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I'm working with 3/32" 15n20, and I tempering at 425F which is 58-59 HRC as I understand. Usually the oxide color is a light straw at this temp., but my latest blade is coming out brown/purple. Both my oven and thermometer indicate I'm at 425F. I'm using the same quenchant, oven, forge, steel, everything is the same.

 

Can anybody shed some light on what's happening here?

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Must have forgotten the traditional sacrifice of chocolate to the domestic goddess for the use of her oven!

 

All kidding aside, the steel has to be absolutely clean, with no residue of any kind, especially oils. A freshly sanded surface is best. You probably had some oil on the blade. Did you wipe it off with a rag before putting the blade into the oven? Perhaps baked chicken was for dinner last night and oil from the interior of the oven is to blame. Not really likely but, I'm trying to make a point. Consider any possible source of contamination.

 

Check your oven and thermometer. Turn the oven on and watch it over the same amount of time that you normally bake a blade. When the element in the oven turns on, does the temperature spike before the element shuts off? Most ovens operate in this way and it is important to know how hot the temperature goes before the element switches off. One safeguard you can use is to place a pan full of sand in the oven ahead of time. Let the pan soak for awhile after the oven reaches temperature to make sure that all of the sand is heated. Blades can be buried in the hot sand and the added thermal mass helps to even out the temperature that the blade is exposed to when the element cycles.

 

~Bruce~

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Bruce,

 

I always clean my blades with hot water and soap after HT. It's possible there was some contamination from the oven side: I cleaned the glass window several hours before using the oven. This is currently my only suspicion.

 

Once the oven reaches temperature it doesn't cycle more than +/- 5F so I doubt that's the issue. I always put a soft firebrick in the bottom to increase thermal mass, and it's a smallish oven at 15"x15"x21". However I like your sand idea, I'll remember that for tempering in wild ovens!

 

My main concern is the blade's hardness...however I am curious about magnetite oxides now! I don't know what cause/effect relationship oxide colors have with temperature and how they are affected by environmental factors, e.g. glass cleaner.

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I have to assume that you've used the 15n20 before and got straw at the 425 temp. If not I wonder if some alloying element in the steel caused this. I don't know, just throwing suggestions out, I've always used plain carbon steel and don't know how other alloying elements may affect oxide colors. Time also plays a role in oxide colors, if you left the steel in the oven longer than normal that could cause colors that would normally be seen at higher temps. Or something on the blade or maybe in the oven. Or something caused the oven to spike in an unusual manner (maybe someone opened the door).

 

I picked up an old cast iron dutch oven some time ago at a pawn shop for a couple of dollars. I've filled that with sand (probably 15 pounds or so) and use it for my thermal mass. It is big enough to bury most knives in and adds significantly to the thermal mass of the oven.

 

ron

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son_of_bluegrass,

 

Yes, this is the same batch of 15n20 I've been using for several years. The steel was in the oven for my standard 1-hour temper before hitting the freezer for the night, which is followed by a second 1-hour temper. I cleaned the blade again after the first temper, but I got the darker oxides during the second cycle as well.

 

I'm thinking it must be external contamination, because there really is no other explanation given my level of knowledge. I find it too unlikely that both my digitally controlled oven and analog thermometer are off by 100F+, but still agree with each other.

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son_of_bluegrass,

 

Yes, this is the same batch of 15n20 I've been using for several years. The steel was in the oven for my standard 1-hour temper before hitting the freezer for the night, which is followed by a second 1-hour temper. I cleaned the blade again after the first temper, but I got the darker oxides during the second cycle as well.

 

I'm thinking it must be external contamination, because there really is no other explanation given my level of knowledge. I find it too unlikely that both my digitally controlled oven and analog thermometer are off by 100F+, but still agree with each other.

Could you have set the temp wrong and not noticed? For me oil tends to turn a turd brown in the oven or just burn off and turn black or, if it is my new HT oil it is red/purple (IDK why, but it seems the HT oil turned red). In my experiance the second temper at the same temp takes the hardness lower then the previous one (ie. the more tempers at X temp the softer it becomes).

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  • 5 months later...

Could you have set the temp wrong and not noticed? For me oil tends to turn a turd brown in the oven or just burn off and turn black or, if it is my new HT oil it is red/purple (IDK why, but it seems the HT oil turned red). In my experiance the second temper at the same temp takes the hardness lower then the previous one (ie. the more tempers at X temp the softer it becomes).

 

I suspect that there is some residual moisture either in the atmosphere or on the steel.

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Brian,

I don't know if anyone else has ever experienced this but I had a knife(my first pattern welded one, 45 layers twisted) and I heated it in the oven to about 500 degrees and each layer was a different color. Yes, I know 500 is to soft for a knife, but it looked cool. It was 15N20 bandsaw blades and banding straps. So it is possible that diferent alloys turn diferent tempering colors also.

Dion Grethen

5049645953_2617d5c912.jpg

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