48 MILD STEEL. 



have to pass a further public survey or not ; and in the case 

 of nearly all large contracts for steel, the material is still 

 further subject to the tests laid down by the Board of Trade, 

 the Admiralty, Lloyd's Committee, or the purchaser's own 

 surveyors and experts, and thns the constructor is enabled 

 to place almost absolute reliance upon the quality of his 

 material, and in consequence it has been stated on the best 

 authority (Mr. Samuel Adamson), that whereas in the old 

 days when using wrought iron the return of bad plates to 

 the makers was at the rate of 12 %, that now with steel the 

 returns do not exceed 0*25 %. 



The mild steel now so much in nse contains, on the 

 average, 99 % iron, 0'15 % carbon, 0'63 % manganese, 0*03% 

 silicon, 0*06 % sulphur, and 006 % phosphorus, and each 

 of these constituents has a special effect upon the temper 

 and quality of the steel. 



The presence of carbon within certain limits has the effect 

 of reducing the malleability and ductility of the steel, and 

 increasing the tenacity. It also imparts the property of 

 hardening and tempering, when after being heated the steel 

 is suddenly quenched in cold water. This latter quality of 

 tempering is generally understood to mark the difference 

 between iron and steel. 



Thus the hardest steel found in commerce and used for 

 razors and tools required to cut chilled rolls and such-like 

 hard materials has as much as l'o% of carbon, with an 

 ultimate tensile strength of 60 tons and an elastic limit of 

 30 tons, and owing to its brittleness and want of ductility is 

 not adapted to constructive purposes. 



On the other hand, steel used in shipbuilding, boiler- 

 making, etc., contains only from 0'15% to 0*20 % of carbon, 

 and has an ultimate tensile strength of from 24 to 32 tons, 

 with from 16 % to 25 % extension in 10 inches ; whilst inter- 



