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ON THE TREATMENT OF STEEL TOR THE CONSTRUCTION OF ORDNANCE. 403 



is of paramount importance in wire that has to be coiled at a high tension 

 on a cylinder, and for this reason wire tempered a-fter instead of hefore 

 finishing is safer, though not so strong. If this wire be thick, judicious 

 annealino-, though it lessens the ultimate strain which the wire will bear, 

 raises in a very marked degree the limit of elasticity. I have found flat 

 steel wire of about -2 of an inch thick, and of great ultimate strength, 

 begin to stretch permanently at a tension as low as 25 tons per square 

 inch, Avhile after being properly annealed it would bear 35 tons before 

 permanent movement. The explanation of this curious fact is probably 

 to be found in the removal by the annealing process of contending states 

 of tension produced by the drawing or tempering on the inner and the 

 outer portions of the wire. This view is confirmed by the fact that, when 

 the thickness of the riband was reduced to somewhat less than half, this 

 advantage of the annealing process almost entirely disappeared, and the 

 wire was simply softened and rendered more ductile. 



Castings of steel, unhammered, are improved by being tempered in 

 oil in much the same degree as the foi-ged material. Test pieces cut 

 from a cast trunnion of steel gave the following results :— Before temper- 

 ing the elastic limit was 16 tons per square inch and the breaking point 

 27-8 tons, the elongation in 2 inches being 7-5 per cent. After tempering 

 the elastic limit was 23 tons, the breaking point 377 tons, and the 

 elongation was 12-5 per cent., showing a great improvement under every 

 bead. The quantity of combined carbon contained in this specimen was 

 •36 per cent. The objection to the use of cast steel in the unhammered 

 form is that it is liable to unsoundness from air-bubbles ; but this, I 

 think, ought not to exclude its use for trunnion rings, which, from their 

 peculiar form, can only be very imperfectly forged. The unsoundness 

 from this cause would be greatly mitigated by casting under pressure, as 

 advocated by Sir Joseph Whitworth. 



There is much less sacrifice of ductility or toughness where increase 

 of strength is obtained by tempering than by increase of carbon, and, in 

 fact, the advantage of tempering in oil is so apparent, both in the case of 

 steel castings and of steel which has been either rolled or forged, that 

 there is strong inducement for engineers to avail themselves of the 

 process for increasing the efiiciency of the material in nearly all its 

 applications. The saving of the weight of material necessary for a given 

 purpose would amply repay the cost of the tempering, and in the case of 

 bridges of great span, where the strains are chiefly due to the weight 

 of the structure, independent of its load, the economy effected would be 

 far more than proportionate to the increase of tensile strength in the 

 material. 



My experiments are not sufficiently extended to enable me to speak 

 definitely as to the best proportion of combined carbon for steel to which 

 the tempering process is to be applied, but excellent results can certainly 

 be attained with steel containing "35 per cent, of carbon. If the masses 

 to be dealt with are thin less will suffice ; if thick, more will be required; 

 but it is quite possible that the mode of applying the oil in the tempering 

 process might be improved so as to render it more efficacious where the 



balk of the steel is large. 



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