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Using a magnet to heat treat


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After reading many posts in this website on heating steel to nonmagnetic for normalizing or quenching, please be aware that steel (actually iron) loses magnetism around 1400 degrees (Curie point). The lower critical temperture for steel is 1333 degrees, and the upper critical temperature varies with carbon content. For pure iron, and low carbon steels, the upper critical is well above 1650 degrees. The upper critical temperature decreases as carbon increases, and for 0.83% carbon, the there is only a single critical temperature (1333, the eutectoid point).

Between the lower and upper critical temperature, steel is transforming from ferrite & typically pearlite into austenite. This transformation is dependent ON TEMPERATURE, not time. That is, just above the lower critical (i.e., 1400 degrees) only a small amount of ferrite is transformed, and holding at this temperature does not convert any more ferrite to austenite. (There is a method called the 'lever-rule' that can be used to determine how much transformation has taken place at each temperature.)

The point of this is -- you can only harden or normalize austenite. If only 10% of the steel has transformed to austenite, then at most only 10% of the part could (assuming quench conditions are right) harden. Since the Curie point is only about 100 degrees above the lower critical for most steels, heating only until the steel is nonmagnetic, only a portion (and in some cases, only a very small portion) of the ferrite has transformed to austenite. And the part will not be able to harden properly. To properly harden you really need to be about 200 degrees to 300 degrees hotter than the Curie point. Manufacturers sometimes recommend slightly lower temperatures for the higher carbon steels, but heating to 1600 degrees will not cause a problem. Use a Tempil stick or something similar to test for the right temperature.

Edited by GrayR
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I am a big fan of Tempilaq- used it for stress relieving tigged 4130 tubular joins with OA. It is cheap for the relaxation and consistency it provides. One has to get pretty fancy to need hardening and conversion of austenites to martensites to standards that render it, Temp.., inexact. The crayons are one handed but the liquid with little brush is a better more even application. Using two temps can give that slot of confidence noted by GrayR---AND WELCOME!!!!mike

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GrayR, I've been preaching that gospel for several years. However, since close is good enough to most folks, be content with the fact that most steels used for cutting instruments are within +/- .2% of the eutectoid compostion (.60% to 1.00%). That means MOST of the iron has been transformed to austenite at non-magnetic. No, it will not get you 95% martensite but then, nobody soaks the steel long enough to dissolve the carbides properly either. That means they would never get maximum hardness out of the steel even if they heated hotter.

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Probably OK for the .80 - 1%, but if you are using 4130's, 4140's, 5160's etc., then it may harden poorly. If someone is investing $ in alloyed steel, then it seems to me that they should invest a couple bucks to know where they are quenching from.

Likewise, I have seen many on this site that are trying to make quick, single use tools from lower carbon steels, so it would even make more sense to get it up around 1700 degrees.

As far as carbide dissolution, in most alloys, the carbides dissolve very quickly. In tests that I've seen on moly/chrome alloyed steels (trying to get 50% martensite at mid-thickness, >1" thick) the carbides dissolve in a few minutes. Some of the tool steels need a longer time, but 10 minute soak at temp. will probably get almost everything. But in reality an extra 50 to 100 degrees will do more than another 20 minutes at soak.

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If you are using low to medium carbon steel, yep, you need to go to a higher temperature. And yes, an iron carbide will dissolve quickly compared to Chromium or Vanadium carbides. However, the diffusion of carbon in steel is fairly slow so to get maximum effective distribution of carbon, some holding time is usually recommended. Also, it take some time to get the heat to penetrate through a cross section so the time used for soaking is well spent. Finally, remember, this is blacksmithing not Metallurgical Engineering. Most people who read here don't even know what steel they are working with. Most of it is "found steel scrap" and some generic processes must be employed. If you actually buy steel, one must assume you posses the equipment and understanding of how to process it properly.

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But in reality an extra 50 to 100 degrees will do more than another twenty minutes at soak.

True, but the center of the metal in question will not get 50-100 degrees hotter unless it soaks long enough anyway. Long enough soak time is equally important as temp, for if either of them is inadequate, proper hardening will not be achieved.
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OK, makes sense for the average guy, but if someone is selling a knife blade for a few hundred or a thousand bucks then it doesn't make sense anymore. There are probably some pretty good bladesmiths or toolmakers that are trying to make a living with their products and they don't understand why they get inconsistent results.

What I don't see in the postings or the recommendations is, 'steel will lose its magnetism as it begins to transform the ferrite to austenite, but in many cases this heating point is too low to harden or normalize.'

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Re: metallurgical eng. vs. blacksmithing -- know the difference. The blacksmithing started years before the metallurgy.

I think at some point, we need to get away from the incorrect information (not necessarily in your work, Quenchcrack) dispensed by many smiths. The Curie point doesn't indicate the change from BCC to FCC, but is a separate phenomena -- what is so hard about explaining that to the average guy that wants to get better?

Re: BP0078 -- have looked at it before and overall very good and correct*, and gets to the point I was making before about providing factual information to the blacksmith. Good job of explaining the metallurgy in a simple manner. Linking the Curie point with BCC-FCC transformation for .5 C steel just confuses the metallurgy. But, again, very good.
(* The description along with Figure 6. True hot forging occurs above the Ac3 temperature because the FCC structure deforms much better than BCC because of more slip planes. Since recrystallation begins quickly above Ac1, anything that is hot forged recrystallizes almost immediately (seconds -- again, I don't have a ready reference), so there should be no deformed grains as shown in Fig. 6 after hot forging.)

Re: carbide dissolution. I don't have a reference at hand to disagree with your point, but I don't think carbon diffusion is as slow as you are indicating. FCC iron is pretty accomodating to movement of carbon. I have seen instances of increasing the austenitizing temperatures 50 degrees, or lengthening the soak time a few minutes eliminate any of the dissolution problems. I think the bigger problem is breaking up of alloy carbides. Again, I would be interested if you have a reference (may help me in another area).

Re: Soak time vs. section thickness -- again, don't disagree, but someone else brought up cutting instruments, so I was going along with mostly thin parts not requiring lengthy soak times.

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Quench: "...Finally, remember, this is blacksmithing not Metallurgical Engineering. Most people who read here don't even know what steel they are working with. Most of it is "found steel scrap" and some generic processes must be employed. If you actually buy steel, one must assume you posses the equipment and understanding of how to process it properly...."

Hmmm.... you might be correct, but I'm not convinced. I, for one, have a pretty good idea what I'm using at all times.

Regardless, I think that the majority of the readers are more than intelligent enough to follow this thread and apply the information if they want. Eventually, every beginner realizes the need to understand his material thoroughly, both treatable steel for tools and raw material for forging... or else he simply remains a beginner. There are some good points worth absorbing in this thread.

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Ed, you are a professional and you make very sophisticated products. I would expect you to know what steel you have and have the equipment and understanding to use it. My point was that most amature smiths have a pile of scrap of unknown compostition. Furthermore, most of us do not own a pyrometer or even a few tempil sticks so talking about trying to adjust the temperature of the workpiece by 50 or 100 degrees is asking a lot. Finally, tell the men in the rolling mill that you can't crush austenite and then water cool it to fine ferrite. It is called Recrystalization Controlled Rolling. Gray, if you want to take issue with my Blue Print, I suggest YOU write one of your own. I know you are new here but this horse has had the flesh beaten from it's bones for YEARS before you showed up. There are still a few out there who still think you have to point the blade north before you quench it.

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Hey, GrayR, glad you are here. I asume you are grayer than most, but my beard is white too. It is tough to join a new group, and get the instant recognition you most likely deserve. I know, I joined this one recently. Sounds like you've got a darn good grasp of metallurgy and this is a good place to share it. Ed, as an example of what I think quench crack means, I once found a drive shaft from something, 1.625" by about twelve feet. I knew it was tool steel from the drive spline. I have made a great many blacksiths tools from that one bar, and still have about 3' left. I am not sure what it is but am able to harden it readily. In another case, a customer comes in with a tool a freind of mine made 20 years ago, just before he died suddenly. The customer claims the tool is too soft and wants me to harden it for him. After checking the hardness and thinking for a while I agree, but I have no idea what this stuff is, as my freind died before he ever told me what he used. I agree with quenchcrack, write a blueprint. I make my living doing this and the more we all know the better.

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Umm . . . actually I was not taking issue with your blueprint, more below.

Re: Recrystallized Controlled Rolling -- yes, it is done, but as you said to me earlier, blacksmithing vs. met. eng. -- RCR was not mentioned in the Blueprint concerning the deformed grains in Figure 6 right, so I was providing was happens during the normal forging process.

Again, it wasn't an issue with the Blueprint, I think that everybody learns from everybody else, just like you did. That is the reason why I do come to this website (it wasn't my first time here . . .) You took the time to write it down on paper, so that is good for you (and for the blacksmithing community). But to be honest, concerning the Blueprint, my metallurgy classes didn't identify Fe-C phase diagrams and ITT/CCT diagrams as maps (maybe yours did), but I did go back to my 1994 Anvil's Rings (21 - #3 & #4) and there was an article on heat treating that does refer to metallurgy maps -- so maybe we all learn from each other -- and that was the point of my original post.

BTW -- Gray is not a name, but my hair color; maybe you don't have as many years as you think.

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Re: ApprenticeMan. Thanks. No axe to grind . . . just responding to the comments, and trying to get the right information out there for reference. Again, QC has good information on his posts -- and, if I didn't say it earlier, the Blueprint has a lot of good (and accurate) information. But I do agree that better information would raise the bar.

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The introduction to BP978 says:
"Other metallurgists will note that some topics have been greatly simplified in an effort to illustrate certain basic principles. The purpose is to present some of the basics of ferrous metallurgy that will give the Blacksmith some insight into what is happening on a microstructural level during normal smithing processes. It is hoped that this will aid him or her in utilizing enough theory to avoid some trial-and-error disasters."

"Fig. 6. Normal shaped grains, on the left, and distorted grains, on the right. When steel is forged hot, the crushed grains will form new grain boundaries and look like the grains in the right drawing."

GrayR, I think the comment under figure six clearly states that crushed austenite will form new grain boundaries. I simply could not draw the fine new grain boundaries that form from the crushed austenite using the simple tools in Powerpoint. Please excuse me for being just a Metallurgical Engineer and not a PowerPoint artist.

Let me explain my frustration. I have been posting on both blacksmithing sites and bladesmithing sites for over 5 years. I was asked to be one of the Guru's on the Anvilfire site. Don Fogg personally asked me to join his forum for the express purpose of clearing up the misconceptions about heat treating. I brought up the fact that the use of a magnet to determine the transformation from BCC to FCC was unreliable for any composition other than the eutectoid point. I posted this on Foggs site, the various knife forums, this site, and others. What you bring up is not new. Your mistake was not understanding your audience. Knifemakers in general are a bit more sophisticated in their understanding of metallurgy. Blacksmiths, while none the less intelligent, are less concerned about metallurgy and more interested in the techniques of smithing. These very same issues will be discussed next year by the new people who come here. While there are plenty of good technical resources to be read, it is simply easier for many of them to simply parrot back what they hear lacking any real depth of understanding. I grow weary of repeating myself and then have to defend my attempt to dispel the misconceptions. I am reminded of what Davy Crocket said to the people of Tennessee when they did not re-elect him to the Congress for a second time: "You may all go to xxxx and I will go to Texas".

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GaryR, please consider putting your knowledge into a blueprint, or two, or more if needed to cover the subject. Maybe one for blacksmiths who are more apt to grab any available piece of metal and bladesmiths that are much more selective in the metal they use.

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Re: Matherton Forge -- great site, nice work. I haven't seen it before. I will spend some more time there when I get a chance.

My original posting came from a discussion a couple days ago, and then some late-night searching at this and bladesmithing websites and the magnet trick comes up frequently.

Re: QC frustration. OK, I've seen your name many times, and your posting quota indicates that you provide frequent advice. I posted a straightforward comment, one which you agree with in concept. You provide a comment in disagreement on what smiths need to know ("since close is good enough to most folks. . .) . . . and I disagreed with that (and still disagree).

After that its gets pretty frustrating for me too -- "remember this is blacksmithing . . ."; I'm shown the Blueprint on metallurgy (I guess so that I can understand what blacksmithing metallurgy really means); my experience is questioned; then it is recommended that I write my own Blueprint; and finally "your (my . . .) mistake is . . ". I'm not trying to put you on the defensive (if that was your point in your last posting), and have even praised your work in these postings, but . . . .

Re: "very same issues will be discussed next year by the new people . . ." that was my reason for posting in the first place. I know that it isn't new information . . . I never said that I invented metallurgy. New people probably come to this site daily(?), and look for advice. If they did a search, they would see my posting (or one of yours, or someone elses); or if they post, then someone who previously had misconceptions about heat treating could respond accurately. I think first grade teachers and college professors get the same questions every year, but at some point, some of their students get it, and take it to another level. I guess I didn't know the rules for this Forum -- don't post a fact or advice until someone asks for it.

Re: Knifemakers understand metallurgy better than blacksmiths. I disagree with that as well. Cashen (based on his website) and some others probably know better; but I see off-base advice on bladesmithing sites as well.

Good luck.

Edited by GrayR
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I kinda figure it all depends on what kind of smithing you do. Is blacksmithing you primary source of income, are you a bladesmith, or artist, or are you like me, just a dodabber that enjoys beating hot iron/steel into something usable around a living history site or the home?
I make no claim to being a professional blacksmith and have no desire to become one. I am retired military, I take my retirement very seriously and have a number of intrest. All of which my wife allows me to indulge shamelessly. :)
My wife and I are currently in the process of moving into a pole barn converted into a living space. I plan on making the cabnet pulls, hinges, etc myself. I could care less about the metallurgy of the stuff I'm using.
Do not misunderstand, I have the highest respect for those of you that sell your work or make your living this way and spend long, hot hours at the forge.
This is just not my primary area of intrest. 1814 living history is and this is just one area of that history.

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methinks this is a lot of bickering about nuthin'. Those who wish to pursue a PhD in metal studies need a different forum. Those who need a layman's explanation of metallurgy to accomplish something in their own shop can find it here without the quibbling. And those who are ready to take it to the next level already know where they have to go.

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First-Gray- the suggestion of a blueprint from you is a recognition that you have valuable information to offer. Frequently, knifemakers are a bit more attentive to more sophisticated hardening and tempering and they often use more complex alloys that require greater precision to get a commercially suitable result for sale to the public. That still leaves great room for shade tree metallurgy.( As I have noted ad nauseum, I take greater pains because I built and drove, betting my life, the most deadly form of racecar extant) Second-rthibeau, all levels of sophistication benefit us, some do want more "scientific methods." Respectfully submitted, JET/Mike T.

Edited by racer3j
found benefit typo
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