PROPERTIES AND THE PLASTIC EXTENSION OF METALS. 
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with time ; and this change from unproportional to proportional elasticity or from 
imperfect to perfect elasticity is accelerated by boiling. 
In fact overstrained iron or mild steel is restored to its perfect or proportional 
elastic state with remarkable rapidity by mere boiling. This point has been 
established by Sir Alfred Ewing. # 
I have found, however, that overstrained high carbon steels and the alloy steels do 
not recover proportional elasticity either by resting or by boiling. 
The elastic line of a 3 per cent, nickel steel is seen in fig. 4. It is lettered A. 
The limit of proportionality is reached at 8 tons, 26 tons per sq. inch, and the yield at 
9 "25 tons, 30 tons per sq. inch. 
The piece was stretched 2 per cent, and then the diagram, line B, was taken. 
Proportional Elasticity has disappeared. 
Curve C is the record after a 6 per cent, stretch. 
Curve D is a repetition test after turning the bar to a slightly reduced diameter. 
The interval of time between C and D is 24 hours. No restoration of elasticity has 
taken place. It has been established by other experiments that a lapse of many 
months has no effect in restoring the proportional elasticity. 
The piece was then boiled for 1 hour, and curve E shows that elasticity has not 
been restored. 
Finally, the piece was heated to 550° C. in a muffle furnace for about half an hour 
and was then allowed to cool down with the furnace. Line F, taken immediately 
after this treatment, shows perfect recovery of proportional elasticity and a slight 
raising of the limit and the yield point. 
I have confirmed these results by other experiments on nickel steel test pieces and 
on high carbon steel test pieces. 
* ‘Phil. Trans. Roy. Soc.,’ 1899. 
