Mete - Another Question Please -

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(assuming primary temper is minimum 400 F for recommended time in temper)

Suppose upon first temper the steel is harder than I wish. Is it just as well to up the temperature and re-temper until desired hardness is acheived or is it better to repeat the heat treat?

It is something I have wondered about for some time now and I encounter this situation occassionally - where I have to temper the second time at a higher temperature than the primary temper.

I suppose what I am asking is: it it better for the life of the tool if the primary temper correctly hits the desired hardness or does it work out just as well if we have to increase the temper until the aim hardness is acheived.

Thanks.

RL
 

Stacy E. Apelt - Bladesmith

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Temper is a result of the maximum temperature at draw.Multiple tempers are common and preferred.Temper to the calculated temp,test edge,raise temperature 25 degrees and re temper,re-test,etc.You actually improve the temper this way.If you overshoot - it's back to the quench,and start over again
 
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Rodger, what Bladsmth said except on steel like 5160 (the king), it's very easy to over shoot the desired hardness so I start at 325 and if needed go up in ten degree increments.

25 degrees can ruin your day when your close. I generally use at least two temper cycles anyway so going up a little at a time is just natural. It also lets you polish a section of blade so you can check the color.....charts are nice, thermo couples are great and slide rules are handy but the color don't lie! :eek:
 

Stacy E. Apelt - Bladesmith

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The great thing about 5160 is it is so forgiving.(Like most knife makers wives).A little too much doesn't ruin it.That said,I agree with Peter,for the perfect edge small steps are preferred.The original question was more hypothetical than practical.All this aside,if you don't know the EXACT temperature of your oven,it doesn't mater anyway.Color is a good way to determine temper.It is the only way to determine differential temper.
 
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OK, I have a question. If you all think color is the surest way to tell tempering temperture. I can temper a 52100 blade at 400 for 2 hours, pull the blade out and its just starting to turn a light straw color. Place the blade back in for 2 more hours and it comes out a darker brown, temper the 3rd time for 2 more hours and the blade is a dark brown. This is done in a toaster oven that has 2 oven thermometers in it, the blade sits on a sheet of aluminium to protect it from direct heat off the bottom elements and has a sheet of stainless foil over the top to protect it from direct heat from the top elements. Why is the blade color getting darker? The thermometers show 400 each and every time I look though the glass and I will check the temp about every 15 minutes.

Bill
 
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Bill,

Tempering is both a function of temperature and of time. While temperature has a more course effect time has a linear effect. So the longer the more.

RL
 
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Don't reharden just keep on tempering.I might add to this when I'm awake !!
 
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Wonderful. Thanks for that and I look forward to anything else you may think necessary to add.

That makes me feel better. There are occassions when the first temper does not hit the aim hardness desired after first temper.

RL
 
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Bill, without getting too far OT, let me correct the impression that I may have made. I don't think color is the surest way to determine temper. Like everyone else I hope, the surest way is to follow the specs regarding time and temperature. Color is the way to both double check yourself and to use as the final resort when the more scientific ways are not working.

It is ...for lack of a better phrase, an old friend that is always there when you need it and will always give you sound advice.
 

Stacy E. Apelt - Bladesmith

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Bill,Like Peter I was not saying that it is the best way.But it is a good indicator of how far the temper has progressed,(ie. your 52100 blade.)When I differential temper by torch,the color is the only thing I have to go on.
Stacy
 

AwP

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You mentioned the steel being harder then you wanted after the first temper. You really shouldn't even check until after the second. Depending on steel, the first temper can convert retained austinite into untempered martinsite. The second temper will temper down the new martinsite that didn't get tempered the first time.
 
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Color is not the way to do it though as Stacy says sometimes it's the only way. The color is the color of an oxide on the surface and is dependent on things like surface preperation and the alloy. Since it takes time to temper ,quickly bringing it to color doesn't permit the proper diffusion to take place. Think of the martensite crystal as an elongated cube with carbon stuffed into the elongated edge of the cube. as you temper at higher and higher temperatures more of that carbon leaves the crystal and forms carbide. It does take time , one hour minimum is a good rule , two hours better, longer than three [oops I forgot to take out the blade last night ] has little benefit. But temperature is the prime thing and if you want to go back and temper at a higher temperature there's no problem. You'll just precipitate more carbon from the crystal.
 
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Thank you Mete. That sets me at ease. I was wondering about optimum heat treat as related to hitting the primary temper most correctly on the first attempt. My question has been answered.

RL
 

Don Robinson

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B . Buxton said:
OK, I have a question. If you all think color is the surest way to tell tempering temperture. I can temper a 52100 blade at 400 for 2 hours, pull the blade out and its just starting to turn a light straw color. Place the blade back in for 2 more hours and it comes out a darker brown, temper the 3rd time for 2 more hours and the blade is a dark brown. This is done in a toaster oven that has 2 oven thermometers in it, the blade sits on a sheet of aluminium to protect it from direct heat off the bottom elements and has a sheet of stainless foil over the top to protect it from direct heat from the top elements. Why is the blade color getting darker? The thermometers show 400 each and every time I look though the glass and I will check the temp about every 15 minutes.

Bill

Bill, as mete said, the oxide color builds up thicker with each cycle. To keep judging by color, simply polish off the oxide coating until you get a bright steel color, then repeat the temper again if necessary, but each time, you must remove the oxide. :)
 
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That was my point,I guess I shouldn't have put it in a question form. To judge tempering by color is not a good idea. I could care less about steel color when tempering unless I start seeing a purple or blue and then I would start checking my oven for heat spiking. The best route, in my opinion, is to test the edge with the rod test. If it doesn't pass it to my liking then I test until I get the results I want. It gets me how many people come in here and ask what temperture they need to temper a certion steel at, when the temper temps can very from so many different factors, one being the quenchent used. Each shop and technique is different and to just assume, that my temper tempertures will work for you are absurd. Test, test, test is the only way to be sure of what your doing. But this too is just my opinion. ;)

Bill
 
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mete said:
Color is not the way to do it though as Stacy says sometimes it's the only way. The color is the color of an oxide on the surface and is dependent on things like surface preperation and the alloy. Since it takes time to temper ,quickly bringing it to color doesn't permit the proper diffusion to take place. Think of the martensite crystal as an elongated cube with carbon stuffed into the elongated edge of the cube. as you temper at higher and higher temperatures more of that carbon leaves the crystal and forms carbide. It does take time , one hour minimum is a good rule , two hours better, longer than three [oops I forgot to take out the blade last night ] has little benefit. But temperature is the prime thing and if you want to go back and temper at a higher temperature there's no problem. You'll just precipitate more carbon from the crystal.

if I can add
"Color is not the way to do it though as Stacy says sometimes it's the only way."
I would go as far as to say telling where the heat is during draw back in
differential temper or differential heardening, not to be used as a temp judgment but the line of transisation. most of us know this anyway I'm mentioning it for the ones that don't know.
roger if you think about the muiltiy tempers you'd know you were right in your thinking anyway.
 
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mete said:
Color is not the way to do it though as Stacy says sometimes it's the only way. The color is the color of an oxide on the surface and is dependent on things like surface preperation and the alloy. Since it takes time to temper ,quickly bringing it to color doesn't permit the proper diffusion to take place. Think of the martensite crystal as an elongated cube with carbon stuffed into the elongated edge of the cube. as you temper at higher and higher temperatures more of that carbon leaves the crystal and forms carbide. It does take time , one hour minimum is a good rule , two hours better, longer than three [oops I forgot to take out the blade last night ] has little benefit. But temperature is the prime thing and if you want to go back and temper at a higher temperature there's no problem. You'll just precipitate more carbon from the crystal.

if I can add
"Color is not the way to do it though as Stacy says sometimes it's the only way."
I would go as far as to say telling where the heat is during draw back in
differential temper or differential hardening, but not to be used as a temp judgment but where the line of transition is. most of us know this anyway I'm mentioning it for the ones that don't know.
Roger if you think back about the mutable tempers you'd know you were right in your thinking anyway.
 
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