Recrystallized zinc polycrystals of commercially pure grade were subjected to hydrostatic pressure of the order of thousand atm, and its effects on their structure and dislocation arrangement were studied. Etch-pitting techique was used for this purpose. Besides twinning, slip bands and dislocation multiplication, dislocation piling-up along both the grain and twin boundaries were observed. It is found that the movement of grain boundaries and twin growth are suppressed by this dislocation lining. Implication of this microscopic change on macroscopic mechanical behavior was studied. The increase in stiffness, the reduction of stress relaxation, and the heterogeneity in hardness were observed, but no difference was seen in tensile strength and elongation. The increase in tensile strength and elongation, which was seen when the test was performed under the confining pressure, was also seen when the specimen was pressure-treated after it had been severely twinned.
During the aging of the solution treated 21-4N valve steel, the interstitial elements form carbides and nitrides by precipitation reactions in the same manner as in most Cr-Ni austenitic heat-resisting steels. The morphology and distribution of these phases will have a marked effect on the mechanical properties of this steel at both room and elevated temperatures. Particularly in mechanical properties, the toughness and the fatigue strength seem to decrease remarkably by high temperature aging of 900∼1000°C at which the lamellar nodules are formed at grain boundaries. In this study, the effect of low temperature pre-aging at the temperatures of 700∼800°C on the precipitation reactions during the subsequent high temperature aging has been investigated for the commercial exhaust valves because of the“double aging process”was found to be effective for the test specimens. The effect of low temperature pre-aging on the mechanical properties after high temperature aging has also been investigated at both room and elevated temperatures. The results obtained were as follows: (1) If this steel was pre-aged at 700∼800°C for about 3 hours after solution treatment at 1200°C, lamellar grain boundary precipitates were scarcely formed and general precipitation was predominant during the subsequent high temperature aging. (2) The employment of this low temperature aging seemed to be also effective to retard the formation of lamellar nodules at grain boundaries in the commercial exhaust valves made of this steel. (3) If the solution treated steel was pre-aged at low temperature before high temperature aging, the fatigue strength of this steel was considerably increased at both room and elevated temperature, and its toughness was also improved at elevated temperatures, compared with steel not pre-aged, because lamellar grain boundary precipitates were detrimental to these properties. (4) On the other hand, the tensile strength at the temperatures up to 800°C as well as the creep rupture strength at 600°C seemed to be scarcely changed by the employment of this“double aging process”because the effect of the two types of precipitation reactions, general and lamellar precipitation, on these properties was only secondary.
The influence of microstructural parameters on the true stress-true strain diagrams between 0.02 strain and the necking strain was investigated in the steels whose microstructure was constituted of tempered martensite patches and free ferrite. The results obtained are as follows: (1) Under some conditions, the logarithmic true stress-logarithmic true strain diagram does not make a straight line but it consists of two straight lines having different slopes. (2) The appearance of the knee, which is the intersecting point of two straight lines, is controlled by the value of the volume fraction of tempered martensite f, the mean free path between tempered martensite patches α2, the tempered martensite patch size β and tempering temperature. (3) The stress at the knee is closely related to the yield strength of tempered martensite structure. It is supposed from this that, at the knee, the macroscopic yieldings of tempered martensite patches start. (4) The lowering of α2, β and tempering temperature and the rising of f introduce the increasing flow stress at a constant strain. (5) To explain the above experimental results, a model representing the microstructure of steel used in the present work was proposed. In the model, all the tempered martensite and some portion of free ferrite were connected in parallel and furthermore this unit and remaining portion of free ferrite were connected in series as shown in Fig. 6(c).
Effects of solution treatment (1000∼1250°C×1hr) and aging treatment (650∼800°C×2hrs) on smooth and notch creep rupture strengths (600°C, 22.5kg/mm2) of 18Cr-12Ni stainless steels with carbon contents varying from 0.018 up to 0.386 weight percent were studied. All steels except the one with the lowest carbon content showed a maximum strength at the solution temperature of 1080°C. The appearance of the maxima was explained in connection with the increase of age hardening and the grain growth, both of which were brought about by raising the solution temperature. The former had the effect of decreasing creep rate (increasing rupture life), and the latter caused premature intergranular rupture (decreasing rupture life), thus they balanced each other, yielding a maximum in rupture life at some solution temperature. With increasing rupture ductility (of smooth specimens), the ratios of notch rupture lives to smooth rupture lives increased, though the relation showed a considerable scattering. Notch weakening was observed in the highest carbon steel specimens which were solution treated at higher temperatures, i.e., in the case of the lowest rupture ductility. The effect of aging temperature was negligible as compared with that of solution temperature. Aging at 750°C gave the best results, 800°C giving an over aging. By increasing carbon content the rupture lives inereased. Beyond 0.15% of carbon, however, there was not much tangible improvement but rather an adverse effect appeared on notch rupture lives. The grain growth of the highest carbon steel was retarded because of the existence of retaining undissolved carbide thus the rate of decrease of rupture life due to the rise of solution temperature was slower in the highest carbon steel than in the lower carbon steels.
This paper aims to present evidences for the changes of work hardening phenomena that can occur in tensile pre-strained coppers when they are subjected to fatigue combined stresses. The present author believes that the phenomena are the result of the origination and propagation of micro-crack. From the fact that the intrusions are found along the slip bands, the author surmises that they develop into persistent slip bands and fissures, and that these bands and fissures are kinds of micro-cracks. Changing the grade of tensile pre-straining (K), the fatigue stage of the first origination of slip band was studied in connection with the generation of the intrusion, and that of the first origination of the persistent slip band was investigated in relation to the occurrence of the micro-crack. The main conclusions from this study are as follows: (1) In the case of uniaxial tensile pre-strained copper the changes of the work hardening phenomena appeared slowly during the fatigue test. These results differ remarkably from that obtained by Professor Wood with regard to the fatigue of complex pre-strained copper. (2) In the case of the fatigue of the tensile pre-strained copper the work hardening was saturated at an early stage of the fatigue test, and the appearance of the stage of saturation was delayed by the increase of pre-strained grade K. Concerning the fatigue of annealed copper, the saturated fatigue stage was about 20 to 30% of its life. (3) The occurrence of the initial and decisive fatigue damages was delayed by pre-straning and also by the increase of the normal stress σx caused in the longitudinal direction of the specimen. (4) In view of the fact that in the case of the pre-strained copper the number of cycles from the origination of the micro-crack to the failure of the specimen is less than in the case of the annealed copper, it is believed that the micro-crack in the former case propagates faster than in the latter.
A high-temperature vacuum tensile-fatigue tester (vacuum pressure 10-5mmHg, testing temperature room Temp. -700°C, testing load 0-50kg, and load frequency 1-10sec-1) is constructed, and the high-temperature vacuum tensile-fatigue test of Cu-Al (5% Al) alloy is performed as a test of the tester. The main remarkable points of the tester and the results of the test are as follows: (1) By the use of vacuum bellows, a cyclic load can be applied to a specimen in the vacuum vessel of the tester. (2) By radiation of heater around the specimen, the temperature can be raised to 700°C. (3) Deformation to rupture increases in proportion to the increment of the cyclic load. (4) As to the relation of the cyclic number to rupture N and the temperature T, (-d log N/dT) generally increases in proportion to the increment of the cyclic load and decreases in proportion to the increment of the temperature. (5) Short slip lines appear below about 400°C, and the length of these line and the deformation, made by those slip, to rupture are proportionally large to the increment of the temperature. (6) Large slip lines appear above about 400°C, and the space between those lines is constant. Considerable deformation is caused by those slips to rupture. (7) [cyclic number/cyclic number to rupture] n-[deformation/deformation to rupture] ε curve coincides with a curve ε(n) ε=0.132·n-1/3·exp(2.03n).
It is well known that 7-3 Brass is used as Standard Block of Hardness. Recently it has been pointed out by many investigators, in consequence of the progress made in testing and extension of application field, that its hardness is not sufficient in homogeneity. The authors have studied on the improvement of homogeneity of the block by means of heat treatments and cold rolling, and by admixing foreign elements. It has been found in the experimental results that the most effective factor for improvement of homogeneity is cold rolling, and the factor next in being effective is mixing additional elements. The most effective element to be added into is Be, and Al, Mn etc. to bellow it. However, Be is not reduced to low hardness but Al obtains low hardness by means of heat treatment Hardness loses in homogeneity by being annealed.
In fatigue test strain usually arises a little later than stress applied to the specimen. Owing to this strain lag, in rotating bending fatigue test, the specimen is bent in the direction otherwise than what it was at the moment stress was applied. However, since the conventional rotating bending fatigue testing machine of uniform moment type loads up the specimen through only the knife edges, the specimen can be bent only in the direction at the moment stress was applied and not at right-angle to it; therefore the specimen is given an unknown restrictive load and the test will be futile. On the other hand, the new rotating bending fatigue testing machine of uniform moment type described in this paper loads up the specimen through the knife edges and a pair of pivots, and the sustaining frames of the rotating axis which keeps the specimen supported with a pair of angular contact radial ball bearings. So the specimen can be bent in any direction and therefore no unknown restrictive load is applied to it, and consequently our test will not be futile. Fatigue strengh of S55C steel specimens has been tested by the new testing machine and compared with the results obtained by the conventional testing machine. It has been found that the fatigue limit obtained by the former machine, equal to the one by the latter, but the fatigue strength at N cycles obtained by the former machine is smaller than the one by the latter. The difference between these fatigue strength values at N cycles is considered to be due to the unknown restrictive load mentioned above and this difference is to increase with the applied stress.