Tempering Colors of Steel

Good photo of samples....but generally of little use in tempering a modern knife blade post-HT.

The blade would have to be cleaned spotlessly, and completely free of any surface oil or other contaminant. Even the soap used can make colors show up. After an hour or more in the oven, all sorts of colors can/will show up on the blade.

When temper colors are used ,as in the chart, it is in flame tempering a blade by heating the spine and "drawing" the temper down the blade bevel to the edge. You watch the colors run down the blade, and when the straw/bronze color hits the edge, you dunk it at bonce in a pan of water. The whole temper process lasts less than one minute. This method will work, but is far less quality than a full oven temper for two cycles lasting an hour or two each. There are reasons to flame temper a blade, but the best method to do it is to first temper it properly at around 350F, then clean and sand the blade to a bright steel surface and draw the colors as desired. This is done in most cases to get a soft spine and a hard edge. The spine will end up blue to turquoise, and the edge will be straw/bronze.
 
this is kind of concerning me. I always clean my blades off really well before putting into the oven at 400* I place them on tin foil, and they come out a mix of the straw and purple. I let the oven pre-heat for about 20- 30 minutes and I place the blades on the second from top rack, one level above middle. I guess I should not have assumed the temperatures were accurate without testing it first.
 
mccandmatt said:
this is kind of concerning me. I always clean my blades off really well before putting into the oven at 400* I place them on tin foil, and they come out a mix of the straw and purple. I let the oven pre-heat for about 20- 30 minutes and I place the blades on the second from top rack, one level above middle. I guess I should not have assumed the temperatures were accurate without testing it first.
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From what I have read, you do not need to worry about those colors. The colors come from different oxides of iron. Heat treating a blade in the old days (@ 150 years ago) would involve normalizing, austenizing, quenching the blade, a quick clean to expose a clean surface across the blade, then the tempering would involve laying the blade along a hot piece of steel. The process would be done in open air. The oxide colors would be regulated by sight and moving the blade on and off of the hot surface. As Bladsmith said, a blue back and a straw edge would have been a typical goal (often referred to as differential tempering). It only takes a couple of minutes. The blade is immediately cooled to keep the tempering process from going too far (auto tempering??). The oxidation process seems to have a kinetic or contaminate component as many report the blue color in ovens regulated at a much lower temperature than the color indicates. It is doubtful that your blade would vary across its length by 150 F during your tempering process. In more modern times a torch is used to temper the blade. A gentleman by the name of Albert Cravens, one of the last forgers to contract to Sheffields, tempered his blades to a purple color using the hot steel differential technique in a video of the process.
 
That color for 400° looks exactly like what my last blade came out of the oven looking like.

I quenched in water and tempered at 400°. If that actually means anything I don't know.
 
mccandmatt said:
this is kind of concerning me. I always clean my blades off really well before putting into the oven at 400* I place them on tin foil, and they come out a mix of the straw and purple. I let the oven pre-heat for about 20- 30 minutes and I place the blades on the second from top rack, one level above middle. I guess I should not have assumed the temperatures were accurate without testing it first.
Click to expand...

If using your household oven for tempering, put some large heat safe mass in there with it, it will even out the temperature swings greatly. Old cast iron pans, large granite blocks, ceramic tiles, piece or RR track, etc...

Ignore the colors (colours), as has been said. Also, use a seperate thermometer to verify temps. I use an inexpensive DMM with type K thermocouple.


-X
 
For tempering, a type J thermocouple is fine.

I don't use it anymore, and don't even know where it is out in the shop, but years back I took a screw stud type J thermocouple and screwed it into a 3/4" thick aluminum plate. It was on one side in the center. I placed the plate in the oven ( TC down) with the knife blades on top of it. The TC was hooked to a cheap PID. The reading of the aluminum's thermal mass was probably nearly exactly the same as the steel blades sitting on it.
 
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I have found that if I grind the blades at all or even somewhat clean them up that the main color of the blade after tempering is very close to the pics. Although there will be some other colors mixed in at various spots from contaminants and what not. Still pretty close. When drawing a spine back with a torch the colors are helpful in finding that spring temper.
 
bladsmth said:
Good photo of samples....but generally of little use in tempering a modern knife blade post-HT.
Click to expand...

I posted the pic not as a guide for oven tempering blades, but rather as a reference for approximating the use of heat in our craft. I've used the information for tempering spines (as you mention and as I show in the below pic) and heat colouring fittings & liners. Simply being aware of the order in which the colours occur may be of use to some, ie: you can heat from purple to blue but not from blue to purple. I leave it to each individual to use (or not) as they see fit.



Thanks to all for your comments and Happy Holidays to everyone.
 
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