Catching the temp, on the way down?

L

lcf

Joined
Nov 28, 2006
Messages
57
When using a gas forge and trying to keep the temp of the steel at a constant (soak), if the temp rises above the required temp, (and forms nasty large grains::eek:), as the steel cools back to the required temp for the steel of choice, can you pull the blade at that time, (back to small grains) and quench? Or does the steel need to be thermal cycled at this point? Wouldn't the grain size be the same on the way down as it is on the way up?
The reason for the question stems from a lot of readings taken with the pyrometer, and 50-75deg comes and goes very quick. Also the readings are different when the gas is on VS. gas off. Steel in the forge VS. steel right out of the forge. Can't seem to find anything in the text books that addresses this. Thanks guy's!!!!!!
 
If you have overheated the steel and grown large grains you have to start all over again.Just quench, temper at 1200 F, cool then harden again.
 
Also consider that, although grain grows quickly, going a small amount over required temp shouldn't cause tremendous grain growth as long as it is not held there for any extended period of time. Of course you'll never really know if the grain has grown unless you break it and take a look. Catch 22!! I really need to get a heat treat oven!

--nathan
 
Ok, but when we normalize the grains grow large then go back to small in the cooling down phase, and also for annealing. So what makes the grains stay larger if you go past the temp range then wait for the steel to cool, as it cools don't the grains also reduce?
I don't mean to be a pain, but this is one of those urban ledgons that has so many different takes written on it.
 
Mete, Stacy,and nathan, I'm looking in the ASM. HEAT TREATER'S GUIDE, under the diffinitions and specific areas for quenching. Do you off hand know of a section to refer to to help explain this question for me to read.
Thank's Larry.
 
lcf, you are a little off in the picturing of the process. Austenite grains form and then grow, but they do not shrink. To make them smaller you must make them all over again in a smaller form. When cooling they do not get smaller they transform into other products such as cementite, ferrite and pearlite. From this you reheat and make new baby austenite grains between and at the corners of the other stuff, these turn into fresh new grains that will maintain their size as long as there is undissolved material in the grain boundaries.

If you heat to the temp at which you dissolve the material in the grain boundary there will be nothing stopping the two adjacent grains from joining to form a single larger grain. If you have grain boundary stabilizers holding things there you will have to go very high in temp to dissolve them, this temp will be much higher than required for simple grain growth, so grain growth will commence very quickly as soon as this point is reached, with such a steel there is no grace period or margin for error above the recommended temperatures.
 
Ok I'll take the hit for the "being off alittle bit" since I asked the question here in the forum. But it is good to have a clear difinitive answer, if you read the post above, the question was raised due to what is being currently written (else where). It's better that I ask and get the right answer than let several others who may read (the new style) and be lead astray.
Your right Kevin the hype is overwhelming, I'm thinking of throwing all of the magazines away.
Thank's again guy's for the right information!!!!!!!! Larry.
 
So, Kevin, am I thinking about this right: that the temperatures required to disolve material in the grain boundaries and cause precipitous grain growth is usually a good bit above the temperatures required in normal heat treating processes?

Thanks for all ya'lls input. I'm absorbing this stuff as fast as I can as well.

--nathan
 
lcf said:
Ok I'll take the hit for the "being off alittle bit" since I asked the question here in the forum. But it is good to have a clear difinitive answer, if you read the post above, the question was raised due to what is being currently written (else where). It's better that I ask and get the right answer than let several others who may read (the new style) and be lead astray.
Your right Kevin the hype is overwhelming, I'm thinking of throwing all of the magazines away.
Thank's again guy's for the right information!!!!!!!! Larry.
Click to expand...

Actually you don't need to take the hit at all, since as you point out it is not your fault if most of the readily available information is so wrong. It is one of the most common bits of missinformation in print that forging "breaks up" or "shrinks" austenite grains. One does not "shrink" austenite, and while you may be able to fracture a grain, it is not at all desireable. But it is interesting to hear and alluring to think about so it works, better than the bit more technical facts.

As for magazines just remember that their number one goal it to sell, to sell issues and most of all to sell advertising (which is driven by the ammount of issues sold). Colorful and wild @$$ed descriptions and theories or interesting little anectdotes sell, facts don't. Facts are boring, that is why you need to spend at least ten times more for a good heat treating book as you do for a magazine.
 
So then tell me ob1 when are you going to print?
AND I don't want this stuff about moral's.
If you write it they will come.
Just do us a favor, and give all the information instead of what the rest of them do and give about half of what is needed.
So far if any one does enough research with this forum, all of the information is there but it is becomeing quite fragmented.
I just recently read a post you did in 2004, that is in the archives. Somehow all of the information needs pulled together. FEEL THE FORCE LUKE?
 
Just to recap Kevin:

When you overheat steel and grow large austenite grains, and then quench it, you are going to lock those grains in at the size they are, as martensite and retained austenite.

When you anneal steel, you heat it up above critical (the grain size is really immaterial) and allow it to cool at a very slow rate, allowing lamellar pearlite (layers of ferrite and cementite) to form. This gives the softest and most workable state.

When normalizing, you are going for reducing grain size ( and relieving stress) ,ending in fine pearlite (small grains of ferrite and cementite mixed).

Spheroidization gives a spheroidal structure of carbides, to improve machinability.

Stacy
 
Thank's Stacy, Kevin,Nathan, for the help in clearing the latest "myth" in blade smithing.
As said in the begining of the thread I have been taking alot of readings with the pyrometer and find a second concern that needs addressed. Watching for shadows?
The room is dark, only the glow from the forge, and the moon is full, when the shadows are gone the temp is still 100 to 150 deg below the required temp according to the manual. Does this 150 deg make that much difference? To obtain full hardness?
 
ICF, you really should fill out your profile.

There are several variables in your setup that could be a problem:

"Shadows" (decalescense) are what you see. Someone else might see something different. If you weren't taught how to read them by a knowledgeable smith, you might not be looking for the right thing.Reading temperature by color and shadow is a very personal thing and not accurate at all. An experienced smith can look at the steel and tell if it is ready, but that is just for his setup and location.It changes with every forge. For HT it is not close enough to accurate.+/- 100 degrees is fine in forging, but not in HT.

"Taking a lot of readings"? The pyrometer should be taking a reading all the time. Are you sticking it in and out? Depending on the type of pyrometer, probe, position in the forge or oven,etc. it might not be accurate. It is best to test it (and calibrate it) with a known source. Temp-stiks are good here, as are the kiln test cones that potters use.
Stacy
 
bladsmth said:
...Reading temperature by color and shadow is a very personal thing and not accurate at all. An experienced smith can look at the steel and tell if it is ready, but that is just for his setup and location.It changes with every forge. For HT it is not close enough to accurate.+/- 100 degrees is fine in forging, but not in HT...
Click to expand...

Well put, but I have also pushed decalescence in conjunction with the magnet, and even on its own, when a person may want to austenitize somewhere other than 1414F. The magnet is very accurate but only for one temperature, and every steel works best at a slightly different temperature. Decalescence is a guideline that is specific to the steel you are using at that time.

That being said, Stacy is right on the subjective part, to be honest I often have a hard time seeing the exact endothermic to exothermic transition at the tail end of decalescence. Instead I have come to rely heavily on the color that my salt thermocouples have definitively shown me is 1475F to 1500F.

I have tested various soak times and temperatures with many steels until my shop is littered with samples and I have lost track of them all. In the big picture I have determined that that slight variation you are seeing in temperatures is negligable for knives. The exact temperature you quenched from may not even be discernable by the Rockwell tester in some cases and only dispersed microscopic features could reveal it. One of the dissenchanting things I find as I continue my research, is how little all this matters in the big picture of knife use, and that I am agonizing over the last 5%.:(
 
lcf said:
So then tell me ob1 when are you going to print?
AND I don't want this stuff about moral's....
Click to expand...

The morals thing has me curious, what excuse have I presented that had a moral overtone? Believe me I am loaded with excuses on this:D, I am just wondering if the moral idea could have come from excuse number 154 or excuse 287. I soon hope to expand my excuses to over 400 when I merge some of my excuses for the "Honey-do" list with this one;)
 
In one of your past post's you had made a comment on (not feeling right selling information to people, that you are happy to give away), That is good and honerable, but now that the information is out there, pull it together in an understandable form, instead of multiple threads on a web site.
Think about it, how long did it take you to read this?
 
Stacy and Kevin, between the last posts I think that has been with out a doubt the best explained information for decalescence I have read!! It now makes alot of sence on what is going on and what to look for.
Stacy I have been taught by good smiths, but it wasn't untill now that it all clicked and made sence (I'm slow and stubborn). I also keep asking questions, and re-read the ASM, untill I'm sure I've got it right.
Stacy, when I say pyrometer, I am refering to a lazer heat reading "gun" type. And reading the steel out side of the forge. Is this not correct for what I want to achieve? Useing the color and the "gun" together?
Also I did fill out the user information. (sorry).
 
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