408 APPLIED SCIENCE 



or blow-holes below the surface of the metal which cannot 

 be seen. When they are on the surface they can be elim- 

 inated in machining. 



466. Mild Steel. Mild steel, a grade of steel that does 

 not harden when heated and chilled with cold water, is 

 made from Siemens, open-hearth, or Bessemer ingots. The 

 ingots are heated and hammered into slabs. These slabs are 

 reheated and rolled, into plates or bars. They resemble 

 wrought iron because of their low percentage of carbon (.15 

 to .5%) and can be easily welded. In welding, care must 

 be taken to see that the pieces to be united contain the same 

 proportion of carbon. Otherwise, the welding temperatures 

 will be different. 



Mild steel has now taken the place of wrought iron for 

 many purposes, especially for boiler plates and stays, bolts 

 and shafting, engine parts, etc. The lower the strength of 

 the mild steel the higher is its elasticity. Boiler plates must 

 be ductile. 



Engine parts such as pistons, connecting rods, shafts, and 

 valve rods, are made of mild steel forged from Siemens or 

 Bessemer ingots. 



467. Influence of Impurities on Steel. Impurities, simi- 

 lar to those found in iron, are present in steel. Sulphur in 

 the steel tends to make it red-short and to interfere with its 

 welding and forging properties. Steel should not contain 

 more than .1% of sulphur, and when possible the amount 

 should be reduced to .03 or .04%. Manganese tends to 

 counteract the effects of the sulphur. 



Phosphorus increases the tensile strength and raises the 

 elastic limit of low carbon or structural steel, but detracts 



