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Tri-iron tetraoxide finish on steel

We have a continuous belt furnace for our heat treatment process. The temper furnace atmosphere is exothermic gas. The purpose of this is to create a Fe3O4 finish on steel parts. Low to medium carbon steel (say 10b21~4140).

My problem is I get a "reddish" finish instead of black. I have checked extensively for air leak, water leak (any O2 contamination). I'm using a rust preventative oil to quench after tempering (not a blackening oil).

I can give more info, but has anyone had experience with this process and been successful?

Thanks

Ken Hopkins
automotive fasteners - Rockford, Illinois
2003

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Hi,
I had faced similar problem before in blackening the shadow masks here, we overcome the problem by putting sus curtain at the input and output area to prevent air exposure.

Pasupathy
- Malaysia
2005