flame coloring stainless steel : surface oxide layer question

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DB731

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If I understood correctly, higher temps and/or longer heating times lead to deeper colors.

Is there a limit to the thickness of the surface oxide layer that forms, other than the (obvious) theoretical limit that is the thickness of the heated piece? As of now, I can easily remove the colored layer (shades of dark yellow/purple/blue) with sandpaper, so I am wondering if I can make it any thicker.



Thanks
 
No , there is a definite thickness as faras heat coloring..It is after all a surface layer not something like carburizing. Corrosion resistance in many cases involve an oxide of the metal .Remove the oxide and you get corrosion. A proper
Galvanic Series Chart gives metals in the order of corrosion resistance including stainless steel with and without an oxide . The two are different .
The one way I know of off hand is to build up the oxide layer electrically . This is done with aluminum during anodizing !!
 
The heat oxidation layer is a few millionths of an inch thick. It won't be made any thicker by additional heat cycles. Anodizing will build up a layer of oxides.
 
It wouldn't give the desired result if you could make the layer thicker. It is the thickness of this layer that creates the color, and this is the mechanism by which coloration changes with temperature. As the temperature goes up, the oxide layer (Fe3O4) gets thicker. As the layer thickens, the color of light reflected back at you changes through the spectrum, until you get to layers thick enough to just give you grey. After you get to that point, it doesn't matter how hot you get it, the oxides stay grey.
 
Follow-up: if you get into the deep blues and were to heat the item again, can you possibly lose that shade? Or, once you get it, are you set in?

Is this process essentially case-hardening?

Thanks again
 
mete said:
You might change color but you won't get a thicker oxide.
Click to expand...

Sorry, I didn't express myself well. If I get a nice deep blue shade (that I prefer to yellow, let's say), and if I see a spot that I missed and would like to redo (make darker), do I run the risk of ruining my work by heating the item again? It takes me more time to get to blue than to get to yellow. In other words, portions that are blue, will they remain blue no matter what I do with heating?

http://www.bssa.org.uk/topics.php?article=140

Also, I read somewhere online "just wait until you get to the shade you want and then stop". That has not been my experience. I see light yellow going to orange, and that's it. Then, as soon as the cooling starts, I know what I am ending up with.
 
Sort of. Depends on what you are using to heat the metal. It won't change from the blue color unless you get the metal hotter than the associated temperature (mid 500s Fahrenheit for the blues). If it gets hotter it will start to turn grey. If you are using a torch, it can be tough to nail your temperature. Also keep in mind that these temperatures will reduce the hardness of a knife blade to a point significantly softer than you want for edge retention.

Here is a link that is a guide to the colors and associated tempratures. Use a heat treating database to tell you what that temp will do to any particular steel's hardness.

If you are using a torch for heat bluing, checkout the youtube channel "Clickspring" for a good technique to evenly heat a piece and get color uniformity.

If you want to retain hardness while getting a nice colored surface finish, look into caustic salt bluing (basically making a solution of caustic salts and boiling your piece in it). This produces an almost black color on your steel. Brownells sells Oxynate No.84 which works well on stainless steels.

Finally, and I am unsure if it works well (or at all) on stainless, but fume bluing can produce some interesting results, and can get you some iridescent blues that you cant achieve through caustic salt bluing processes (as far as I am aware).

All bluing processes other than temper/heat bluing will preserve the hardness of your metal.

Joe
 
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