That's what the photo was part of in the last post. I was heat treating the ejector rail for an AK project.
The steel is 4130. I heat it up to cherry red, which is about 1600 degrees until it looses its magnetism, then quickly quench in oil. This makes it very hard, to hard in fact. So I have to anneal it. I need to heat it again to around 700 degrees and let cool slowly. I do this, as in the photo, by pouring brake fluid in a small lid and laying the parts to be annealed in it. Then burning it off. The flames are about the right temp. Then let air cool.