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Metal Hardening
Name: Peter
Status: other
Grade: other
Location: WI
Question:
Why does steel seem to get tougher or harder to drill as it gets old?
Replies:
Hi Peter,
Steel, in fact, does not get harder with age. It is true that some
alloys (notably some aluminum alloys) do undergo an "age hardening"
process (more correctly called Precipitation Hardening), but the time
required for this is only on the order of hours or days, not weeks or
longer.
Heat treating of high carbon steel to harden it is an instantaneous
process. The steel is heated red hot, causing the formation of
crystals of very hard type of Iron Carbide called "Martinsite". If
cooled slowly, the Martensite reverts to iron and carbon again and the
steel remains soft. But if cooled rapidly by plunging the red hot
steel in water or oil, there is insufficient time for the Martinsite
to break down to iron and carbon, and it remains as crystals of very
hard Martinsite, imparting hardness to the steel. Once this process is
complete (in seconds), any further time makes no difference at all.
Note that in order for this hardening process to work, the steel must
have at least 0.8% carbon in it, and preferably 1% to 1.2% carbon
content. This is called "high carbon steel". Steel with less than this
amount of carbon cannot be hardened, and remains relatively soft, even
if hardening is attempted. This type of steel is commonly called "mild
steel", and is the most common type for most applications such as
sheet steel, and other steel objects that do not specifically require
extreme hardness.
Note also that to improve the "machinability" of some steel (for
example, steel rods that is machined in a lathe or a milling machine),
a special alloy of mild (low carbon) steel is commonly used that
includes a small percentage of lead in its makeup. This makes
machining much easier, and is generally referred to as "free machining
steel".
But to repeat, age has no effect whatsoever on the hardness of steel
alloys.
Regards,
Bob Wilson
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Update: June 2012
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