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  Black Or Bright Bar  
  Black refers to the way it is finished after rolling down to size from a 5" billet.  The black bar has been cooled in air from rolling temperature - in modern mills in a cooling rack and in the old days on the mill floor.  This allows the outside to oxidise - which gives the blue-black oxide form (this turns to the red oxide form if you let it).  The description for this simple cooling is 'normalised'.  If for metallurgical reasons you want the material dead soft  you could anneal it by decreasing the cooling rate further eg by cooling in a controlled furnace.  
  The softness of the bar depends on the heat treatment effected by the
cooling process and the chemical constituents of the steel.  So called
mild steel has very little carbon and significantly more manganese.
It is relatively unaffected by heat teatment so the normalised
material is not far different from the annealed .  It became the
material of choice for boilers and structural steelwork only after the
French discovered how to produce low carbon ferro-manganese in about
1880. Prior to this, bulk manufactured steel had a middling  high
carbon content, which restricted its structural uses, although it made
good railway tracks.
  Bright bar has had the black oxide removed, usually by pickling in
dilute hydrochloric acid; although they used to do peeled bar at one
time.  The bar can then be cold worked to a precise size by drawing it
through a die (wire drawing), reeling or cold rolling. This cold works
the material and induces residual stresses, particularly near the
surface of the material.  If you then machine the material
assymetrically, the stresses on the side which still has the material
will pull relative to the neutral axis and bend the workpiece.

So :
mild steel is low carbon
bright bar has been cold worked and may be susceptible to residual stresses

black bar is usually slightly softer and less ssusceptible to residual stresses black bar should be cheaper.