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.