does forging too cold create "stresses?"

[soundmind]

I think I've been forging at borderline low temperatures had a couple of questions. Correct me if I'm wrong but does cold forging create stresses in the steel? I was also wondering if it is acceptable to straighten at a black heat or does that also create stresses?

Are stresses in the steel potential areas of weakness in the blade after heat treating? And say there were any stresses, can they be cycled out in thermal treatments before hardening?

Another issue was if I did forge the narrow tang and the points at too low heat will it create a void or empty spot or something like that? This steel is 1.5" wide. I think what I'm imagining would be something like a "cold shut" but I'm not sure if that name only refers to welding.

 

[Brad Lilly]

Cold forging causes cracks.

 

[EdCaffreyMS]

Forging will always induce stress into steel. Hence the reason for a stress relieving step in the forging process. When it comes to forging too cold, it will often cause what I call "micro fractures". These are tiny cracks you will not be able to see with the naked eye, but will sometimes become clear when more stress is induced.... such as during heat treating. In other cases, the fractures will not become visible until you are at the hand sanding stage....then you may see something that looks like a tiny hair on a blade..... but the more you sand, the larger it becomes. It's always best to educate yourself on the "forging range" temps of the steels you choose to use, and go from there.

 

[Kevin R. Cashen]

Some of the effects that can arise have already been discussed, but to complete the picture I thought I would share some of the causual mechanisms. Proper forging temperatures occur above the recrystallization temperature of the steel and thus leads to much less stored energy from the strain. Every hammer blow introduces strain energy that is absorbed by the process of recrystallization to spontaneously generate new grains, this is known as “dynamic recrystallization”. By matching your rate of deformation to the heat that drives the grain growth mechanisms you can keep things refreshed and evened out; this is also a reason why it is a good idea only to heat the area that you are working at the time and is one advantage that coal has over a gas forge.

The cooler the temperature that you forge at, the more you can outpace the recrystallization and slip systems and create a build up of strain effects. Once you drop below the recrystallization temperature of the steel you are now only deforming the same grains with no recrystallization to reset the system and all of that energy is being stored it the steel. Compounding this issue is the fact that the atomic stacking at these temperatures has limited slip systems available to accommodate the deformation. Slip is how steel deforms, by planes of atoms “slipping” past each other. This is why steel is stiffer at room temperature, but it forges much more easily when glowing orange. When it is at forging temperature the atoms are stacked in a face centered cubic arrangement, which has many more slip possibilities. Gold atoms, for example, are face centered cubic at room temp and this is why it is so malleable.

So, when you work steel below the recrystallization temperature and do not have enough slip systems to accommodate it, there is a pile up of misalignments in the atomic stacking known as “dislocations”. This all equates to more stored energy in the steel. If you continue to push the steel when it has run out of mechanisms to handle this pile up, it begins to come apart. This is exactly what happens when you bend a wire back and forth until it breaks.

In light of all of this, one could say that forging cold is virtually the definition of stressing the steel.

 

[C Craft]

Kevin I must say I use read something like the post above and go, "what the hey did he say" but I only had to go back and re-read one part of that explanation!!

I think I am getting better at interpretation, or then I could just be delusional !!

 

[Doug Lester]

Black heat covers a lot of territory. I don't know at what temperature color goes out of a heated piece of steel but if you catch the steel between the nose of the cooling curve and the Ms point where martensite starts to form. As the temperature falls below the Ms point more and more untempered martensite forms and the steel will become more brittle and trying to straighten the blade cold may cause it to break. It might be better if you heat the blade back up and then straighten.

As far as forming the tang goes, yes you can get into trouble if you try to push forging the bar down to form the tang. You need to forge the bar down a bit until you just start to see a "gutter" start to form and then turn the blade and form it flat again. Just work back and forth and you will form the tang and draw it out. Experience will show you how much of the bar you need to start with to form the tang.

Doug

 

[C Craft]

If you don't believe that cold forging causes stress fractures or just plain stress!!

Go get you a piece of blade steel and at 2" from the end of that bar, beat the bejesus out of that one point until it begins to thin and move.

You will be able to see the stress that cold forging produces! It will begin to stretch and move but as it gets thinner you will be able to see the stress fractures beginning with the naked eye!

Any blade steel will do the same thing if the temp is not correct. That is why there is a optimum temperature for working each and every steel!

Think of it like this. If you have ever tried to bend a tight bend, even in mild steel; without heating that area?? You may be successful in bending a hook in the steel. However all the edge and the center of the bend will have tiny fractures!

If you want to know the technical jargon re-read Kevin's post!!! If that doesn't convince you take a look at this information from the same source, Kevin Cashen!! http://www.cashenblades.com/forging.html It will give a new insight into your own question!

One thing you don't want in a forged blade is stress or stress fractures cause that is a guaranteed failure at some point in time!!

 

[Kevin R. Cashen]

For any metallurgist that may read the thread, I should also add that slip is the mechanism by which steel deforms the most, and more at elevated temperatures. I did leave out twinning, which is what the steel has to resort to at lower temperatures. You can also see twinning effects on the surface of clean steel when you deform it. I see it often on ABS test blades that have been bent. There is really no effective way for the atoms to move, so they sort of accordion up in their planes instead. The steel will only take so much of this.

 

[Doug Lester]

I don't know if I caused confusion with my answer on forging in a stick tang or not but I was not referring to cold forging the tang. I was meaning that the tang should be formed at forging temperatures not at a black heat.

Doug

 

[C Craft]

I see it often on ABS test blades that have been bent.

Kevin I am assuming that you are referring to a blade that has been bent during ABS testing!!

 

[soundmind]

Sorry for the late reply. I saw your replies yesterday but I was super busy (dealing with mold in the house). But it gave me some time to think about it.

C, I realized I have a blade I tried to bend cold which cracked where I hit it. It was Damascus and it cracked at a weld. That experience was off my radar b/c I was doing something different with that one... new at knifemaking and experimenting... I was trying to bend the handle down without affecting the HT on the blade and it cracked in the ricasso. Now it's sitting in the shop and rust is forming in there like a bad cavity.

It sounds like I need the control to stop hammering away when the steel is hard to move. Only try to move it at the right temperatures. Ed is right, I need to look harder to find out what the right forging temps are.

I'm going to change a few things on my set up to attempt a more accurate reading on my thermocouple and shade my anvil from the sun.

Doug, thanks for the forging tips. What you and Kevin said gives me a better understanding of what I'm doing during this process. The heat is so important, but I guess that's why it's called forging. My only concern is being slow and unquardinated with the tongs so I end up putting the steel through a lot of heats. What's normal, like 20 heats for a blade? I think I did that much just for the tang.

Anyway thanks for making sense out of what I was asking. It's obvious now that this isn't a mistake that can be cycled out. And the important lesson for me is don't hit the steel below the right forging temp even if it's still super hot.

 

[soundmind]

Now I'm wondering if these blades are even any good anymore.

They are still pretty thick. Is there any chance I could grind some of the surface away and check for cracks at this point? Or should I expect there are cracks all through it and start over with new steel?

 

[soundmind]

I ground the surface scale off these and found little black lines in the tang and tips.

Not sure if it's twinning from deforming the steel or big cracks with micro-fractures hidden all around. The steel wasn't super cold, just a little not hot enough.

I'm going to grind some more just out of curiosity. But I probably won't finish these out.

Just wanted to edit and say thanks again for the answer.

 

[Kevin R. Cashen]

Twinning will not be black, and you will need to know what to look for to find it. I know the lines you are seeing, it comes from reducing a cross section too drastically during forging, some alloys are more prone to it than others, what steel is this again? I have found that chromium bearing steels are prone to the black stress lines during reduction when pushed too hard. Sometimes they are only a surface feature, but even then the strain effects are hidden under the surface. Try normalizing and these cleaning the area up to see if there is any disturbance in the metal still.

 

[soundmind]

Try normalizing and these cleaning the area up to see if there is any disturbance in the metal still.

ok I will.

some alloys are more prone to it than others, what steel is this again? I have found that chromium bearing steels are prone to the black stress lines during reduction when pushed too hard.

The steel is 80CrV2. Might have been a mistake to use this steel as beginner steel.

it comes from reducing a cross section too drastically during forging,

I could see this might be what I might have done if the steel was too cold or if I might have too light of a hammer. The steel is 1.5" wide - not sure if that matters or not.


Thanks Kevin

 

[soundmind]

I've been thinking I need some forging tutorials so I watched several videos. No doubt my technique could be improved but I was essentially following the same steps. The major difference I noticed in the videos was how long the bar of steel held heat. I saw guys getting in a lot of hammering before the bar got too cold. For me I'm lucky if I get ten strikes.

I'm keeping my anvil, tongs and hammer warm. Does anyone think I could be loosing heat due to air temperature? It was about 20 -30 above at the time. Which I thought would be alright - at least manageable. Anyone else doing this outside in the colder states?

 

[Doug Lester]

I would consider 80CrV2 to be a good beginner steel. The little bit of chromium that is present there will give a little more time to get under the nose of the cooling curve and the vanadium will help pin the grain size and slow grain growth.

I usually don't forge at the temperatures that you mentioned but, yes, that will cause the steel, and everything else, to cool more quickly.

Doug

 

[soundmind]

I usually don't forge at the temperatures that you mentioned but, yes, that will cause the steel, and everything else, to cool more quickly.

That's probably obvious and I should have figured it out. Having this question about the cooler temps made me hesitant to normalize right away, too. I thought maybe it would cool too quickly between cycles but then again maybe not enough to really matter. Anyway its warm enough to rain now. I hope to get back to these this week.

 

[Doug Lester]

Ok, we're talking about normalization or thermal cycling here, not air quenching. Even if the colder ambient temperature does cool the steel from an austenized state faster it's not going to have the effect of air quenching. Not even the steels that tend to air quench a bit in thin sections, like L6, are going to show much difference. If anything the colder ambient temperatues might drive a little more retained austenite (and there's always just a little retained austenit after quenching) to convert to untempered martensite.

Be aware that what I said does not apply to air quenching steels because air quenching steels can't be normalized. You have to anneal them.

Doug

 

[soundmind]

I know the lines you are seeing, it comes from reducing a cross section too drastically during forging,

As far as forming the tang goes, yes you can get into trouble if you try to push forging the bar down to form the tang.

So, when you work steel below the recrystallization temperature and do not have enough slip systems to accommodate it, there is a pile up of misalignments in the atomic stacking known as “dislocations”. This all equates to more stored energy in the steel. If you continue to push the steel when it has run out of mechanisms to handle this pile up, it begins to come apart. This is exactly what happens when you bend a wire back and forth until it breaks.

I have found that chromium bearing steels are prone to the black stress lines during reduction when pushed too hard. Sometimes they are only a surface feature, but even then the strain effects are hidden under the surface. Try normalizing and these cleaning the area up to see if there is any disturbance in the metal still.

My job and new baby (boy!) has had me away from knifemaking the past several weeks.
I snapped this photo before I left town. At the time I had 4-5 knives I forged that all turned out with some variation of this:

There are the black lines I mentioned on the spine of the first. The knife is upside down.

The steel looks rolled over and folded on the second specimen which is a tang I cut off. ("Dislocations"??)

The same thing was happening on the third. That's when I decided to stop. The bar stock is 1.5"

Thanks all. I know I hadn't followed up on your advice yet and it has been awhile. I hope to be home soon. I plan to try this again at warmer outside temps with fresh steel. I also plan to normalize and grind the original batch. Only problem is I don't know if I can take these knives through a true test of heat treatment if already they're weakened.