tempering color

franklin

franklin

Well-Known Member
Is it me guys if you take the same steel 1095 and heat treat say one in parks and it comes out 67 rc
Now you do another in same or motor oil or what ever and only get 63 rc.
When you temper say at 375 or 400 it seems the lower one turns straw yellow and even a little shades of blue purple
will start to show in one temper cycle and the higher rocked knife barely gets to very light shade straw yellow.
Is this me or does that always happen and can you walk in a temper by its color? If your blade goes in clean the same way every time?
Or does this mean nothing at all or can this happen to same rc hardness of a knife? ( if put in same way every time)
Seems like I got very different colors back before I got a heat treat oil made for 1095 (using DT-48) and am still using the same
home oven with 2 thermostats in it that read same as the oven itself.
 
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Sorry warren I really don't want to temper just by color, what I mean is to walk your temper in if you don't have a rc machine.
If you tempered at a given temp then by color go up by 25 dagrees. But if you had a idea or a head start to now about were your first temper
came out it would help you decide if you want to temper higher, I would never go by color alone.
 
Warren Krywko said:
Tempering by color is unreliable.
Click to expand...

Exactly. Take a fully hardened blade of 1095 at a given RC value, let's say you nailed it and it is at 66. Put that in an oven at 400F for an hour, and it will have a light straw color. Put it back in for another hour. It will be even deeper straw. Put it back in for another, and it will start to turn blue and purple. The temper of 400 might bring the RC down to 61 or so from 66, but you can temper over and over at 400F (within limits I'm not talking about hours upon hours upon hours), and the RC will be the same, but the colors will change. So judging by color is out the window.

One thing I make sure to do during tempering is leave my wife's large stone bakeware in the oven as it warms up and during the temper cycles. It takes longer for the atmosphere in the oven to reach 375F or what have you with the stone in there, acting as a heat sink, but helps to maintain an accurate temperature, as the element will not kick on and off near as often. I highly recommend using some sort of cooking stone, or even a brick or rock, to help negate the possible wild temperature swings inside a kitchen oven.
 
While I've seen enough variation to agree with Warren, I'd still be curious as to "why". Franklin asks about hardness. What about alloy? Perhaps oxygen exclusion by contaminants such as oil or buff compound? Good question Franklin. I'll be watching for answers too.

Rob!
 
Yup I now what you guys are saying but I said a blade that's the same each time that was clean and only one temper the first time.
This would just be a head start for a guy that's got now rc machine. yes you can have a recipe but things very each time and if the color coralated to the rc at quench it could help. just wandering if this would be a way to help a guy figure out were his blade came out of quench and were to go after his first temper. I have came pretty close to figuring out were my set up is and what temp it should be at but took a lot of trial and error to get were I wanted, I just get the same color each time and was thinking maybe it would help some one out.
 
I think the problem is that cleaning the blade is only one variable. What else has been in the oven, and will it react with the steel? What alloying elements might skew results, if at all? Is it the same soap, or solvent? Dis you use a rag to wipe, and how do we know what trace elements might be in the rag?
 
If I follow you right, Franklin, you are asking if a given color can tell the knifemaker what his RC is after temper. One cycle, same steel, etc. In theory, yes. In application...no way.
 
I believe Franklin noticed a feature that could deserve further investigation.
i consider a possibility that the tempering carbides could play a role in the oxides color during tempering, and you can have different colors (same steel, same temperature, only one tempering cicle of the same time) depending on the martensite quality and carbides array...differences between tempering carbides and pearlite leftover from a not perfect quench.
The variables though are many and an hardness tester may prove to be the right tool for nailing the tempering. ;)
 
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