Like the forms of bluing (rust and salt), strawing is a method for forming a protective oxide layer on steel. Strawing is done by using heat. When the steel reaches the right color, it's quenched in oil. This process is related to "tempering" steel.
There is a nice writeup about tempering steel here:
http://www.tpub.com/steelworker1/11.htm
Marc
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Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.