Noob question about forging heat

M

Makermook

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Feb 28, 2011
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Of course temperatures at hardening quench and tempering temp are critical, but what about the temperature while you're actually forging?

I keep reading about grain growth being related to high heats, but then I'll read about guys who prefer to forge at bright yellow heat.

Provided you do the proper thermal cycling and annealing prior to heat treating, does forging temperature matter that much? (And provided, of course, that the metal doesn't get so hot it burns and sparks...)
 
The forging range for most common blade steels is 1600-2200 F. The grain growth isn't a problem because it gets reversed in the grain refinement steps in HT. At the end of a days forging, I usually give the steel a down cycle normalization, just to reduce the grain a bit.
You do that by heating it to about 1500F and quenching it. Then you heat to about 1400F and let air cool, lastly you heat to 1200F and allow to air cool. The first step refines the grain to small size, and the last removes the stress and brittleness from the quench.
You can just air cool at each step, but I like the finer grain that you get from the quench.
 
In my experience forging temperature of properly prepared steel is very significant.
When you have large sheets of steel coming off of your billet you are way too hot, grain will grow and performance will decrease.

Our favorite forging temp is 1,650f, you can tell when you are at this heat when the slag coming off of the billet are about the size of snow flakes. When forging at this temp each and every thermal cycle results in finer grain, once you have lost it you will never be able to recover the fine grain you could have had.

The first heat, the scale will be dependent upon time and temp. if you try to forge as soon as you reach 1625 the steel will be hard to forge, let it soak one hour for every linear inch to the center and it will forge easily, the first slag will be larger, but the rest of the forging cycles at this temp will not result in the heavy slag and you will develop a nice fine grain, don't worry about too many forging heat cycles, the more the better and at this temp you will not loose carbon.

Good luck
 
I don't have the years of experience that Ed and Stacy posess but I do know that with regards to bladesmithing, 80% of a good heat treat is "set up" prior to the quench. If you are moving a lot of metal, forge hot (1800-2000F). As you get closer to shape subsequent heats should get cooler to help reduce scaling and decarb. I get down to 1200-1300F for straightening and tweeking. My set up is similar to Stacy's and Ed's, too.

-heat to 1600F and air cool to 400-500F
-heat to 1500F and quench (twice? sometimes)
-heat to 1400F and air cool to 400-500F
-heat to 1250F and air cool to room temperature

At this point it is ready to go and the final heat treat is a snap... well, not literally.

Rick
 
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Another little tip you won't read very ofter:
After you have your blade forged to shape, heat to above critical and quench in room temp quenching oil for 35 seconds, heat it back up again above critical, quench again for 35 seconds, then reheat to just above critical and quench again for 35 seconds in the oil.We have not had one blade warp after these steps.
Then do your normalizing cycles and anneal.

You are ready to harden, again I only work with 5160 and 52100 the above applies to these steels. Works for us.
 
I'll just say that I disagree with Ed's first post and agree with his second one.

If you follow his down cycling multiple quench normalization you will get fine grain and reduce the stresses.
However, the final grain structure and stresses will be determined by the last above critical treatment, and any earlier treatments or soaks in forging will not affect the final results.
If you carve a eagle in ice and melt it, it won't matter how fast or slow you re-freeze the water...it will never look like an eagle again ....unless you re-carve it.
 
bladsmth said:
If you carve a eagle in ice and melt it, it won't matter how fast or slow you re-freeze the water...it will never look like an eagle again ....unless you re-carve it.
Click to expand...

Great analogy, Stacy. You can do all the grain refinement you want but overheat the steel before the final quench and it's all for nothing.

With regard to the OP's comment on some folks forging at bright yellow heat. If the stock is thick enough (ie. drawing out large rounds, pattern welding billets) you can work in those temperatures without ruining the steel. It's when you get down to thin cross sections that you need to pay closer attention to heat.

Rick
 
Ed Fowler said:
Another little tip you won't read very ofter:
After you have your blade forged to shape, heat to above critical and quench in room temp quenching oil for 35 seconds, heat it back up again above critical, quench again for 35 seconds, then reheat to just above critical and quench again for 35 seconds in the oil.We have not had one blade warp after these steps.
Then do your normalizing cycles and anneal.

You are ready to harden, again I only work with 5160 and 52100 the above applies to these steels. Works for us.
Click to expand...

I've done that three-cycle thing, but never for as long as you say. Generally, I do it for just a quick dunk, a second and no more. I'll try it for a longer time from now on.

Thanks!
 
All I can do is suggest and relate aspects we have found significant. It is up to the blade smith to try them and test his blades on his own, comparing one method to the other one or ones. You will only know if you test your blades in your shop using your materials and equipment.

Some university folks are working on some blades developed our way, so far they are very interested and we will have more information in the future, I hope!
 
Ed Fowler said:
When forging at this temp each and every thermal cycle results in finer grain, once you have lost it you will never be able to recover the fine grain you could have had.
Click to expand...

If you get finer grain with each cycle at the proposed temperature, and if grain growth were to happen, why can't you just do more cycles to get back to the size you want?
 
me2 said:
If you get finer grain with each cycle at the proposed temperature, and if grain growth were to happen, why can't you just do more cycles to get back to the size you want?
Click to expand...

As I'm beginning to understand it, it seems like you CAN do that, potentially as often as you'd like to. The trick is to cycle it so the grain is as fine as possible, then NOT overheat it during the final heat to hardening temp and ruin all that work by causing grain growth at the very end of the whole process.

Does that sound about right?
 
Me2- when we test blades to destruction those that we over heated fail in destructive testing at lower levels of performance.

A little more food for thought, try testing blades outside when it is -30f. This brings a new dimension to both testing and reveals that an over heated blade is just a little shy of what it could have been.

I enjoy your questions!
 
But why do they fail? Can the grain size be re-reduced by further cycling? If every cycle at the referenced temperature reduces grain size, I would say it could.

Are these failures directly attributable to grain growth that was repaired by further thermal cycling, or attributable to the additional thermal cycling itself? Do you quench after tempering? I've read this helps with low temperature brittleness issues in steels.
 
I respect Ed and his testing.

However, as I understand it and based on modern metallurgy, every time you refine the grain by cycling through descending quenches, he slate gets erased and you start over with new grain. There can be some residuals of other things, but they have nothing to do with previous grain size. The final HT, done at the proper temperature and soak times, should reverse all internal structures and give a consistent final grain size.

I'll drop out of this thread now, as I think all that is pertinent has been said.
 
Does this mean that one needs to find steel that was rolled to size from billets at 1650?

I believe most mills use temps significantly higher than 1650 when the size down the original poured billet.

So, what is the process they use to get the steel back to the point where keeping it below 1650 in future forgings would be beneficial and where heating above 1650 it would be impossible to get the steel back into the same shape it was from the mill?

Or is the heating and forging done at the mill in some way different than in our shops that it doesn't cause irreparable harm to the steel?
 
Me2 - Why do they fail? They chip, they break with only one quench, way too hard in the cold air. This is why when we develop a blade for use at very cold temperatures we temper 15 degrees higher.

Why does this happen, I feel I understand, but don't know how to put it in simple language. Our last student is in his senior year studying to be a metallurgical mining engineer, he said he spent an entire semester studying events like this.

I never quench after tempering, always let it cool to room temp. in my Paragon with the door closed. This is one of the the reason Rex joined the team, he had to come to the ranch and watch how we developed what he could see in his microscope.

I do not believe you can repair an overheated blade to peak performance.

Stacy: the steel is never the exact same, each hammer blow, each thermal cycle is recorded in the steel.

Jim: The high temp used in rolling steel is used in the mills because rolling at low temp is hard on equipment and not efficient. This reduces profit and ultra high levels of performance is not demanded by industry most of the time. One outfit buys steel tubing that does not last as long as the better tubing - why? because it is cheaper to replace than the price of the better tubing.

Low temp forging seems to always improve performance in relation to the rate of reduction by forging. In other words start with the largest billet you can work and you may be able to increase performance.

Will write more physical therapist just arrived.
 
Interestingly, LOTS of branches of engineering and industry have interest in pushing the envelope of performance with the materials they examine - cost no object! They've determined all kinds of really cool facts about metal, with total abandon of expense.

I urge everyone interested in understanding these concepts to discuss it with professionals. You'll find that they're just as passionate about these things as knifemakers are, and that they aren't the silly bean counters that they're often portrayed as. Most will tell you that they are intrigued by the thought of anecdotal evidence, but never stop there - and always try to avoid taking a position and maintaining it by only accepting evidence that supports the position.

There are lots of industry engineers in my neck of the woods (Moog, Praxair, Northrup Grumman, etc). Some of them work in advanced projects where lots of money is invested to move processes toward better performance. By their own admission, most of the initial work is in finding out how, then it's determined if it can be done less expensively. Regardless, they are perpetually trying to push the limits of performance.
 
Ed
Am I to understand that the best steels are never heated above 1650 after they are poured into a billet?
 
Continuing the last post:
Every thermal event, every hammer blow is recorded in the finished blade. Naturally some events are more significant than others.

Every billet of steel we work on will know one first step, it will receive a thermal cycle at a little above critical for one hour for every linear inch to the center of the billet, heated from room temp at 1,000 an hour, held for at least an hour then allowed to cool down slowly in the Paragon at room temp. This is just one step in starting every blade from the same place as possible and is responsible for correcting one unknown variable that almost derailed our quest.

Matt: that was a good post! The only thing I would change is the thought that most of the metallurgists are interested in pushing the envelope, some only regurgitate what they have learned in school and look on any potential improvement or change with disdain, almost like all learning stops when they graduate. Many articles in technical journals are similar, but it still pays to read what they have to say, sometimes a comment or a footnote can be very significant if we ask why.

There is a lot of safety when you have the backup of traditional textbooks and for some it is not easy to stick your neck out when talking about something outside of the box for some.

Most significant is our own individual testing of or our work in our shops, you never know how far a knife can go unless you test it to destruction.

And be sure to have fun with it.
 
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