Mete or Kevin - Here is Another One of Importance to Us -

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From Bill Bryson's book on heat treating tool steels I have gotten that it is best to not preheat steel greater than 15 minutes prior to ramping to austenitizing. His book is not specific in explaining why and I admit I do not remember if he speaks specifically of steel already equalized but presume he speaks of a slow enough ramp to the preheat to cause that. IF HIS ADVISE BE CORRECT, can you please explain in terms to this old mountain boy and to the bulk of the rest of us here can understand why it be? I will say that through some experience I notice a beneficial difference in warping by preheating. However, I can not definately point that difference to the preheat but do believe in it because of other things described in another paper. Forgetting that statement, my question is about time at preheat. His writing indicates nonbeneficial results from preheating too long.

As a note of possible interest I have pretty much settled, for my liking, a equalized preheat of about 7 minutes.

Thanks again.

RL
 
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I am not familiar with Bryson's book. There are things that you see about heat treating tool steels that pertain to complex shapes and large sections.In large sections it takes time for the center to heat to the furnace temperature while the outside may already be at temperature .The outside then is exposed to austenitizing and therefore decarburizing temperatures longer. So the prime reason is to reduce decarburization and a second reason is that it's better as far as warping to have the change to austenite more uniform throughout the piece....This is not nearly as critical when dealing with blades which are simple and small shapes.15 minutes seems reasonable for blades though I don't see harm in going a bit longer.
 
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Thank you Mete for that and you did explain it in terms understanable by even me. Now, the other paper I previously refered to may relate to what you just explained. My basic understanding of it is that steel will tend to some extent expand as it proceeds through the preheating temperature(s) and then tend to contract during austenitizing. My feelings from that reading left me to believe it best to allow the steel to completely equalize at the expansion heat - the preheat - (so as to allow all thickness to expand equally) and then ramp quickly to austenitizing, where the opposite may occur. Is my assumption in error or with foundation? With that question, of course we speak in terms of blade steel cross sections only, say up to 1/4 inch or occasionaly a bit greater.

RL
 
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It would be best to listen to anything mete has to say here. I have very little experience with preheating since, as mete mentioned, knives are very simple cross sections, and there is no preheating with a single tube of high temp salts; it is straight to austenite, and pretty darned quickly ;)
 
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I sense the same thing I think I did the other day. Little zingers flying over head.

I will take that as a yes on what Mete advised and now proceed to get out of the way.

Thank you.

RL
 
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1/4" ? that's just baby stuff, when you get to 12" then you're really talking !Don't worry about it . You have been using protective foil so that pretty much eliminates decarburization.A proper soak at austenitizing and you're done .
 
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Okay Mete, I'll step in it now. !/4 inch is baby stuff? Well I'll tell you what, ha ha , yeah I guess you're right.

Thanks.

RL ;)
 
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Most steel manuals center on a specification of 1" in thickness. That is not a knife. The reason I soak before attaining critical temperature is stress relief. Though a knife is not a complex shape, the heat treatment process creates tremendous stress in sharp or angular areas of machining. the edges of drilled holes, the corners of complex filework, a radical radius of a sculpted choil: all these areas are potenial sources of internal residual stress. These may not show up as a problem until a knife in use cracks! A stress relieving presoak allows the steel to "relax" after machining, cold working, welding, or normalizing.
10-15 minutes is plenty of time to soak a thin cross-sectional area of bladework. Always refer to the manufacturers guidlines on these exotic and expensive steels.
 
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Jay, I take your points and believe I understand them. I may take small exception at the way you use the term 'stress relieve'. Of course I may have misread you on that but I understand stress relieving as a separate proceedure not normally performed just prior to ramping to austenitizing, Please correct me if I misunderstood your writing. But that is different fom this thread, which deals with preheat. The reason I posted this thread is because, while not knowing better but believing in it, I wanted a true feeling of the subject from a couple of people that I do believe in. This particular subject, of course. metallurgy. The reason I am sold on preheating as per vendor specifications (so far as equalized temp. goes) is from a document a vendor put out that describes why. Within that sheet I believe I learned - well, what I have been preaching - contraction and expansion separately occur at basically two different temperature ranges and that it is best to allow all contraction throughout the cross to occur before ramping to a temperature which causes/permits expansion. In doing so, theoretical in my mind but based on vendor advise, both contraction and expansion is less likely to occur simultainiously throughout the cross section.

But even that was not part of the question. The question was of time at preheat, pretty much only. I think Mete answered that and my take from his answer is 'it is not a critical choice for the steel thicknesses we normally work'. That was my take on his answer. That is good enough for me and not any indication that I should change the way I have been doing it.

RL
 
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My, mistake, I didn't know you were talking about partial or complete austentizing.
Jay
 
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No zingers that I am aware of, just simple, thin, cross sections. One does have to consider that there will be slight expansion up to Ac1 and then there will be a more drastic contraction. Complex and heavily varied thickness and cross sections will present greater problems with this. Air is an insulator, it requires a little time for things to equalize (particularly if one part may be closer to a heating element than another). Salts are conductors, heat moves very quickly through them and fairly evenly in comparison. I am still fascinated whenever I pull a blade out, that just went in, in order to re-adjust something. The spine rarely has the great heat differential from the tip or edge that I would see if I were doing in an oven or a forge. The whole thing is very close to the same shade of red. And within a couple seconds of immersion at that, so I would have to have two tubes of high temp, going at different temps, in order to do a preheat. So this is outside of most of my experience.
 
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Having you and Mete is a pure gift to any of us serious in wanting to learn proper heat treating of steels. Thank you for your concisenes .

I went back after posting and re-read more attentively. Am I correct, in your opinion, it best to preheat even our relatively thin cross sections just prior to quickly ramping to austenitizing? I ask that mostly for the benefit of others instead of only myself. I am sold on preheating. It is only the question of time at preheat I was asking about and to that I believe I have been given an answer.

Yes, I have also seen things I did not expect coming from the heat. My latest unexpectation was a trip from the tempering oven back into the austenizing furnace.

RL
 
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I would say, for knives , not essential but good practice, better practice the more complex the steel.
 
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That's gospel speak to me Mete. I'll take it. Thanks. And, also, I believe I understand you properly that equalized preheating a few minutes this way or that is not necessarily (wrong word for me to spell correctly) a big concern(?)

Thanks both Mete and Kevin and anything else pertaining is most appreciaited.
(as for my zinger comment earlier, I really did think you two were throwing fun barbs back and forth. Sorry if mistook.)


RL
 
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