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内容記述 |
As is well known, tensile properties, expecially the elastic limit of the cold-drawn steel wires increases remarkably, when subjected to the low-temperature heating after cold-drawing. There is, however, no definit study on the mechanism of the above-mentioned problem, including further that on the change of properties of cold-drawn wires at room temperatures, for instance, the strain-aging. There have been found a number of experiments which contradicted each other even in their results and in their formations of conclusion. The authors here investigated the effect of low-temperature annealing on mechanical properties of cold-drawn Cr-V-steel wire, the material for exhaust-valve springs of aircraft engines, with special reference to the increase of tensile properties and the recovery of elasticity and then discussed the hardening phenomena of wires due to the low-temperature annealing in relation to the cold-working prior to it. In the present report the experimental results containing some remarkable points are informed. The preperation of samples was as follows. Wires fully annealed at 700℃ for 1 hour were cold drawn through the "Dialloy" or "Tungalloy" dies applying a special solid lubricant at a considerably slow speed of drawing. Samples of about 200mm length were taken from the cold-finished wires, all of them having the diameter of 2.6mm and subjected to the successive heating at different temperatures for varying hours in the electric tube furnace of ordinary atmospheric condition. The material tested contained about 0.5% C, 0.5% Mn, 1% Cr and 0.18% V as the main elements. Therefore, it may be suggested, that in the full annealing has accompanied a quenching effect as the result of air-cooling of annealed wire. The mechanical testing of the wire was performed first by investigating the recovery of elasticity accurately. And so the tension tester was equipped with a Martens' mirror extensometer and from the stress-strain curves gained the elastic limit and yield point were determined at the residual strain of 0.03% and 0.2% respectively. Besides, the "Vickers" hardness numbers were measured for all series of samples; and Young's modulus of elasticity and torsional value for some series. The experimental results obtained and their explanation are presented in Chapter I and II with 9 Tables, 14 Figures and 6 Photographs, from which some remarkable points may be abstracted as follows. (1) As is shown in Fig.1, the general effect of cold-drawing is the increase of hardness and strength and the decrease of plasticity, while, the elastic limit, unexpectedly, deviates more from the yield point and breaking strength as the amount of cold-reduction increases. (2) The effects of low-temperature annealing on properties of cold-drawn Cr-V-steel wires were in various characteristics being affected by such factors as amount of cold-reduction, annealing temperature and annealing time. Most outstanding example of the characteristics (Fig.2) was for the sample with 19.6% cold-reduction at which by annealing at 300℃ the hardness and the strength come to maxima, while, the breaking strength, yield point and the elastic limit nearly coincide together and the elongation drops to minimum. The change of Young's modulus of elasticity occurred by annealing at 100-200℃, displaying a sudden recovery which was followed by the gradual increase. (3) For the higher amount of cold-reduction, for instance 75.0% or more, various properties were affected in different ways. Their maxima were shifted towards the higher temperature side when annealed for 1 hour, but the annealing for longer durations brought the maxima towards the lower temperature side (Fig.3-Fig.7). (4) In this respect data on the relation between the amount of cold-reduction and the degree of increment of elastic limit, also the annealing temperature at which its maximum points occurred, as is given in Fig.8, were of most significance so far as concerns our considerations. (5) The effect of annealing time at elevated temperature (for example at 300℃) and the aging effect at room temperature were almost the same as expected (Fig.9 & Fig.10). (6) As stated before it was needed to discuss in the present problem, the low-temperature annealing procedures of this wire, whether the softening treatment prior to the cold working has been completely performed or not, in other words, whether some residual compounds capable of precipitation due to the low-temperature tempering alone, have still existed or not. So the authors examined the effect of different cooling rates after softening treatment (700℃×1 hr.), i. e. the air cooling, the furnace cooling and especially the "slow" cooling which was as carefully accommodated as possible to the change of equilibrium state. The results, however, were just the same as for each series and hardening phenomenon was never established resulting from the low-temperature tempering for 1 hr. after any cooling processes. Only a slight cold-working after the softening treatment produced a maximum hardened state by annealing at 300℃ of which process was the same as tempering one (Fig.11). (7) Futhermore it was determined that the low-temperature tempering for longer durations, i. e. about 900 hours or more, showed few temper-hardening phenomena in any samples subjected to the above-mentioned softening-and-cooling processes. Therefore it may be almost perceived, that no precipitation hardening due to the simple "quench-aging" effect could be introduced. Yet the materials subjected to the long-time tempering and consequently some stabilizing-treatments up to their equilibrium states could be more hardened, when subjected to the low-temperature annealing for 1 hour after cold-working quite similarly for cases without the longtime tempering (Fig.12). (8) The effect of repeating the cold-working and the low-temperature annealing alternatively was seemingly equivalent to increasing the cold-working degrees in the result. So in the processes of repeatings, -the cold-working, the low-temperature annealing, the cold-working and then the low-temperature annealing-the annealing temperature at which the maximum hardening occurred was shifted towards temperatures higher than 300℃. Further in the progressively repeated stages such a quasi-saturated condition was perceived that the work-hardening has no more taken place in the cold-working process, and the final annealing at 500-600℃ showed a considerably intense decrease of hardness, perhaps due to a drop of recrystalization temperature (Fig.13). (9) From the above-mentioned points it was ascertained that for the occurrence of the hardening phenomena resulting from the low-temperature annealing some internal stresses were necessary due to the cold-working prior to low-temperature annealing. In the experimentally determinable range the lowest amount of this necessary cold-working was about 8.72% (Fig.14). (10) The microstructure of this wire, when subjected to the low-temperature annealing after cold-working, displayed no more changes than those simply cold-worked. At most a disappearance of fibrous flow due to the cold-drawing was partly recognized. Hence, it was impossible to correlate the hardening phenomena directly with the changes of microstructures (Photographs 4, 5, 6). (11) The object of this study, namely the mechanism of the hardening phenomena due to the low-temperature annealing, will be theoretically informed by the authors later in the 3rd report. |
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