PDA

View Full Version : Heat treating mild and low carbon steel.



Forrest Addy
10-16-2013, 02:06 PM
Many times you need a little extra durability for surface wear for a piddling project but the only material you have handy is low carbon steel. There is case hardenng but who wants to mess with it for a teeny batch of nuts or the threads on a shaft for a bench grinder?

Carbon is a big player in steel. A little carbon in iron makes steel; i samm proportions it enhances the properties of iron. Heat treat 40 point (0.4 percent) carbon steel and it will harden right up. Less than 20 points carbon steel the low carbon concentrations doesnt do much regardless of heat treatment - but it does to something and that something can be useful.

Some years ago HSM published an article on this very topic. The author discussed heat treating low carbon steels and the use of plain water as a quenching media; the effct of detergents, salts etc has on its quenchng action. "Super quenching" I think its called. He conducted tests and made photos and charts of his results. It was an impressive article.

I've heated and quenched mild steel myself seeking a little more than dead soft properties. It works. A thin "case" a film of enhanced properties can be attained. If one were to heat to red then quench a piece of mild steel in violently agitated ice cold water, and then sectioned and etched the sample, a film of materail that responded to treatmentl can be cearly seen under magnification.

This is not to say the strength of the bulk of the material is significantly altered but some enhancement to surface durability and hardness in thin parts is very possible.

The second point I wish to make is to suggest, urge, plead, nag, beg, whine HSM to publish their past issues as a resource library on their website. I went looking for the very article I referenced in my library (boxes of stuff) and can't find it.

Arthur.Marks
10-16-2013, 02:50 PM
I went searching through the article index at the HSM mag. webpage. Is this it?

Heat Treating Steel - An Amateur's View
Constantine, Randolph
Subject: Metallurgy
Issue: HSM Vol. 22 No. 6 Nov-Dec 2003

OR

Heat Treating Basics
Acker, Steve
Subject: General Machining Knowledge
Issue: HSM Vol. 10 No. 5 Sept-Oct 1991

J Tiers
10-16-2013, 09:51 PM
There may be a laboratory-detectable effect......

I have never gotten any benefit whatever from the microscopic layer of "a-bit-harder-than-chewing-gum" stuff on any mild steels.... I did try it, but IMO it sure ain't worth the fuss of getting out the torch.

There DID seem to be some effect, but if that was real, and not wishful thinking, it was rapidly worn through and over with.

Better to case harden, or use a hardenable material, IME.

CCWKen
10-16-2013, 09:52 PM
I've hardened mild steel enough to use as the shank portion of my planishing hammer dies. The hammer uses 1/2" square shank tooling and I needed to make a couple of hammer dies. All I had on hand was some 1/2" square stock salvaged from a gate. After turning down a round area of the shank for the pin, I welded it to a disk made of W1. The first trial broke at the pin to shank transition (photo below) because I didn't harden far enough down the shank. I only run the hammer between 30-50 psi but I've been using the subsequent dies for nearly 10 years. I see no need to upgrade to tool steel. :)

Added: Note that the pin takes the hammer force and you can see that it held up. The shank started to mushroom and the pin broke at the transition. I don't know how long it took to break. Some time between 0 and about three hours of use when I changed the die.

http://img.photobucket.com/albums/0903/CCWKen/ToolBox/P-Hammer/hammerdie.jpg

Jaakko Fagerlund
10-16-2013, 10:26 PM
Have to try this.

ironmonger
10-16-2013, 11:19 PM
More to consider:

http://www.cvbg.org/superquench.PDF

(http://www.cvbg.org/superquench.PDF)
(http://www.cvbg.org/superquench.PDF)

darryl
10-17-2013, 12:09 AM
I've wondered about quenching under pressure. If you were using induction heating, it would be doable. Seems like it would increase the cooling rate.

Another thing I've wondered about is spraying the quench medium at the part instead of simply dipping it (with the requisite figure 8 pattern). You would design the spray pattern to surround the part equally from all directions, and probably have a mechanical means of holding the part and transferring it from the flame to the quench tank.

Jaakko Fagerlund
10-17-2013, 04:05 AM
More to consider:

http://www.cvbg.org/superquench.PDF

(http://www.cvbg.org/superquench.PDF)
(http://www.cvbg.org/superquench.PDF)
Funny that they advocate it lye-free solution, but when one reads the ingredient label on the Shaklee Basic, it contains potassium hydroxide. So i don't see any difference in the safety of use.

Always wear protective gear and a full face mask when quenching, even water will hurt you when it is boiling if you are not protected.

But, one thing I'm not sure I understand here. What is the point of this, when a regular carbonising takes just about the same time and effort than this? In both you have to heat up the part to the required temperature and in both you quench (with few exceptions). The other gives tough result, the other hard.

ironmonger
10-17-2013, 08:30 AM
Funny that they advocate it lye-free solution, but when one reads the ingredient label on the Shaklee Basic, it contains potassium hydroxide. So i don't see any difference in the safety of use.

Always wear protective gear and a full face mask when quenching, even water will hurt you when it is boiling if you are not protected.

But, one thing I'm not sure I understand here. What is the point of this, when a regular carbonising takes just about the same time and effort than this? In both you have to heat up the part to the required temperature and in both you quench (with few exceptions). The other gives tough result, the other hard.

The Basic I bit struck me as well... just goes to show that the safety nazis were pretty easy to satisfy.

The rapid cooling does not add carbon, it only allows the carbon that is in the metal to achieve hardness by dropping the temperature rapidly enough to lock in the carbon structure.
The cooling or quench rate is what is important. The cooling rate is affected by the boundary layer of steam that forms when you try to quench something. The added chemicals help reduce the formation of the boundary layer. Oil is no good for lower carbon steels as it cools at a slower rate than water.

You have to get the metal below it's critical temperature inside of the time determined by the carbon content. Higher carbon content allow more time to drop from the carbon solution temperature to lock in the hardness structure. Martensite and Austenite... I'm not a metallurgist obviously.

start reading here:
https://en.wikipedia.org/wiki/Isothermal_transformation_diagram

This rapid cooling process results in a hardness that you would not normally expect from 1018ish steels.

My personal observation of the behavior of light weld tacks might be an effect caused by rapid self quenching of a small weld puddle. They seem to be very fragile... it could just be my ham handed expectation of the tack being much stronger than it is...

paul

Jaakko Fagerlund
10-17-2013, 08:39 AM
ironmonger, I think you misunderstood me. I meant what is the reason for doing this kind of hardening, as a harder/better result can be had with carbonising which basically requires about the same time as this method.

I very well know hardening processes and especially carbonising, so there is nothing mysterious in the process itself. I'm just asking why would one do it.

Wayne Sippola
10-17-2013, 08:44 AM
I read the HSM article - seems to me it was around 5 years ago, but I'm not sure. From what I recall, the original quench solution was water, salt and dishwasher detergent. After some discussions with his daughter (?), they decided the salt really wouldn't do anything, and used just the water and detergent. The theory was that the detergent reduced the surface tension of the water allowing a faster quench. I have the article somewhere on the shelf, so if I get a chance today, I'll look around for it.

Wayne

ironmonger
10-17-2013, 08:52 AM
I believe that the rapid quench, if it hardens the metal enough for your intended use, would be faster that carbonizing. No soak time for the superquench. A lot of my bias comes from the blacksmithing side. I'll use 1045 or 4140 for small parts cause I forge them close to shape first and the file, machine and polish before hardening. I have an induction heater in my shop, and many times it's easy to flip the switch, forge to shape and then harden. The heating is rapid and easy to control.

I have nothing against case hardening, only that the most expedient method might be useful. Won't get the pretty colors though... :>)

paul

DR
10-17-2013, 12:23 PM
As for torch heating, does that leave a scale on the part after quenching?

The shop heat treating I've done, O-1 for instance, left a scale when heated to temp in a small furnace. I do have a powder compound (forgot the name), you slightly preheat the part and dip in for a protective coating, but hard to remove if threaded holes are present. I also wonder if the quenching is lessened by the "insulated" coating.

Jaakko Fagerlund
10-17-2013, 02:36 PM
I believe that the rapid quench, if it hardens the metal enough for your intended use, would be faster that carbonizing. No soak time for the superquench. A lot of my bias comes from the blacksmithing side. I'll use 1045 or 4140 for small parts cause I forge them close to shape first and the file, machine and polish before hardening. I have an induction heater in my shop, and many times it's easy to flip the switch, forge to shape and then harden. The heating is rapid and easy to control.

I have nothing against case hardening, only that the most expedient method might be useful. Won't get the pretty colors though... :>)

paul
Yeah, this might have its uses, but then again one could just use prehardened as it is almost the same in hardness and the base metal is better than mild steel. 43 HRC is still drillable with HSS, so it isn't very hard.

And case hardening doesn't bring you any colors, as it is a hardening method and not a coloring technique. What you meant is color case hardening, technically it is case hardening but is much more work and usually uses mixtures of stuff not needed in case hardening to get more specific colors.

PStechPaul
06-13-2014, 06:36 PM
I may need to harden the steel adjusting screws I'm making for my chuck, and I have some 12L14 and 4140 I can use for this. In my search for information, I found that 12L14 is not considered a hardenable steel, but it can be case hardened by carburization or using a hardening compound such as Kesenite or even bone meal. Here is an article in PM that goes into much detail, and also discusses the concept that heat soaking the material in a partially sealed container allows some pressure to build and maintains a higher concentration of carbon to be introduced to the steel:
http://www.practicalmachinist.com/vb/general/how-get-20-30-thou-case-hardened-depth-167146/

I will probably opt for using the 4140 even though it will be a little (or a lot) more difficult to machine, but I will have a lot of time invested in these parts and I want them to last "forever". I figured it would be better to add to this not-too-old thread and maybe discuss some of the information from the PM thread.

boslab
06-13-2014, 07:28 PM
Carbon can migrate from one place to another, even from one peice of steel to another, at work we had a problem getting the carbon levels down to less than 0.0035% at the vacuum degasser, it was a nightmare, i found the problem after quite a while, a sample of the steel was sucked up into a sampling mould, it consisted of two pressed steel moulds held together by a clip, vacuum is applied after sticking it into the molten steel, it turned out that the moulds were pressed from medium carbon steel, carbon actually migrated from the mould to the sample making us think the carbon was too high, when i machined the skin right back i found the core of the sample was less by a country mile, well 0.0010.
Got the steel shells made from ultra low carbon steel, problem solved.
Heating and quenching low carbon steel can and does affect hardness, not a lot but the effect is there, the carbon may migrate to the skin, if you catch it right the grains can be somewhat harder, think it a phase called upper bainite but thats from memory, you can change the grain size with a quench, there may be a bit of manganese in there too, it can get very complicated, nitrogen can get into the skin and make nitrides between the grains, that increases hardness too
All good fun, low carbon steel ion nitrides nicely too
Mark

darryl
06-14-2014, 03:10 AM
I haven't found it to be much of a chore to grind a red hot part into a container of hardening powder. I've done that many times and ended up with a surface that deflects a file. The usual procedure (my procedure anyway) is to heat to red hot, then grind the part around in the powder, reheat and grind again, repeat- then clean it up before heating again and quenching. Clean again, then reheat for tempering, etc. I have no idea of the depth of case, but if we're talking about some undetermined measure of abrasion resistance, this has always worked for me. A spike can easily be hardened to the point where a file will just skip over it.

Obviously it's going to depend on the shape of the part and the area that needs to be hardened.

bob ward
06-14-2014, 06:14 AM
If I have a piece of mild steel that needs to be 'hard' I use Cherry Red case hardening powder.

https://www.youtube.com/watch?v=9tlsq2ESQz0

boslab
06-14-2014, 09:27 AM
Normal case hardening, kasnit, cherry, coal dust, bones and stuff, does the job
Mark

Jaakko Fagerlund
06-14-2014, 11:09 AM
Normal case hardening, kasnit, cherry, coal dust, bones and stuff, does the job
Mark
Please don't use bone, it contains mainly phosphates which do not any good to the steel. Just use plain carbon like used in BBQ's (not briquettes).

boslab
06-14-2014, 12:48 PM
Ok i wont use the bones anymore, means i have to get a doog instead!, i heard kasnit was a closely guarded secret for years, don't know how much truth there is in that though!
Mark

RWO
06-14-2014, 01:12 PM
Seems to me that heating a piece of low carbon steel to red heat in air with a torch is going to decarb the surface so that there is nothing left to contribute to surface hardening. Maybe nitrides are forming to provide whatever hardening some people claim to detect.

RWO

Rich Carlstedt
06-14-2014, 02:14 PM
I may need to harden the steel adjusting screws I'm making for my chuck, and I have some 12L14 and 4140 I can use for this. .................................................I will probably opt for using the 4140 even though it will be a little (or a lot) more difficult to machine, but I will have a lot of time invested in these parts and I want them to last "forever". I figured it would be better to add to this not-too-old thread and maybe discuss some of the information from the PM thread.

good choice Paul !
100 % !
No worries whether it will harden, it will !
Just remember to temper it after HT to about 375 F as I recall .
You can always take it to a HT shop as well, they do lots of 4140 HT and may give you a bargain.

Rich

planeman
06-14-2014, 02:18 PM
One easy method to add carbon to the surface of steel IF YOU HAVE AN OXY-ACETYLENE TORCH is to "flame harden". Flame hardening is akin to case hardening as it adds carbon to the surface of the steel. Flame hardening is easily done by adjusting the flame of the torch from the usual balanced flame (where the two cones of the flame are adjusted to meet) to where there is an excess of carbon produced by the burning flame (too much acetylene). Oxy-acetylene torch users often see this excess carbon produced when lighting off the flame. It is the fine black powder that wafts through the air. Heat the surface of the steel with the torch in this condition until the surface becomes shiny (begins to melt) and continue passing the flame over the steel when it it is like this. You can do a small local area or a fairly large by slowly shifting the torch to a new area. Do this for about 30 seconds to a minute after the steel reaches the shiny stage. The longer you play the flame over the steel the more carbon you add to the steel surface. I watched a huge gear about 5 ft. in diameter have its gear teeth hardened this way back in the 1960s at a LeTourneau-Westinghouse plant that makes gigantic earth moving equipment. So obviously it works well enough for them. Planeman

boslab
06-14-2014, 09:36 PM
You can look at your bits of steel or whatever a little more scientificly using a bit of basic metallography, not difficult and quite interesting too, at least you can see whats going on, does deed a bit of 400 grit silicon carbide, a peice of glass and a trickle of water, no machines needed, will need etching with dilute nitric acid, but there are loads of other etchants like citric, vinigar, lemon juice, tea!, at least the way i make it, theres a whole hidden world there, no microscopes required!
http://youtu.be/fc8zrgYJCJw
Mark

darryl
06-15-2014, 01:26 AM
Hmm- very interesting-

PStechPaul
06-15-2014, 02:05 AM
The microscopic examination is very interesting as well. It's in German so I did not understand much of what was said, but I got the general idea. There are many more steps to the process, mostly progressive grinding with fine abrasives and then etching. I think the guy made a mistake by leaving the water running at one point:


https://www.youtube.com/watch?v=UmH5QcT7Vnk

Jaakko Fagerlund
06-15-2014, 05:02 AM
Seems to me that heating a piece of low carbon steel to red heat in air with a torch is going to decarb the surface so that there is nothing left to contribute to surface hardening. Maybe nitrides are forming to provide whatever hardening some people claim to detect.

RWO
It takes quite some time to decarb it, especially since it is inside an oxygen free flame. The harder skin to a steel like S355 comes from actual case hardening due to the carbon content, not from nitrides if you heat it red and drop it in ice cold lye water. Also have been hardening W.Nr. 1.2510, a basic cold working tool steel without any issues of decarbing due to open flame or non-vacuum oven.

planeman, you are correct in that oxy-acetylene can be used to induce more carbon to a steel and it is flame hardening, but flame hardening usually has meant heating and quenching continuously a steel work piece that already has enough carbon in it to case harden. For example cast iron lathe beds are frequently flame hardened (though today they use induction hardening as it is cleaner & cheaper), meaning they pass a flame over the area and have a water jet trailing behind which quenches the heater workpiece.

If you use oxy-acetylene as your only source of extra carbon for the workpiece, expect nothing but a superficial surface hardening, meaning you can't grind it afterwards or you'll lose the hard outer skin as it is so thin.

Mike Hunter
06-15-2014, 10:16 AM
Forrest is one of those guys that when he starts talking I just usually just shut up, sit back and listen.

Low carbon steels with less than .02% carbon don’t have enough inherent carbon to self-harden. I have not read the HSM article that Forrest is referring to, but looking at the article by Robb Gunther, I have to raise the BS flag.

As we all know the old pack hardening process of case hardening involved packing low carbon steel with wood and bone charcoal, which is pretty much pure carbon, then heated. At around 1100 deg F steel begins to have the ability to absorb carbon, this carbon is in the form of carbon monoxide and carbon dioxide, which the wood and bone char off gasses at higher temps.

There are direct relationships between temps and the steels ability to absorb carbon, the higher the temsp the more readily the steel absorbs gaseous carbon. It takes several hours for the steel to absorb the carbon to any depth. I don’t see how steel can absorb any carbon in the fraction of a second it takes steel to cool during quenching.

Looking at the “super quench” recipe, the lye, detergent and salt; in this application I would call them wetting agents, making water wetter, allowing the water to cool the part more rapidly than just plain water. There is no carbon in the quench, so where does this additional carbon come from, it didn’t just magically appear.

So the three areas that I have questions are:

Where does this additional carbon come from?
How is it that the steel can absorb carbon during quenching?
How is it that steel can absorb this carbon so quickly?

Mike

Jaakko Fagerlund
06-15-2014, 12:31 PM
Looking at the “super quench” recipe, the lye, detergent and salt; in this application I would call them wetting agents, making water wetter, allowing the water to cool the part more rapidly than just plain water. There is no carbon in the quench, so where does this additional carbon come from, it didn’t just magically appear.

So the three areas that I have questions are:

Where does this additional carbon come from?
How is it that the steel can absorb carbon during quenching?
How is it that steel can absorb this carbon so quickly?

Mike
You are absolute correct, the term wetting agent is the key in the recipe.

The method doesn't increase the carbon content, it is at the low 0.2-0.25 % as is usual for structural steels like S355 (not 0.02 % like you said, possible typo?). It just quenches fast enough, and that's it. The hardenability of a material depends not only on the alloying components but on the cooling or quenching rate you can get.

For heavily alloyed steels like HSS are, only an air blast is sufficient to harden. For most tool steels you only need light oil to quench fast enough. For case hardening medium to high carbon steels water is necessary to achieve fast enough cooling rate.

As you can see, the less carbon or alloying elements the steel has, the more steep the cooling rate curve has to be. Low carbon steel thus has to have an extremely steep/fast cooling rate for it to harden. The only cheap & easy way to achieve that is with wetted water that is as cool as possible. Basically meaning that the best option is lye in water and the water having a temperature of 0 įC.

Of course the result will not be as hard as properly case hardened material, but it is way harden than the material in as-supplied condition.

Please do test it out, last time I just used as cool tap water as I could, mixed in some ice slush and poured in lye about 5-10 %. Heatedd the S355 steel parts in a wire hanger, hold them red hot for a few moments and dipped them quickly and moved them around until they were cooled down enough. Quick test with a file confirmed that a hard layer had formed.

It is so easy, cheap and fast to test it out, that I urge anyone to try it. It doesn't replace proper case hardening (which doesn't require bone in any shape, size or form), but provides a nice addition to quickly getting something more durable than plain steel. Have used this tehcnique for several small rolls and pins that benefitted from the hardness.

boslab
06-15-2014, 07:24 PM
I would think that there will be a phase change between alpha ferrite to austinite at the surface, similar to a peritectic transformation, the face centred cubic goes body centred, the amount of carbon that can be held by the crystal decreases releasing more for other crystals so a slightly harder layer occurs, there is also a shrinkage, thats what causes quench cracks too
Mark

Mike Hunter
06-15-2014, 10:35 PM
Jaakko

I fully concur that water temps have a significant impact on case hardening. I will often adjust the quench water temp depending on the part and its potential to warp or crack.

I donít add anything to the water when I case harden parts as I donít know the impact of the additives. Plain water has a known cooling rate, water with a cup of added salt is unknown, much less the impact of dish soap, lye, and salt.

I have seen the effects of case hardened parts quenched in a brine solution, they tend to be extremely brittle, and as Mark noted, cracking.
With the addition of salt, lye and other ingredients you can lower the freezing point of water to well below 0 C. But not something Iím willing to try on any part that I want to use.

Proper case hardening of low carbon steels is well understood and documented, super-quenching Önot so much.

Jaakko Fagerlund
06-15-2014, 11:47 PM
Jaakko

I fully concur that water temps have a significant impact on case hardening. I will often adjust the quench water temp depending on the part and its potential to warp or crack.

I don’t add anything to the water when I case harden parts as I don’t know the impact of the additives. Plain water has a known cooling rate, water with a cup of added salt is unknown, much less the impact of dish soap, lye, and salt.

I have seen the effects of case hardened parts quenched in a brine solution, they tend to be extremely brittle, and as Mark noted, cracking.
With the addition of salt, lye and other ingredients you can lower the freezing point of water to well below 0 C. But not something I’m willing to try on any part that I want to use.

Proper case hardening of low carbon steels is well understood and documented, super-quenching …not so much.
Yup, that temperature is a one thing that an old (1920's) book here in Finland about case hardening warns about, especially saying that it shouldn't be below 10 įC or the parts can become too hard and crack. The super-quenching is one of the things listed in the book, along with some recipes and warnings and also stresses out that it only provides a small layer of hardness, not as good as proper case hardening will but something that makes a part last better.

Our own national epic "Kalevala" describes a lye solution that was used for hardening low carbon steel used in agricultural tools or some such. That book of course doesn't go into more details as it is just a collection of historical poems and such, but just provides a nice window to see in to the past :)

boslab
06-16-2014, 01:29 AM
Somewhat related my dad was a blacksmith and insisted that the quench bath improved with age, it was added to with a handful of lime, quicklime that is, apparently to stop the scum and keep it sweet?
Also how you stuck things in there was considered important, just poking it in, no good, wafting it about vigorously good as the bubbles that formed by steam on the surface got displaced, scythe and sickle blades at an angle upside down as they did a natural curve up, a curved down scythe digs in all the time
My mum was quite handy with a scythe, used to cut the grass verges for free hay!
Don't times change lol
Rate of quench is important, there is no such thing as heat to red hot and quench!
The transformations in steel, wait, metals in general take time to occur hence a Time Temperature Transformation graph for just about every known commercially available steel exists, TTT for short, the heating of the steel is important, give it time, the cooling more important.
http://www.bondhus.com/metallurgy/body-6.htm
An interesting site, not bad allen keys either!
Mark