EFFECT OF ANNEALING. 315 



300 resulting in a fall in the limit load of some 

 6 per cent. 



165. Effect of Annealing. The effect of annealing on 

 an overstrained steel bar is to bring the metal back to its 

 original state. This is shown very forcibly by the results 

 of some endurance tests made on two steel bars by 

 Mr. Coker. In these tests, the bar was taken and loaded 

 to just beyond the yield point, the permanent set being 

 a quarter of an inch. The bar was then annealed 

 by heating to a dull red heat arid cooling slowly in 

 lime. After this, the bar was again loaded until the 

 extension was a quarter of an inch. This process was 

 repeated 37 times in the case of one bar and 39 times in 

 the case of the other. It was found that the yield point, 

 instead of being continually raised, as would be the case 

 without annealing, was approximately constant through- 

 out the whole series, fluctuating about a value of 17 tons, 

 and generally keeping between 15 and 19 tons per square 

 inch. It was also found that the total extension of the 

 bar was about four times what it was for a bar of the 

 same material tested in the usual way.* 



1 66. Effect of Annealing on Wire. Cold-rolling and 

 wire-drawing give rise to effects which are analogous to the 

 overstraining of a test bar. The elastic limit is raised, 

 and also the breaking strength, while the ductility, as 

 shown by the extension and reduction in area, is reduced. 



These points are shown very well by the results of 

 Mr. Keigwin'sf tests on unannealed and annealed steel 

 wire. Here the strength of the wire when annealed was 

 found to be lowered to that of the billet from which it 

 was drawn, or a reduction of 39 per cent. ; the ratio of 

 yield point to breaking was lowered by annealing ; while 

 the elongation and reduction in area were greatly 

 increased. 



Another effect of loading a ductile material by exposing 

 it to a steady load not far removed from the breaking load 

 for a considerable period of time, shown by Lord Kelvin, 

 is an increase of strength and loss of ductility. It was 

 found that the effect of this long-imposed stress was to 

 cause the specimens to break at a much higher load than 

 the original unstrained bars. 



* Min. Proc. Inst. C.E., vol. cxxxv. 

 t Min. Proc. Inst. C.E., vol. cxxxvi. 



