THE MAKING AND WORKING OF STEEL 409 



from its ductility and strength to resist shocks. Phosphorus 

 is, therefore, considered a dangerous ingredient, and should 

 never exceed .2%. 



Silicon influences the form which carbon takes in cast 

 iron and its rate of cooling; it increases the tensile strength 

 and reduces the ductility of steel. The process of manufac- 

 ture, however, usually removes all silicon, and therefore that 

 element gives very little trouble. 



Manganese increases the elastic limit, the tensile strength, 

 and the ductility of steel. It also counteracts the sulphur 

 and phosphorus. It is thus an important factor in prevent- 

 ing red-shortness. The proportion of manganese necessary 

 to produce good results is between .2 and .5%. 



468. Influence of Nickel on Steel. Nickel gives great 

 strength to steel, and is, consequently, widely used for shaft- 

 ing, rods, engines, etc. The addition of chromium to nickel 

 steel gives an exceedingly hard steel that is used for gears 

 and springs. Vanadium chrome steel is used for some auto- 

 mobile parts. 



469. Case-Hardened Steel. Case-hardened steel is pro- 

 duced by placing bone-dust, specially prepared for the pur- 

 pose, or burnt leather scraps in a cast iron box together 

 with the article to be hardened. The top of the box is covered 

 with plenty of the hardening material to keep out the air. 

 The whole mass is slowly and uniformly heated from two 

 to five hours until it finally attains red heat. A few iron 

 rods about -fg in. in diameter are packed in the box, one 

 end of the rods reaching to the middle and the other project- 

 ing through the hardening material on the top. When the 

 heat appears to be about right, these rods are pulled out one 



