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Normalisation after forging


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Hello everyone,

I'm from Italy.

I have a question about triple normalisation after forging operation.

 

I know that this operation is do and repeat for stress relieving and for refine the grains of steel. But every time I (or you) heat the steel over crtical temperature (Ac1 or Ac3 depending if my steel is ipo or iper eutectoid)start recristalization and there are the risk to lose the advantage of forging operations (refine and orienting structure of steel. Grains and eventually undissolved carbides and non metallic inclusions)?

 

So, the question is: -" If I normalise for two or three (or more) time my forged knife I lose (totally or partially) the advantage due to forging operations (mainly orientered structur of grains)?

 

Excuse me for my bad english. I will try to emprove it.

 

Regards

Francesco

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Francesco, forging puts strain on the grain of the steel by distorting the bonds between the iron crystals. Normalizing relieves the stress on the structure of the steel by allowing the iron crystals, which are the matrix of steel, to reform. Also when you normalize, if you keep your temperatures down, you also correct for any grain growth that may have occured during heating at the forge. If you don't keep the temperature of the steel to just over critical, you could actually increase the grain growth. I'm not sure that the advantage of forging lies in the orientation of the steel grain.

 

By the way, don't worry about your English. You're understandable, and if there is a problem with understanding anything that you post, we'll work it out.

 

Doug Lester

HELP...I'm a twenty year old trapped in the body of an old man!!!

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Thanks.

 

Oh, if the steel is mantain below Ac1 i don't lose orientation.

 

But for normalisation I have to heat steel over critical temperature (Ac1 or Ac3) to obtain an austenitic structure.

 

If I heat below Ac1 i don't make a normalisation but i do a stress relief.

 

So, when the knifemaker talk about normalization, I think is necessary heat above Ac1, not below.

 

Regards

Francesco.

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Francesco, forging puts strain on the grain of the steel by distorting the bonds between the iron crystals. Normalizing relieves the stress on the structure of the steel by allowing the iron crystals, which are the matrix of steel, to reform. Also when you normalize, if you keep your temperatures down, you also correct for any grain growth that may have occured during heating at the forge. If you don't keep the temperature of the steel to just over critical, you could actually increase the grain growth. I'm not sure that the advantage of forging lies in the orientation of the steel grain.

 

By the way, don't worry about your English. You're understandable, and if there is a problem with understanding anything that you post, we'll work it out.

 

Doug Lester

 

Hello Lester.

 

I confirm you that forging allows to orienting and refine the structure of steel (lamination, for example, allows to orienting structur of steel but not to refine it. Soure Toll steel, 4th edition and John Veroheven e-mail ).

 

So, I know that during forgin there are the risk of grain growth. I think that, for minimize that and for refine granins during forging we have to control the temperature and don't overheat. I do (or try to do) heavy forging operation at salmon-orange and other operations at orange, red.

 

Best Regards

Francesco

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Yes, you do have to meet or slightly exceed Ac1 to achieve normalization. What I meant was, do not hold it at that heat for too long and you won't get appreciable grain growth. You need to hold it just long enough for total transformation to take place. For simple carbon steels up to 0.1% C, that's just a few seconds. For higher alloys with stong carbide forming elements like L6, it can be a few minutes at or slightly above Ac1.

 

It sounds like you are on the right track with your thinking, so as long as you get the results you want there is no need to worry. :)

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Francesco, I'd like to suggest that you read this article.

 

First, at least as I understand it, "grain" orientation in steel is largely unrelated to the crystalline structure that can be manipulated by various heat treating procedures.

 

Second, grain growth is far more sensitive to temperature than to time. In other words, down around the Ac1 (or even Ac3/Acm, AFAIK) range, grain growth does not happen nearly as fast as many people think. Proper normalization at those low temperatures will reduce grain size, not increase it. (See here for a discussion of some O1 steel that was held at around 1500 F for five hours with basically no ill effect.)

 

Third, proper heat treating can correct grain growth (as metnioned above) and grain boundary carbide segregation.

 

Finally, although forging steel will greatly improve its properties compared to an as-cast ingot, that's of almost no significance unless you're making your own steel. (But some people here are doing that!) If you're like most of us who buy our steel from commercial sources, your steel has already been subjected to massive changes in cross-section at the rolling mill, long before you ever stuck it in the forge. Whatever little taps you may give it with a hammer after that are highly unlikely to improve its quality, although there is a good chance of substantially reducing its quality if you're not careful. :)

Edited by Matt Bower
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Say Matt, thanks for that link on metallurgy. Nicely summed up! Might be a good idea to post that link in the section "Beginners Place".

 

Cheers

Two roads diverged in a road,

And I,

Took the one less traveld by.

-Robert Frost

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Francesco, I'd like to suggest that you read this article.

 

First, at least as I understand it, "grain" orientation in steel is largely unrelated to the crystalline structure that can be manipulated by various heat treating procedures.

 

Second, grain growth is far more sensitive to temperature than to time. In other words, down around the Ac1 (or even Ac3/Acm, AFAIK) range, grain growth does not happen nearly as fast as many people think. Proper normalization at those low temperatures will reduce grain size, not increase it. (See here for a discussion of some O1 steel that was held at around 1500 F for five hours with basically no ill effect.)

 

Third, proper heat treating can correct grain growth (as metnioned above) and grain boundary carbide segregation.

 

Finally, although forging steel will greatly improve its properties compared to an as-cast ingot, that's of almost no significance unless you're making your own steel. (But some people here are doing that!) If you're like most of us who buy our steel from commercial sources, your steel has already been subjected to massive changes in cross-section at the rolling mill, long before you ever stuck it in the forge. Whatever little taps you may give it with a hammer after that are highly unlikely to improve its quality, although there is a good chance of substantially reducing its quality if you're not careful. :)

 

 

Hello Matt.

 

The grain orientation mainly depend to the cold or hot deformation of the steel. Cold or hot rolling or forging.

So, this operation, if properly esecuted, increase strenght and resistenace of steel (nad I thinck density, despite your article suggest the contrary). An exemple is drawn steel, that is more harder and difficult to grind respect annealed steel. Same thing to raw stainless austenitic steel respect annealed stainless austentic steel. This is do to dislocation that form during deformation process.

So heat treatment can restore the "normal" alignment and size of steel's grains and eliminated or decrese the amount of dislocation, but can't align or orientate grain of steel.

 

So, I'm not worried about grain growth. A control of temperature forging and a proper heat treatment can correct a eventual grain growth and in some case allow to obtain a fine grain. Forging operation may reduce the size of grain, if properly conduced. So, don't forget that when you heat the steel at orange and hammering it, the steel undergoes a sort of normalization. So, obviously, if the temperature forging is too high (yellow or white for example), grain growth can occur.

 

So, grain growth is sensitive to time and temperature, but, if you are below critical temperature, (so below Ac1), grain growtu is VERY slow. Grain growth "start" above critical temperature, when are involved ricristalization phemonena. So, over Ac1, the grain growth depend by temperature and time. More high is temperature more time occour that grain growth. So grain gorwth depend also by chemical composition of steel (that involve solute and particle drags phenomena), but is anothet thing.

 

I'm agree whit you and the article that forging can decrease the property of steel, if forging is not properly executed.

Overheating (that cause grain growth) and decarburation are the mainly risck that can occur during forging.

But, in my opinion, that no signify that forged knives are worst respect that obtained with stock removal, but only that forging a knives is not simply operation and request great amount of abily for obtain a better product.

 

And is true that with hammer the deformation of grain and lattice of steel is not very big, but I thinck occur. More big is deformation (related to the force of hammering) more big is the orientation of structure.

 

 

So, on every case the benefit of forging can't be predict with precision, because there are too variables that can't be measured.

 

My question is: "If I do a triple normalization, heating the steel above Ac1 or Ac3, can I lose the eventual forging benefit"?

 

Best Regards

Francesco

Edited by FrancescoB
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Hi Francesco,

 

I think I understood your question the first time. I also think I already responded to it. My understanding of metallurgy -- and I admit that I am not a metallurgist -- leads me to believe that when dealing with good quality, commercially prepared bar stock, hot forging to final shape does not produce any benefits in terms of the mechanical properties of the blade. Therefore, no, triple normalization will not eliminate the "benefits" of forging -- because there are none.

 

Exactly what benefits of forging are you afraid that you're going to lose?

 

I should point out that our terminology has gotten a little mixed up here. A1 is a constant value that marks where austenite formation begins. A3 and Acm are the lines that mark where full transformation to austenite occurs for hypoeutectoid (A3) and hypereutectoid (Acm) steels. Normalizing temperatures are above A3 and Acm, and well above A1.

 

Solid materials cannot be compressed under any conditions likely to occur in your or my shop. A porous solid, or small, loosely packed particles can be packed more tightly together to eliminate empty space, but that doesn't apply to steel.

Edited by Matt Bower
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Hi Francesco,

 

I think I understood your question the first time. I also think I already responded to it. My understanding of metallurgy -- and I admit that I am not a metallurgist -- leads me to believe that when dealing with good quality, commercially prepared bar stock, hot forging to final shape does not produce any benefits in terms of the mechanical properties of the blade. Therefore, no, triple normalization will not eliminate the "benefits" of forging -- because there are none.

 

Exactly what benefits of forging are you afraid that you're going to lose?

 

I should point out that our terminology has gotten a little mixed up here. A1 is a constant value that marks where austenite formation begins. A3 and Acm are the lines that mark where full transformation to austenite occurs for hypoeutectoid (A3) and hypereutectoid (Acm) steels. Normalizing temperatures are above A3 and Acm, and well above A1.

 

Solid materials cannot be compressed under any conditions likely to occur in your or my shop. A porous solid, or small, loosely packed particles can be packed more tightly together to eliminate empty space, but that doesn't apply to steel.

 

Hi Matt.

I refer Ac1 or Ac3, the temperature at the steel change to a austenitic matrix on heating (to ferrite-austenite at usatured austenite for Ac3 and to perlite-secondary cementite at satured austenite for Ac1). Ar1 abd Ar3 are the temperature when austenite transform to perlite or perlite and ferrite (Ar1) or austenite and ferrite (Ar3, for hypoeutectoid steels), on cooling obviously.

 

So exist also Accm and Arcm.

 

So, I refer to Ac1 and Ac3 and not A1 and A3.

 

The benefit of forging whith hammer, in my opinion, are to orientate the grain direction of steel. This is due to mechanical drag of steel, that involve orientation of grains. So, the orientation can be more or less bigger and evident, dependig the amount of deformation and temperature of forging.

 

Other possible benefit of forgin are to decrese the size of grain and non metallic inclusion or undissolved carbides. For decrese the size of grain I think is more simple er efficient use an appropriate heat treatment.

 

So, I think that triple normalisation, heating the steel above Ac1 or Ac3 (or Accm), can eliminate the orientation of grain, becouse involve recristalization phenomena.

 

Regards

Francesco

Edited by FrancescoB
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I have asked a opinion at professor John Verhoeven about advantage of forging, specially if is true that density of steel decrese (and not increse), as write in the article posted by Matt.

 

That's reply of professor Verhoeven:

 

" Francesco,

 

 

Some ideas:

 

 

Forging is done hot, not cold as the article requires. So the dislocation argument does not apply because hot forging causes the grains to recrystallize which removes the dislocations caused by the deformation.

 

 

Secondly. forging has the advantage that it closes up any voids that may be in the metal from its original method of preparation. This definitely reduces brittleness and increases density.

 

 

The effect of dislocations produced on cold work at increasing void space and reducing density is extremely small.

 

 

The only problem with stock removal is that the mechanical properties of the resulting knife will depend on the soundness of the starting material. If the original stock is brittle the knife will also be. Proper forging can insure that brittleness is removed."

 

So, forging increse density, not decrese it. So, I think that the amount of that increse is great if deformation is bigger.

 

Best Regards

Francesco

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I believe what Professor Verhoeven is saying is that if the steel contains voids as received from the mill, then forging can close those voids and thereby make that piece of steel more dense. However, high quality rolled steel should not contain any voids as received from the mill. Assuming that it does contain voids, forging may have some advantage, but that advantage will not be undone by normalizing. (And you will see no increase in density beyond the amount of deformation necessary to close the voids.)

 

As I understand your original question, you are asking whether normalization will eliminate the benefits of forging -- not the benefits on the fibrous structure of the steel caused by elongation of voids, inclusions, etc. (which happens during rolling at the mill) -- but the elongation of individual steel grains that can occur through forging (normally by cold working., though sufficient deformation during hot forging may have the same effect). My understanding is that any benefit to the mechanical properties of the steel that might occur through deformation of the grains of the steel will be undone when you heat the steel above Ac1, which you must do when you austenitize it in preparation for hardening. So I continue to believe that, no, normalizing will not cause you to lose any benefits that you may have gained through forging -- at least not any benefits that you will not lose anyway, when you harden your blade.

 

Since you are in direct contact with Prof. Verhoeven, why don't you ask him your question about triple normalizing, and post the answer here?

Edited by Matt Bower
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Hello Matt,

I have asked to Professor Verhoeven the same question of this topic,but he tell me that is difficult to say and predict.

 

I think is because the effect of recristalization process that occur above Ac1 on the eventual elongated structure of grain dependig to hold time, temperature and how bigger is elongation of grain.

 

In every case, void and other defect are always present in the steel (the crystalline structure, formed by millions of lattice, are always imperfect).

 

So, despite I have the opinion of professor Verhoeven and I have a idea that how happen when heat the steel above Ac1, I was curios to know the opinion of some bladesmith. Thera are always something to learn. :rolleyes: .

 

So thanks at all for your opinions :) .

 

Best Reagards

Francesco.

Edited by FrancescoB
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I think is because the effect of recristalization process that occur above Ac1 on the eventual elongated structure of grain dependig to hold time, temperature and how bigger is elongation of grain.

 

In theory, you're right that the degree of recrystallization depends on how much deformation has occurred, the temperature to which the steel is raised, and how long it is held there. In fact this is graphically illustrated in Dr. Verhoeven's book on metallurgy for bladesmiths, in a pair of before and after photos of steel that had undergone severe reduction and grain elongation by rolling, followed by annealing. (For the record, I printed a copy of his book years ago.) Without knowing those parameters, we may not be able to say whether any elongation of the steel grain may remain after a thermal treatment. I suspect that this is also why it's difficult for Prof. Verhoeven to give you a concrete answer to your question; in order to give you a solid answer he'd need to pin down some variables.

 

But in the real world in which bladesmiths operate, I would think it should be possible to make some reasonable assumptions about things like forging temperatures (mainly above Ac3/Acm), the amount of reduction in cross-section per forging heat (maybe 10%-20%, as a guess, for me, using hand hammers, perhaps significantly more for people who use power hammers and presses), austenitizing temperatures (approximately 1500 F for the simple, roughly eutectiod steels that I use) and hold times (minimal in my case, which is why I use roughly eutectoid steels, but often much longer for bladesmiths who have good, tight temperature control).

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Francesco, I think that you are trying too hard here and your agruements are starting to get circular in nature. Maybe you might do better by putting the issue aside for a little while. Without reguard to what the process will or won't do to any effects of forging there are some things that must be done in heat treating a steel blade. One is that it must be heated to where the steel is at a stable austinetic state throughout and be in that state when it goes into the quenchant to achieve conversion to martensite. Another is that the steel must be austinized and allowed to cool slowley to reduce the stresses in the steel caused by forging. That last process, refered to as normalization, also has the advantage of causing the steel to form pearlite instead of martinsite and, as a result, be in a "softer" state for tooling. I know that steel metalurgy can be a bit to get one's mind around. That's one of the reasons that I have one of Dr Verhoeven's books by my bed and refer to it often. Hang in there and keep hammering.

 

Doug Lester

HELP...I'm a twenty year old trapped in the body of an old man!!!

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For Matt.

 

Matt what I would is not a perfect prediction of the effect of recrystalization on the structure of my forged blades, but only a reasonable opinion about that.

 

So, I can give more information. I use car spring (made on UNI 55Si7 or AISI 1060 or other low alloyed spring steels). I do the "heavy" forging operation, like give the shape to point and tang, at salmon-orange color. So, I forge the bevel at orange or red. More I approach to final shape, more lower is the forging temperature. I do the final forging operation at dark orange or red.

 

I have do some tests, and I have see that at dark orange the steel that I use is magnetic, so, even assuming a strong decarburation, the temperature shold be below 760°C (Curie Point).

 

The thickness of steel bar is around 5-6 mm, and the final thickness of the edge will be of about 1 mm (or, in spome case, less).

 

I try to don't overheat the steel, esapecially during the final operation, for not invoolve grain growth and excessive decarburation. For the last reason i try to put the blade on the mid of hot carbon, at 6-7 cm above the air grate.

 

For Doug.

I can't avoid to do austenization for hardening (a annealed or normalized blade is not a good blade :D ), but I can avoid to do triple normalization. I can do a sub-critical normalization (stress relief) for decrese internal stress, that not involve recristalization process of normalization.

 

 

So, I would know if multiple austenization (for triple normalization or triple hardening or other) after forging can be eliminate the eventual benefit of forging becouse I think there are some heat treatments that can be very effective to reduce grain size and cardibe size, parameter very important to improve strenght, toughness and edge retention, but this process (triple normalization, triple hardening and other heat treatment process) involve austenization.

 

At this point I am very curios to know what are the real benefit of hand forging.

 

 

Another thing. I have a lot of metallurgy books (Metallurgy for non metallurgist of John Verhoven, Tools Steel simplified, Tool Steel 4°Edition, Steel Heat Treatment 2nd edition, and others book on english and italian language), but some information are difficult to find and I would know the opinion of other person about some dubt that I have.

 

Regards

 

Francesco

Edited by FrancescoB
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Francesco, on triple normalizations or triple quenching. Those techniques are use to refine the grain of the steel after forging. There are those of us who don't get too worked up about grain growth during forging because it can be corrected for later and those two methods are what is used. I have used both on the same blades but now I just normalize three times after I finish forging the blade and again three times after grinding immediately before I quench and I've found that this will give the steel that I use a fine grain. Here I use the word grain to signify the degree of courseness that is caused by crystaline structure of the steel. That said, I didn't recognize any problems caused by tripple normaization and tripple quenching together, I just decided to avoid as much as possible the microscopic cracks that quenching can cause. Three more quenches was three more opportunities for these cracks to occure. There is also a feeling out there that with shallow hardening steel that the grain can be over refined which can prevent the blade from holding an edge.

 

Doug Lester

HELP...I'm a twenty year old trapped in the body of an old man!!!

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Hello Lester, thanks for your reply.

My "fear" is that multiple austenization required for triple normalization or triple hardening plus austenization for final hardening can lose some advantage give by forging process.

 

I refer mainly to the orientation of grains (on forging direction) and increase of density that forging can involve.

 

But now is on discussion that forging can have benefits like those on steel structure so I have to understand if that benefit are real or not.

 

Why austenization can lose benefits of forging? Because involve recristalization (nucleation and formation of a new set of grain and redistribution of iron and alloing element atoms during change to BCC lattice to FCC lattice).

 

 

Best Regards

Francesco.

Edited by FrancescoB
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Having done both stock removal and forging of blades, I personally think the only advantage of forging is the ability to change the shape easily...I can't tell any difference in edge holding between the two processes. Also, not normalizing (or insufficient normalizing) tends to lead to serious blade warpage, in my experience. I found I HAD to normalize 2-3 times, especially with forged pieces, or I could not get larger blades to stay straight when hardened.

 

I am no metallurgist, and of course this is just my opinion, but it is based on doing a lot of knife and tool testing. If you are getting good results from using only sub-critical normalization, then stick with what works for you...

My hand-forged knives and tools at Etsy.com: http://www.etsy.com/shop/oldschooltools

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Hi Orien.

Thanks for your reply.

 

A couple of my large forged blade NOT normalized bend (negative sori)but I think it was for differential hardening.

 

A little provocation :rolleyes: . Maybe your forged knives don't show difference than your knives realized by stock removal for the triple normalization, becouse you have lose vbenefit of forging?

 

So, It's is a provocotion, A joke :rolleyes: .

 

I think is very hard understand what are the effective advantage of forging on a blade if tha test are not made using two identical blade, one forged, the other realised by stok removal method, with the same steel and heat treatment.

 

Heat treatment, shape of blade, type and shape of bevels, type and degree of sharpening (etc) are things that influence edge retention and edge holding.

 

So, my question is only for try to have a major number of opinions and information for study and make the better treatment and lavoration for my knives.

 

Best Regards

 

Francesco

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Hello everyone,

nice and interesting forum topic.

I wanted to add: a heat treatment to refine the grain can only return to the grain size of origin, can not go below that size. The grain size is determined by the type of steel.

On the specific question I can not answer with precision. I can only say based on my knowledge and experience that the benefits of the arrangement of the grains had to be operations like rolling or hammering it should not be removed.

A manufactured good is cut along the "fibers" and rolling along the beam. I'm not so sure that normalize for two or three times is better, remove the internal stresses and to refine the grain first time I think that's it. Some types of bolts used in airplanes or in hydraulic hammer for construction equipment, obtained the head of the hot rod rivetted to the press. The reason is that the arrangement of "fibers", an inexact term, but in common usage, increases by 20% the strength of the rod. Following normalization and then tempering treatment. If you are not running this cycle, the head of the rod may break. It is convenient not to cut the fiber with machine tools, in fact over-the metal is very little left. Risks are the forge that I see others, decarburization is a killer for the steel, precise temperature quenching and manual cycle, I would be more afraid of these operations. Above Ac1 eliminates tension, above Ac3 you homogenized and redistribute the structure, but the balance is getting better, too homogenize means losing some benefits, money for example :)

In maraging steels to increase hardness and strength just use the hammer, no machine tools.

In the end I think the arrangement of the fibers, is a process of physical deformation of the material, this process is irreversible with heat treatment. A sword is always beaten with a hammer better than a sword cut to machine tools. Refine the grain structure and redeploy the three times is not necessary.

 

All Best

Maurizio D'Angelo

Edited by Maurizio D'Angelo
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  • 4 weeks later...

Personally for many industrial applications and for control of distortion and residual stress, I happen to prefer a spherodation anneal. Nice fine grain, uniform response to heat treatment and low distortion. I think several people here have tried it and liked it. One of the really nice things is that it removes the forging residual stresses.

D. Scott MacKenzie, PhD

Heat Treating (Aluminum and Steel)

Quenching (Water, Polymer, Oil, Salt and Mar-Tempering)

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