HARDENED BY OVERSTRAIN. 
29 
effected by warming to between 100 and 200° C., and then the specimen was heated 
to 500° C. to see if the yield-point would thus be brought to approximately the same 
position as in Curves 8, 9, and 10, which were obtained after the specimen was pre¬ 
viously heated to this temperature. Curves Nos. 16 and 17 show that the yield- 
point occurred at a much lower stress than in the Curves 8, 9, and ] 0, so that the 
material was in an entirely changed condition as regards the effects of temperature. 
A further hardening of the material was next attempted, but fracture inad¬ 
vertently occurred, as illustrated by Curve No. 20. A temperature of 200° C. had 
been employed to effect recovery from the overstrain illustrated by Cuiwe 19, l)ut 
Curve No. 20 seems to indicate that this temperature had been sufficient to produce 
annealing action, because large yielding and then fracture occurred at a load distinctly 
lower than would naturally have been expected. This test was perhaps not quite 
conclusive as sliowing tempering by 200° C., for the specimen after so many over¬ 
strains was not very uniform in section, and the fracture occurred at what was known 
to be a rather thick part of the bar. It is thus just possible that the fracture was 
due to the bar not liaving been thoroughly overstrained by the precediiig loading, 
although in that case yielding might have been expected to have started below 
26 tons per square inch instead of at 27 tons per square inch. On measuring the 
fractured specimen it was f)und that the length, which had been originally 4 inches 
long, was now 5'80 inches. 
Had fracture not inadvertently occurred, as explained above, there seems t(j be no 
reason wliy this specimen should not have been overstrained and tenq^ered or 
annealed an indefinite number of times, and so tlie material have Ijecome drawn out 
into the form of a wire. 
The history of another specimen of the same Lowmoor iron is giveii in Diagram 
No. 8, and the results recorded in that diagram serve generally to corroljorate those 
illustrated by Diagram 7. The specimen employed has already been referred to in 
the preliminary examination of the Lowmoor iron described above. It was the 
s|)ecimen which was subjected to prolonged annealing at 750° C., and then hardened in 
the manner illustrated by the curves marked B in Diagram No. 5. In Diagram No. 8 
there are illustrated the first and last overstrains in this hardening of the specimen, 
and also the position of tlie four intermediate yield-points. 
Curve 8, Diagram 8, shows that 260° C. was too low a temperature to produce 
annealing, while Curve 9 shows that 310° C. sufficed to lower the yield-point by 
almost 2 tons per square inch. It is just possible that in the preliminary hardening 
of this specimen a seventh yield-point at a stress of about 29 tons per square inch 
might have been safely passed, and so the material after restoration of elasticity 
brought into a condition capable of bearing a load of 31 tons. Had this been safely 
accomplished, then no doubt a temperature lower than 310° C. would have sufficed 
to produce a tempering of the material. 
A comparison of Curves 11 and 12, Diagram 8, indicates that time has some 
