Why is the surface of stainless steel screw black?

Often see some stainless steel screws, cross groove is black, or tooth thread is black, can’t wash them clean. So what is the cause about the surface of stainless steel screw in black and can’t wash them clean? Simple will analyze what causes the stainless steel screws to turn black and not clean? 
Blackening of Stainless Steel Screws is usually caused by the heat generated during cold heading or tooth rubbing. It usually occurs at the apex of the cross and snail teeth. So through the supply of lubricating oil to achieve timely heat dispersion. The speed of the machine can also be reduced appropriately. Add drum polish when cleaning, and then use organic solution in cleaning (pay attention to fire prevention). 
In the production of stainless steel screws, this encountered a number of problems. Stainless steel screws’ surface is black, inner cross groove is black, tooth thread is black these are the problems encountered in the production of stainless steel. Stainless steel shows in black in addition to technical reasons, there may be a problem with the wire material of the stainless steel screw itself, raw materials have impurities, material is not very good and so on the reasons will make to the surface of stainless steel screw blackening.