Ni-Cr-Mo steel were tested axially under completely reversed constant strain range and completely reversed constant stress range from monotonic tension to 130 cycles. Hysteresis loops were recorded at intervals throughout the life to failure. The mechanism of tests were explained by true stress considering the change of test section. That is, completely reversed straining test was completely reversed as to true stress. The completely reversed stressing test was partially reversed as to true stress, and then the mean stress caused the increase of mean strain. Stress and strain cycling results were compared on the basis of the condition for the first cycle. For the same condition for the first cycle, cyclic stressing test shows relatively shorter life than cyclic straining test. The mechanism of fracture were found a conventional fatigue during cyclic straining but a necked-out type with the increase of mean strain during cyclic stressing. The total plastic strain to failure of cyclic straining test was larger than that of the cyclic stressing test. This seems to show the difference of modes of fracture of the tests.
The salt-bath nitriding is used widely today because it has an advantage of shorter treating time than the conventional nitrding process. We studied the effect of this process on the improvement in fatigue properties of high Cr steels, and clarified various features of hardened case by metallographic and X-ray diffraction tests. The results obtained are as follows: (1) The extremely hard compound layer produced in the surface of austenitic high Cr steel (SEH 4 or 21-4N) during the salt-bath nitriding at 570°C contains a large amount of Cr-nitrides and Cr-carbides. In ferritic high Cr steel (SEH 3), however, the formation of Cr-compound is less than in austenitic one, and it is found by X-ray diffraction analysis that the lattice in surface zone is distorted considerably after this treatment. (2) The salt-bath nitriding improves remarkably the rotating bending fatigue strength of SEH 3. But this process is not so effective for the improvement of fatigue strength of SEH 4 or 21-4 N. (3) In the welds of SEH 3 and SEH 4, in which the rupture occurs usually in SEH4 side during the fatigue test, the fatigue behavior of the welds before and after the nitiriding is almost equal to that of SEH 4 base material. On the other hand, it is found that this process is not always effective for improving fatigue characteristics in the welds of SEH 3 and 21-4 N, in which the rupture occurs usually in the weld zone. (4) Tufftride process which employs the salt-bath consisting mostly of potassium cyanide and cyanate has a more pronounced effect on the improvement in fatigue strength of high Cr steels than the domestic process.
Comparison has been made between the results obtained from the fatigue tests with several types of fatigue testing machines under different testing conditions with the same material. As the results of it, it has been disclosed that the effect of the force of inertia caused by the interposed mass between the dynamometer and the specimen becomes large enough not to be negligible. Correction factor for this effect of the force of inertia has been calcurated from simple vibration model. The corrected S-N curves of the different types of testing become in good agreement. Calculation for this correction factor has been shown for the several types of testing machines under several conditions.
To investigate the influence of loading frequency in high speed fatigue of metals, a pneumatic machine for fatigue testing at frequencies ranging from 150 to 2000 cycles per second has been designed. Method for excitation used is an air nozzle system of so-called flutter type, in which the specimen vibrates in a direction transverse to the air stream, and it does not require complicated control system to maintain resonant condition. Specimens are thin reeds of 1.2mm in thickness and 4 or 5mm in width at the test section, and are vibrated in the fundamental mode of cantilever beam. Strain amplitude of the loaded specimen was measured by electrical resistance strain meter, and certain considerations about measurement were made with respect to gauge length, thickness of gauge base and temperature increase during test. Cyclic strain fatigue tests were made at room temperature on an annealed 0.23% carbon steel, and it was found that the endurance limit increased with increasing test speed in this range of frequencies.
The variation with temperature of the magnetic susceptibility of sodium silicate glasses containing various amount (0.5∼17.0%) of iron oxide has been studied over the temperature range -180 to 400°C. As a complementary study, the optical transmission properties of the glasses used in the magnetic work were measured in the visible and near infra-red regions of the spectrum. The mass magnetic susceptibility χb of the parent glass was found to be temperature invariant and negative. The glasses containing the iron oxide additions (0.5∼2.5%) showed a relationship between the value χg-χb and the absolute temperature T of the form χg-χb=C/T, where χg is the mass magnetic susceptibility of the glass and C the constant. Three forms of iron are present in the glasses, namely, as fine particles of ferric oxide, Fe3+ and Fe2+ ions. The proportion of iron present as fine particle of ferric oxide seems to increase with the increase of total iron concentration.