Scott. — Resistance of Steel to Mechanical Shock. 527 



That the impact test on notched bars is of marked value 

 has been fully demonstrated by the clearing up by this method 

 of testing of much of the mystery which has surrounded the 

 results of the oil hardening and tempering of steel. 



Oil tempering as generally practised for guns, axles, &c, 

 consists in heating to about 850° C, then quenching in oil at 

 80° C, after which the material is reheated to between 550° 

 and 650° C. By the first heating (to above the Ar 3 point) fine- 

 ness of structure is obtained, and by the quenching retained. By 

 the second heating (to below the Av 1 point) any mechanical 

 stresses due to the quenching are removed, the hardening carbon 

 converted into cement carbon, but the fineness of structure is 

 unaltered. Practice has proved that steel so treated is safe 

 for gun-construction, and when not so treated unsafe. Yet the 

 tension tests show only an increase of strength and ductility by 

 oil tempering of something between 5 and 30 per cent. 



The improvement in the material is out of all propor- 

 tion greater than that indicated by these values ; and Mr. 

 Seaton, applying his test, finds that by so tempering, the im- 

 pact strength of the steel, as measured by the number of 

 blows required to produce fracture, is increased to between 500 

 and 600 per cent, of that of the untempered steel in its best 

 condition. 



Messrs. Seaton and Jude also find that oil tempering has the 

 property of levelling up the " shock " strength of steel to a 

 fairly constant quantity. Two articles may have been made 

 from the same grade of steel : owing to difference of heat treat- 

 ment in manufacture one may be coarsely crystalline and 

 dangerously brittle, the other fine-grained and tough. Oil 

 quenching will bring the shock strength of these up to a common 

 value — the increase in one case being measured by thousands 

 and the other only by hundreds per cent. 



Since it would appear that the failure of steel members 

 may be brought about by (1) statical overloading, (2) fatigue, 

 (3) shock (the effect of which is apparently cumulative), it 

 would seem rational that for all practical purposes material 

 should be tested in the same manner as that in which it is to 

 be loaded when in use. Thus, for structural work, where onty 

 a dead load is to be carried, a tensional test is sufficient, for 

 from this in all but special cases the compressional, cross-bend- 

 ing, and shearing strengths can be computed. For structural 

 work with gradually applied live load the tensional test is again 

 required, as from this and the ratio of live to dead load the 

 working-stress permissible, with security against fatigue, is ob- 

 tained . 



