30 



MR. J. MUTR ON THE RECOVERY OF IRON FROM OVERSTRAIN. 



under 40 tons per square inch of original area. The total elongation in this case \\.iw 

 found to be fully 16 per cent, on the 8-inch length. 



In order further to call attention to one or two features of this recovery from 

 overstrain and the effect of temperature on it, the history of another specimen is 

 given in Diagram No. IX. 



The steel rod from which this specimen was cut differed but slightly from preceding 

 ones. The rod was 1 inch in diameter, but the specimen was turned down, except at 

 the ends, to a diameter of about 0'8 of an inch. The yield-point occurred at a stress 

 of 23 tons per square inch, and was well defined like those shown in Diagrams III. and 

 VIII., unlike those in Diagrams IV. and VII. The position of the yield-point was, 

 however, sometimes found to vary even with specimens taken from the same rod. 

 Thus, the specimen of the present diagram (No. IX.) gave apparently a perfectly 

 steady extensometer reading after 22 tons per square inch had been applied steady 

 for, say, half a minute. The addition of the next half- ton produced rather greater 

 elongation than was in accordance with the elastic law, but the reading was still 

 steady. With 23 tons, however, creeping set in shortly after the extensometer 

 reading a rather large one had been observed. This yielding continued, becoming 

 greater and greater, and the skin of oxide began to spring off in the manner 

 characteristic of the yield-point. Another specimen taken from the other end of the 

 same bar (a 10-foot one) showed creeping and the springing off of the oxide after 

 22 tons of stress had been applied. 



After the passage of the yield-point, illustrated in Curve No. 1, Diagram IX., the 

 specimen was put into boiling water, and kept there for over 12 hours. This was to 

 see if the position of the second yield-point would be affected by such prolonged treat- 

 ment. As was expected, on cooling and re-testing the specimen a yield-point occurred 

 at a load which agreed with that obtained from an adjacent specimen of the same rod, 

 which had been immersed in boiling water for 3 minutes only. A third specimen 

 from this same rod, after being overstrained, was put in a sand-bath, and kept at 

 250 C. for half-an-hour. On slowly cooling and then re-testing, the material behaved 

 exactly as in the case of the comparison specimen, which had been restored by 

 3 minutes' immersion in boiling water. Had the specimen been annealed by heating 

 to redness and slowly cooling, then, of course, the effect of overstrain would have been 

 entirely annulled, and a yield-point obtained at a load corresponding to the stress at 

 which the primary yield-point occurred.* It was found, however, that no effect 

 (other than the recovery from the temporary effect of overstrain) was produced, until 

 a fairly high temperature was attained. 



To return to Diagram No. IX. After the second yield-point had been passed, the 

 bar was re-measured and re-tested in the usual manner, Curve No. 3 being obtained. 

 In this test the maximum load was kept on over night, and the creeping which 



See paper by UNWIN, "On the Yield-point of Iron and Steel, and the Effect of Repeated Straining and 

 Annealing," ' Roy. Soc. Proc., 1 vol. 57, 1895. 



