Differentially Hardening question

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KyKaMo

Joined
Mar 31, 2012
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Hi,

So I have made a few knives and am now going to be making a sword (a wakizashi). I have been trying to decided on the best differential heat treating method for it.

On my knives, I used the soft back draw with a torch after through hardening and tempering in the oven. I like this method because it leaves the back(spine) at around 55rc so when i need to smack the back of the blade with a hammer like object to help chop through something it doesn't leave my blade dented. Unlike the super soft 40-45Rc backs you get from differentially hardening a blade. But i heard that, that technique can just differentially temper a "skin" and not through the whole spine thickness. Giving me a false perception that I can bend my knive 90 degrees if i ever needed to for some reason (it's not like i have the balls to try that test on one of my hard worked knives yet so this is an issue).

So this is my question; can you control the hardness (say to 55rc) of the spine using the differential hardening clay technique, by the amount or consistency of clay applied? Or is there any way at all?

Thanks
 
I will first temper my answer by stating that you have a lot more practical knowledge than I do about knife making. I am doing this as a hobby and I am just at the point of starting my heat treatments. I have thought about the same question and here is what I came up with; the clay coated simple high carbon steel blade leaves the back soft due to the pearlite and a hamon along the area of martensite and pearlite. This also results in the characteristic upward of the blade because of the different densities (volumes) of martensite and pearlite. It seems that the pearlite can take the compression. If you were to leave the back of the blade uncoated or with less of a coat of clay you could get martensite to form but you would have pearlite in the middle. The martensite on the back would be expanding against the martensite on the edge and the pearlite in the middle would be resisting both. I think the blade would break.

If you were to forge weld a piece of say 1050 steel to a piece of 1084 steel along the length of the blade. You could forge the 1050 as the back and the 1084 as the edge. If you tempered the edge to the high 50s then the back should temper to the low 50s high 40s (HRc). Since the back will have a higher ratio of RA to martensite you might get a bend in the blade. I think you would lack a hamon though.

Again, I am new to this and just trying to put what I have read into perspective. I hope others more knowledgable than myself will chip in to correct any misconceptions I have or verify those that are correct.
 
Hmmm, thats good theory. I have never had a blade bend on me because of martensite and pearlite formation before.

Oh if only we could post this on a forum where people more experienced and intelligent than us could help us out...
 
KyKaMo said:
So this is my question; can you control the hardness (say to 55rc) of the spine using the differential hardening clay technique, by the amount or consistency of clay applied? Or is there any way at all?

Thanks
Click to expand...

Is it possible? Yes. Is it easy to get consistent results? No.

Theres a lot of variables involved... refractory properties of the clay, thickness of the clay, how far down the blade you apply the clay, the heat you quench at, thickness of the steel, quenching fluid, quenching fluid temp, how fast you agitate it, etc.

Getting a consistently accurate rc on the spine of the blade is about as easy as being able to duplicate a hamon...and as you've probably seen, hamons tend to be unique because the variables change from blade to blade.

You can easily get a softer spine with the clay coating method but getting a consistent rc is difficult....the rc of the spine will usually vary a few rc points. With some practice you can get it to consistently be either a soft spine, or a springy spine at will but usually the rc will vary a bit.

I've done probably around 800-1,000ish hamons and they still fluctuate a fair bit.
 
They will happen along. Probably have better things to do on a Saturday night. Oh ecos posted while I was writing this.

You might fill out your profile as many posters seem to be more comfortable replying when they know something about the original poster.

If you get impatient there is a lot of information in the stickies.

" I have never had a blade bend on me because of martensite and pearlite formation before.

"

My understanding is that the clay coating along the spine of a sword slows the rate at which the steel underneath it cools. This allows the pearlite to form instead of keeping the austenite. Along the belly of the sword, given a fast enough quenchant, austenite is retained. As the blade cools to a certain temperature, assuming the cooling rate is rapid enogh austenite starts changing into martensite. The density of martensite is lower than the density of austenite and the volume of the martensite is greater than the volume of the austenite it was made from. I think the pearlite just shrinks a little due to the cooling of the metal. Therefore, the spine gets shorter and the belly gets longer resulting in the, best case scenario, tip raising up and the characteristic upward bend of a katana as an example. This is for a simple high carbon shallow hardening steel. It might be impossible to accomplish the same thing with a deep hardening steel like 5160.
 
There is also various yielding going on as the blade cools. At different temperatures the steel has different yield strengths. The part that cools first gains its yield strength and becomes relatively hard. The part that is hotter and cools later is still wanting to shrink but the steel that has already cooled resists that shrinking and thus you get a curve. And this also leaves you with residual stresses in the steel. You can get the residual stresses out by annealing but you don't want to do that to a blade. I'm not familiar with what you can do with cold treating.
 
The hardness and the yield strength ( breaking point) are different things.

The yaki-ire on a wakizashi is done to form the hamon and leave a pearlite spine. This makes the sword less likely to snap ( pearlite has a different yield point than martensite) and the edge hard but chippable ( martensite has more hardness, but lower yield strength). This is a good marriage for a cutting sword, but not a necessity for a general sword.

You can:
A)Do the standard clay treatment ( yaki-ire) and get a hard edge, hamon, and soft pearlite spine.
B)Do a full quench in oil, temper to Rc58, then set the blade edge in a cookie sheet with 1/4" of water, and draw the spine down to Rc 52-55 with a large torch.
C)Fully harden the blade with an oil quench, and temper twice to Rc55-57.

(A) is for a traditional blade that will be used to cut. It will require more work in polishing and maintenance. This has a lot of work and a high failure rate in the beginning.
(B) is for a tough sword that will be for fun and camping. Fairly straight forward and simple.
(C) is for a nicely polished sword ( sans hamon), using the standard knife polishing techniques, that will work well and look good. This is the simplest method, and good for a first attempt.
 
Thanks for your posts.

Bladsmth: If i did a) and then oven tempered to 58rc, would that effect the spine at all? like make it softer?

And if i wanted to try the tradtional clay method (a)) with more of a springy spring than soft, what would i want to do to best achieve this? even tho i now know it will be very difficult.

And have any of you done soft back draw on your blades by methods other than the torch? like the hot tongs technique? How do they fair in compareson?

Thanks guys
 
KyKaMo said:
Thanks for your posts.

And if i wanted to try the tradtional clay method (a)) with more of a springy spring than soft, what would i want to do to best achieve this? even tho i now know it will be very difficult.

And have any of you done soft back draw on your blades by methods other than the torch? like the hot tongs technique? How do they fair in compareson?
Click to expand...

Getting the spine springy isn't too hard. Its getting it a consistent rc that would be hard. Basically thicker clay=softer spine. You will just need to pick a clay type and learn how thick of coatings you need to do. Some clays are more insulating than others. Also I've found with the 10xx steels I work a thicker blade needs thinner clay than a thin blade, since it already cools slower due to the thickness.

Go too thin on the clay and you won't get much of a hamon and the spine may be too hard. Go too thick and the spine may be soft. Also if you go too thick with the clay and do not soak the steel in the fire long enough then the steel under the clay does not reach temp and you end up with a fully soft spine.

Another option you have available would be to use a laminate/san mai steel. Have a high carbon core with lower carbon pieces welded to the outside.

Personally I've only tried the torch or "place spine in hot coals" method of drawing the temper on the spine. Both worked well if done right, neither worked good if done wrong. I usually use the method bladsmth described with the edge in water and torch draw the spine.
 
KyKaMo said:
Thanks for your posts.

Bladsmth: If i did a) and then oven tempered to 58rc, would that effect the spine at all? like make it softer?
There would be no change in the spine, since it is pearlite, not martensite. The edge would be Rc58, and the spine would be whatever the pearlite hardness was. It might be minutely softer, because there may be some pearlite martensite mix, but the change will be small or nil.

And if i wanted to try the traditional clay method (a)) with more of a springy spring than soft, what would i want to do to best achieve this? even tho i now know it will be very difficult.
It is a lot more complex than that, but short answer - Yes. The usual way is to wipe the clay off the very top of the spine .

And have any of you done soft back draw on your blades by methods other than the torch? like the hot tongs technique? How do they fair in comparison?
Many sword smiths have a special iron with a block of steel welded on the end. It is normally about 2"X3"X1", and on the end of a 24" long piece of 1/2" round rod. You heat this in the forge and run it up and down the spine ridge ( mune), watching the temper colors. When not being used to draw the temper, it is often used to heat the quench oil.
Click to expand...

Hope that is clear.
 
Ah yes, excellent answers, thank you guys.

This may be off topic, but i read something on a knifemakers site (i wont say who's other than, J. Fisher) and they said that 10xx steel (which is low alloy steel right?) is very brittle when through tempered and therefore needs to be differentially heat treated to even compare to higher alloy steels performance. Is that really true? is there that much of a difference in quality (considering they were all HT well)?

I thought 10xx steels were considered spring steels?
 
Edited:
I misunderstood your question about 10XX steel. Yes, 10XX steel is a low alloy steel.
1095 is sometimes called a spring steel, but that is not really a good description as far as knife use for 1095.

The hardening and tempering of any steel are designed to fit the desired use. 1095 could be hardened and tempered to make an anvil...a knife...or a spring.
Differential hardening and differential tempering are two different ways to make a harder edge and softer spine. This has an advantage for swords and large knives, as the blade will resist breaking , even if the edge should chip.
See ecos post below.
 
Last edited:
KyKaMo said:
Ah yes, excellent answers, thank you guys.

This may be off topic, but i read something on a knifemakers site (i wont say who's other than, J. Fisher) and they said that 10xx steel (which is low alloy steel right?) is very brittle when through tempered and therefore needs to be differentially heat treated to even compare to higher alloy steels performance. Is that really true? is there that much of a difference in quality (considering they were all HT well)?

I thought 10xx steels were considered spring steels?
Click to expand...

Theres a few half truths there that have been combined to make an untrue statement.

1095 steel is often considered "brittle" when through hardened at a high rc, many makers choose 1080 in place of 1095 for this reason. But as long as the steel is heat treated properly for its intended purpose the knife should last you well.

"compare to higher alloy steels performance" If you are talking wear resistance performance than higher alloyed steels that have materials like tungsten, vanadium, etc in them will hold an edge longer if processed correctly. But in many cases the steel will be more prone to breaking because the alloys make the steel less able to handle abuse....thats why stainless swords are usually a bad idea. Small amounts of alloy can improve simple carbon steels without negatively affecting it a noticeable amount...O1 comes to mind.

So in general lower alloy steel usually allows the blade to be put through more abuse without snapping, but won't retain its edge as well.

I work a lot of 1095 and I like it. It won't compare to the higher priced super steels when it comes to edge retention but it makes a very serviceable knife. My small knives are through hardened unless I want a hamon for decoration, on my larger blades that will see more stress I will typically differentially harden it.
 
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