# Heat Treating metal.



## Rocket Man (Feb 16, 2012)

I learned something new yesterday about heat treating. Normally you heat Air Harden Metal to a certain temperature for a certain length of time in the heat treat oven. Then the oven is turned off and the metal is allowed to cool naturally for several hours. Then the metal is reheated to about 600 deg F then allowed to cool naturally again.

This is the what I learned. Do the same thing as mentioned above only thing you do different is after the metal comes out of the 1800 deg F heat treat oven put it in liquid nitrogen for several minutes before putting it in the 600 deg F oven. Even after you draw the metal back in the 600 deg oven the metal to keep it from being brittle it is several times harder than it would have been without dipping the part in liquid nitrogen. Wear resistance is increased many times over by the liquid nitrogen cooling process. 

I would love to try this in a car engine it would be really cool is an engine would run over a 1,000,000. miles before it wears out.


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## bob ward (Feb 17, 2012)

This is an interesting subject, but the jury is still well and truly out.

http://www.practicalmachinist.com/vb/general/cryogenics-140883/


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## bezalel2000 (Feb 17, 2012)

Hi Rocket man

There maybe others here that know this subject in more detail than I do, but allow me to share what I know about this.

What you have described is basically the three main aspects of heat treating steel 




			
				Rocket Man  said:
			
		

> <snip> heat Air Harden Metal to a certain temperature for a certain length of time in the heat treat oven. Then the oven is turned off and the metal is allowed to cool naturally for several hours.<snip>



This is essentially the annealing process which softens the steel. By allowing the steel to cool slowly the Crystal structure has adequate time to fully adopt a Body Centered Cube (BCC) lattice. which means a carbon atom in the center of each Iron crystal. 



			
				Rocket Man  said:
			
		

> <snip> after the metal comes out of the 1800 deg F heat treat oven put it in liquid nitrogen for several minutes <snip>



This is the hardening process. When the steel is hot carbon is absorbed into the iron crystals with a carbon atom in each face of the crystal. That it is a face centered cube crystal structure (FCC). The FCC crystal structure is extremely hard and brittle. The use of nitrogen is just a rapid quench to a much lower temperature than ambient air. This ensures the crystal structure remains on the left side of the S curve. 
The S curve is a time verses temperature chart. Where there is an S shape drawn on the chart. If the cooling time is short (seconds), the majority of the crystals remain FCC. If it takes longer to cool to room temperature it will cross to the right of the S curve indicating most of the crystal have had time to revert to BCC (soft) crystal structure. 



			
				Rocket Man  said:
			
		

> <snip> Then the metal is reheated to about 600 deg F then allowed to cool naturally again.
> <snip>



This is essentially the tempering process which toughens up the steel after it has been hardened by quenching. (ie. cooled quickly) at a certain temperature (i.e. 600 F) a known percentage of the crystals will revert to BCC and the remainder will stay FCC this mix of crystal types makes the steel a little softer but much less brittle. 

So we can bang it with our favourate 24 LB hammer without it shattering into a gazillion pieces.

Bez


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