high 1000hr creep rupture strength, that is 26.5kg/mm2 at 700°C. The latter possesses strength of 23.0kg/ mm2, and shows no abnormal drop in the rupture strength even after the test period of 20000hr at 700°C. This property together with ease of hot-rolling and forging make this steel very attractive for high strength applications at elevated temperature.
From the tests of tensile creep rupture in notched plate specimens of 0.098 per cent carbon steel at 500°C, the following conclusion can be derived. (1) There was considerable change in stress distribution on the cross section of the notched bottom during the early stage of the creep, but after a time no remarkable redistribution was observed. (2) The notch-strengthening effect of creep rupture strength of the material tested was examined on the basis of the nominal stress. On the basis of the equivalent steady stress, whose increase of the true stresses with progress of creep was taken into account, the difference in rupture life between the notched specimens and the smooth specimens was found smaller than that on the basis of the nominal stress. However, the behavior of notch-strengthening was still fixed. (3) In order to determine such a notch-strengthening effect, the following stress condition was estimated. This is the case where the level of hydrostatic component of stress at near part of the notch bottom of the notched specimen is lower than that at the middle part on the cross section, and the former level is lower than that at near part of the bottom of the smooth specimen. (4) The initial rectangular anisotropy of the material tested was examined experimentally, but neither the level of the equivalent steady stress nor multiaxiality of the stress was affected by the present anisotropy of the material.
The creep-rupture strength of precracked specimens made of S10C carbon steel was investigated at 450°C as an extreme case of the strength of specimens with a sharp notch. Two methods were employed for making precracks. One was by using a rotating bending fatigue testing machine with specimens having a sharp notch, as had often been used by many investigators. The other was by using a micro-cutter to make a circumferential slit as narrow as 0.15mm to a prescribed depth at the notch root of the specimen. This specimen was compressed in the axial direction until the flanks of the slit became close together. All the specimens were annealed in vacuum after the precracking. The latter method was mainly adopted in this study because it afforded easier control of geometry and size of precracks than the former method. The S10C carbon steel, which had high ductility, showed“notch-strengthening”for all the precracks investigated in this study. The notch-strengthening was higher for deeper precrack, and decreased with increasing rupture time. For specimens with a precrack-depth to outer-diameter ratio higher than about 0.1, the notch-strengthening was found to converge to a certain value at rupture time extending for hundreds of hours, irrespective of this ratio. The cracks were observed to propagate itself in a mixed manner of both the transcrystalline and the intercrystalline modes. No noticeable difference was observed at longer rupture time between the two methods of precracking, either in strength or in microscopic behavior of crack propagation. This means that the method of precracking by the use of a micro-cutter is effective for investigating the characteristics of crack propagation.
Intermittent stress creep tests were performed of low carbon steel, commercially pure titanium, commercially pure aluminum, high purity aluminum (99.99%), tough pitch copper (99.9%) and high purity copper (99.99%) at various temperatures, in order to get informed of the mechanism of the static-to-dynamic transition in creep. Drastic transitions were observed in the materials of commercial purity, whereas transitions were not apparent in the materials of high purity (99.9% or more). With regard to the effect of temperature, there seems to be a critical temperature for each material above which the transition appears and below which it does not appear. The critical temperature is likely to be related to the strain ageing temperature of the material. There was no difference between the transition behaviors in the range of the high temperature creep and the low temperature creep. According to these results, the transition is possibly related to the interaction of dislocations and solute impurities, appearing at temperatures where the diffusion rate of the solute atoms is high enough to get to dislocations readily. This means also that the interaction of creep and creep recovery, which is the cause of the transition, is related to the solute-dislocation interactions.
Hereunder is reported an experimental study made of the effect that the corrosive environment due to high temperature gives on the stress-rupture strength and the fatigue strength of Inconel 700 which is heat resisting Ni-base alloy. The experiments were carried out in such environments as residual fuel combustion gas, vanadium ash, mixture of vanadium ash and nickel sulfide and their combination at 700° and 750°C. The results are summarized as follows: (1) The strength of the alloy showed a remarkable declining tendency in sulfur-bearing environment. (2) The corrosive effect was found larger on the whole in the fatigue test than in the stress-rupture test. This is presumably due to the fact that the alloy is more sensitive to intergranular corrosion and notches under repeated stress than under static stress. (3) The estimation of the stress-rupture life of this alloy under gradually increasing stress caused by corrosion was investigated by using linear damage rule. The estimated life showed critical value when the corrosive environment is due to sulfur content.
For the design and fabrication of the experimental fast breeder reactor it is desirable to obtain the creep and creep-rupture properties of the domestic tubes of 316 stainless steel with nominal dimension of 5.6mm inside diameter and 0.35mm wall thickness, that will carry nuclear fuel. The creep rupture tests under internal gas pressure and creep and creep-rupture tests under uniaxial tension have been carried out on two kinds of these tubes, one of which was approximately 6% cold-drawn and the other was approximately 18% cold-drawn, in unirradiated and atmospheric conditions at test temperatures of 550°C, 600°C, 650°C and 700°C. The results obtained are summarized as follows: (1) The regression curves for the creep-rupture data obtained from the internal pressure tests and uniaxial tension tests showed points of bend at the pressure of 360kg/cm2 and stress of 30kg/mm2 when the Larson-Miller parameter method was applied. (2) Compared with ISO's and NRIM's data, the present data for uniaxial tension tests showed a little greater creep-rupture strength for 6% cold-drawn tubes and definitely greater creep-rupture strength for 18% cold-drawn tubes. (3) The creep-rupture strength of 18% cold-drawn tubes is generally greater than that of 6% cold-drawn tubes. However, the creep-rupture curves for the two kinds of tubes tended to approach each other when the temperature was raised from 600°C to 700°C and the test time was prolonged. (4) The data for creep-rupture tests under internal pressure nearly agreed with those for uniaxial tension tests at stresses less than 30kg/mm2, when the hoop stress of internally pressurized specimens was calculated by the mean diameter formula.
It has been the object of the present study to elucidate the effect of hydrostatic stress on the mechanical behavior of polycrystalline metals at elevated temperatures. In the studies hitherto made by authors, the effect of hydrostatic stress on metallic tensile creep and torsional creep was investigated through the tests under combined hydrostatic pressure at room temperature. The question concerning the pressure effect under the influence of elevated temperatures has, however, been left still for further inquiry. From (The present) analytical and experimental studies on tensile plastic deformation and tensile creep under hydrostatic pressure at elevated temperatures, the following conclusions have been made. (1) The effect of combined hydrostatic pressure on the plastic flow stress of polycrystalline metals at elevated temperature is observable in the region of large plastic deformation. Therefore, it is necessary to consider the influence of hydrostatic stress in the yielding condition at this region. (2) The effect of concentrated pressure on metallic creep results in decrease in the strain rate of second creep stage at elevated temperature as the same behavior at room temperature. The effect of hydrostatic pressure on metallic creep at elevated temperature may be more intensive than that on the static tensile strength at the same temperature.
Strain-controlled low cycle fatigue tests were conducted at four temperature levels; room temperature, 150°C, 300°C and 450°C. The temperature dependence of fatigue strength of low carbon steel was discussed with reference to the result of the tension tests at elevated temperatures. The conclusions obtained are as follows: (1) The stress range increased continuously at 300°C until the specimen failed, while it held almost constant within other temperature levels. (2) The mean stress range had its peak value near 300°C just like tensile strength. (3) The number of cycles to failure of the material decreased with temperature when the test results were plotted both by the total and the plastic strain range. (4) The plastic work, no matter whether per unit cycle or total, was not effective for the arrangement of low cycle fatigue date, though such equivalent plastic work as defined in the study was found fairly effective. (5) The modified Larson-Miller method was applicable to estimation of failure life of low cycle fatigue at elevated temperature: Good correlation was found between the parameter P'=T(logN+C') and the total strain range Δε except for the case of room temperature.
Graphites are subjected to severe thermal shock, to thermal fatigue and to their combined effect, when they are practically used as core material in the nuclear reactor, or as electrode in the steel making furnace at very high temperature. In consequence of the combined thermal shock fatigue, three material parameters to govern the strength have been found in the general equation expressed by maximum temperature difference to resist the fracture. An ultra-high temperature strength testing apparatus has been designed and applied to measure the tensile strength, elastic modulus, fracture strain for four grades of graphite up to 2500°C. The thermal expansion coefficients were also measured by another high temperature apparatus. The high temperature strengths of four grades of graphite have been discussed comprehensively according to the maximum temperature difference determined by the parameters on the basis of these experimental date as functions of temperature, heat transfer and number of fatigue cycle.
The effects of temperature and strain-rate on the strength of polycrystalline pure aluminium and on that of aluminium-magnesium alloy have been investigated within the range of temperature from -195 to 450°C and strain-rate from 10-4 to 3×102/s. The test results have been studied from the standpoint of the thermally activated deformation theory. The parameter Z=εexp(Ht/R.T.) (ε is the strain-rate, Ht the apparent activation energy, R gas constant and T the absolute temperature) is effective in prescribing the flow stress in two limited temperature ranges (The intermediate temperature range and the high temperature range) up to ε=10∼50/s. In the case of pure aluminium, the value of the apparent activation energy Ht increases with the increase of strain (Ht=6600∼19000cal/mol for ε=5∼20%) in the intermediate temperature range (-20∼80°C) and is nearly constant (Ht=34000∼32000cal/mol) in the high temperature range (250∼450°C), while in the case of aluminium-magnesium alloy, Ht is nearly constant in the intermediate temperature range (-20∼80°C) and also in the high temperature range (350∼450°C) (Ht=9000∼12000cal/mol and 42000∼39000cal/mol respectively).
The experimental studies of such refractory metals as tungsten, molybdenum and tantalum have so far been often performed to measure their maximum strength to hold out with their integral hardness against the heat to which they are constantly exposed. In view of the fact that c. 1000°C has been the maximum temperature hitherto used in the experiments, while higher temperature limit is in prospect for the capacity of these refractory metals, a new hardness tester has been designed for temperature as high as 1600°C, and it has been used in the present experiment for the hardness test of tungsten, molybdenum, tantalum and tungsten-molybdenum alloys. The investigations have been carried out with the following results. (1) Tungsten and molybdenum, when work hardened, will gain in temperature strength to certain degrees below their recrystallization temperature. (2) In tungsten, molybdenum and tantalum the temperature required for their transition from the ductile to the brittle is considered to be in correlation with the bend point which represents the rate of hardness integrity of these metals respectively against heat, on the curve of the testing temperature. (3) Molybdenum indicated the bend point at Th≈0.4, where its remarkable softening began, while tungsten showed no clear bend point below Th=0.5. (4) Tungsten-molybdenum alloys, though they ranged between tungsten and molybdenum in maintaining their integral hardness in room temperature, showed even higher rata of hardness integrity against heat than tungsten in certain range of high temperature. (5) The rate of hardness integrity of tungsten, molybdenum and tantalum against heat obtained by the present investigators were found within the range of the scattered data obtained by other investigators. (6) Molybdenum, cast or annealed, appears to have rate of hardness integrity against heat in proportion to its tensile strength in high temperature.