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Mini electric kiln project
Attached you can find an interesting note about grain size specifics.


.pdf   Grain Size and Its Influence on Material Properties.pdf (Size: 396.55 KB / Downloads: 7)

Thank you for this, Mr. Jan! I always like to learn more about metallurgy.

I guess I knew most of the benefits of fine grain. Is it possible to approximate grain size by performance of a blade?

For instance, if you had two bars (about 1" wide, and 1/8" thick for instance), is it feasible to to bend them, while measuring the strength and flexibility (with a torque wrench for strength, and the ability to measure the the angle of flex), and make any determination about grain size?

The reason I ask is because I don't see other knives that are as strong for their thickness, and also as flexible (being able to bend as far, as many times) as my knives.

I pull every trick in the book (that I'm aware of) to refine the grain of my blades. I've always done this, and I've always tested my blades to destruction to see how they perform, and to see how they break, and what they look like inside.

I certainly know they have changed a lot over the years, but I've never actually had the grain size measured, since I reckon it would be very expensive.
Yes, Mr. Mark you are asking a good question. Wink 

As mentioned by prof. Herring in the article quoted above, grain size has a measurable effect on most mechanical properties of steels. Hardness, yield strength, tensile strength, fatigue strength and impact strength all increase with decreasing grain size.

Quantitatively this improvement of mechanical properties is described as:

Example: In table 8.2 on page 69 prof. Verhoeven describes grains refinement from very fine grains with average diameter of 8 microns to ultrafine grains with average diameter of some 2 microns.

In this case we can expect following percentage performance improvement (ppi):
[Image: ppi.jpg?dl=1]
You surely understand that there is considerable uncertainty regarding the detailed mechanism of the grain size effect, and also appropriate definition of the average grain size.


P.S.: Mr. Mark’s question is a perfect opportunity to use the Rockwell Hardness Difference Calculator (

Imagine, that via 3 cycle heat treatment we reduced the grain size and the blade hardness increased from 55 HRC to 60 HRC. The Rockwell calculator tells us that the change in hardness was 23.6% (ppi = 100 + 23.6 = 123.6%). The question is, what happened with average size of grains?

Using the ppi formula above we can calculate that: initial grain size / final grain size ≈ 1.5

Outstanding! I never would have guessed there would be an actual formula! Super interesting!

Now I can run some tests to see how this pans out. Actually, you can run even better tests with your kiln.

The difference is that I probably start with really fine grain, due to the rate of reduction from a cylinder with diameter 3" x 10" length, forged all the way down to a blade. By careful low temp forging, I don't know how fine the grain is, but it must be awfully fine before I do any exact grain refinement heat cycles in my kiln.

With low temp forging, and so much rate of reduction, wouldn't it be a given that the grain would be very fine, post forging?

One thing I question is the difference in hardness from coarse to fine grain. Fine grain is more difficult to get fully hardened than coarse grain, in my understanding, and it seems like I've run into that. I switched from Park AAA to Park 50, because I was coming up with a full hard HRC 64 with AAA, and 66 with P50.

If I remember, AAA at 180°F is maybe 9-10 second, where P50 at 100°F is 5-6 seconds. That might be another indicator of successful grain refinement. At least I think so.
Mr. Mark, the formula for performance improvement does not come from metallurgical textbooks, I have derived it from so called Hall-Petch relation to provide simple answer to your question. Please keep in mind that my formula is simplified and may be even oversimplified. The consequence is that the formula may work with different accuracy/applicability for strength, toughness or hardness.

The Hall-Petch relation describes the grain-boundary strengthening method that uses grain boundaries as pinning points which impede dislocation movement. The smaller grain is stronger because in the larger grain, a dislocation can travel without being stopped by a grain boundary.

The Hall-Petch relation does not consider so-called twin boundaries within the grains, but despite all possible imperfections it is one the most universal relations in mechanical metallurgy. It says, that over wide range of grain sizes the typical mechanical properties increase with the reciprocal root of the grain size.

Your P50 seems to be very fast speed quenching oil with low viscosity. Its quenching power almost approaches that of water or some water solutions. High initial cooling rate is important to get full hardness, but slower cooling is necessary when martensite is formed because stress needs some time to equalize.

What concerns oil quenching I have found several useful information in the attached paper.

.pdf   Oil Quenching Tech.pdf (Size: 986.41 KB / Downloads: 3)


some thoughts from another knife maker.  first, pick a good steel to work with.  O1 or 1.2510 has been around a long time.  the newer versions have 0.20% vanadium in addition to 0.50 chrome and tungsten.  vanadium and tungsten form very small carbides which a dispersed throughout the steel.  if purchased as 'Precision Ground Flat Stock' it is ready to use and heat treat.  I prefer it when doing stock removal, I remove steel using files, sanders and grinders.  I don't forge because I don't have the equipment.  the key is to heat treat at the minimum possible temperature for the shortest time.  Temperature has the most effect on grain growth.  for basic carbon steels like O1 or 52100 or 1.2519, make your maximum heat 1500F/825C.  for material thinner than 2mm, leave in the furnace for 7 to 9 minutes.  quench.  I use canola or rapeseed oil.  this is guaranteed to give you the finest grain and maximum hardness.  
how much steel will flex depends on geometry not heat treat.  here is a good reference
a quote from the article: This physical fact also rules bending tests and test alike when bending is involved. And as Kevin mentioned before, it is material related and doesn't get changed by whatever HT on does on the blade. 
Scott, thanks for all your practical guidance, highly appreciated! Smile 

What concerns the Flextest, I agree with the cautious statement by prof. Landes: "It can be used to get an indication if a blade has its HT done in the right parameters and its geometrical figures are somewhat where you wanted them, but it is no final tool to proof quality."

The blade in the Flextest can be considered as a cantilever beam, where the deflection at the free end is proportional to the bending force and to the reciprocal value of the elastic modulus (Young’s modulus). For some reasons the effect of HT on the value of the elastic modulus is small, usually smaller then 10 to 20% of the nominal value. For this reason many technicians consider elastic modulus as a quantity which is fully independent on HT. This is simplified and not fully justified approach.

From physical point of view the elastic modulus is given by the size of inter atomic forces dominated by iron interactions. Those forces are fundamental and not changed by HT unless the lattice structure is changed. When there is a transformation to martensite than also the elastic modulus is slightly changed. 

On the other hand, plastic properties of metals (e.g. hardness) are a different story, because they are heavily influenced by dislocations, which are significantly influenced by HT.


Mr. Jan, that is a  very interesting attachment! I haven't heard of quenching under vacuum pressure, but it makes perfect sense. Engineered quench oils are designed with additives to eliminate the vapor jacket, because that is critical to cooling the steel. That's why I use quench oil designed for this purpose. I have no idea how to go about quenching a blade in a vacuum. I've never heard of it, but that would be the ultimate way to defeat the vapor jacket!

I agree that faster quenching can result in warped blades, but there are ways to get around it. I rely on multiple thermal cycles, and straightening the blade after each cycle. After the grain refinement heat cycles, the blades stay straight 90% of the time. I would do grain refining cycles no matter what, so a little extra blade straightening during that time is no inconvenience. 

My final proof of heat treat quantification is testing to destruction, which is something I do constantly. My normal hunting/survival blade will withstand elastictic deformation flexing back and forth to 30°. At 45°, I get about 5° plastic deformation. At 90° flex, I get about 45° plastic deformation.

Some steels can be purchased ready for hardening. I know Aldo sells 1084, but hypereutectoid bar stock like 52100 is most often heavily spheroidized to make it easier to work. It takes a very significant heat cycle just to dissolve and distribute the carbon before you can work at refining the grain.

Temperature is one aspect of heat treating, but time is just as important to all heat treating cycles. Too hot for too long causes grain growth, but steel has to be a certain temperature for a certain time to fully dissolve the carbon and other alloys. The higher carbon and alloys, the higher the temp and/or longer the soak. 

Mr. Scott, I will quote your mostly highlighted statement;

(Quote) (sic)how much steel will flex depends on geometry not heat treat.  here is a good reference
a quote from the article: This physical fact also rules bending tests and test alike when bending is involved. And as Kevin mentioned before, it is material related and doesn't get changed by whatever HT on does on the blade. (Unquote)

I think you're taking this out of context or something, because to take this literally is ridiculous. Heat treat makes no difference on a blade's ability to flex? So identical blades, one lightly tempered (still very hard) and one heavily tempered (much tougher) will flex equally? Not to mention Mr. Jan's testament related to the benefits of heat treating to refine the grain. Not to mention the most critical step in my heat treat regimen- drawing the spine.... 

I don't rely on normal tempering to achieve my goals. I temper the blade at 365°F for two hours, twice. In reality, only the edge of the blade remains HRC 62. I figured a way to draw the rest of the blade down to about HRC 50, which leaves me with perfect martensitic spring steel. The performance of my knives has been documented in YouTube videos for many years. Of course I welcome side by side comparison of every aspect.
some quick thoughts
yes grain refinement can be helpful.  yes I read pg. 69 in the Verhoven book and saw the table.  you need to go just a step further and read the reference.  Grange patented his grain refinement techniques in 1965.  this is part of the fine print we may have missed:
In practicing our invention, it is necessary that the heating be done quite rapidly but once the desired rate is obtained on further advantage results from exceeding such rate by extremely fast heating. In thicknesses up to 0.5 inch, satisfactory results can be obtained by leadbath heating but other types of liquid baths, such as salts, or electrical induction or resistance heating may be used. The heating time should be less than 60 seconds and preferably less than 20 seconds. In such thicknesses as .03 to .50 inch, the same ultrafine grain size was obtained upon heating in a lead bath from 10 to 20 seconds. ref:

I am not sure what to make of this, Mr. Scott. I don't know what could be so special about a grain refinement process that it could be patentable, but the patent most likely ran out 30 years ago. If they were actually using molten lead, I assume the risks outweigh the rewards, especially for a one-man knife making operation.

I'm confident my grain refinement procedure is sufficient for me, but I Thank You none the less for your continued research.

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