Carbon Steel Blades and Grain Size

[mete]

I'm catching up here . So you are a fellow metallurgist ! Welcome !

 

[me2]

I've never had enough leather to use that in
testing. I've always used cardboard, as i have a virtually endless supply from the same source.

Was the previous structure pearlite or spheroidized?

 

[Juha Perttula]

I've never had enough leather to use that in
testing. I've always used cardboard, as i have a virtually endless supply from the same source.

Was the previous structure pearlite or spheroidized?

The test knives were forged, normalized, hardened and tempered.

My friend had endless source of veg tan scrap leather. So, we had plenty of free testing material. I think it is important to make real life cutting tests, because the real life cutting situations are very different. When a competent leather professional makes leather cutting test, we can say something about leather cutting knives, but we can not make conclusion that the same knives and steels are good in other cutting situations, for instance, rope or cardboard cutting.

 

[Juha Perttula]

How to attain superfine grain size ?

In order to achieve superfine grain size after hardening, initial austenite grain size shall be superfine and its growth shall be prevented during heating. Carbides, aluminium or vanadinium alloying can prevent grain growth. Initial austenite grain size depends on the previous microstructure.

It is generally accepted that normalized microstructure produces small enough initial austenite grain size. When normalized steel is hardened, grain size of about 0.01-0.02 mm can be attained. However, hardening two times can further reduce the grain size. Double hardening can produce grain size of about 0.005-0.01 mm. Multiple normalizing or more than two hardening cycles does not shange the situation.

The fact that double hardening produces finer grain size than normalizing and hardening is not very well known in metallurgical literature. But I have noticed it many times in my own laboratory tests; therefore, I am sure that it is a real fact. Thus, we can expect that double hardening improves toughness particularly with high hardness levels.

Lauri utilizes my finding in his double hardened blades (Lauri PT blades). At first, the blade is hardened and spring tempered, and after that, the cutting edge is rehardened. The blade won’t break due to relatively soft (50 HRC) spine, but also, due to double hardened edge it has exceptional fine grain size, and for that reason, it resists chipping despite of 63 HRC hardness. Here is a picture of my favorite model of Lauri’s double hardened blades. The faint line in the middle of the bevel is the line of the second hardening. It becomes visible during grinding because hardness difference.

 

[BluntCut MetalWorks]

Thanks for sharing good metallurgical info

IMHO/IME

per green-highlighted line: there are quite a bit of published research & lits on using particle (precip cementite in this case) to refine grain. Micro-alloy and other MC particles can help achieve grain dia to certain size, further refinement - one would need transient particles to nucleate prev-aust-grain. Your Lauri's example (btw - very nice blade), spring tempered = well distributed precip cementites which just large enough dia not to dissolve on 2nd aust, where they are pinning new grain (nucleation points/interfaces).

Here is my light/optical micrograph (1K negative) of W2 microstructure after 1 hardening. I also optimized on gb cleanliness(minimize precip/agg), thereby getting higher cohesion. 99% of my blade in low/no cr% steels have working hardness at 64+rc.

How to attain superfine grain size ?

In order to achieve superfine grain size after hardening, initial austenite grain size shall be superfine and its growth shall be prevented during heating. Carbides, aluminium or vanadinium alloying can prevent grain growth. Initial austenite grain size depends on the previous microstructure.

It is generally accepted that normalized microstructure produces small enough initial austenite grain size. When normalized steel is hardened, grain size of about 0.01-0.02 mm can be attained. However, hardening two times can further reduce the grain size. Double hardening can produce grain size of about 0.005-0.01 mm. Multiple normalizing or more than two hardening cycles does not shange the situation.

The fact that double hardening produces finer grain size than normalizing and hardening is not very well known in metallurgical literature. But I have noticed it many times in my own laboratory tests; therefore, I am sure that it is a real fact. Thus, we can expect that double hardening improves toughness particularly with high hardness levels.

Lauri utilizes my finding in his double hardened blades (Lauri PT blades). At first, the blade is hardened and spring tempered, and after that, the cutting edge is rehardened. The blade won’t break due to relatively soft (50 HRC) spine, but also, due to double hardened edge it has exceptional fine grain size, and for that reason, it resists chipping despite of 63 HRC hardness. Here is a picture of my favorite model of Lauri’s double hardened blades. The faint line in the middle of the bevel is the line of the second hardening. It becomes visible during grinding because hardness difference.

View attachment 731006

 

[Juha Perttula]

Thanks for sharing good metallurgical info

IMHO/IME

per green-highlighted line: there are quite a bit of published research & lits on using particle (precip cementite in this case) to refine grain. Micro-alloy and other MC particles can help achieve grain dia to certain size, further refinement - one would need transient particles to nucleate prev-aust-grain. Your Lauri's example (btw - very nice blade), spring tempered = well distributed precip cementites which just large enough dia not to dissolve on 2nd aust, where they are pinning new grain (nucleation points/interfaces).

Here is my light/optical micrograph (1K negative) of W2 microstructure after 1 hardening. I also optimized on gb cleanliness(minimize precip/agg), thereby getting higher cohesion. 99% of my blade in low/no cr% steels have working hardness at 64+rc.

64 HRC is impressive hardness. If you do not have chipping or breaking problems, you obviously have found a way to avoid brittle grain boundary fracture. The double hardening is not the only way to attain good toughness at high hardness levels.

About Lauri’s double hardening process. He uses 80CrV2 steel, which contain only 0.8% carbon. Thus, it is necessary to dissolve all cementite-carbides to attain high hardness, but the 80CrV2 steel contains about 0.2% vanadinium which binds 0.05% carbon into vanadinium carbides. The vanadinium carbides do not dissolve during hardening, they are very small and thus their number are large; therefore, they effectively prevent grain growth. The first hardening (spring tempering) is made in a furnace, and the second hardening i.e. the edge hardening is made by electricity (induction current). The induction heating lasts only a few seconds at about 900 C. Lauri's process control includes hardness measurement, grain size measurement of fracture surface, and torture test of the edge.

 

[Wowbagger]

So, in unalloyed carbon steel majority of grain growth may take place withing 5 minutes. But with steel which have carbides, aluminium or vanadium micro-alloying, similar amount of grain growth may occur after several hours or days. With overhigh temperatures grain growth is faster.

This is a quote from post #18 by Juha.
I am certainly no metallurgist so please bear with my lameness and semi understanding.
That said I am always taken aback when I hear aluminum is an alloy in steel. Seems like it would just vaporize given the dif in melting point (alu ~1200°F) . . . I just looked up boiling point ~4400°F so that must be why. I'm still scratching my head though.

OK got that side trip out of the way. What I wanted to ask about the above quote:
You say grain growth may occur after several hours or days. Is this at elevated temp or at room temp ? I assume elevated but as I understand it changes in heat treated structural aluminum materials (e.g., 6061) can continue at room temp before stabilizing.

Thanks !
Juha, your thread was just what I needed on a lazy day off. Great stuff everyone who posted here.

 

[Juha Perttula]

This is a quote from post #18 by Juha.
I am certainly no metallurgist so please bear with my lameness and semi understanding.
That said I am always taken aback when I hear aluminum is an alloy in steel. Seems like it would just vaporize given the dif in melting point (alu ~1200°F) . . . I just looked up boiling point ~4400°F so that must be why. I'm still scratching my head though.

OK got that side trip out of the way. What I wanted to ask about the above quote:
You say grain growth may occur after several hours or days. Is this at elevated temp or at room temp ? I assume elevated but as I understand it changes in heat treated structural aluminum materials (e.g., 6061) can continue at room temp before stabilizing.

Thanks !
Juha, your thread was just what I needed on a lazy day off. Great stuff everyone who posted here.

I should say "after heating of several hours or days". Grain growth in steel occurs only at high temperatures. Quenching freeze the grain size, and these grains will never grow again. Reheating of the quenched steel result in nucleation of new small grains and they start to grow.

 

[Greg Obach]

Hi Juha
would it not make sense to do both hardening steps with induction heater ? this would have a very quick heat up time to non-mag
i have an induction heater and heat up time is so short.. that i wonder if too much grain growth is even possible with this machine... (unless you massively overheat it ) but still there is relatively no hold time at the temp....
After the second heating, is the Lauri blade tempered again or is that left as unquenched martensite ??

there were posts on this forum about using triple quenching for steels like 52100 to make a very fine grain. ( heated in a forge )

if grain size is very small... does it reduced the amount of martensite that can be produced by a certain speed of quenchent ? i've tried the double and triple quench with shallow hardening steels and i found after a certain point that the steel didn't harden very well ... stayed somewhat softer

64 HRC is impressive hardness. If you do not have chipping or breaking problems, you obviously have found a way to avoid brittle grain boundary fracture. The double hardening is not the only way to attain good toughness at high hardness levels.

About Lauri’s double hardening process. He uses 80CrV2 steel, which contain only 0.8% carbon. Thus, it is necessary to dissolve all cementite-carbides to attain high hardness, but the 80CrV2 steel contains about 0.2% vanadinium which binds 0.05% carbon into vanadinium carbides. The vanadinium carbides do not dissolve during hardening, they are very small and thus their number are large; therefore, they effectively prevent grain growth. The first hardening (spring tempering) is made in a furnace, and the second hardening i.e. the edge hardening is made by electricity (induction current). The induction heating lasts only a few seconds at about 900 C. Lauri's process control includes hardness measurement, grain size measurement of fracture surface, and torture test of the edge.

 

[Juha Perttula]

Hi Juha
would it not make sense to do both hardening steps with induction heater ? this would have a very quick heat up time to non-mag
i have an induction heater and heat up time is so short.. that i wonder if too much grain growth is even possible with this machine... (unless you massively overheat it ) but still there is relatively no hold time at the temp....
After the second heating, is the Lauri blade tempered again or is that left as unquenched martensite ??

there were posts on this forum about using triple quenching for steels like 52100 to make a very fine grain. ( heated in a forge )

if grain size is very small... does it reduced the amount of martensite that can be produced by a certain speed of quenchent ? i've tried the double and triple quench with shallow hardening steels and i found after a certain point that the steel didn't harden very well ... stayed somewhat softer

Hardened and tempered starting microstructure produces somewhat finer initial austenite grain size than normalized structure. In properly executed induction hardening, initial austenite grain size is nearly equal to final grain size (no grain growth). I am not sure does double hardening always have practical importance, because the grain size difference can be quite small. But in my tests it was useful.

After second hardening Lauri's double hardened blades are tempered at 160-170 C.

Yes, fine grain size can reduce hardenability. Then faster oil or water shall be used to attain 100% martensite. Small amount of chromium alloying helps oil quenching of superfine grained steels. This is one reason why Lauri uses 80CrV2 steel which contain 0.5%Cr.

 

[scott.livesey]

Is there a chart that will convert 1 to 10 fracture grain size to microns? an example, properly HT'd O1 will have a grain size of 9.5, what would that be in microns? ref is http://cintool.com/catalog/Oil_Hardening/O1.pdf
I would guess most tool steel would contain small amounts of aluminum as it is also added to remove free oxygen or "kill" the steel.
most folks don't realize that most of your powder stainless steels were designed for plastic production, and that the blades and knives talked about on manufacturer data sheets are the kind used to turn a bull into bologna or make mechanically separated chicken.

 

[BluntCut MetalWorks]

imo, O1 grain diameter:
good: 8.5-10 microns
excellent: 5-8 microns
exceptional: sub 4 microns

I took(prepared+etched+microscope) a few micrographs for another maker/ht-tinkerer - look like one of the O1 ht coupon has grain diameter ~5 microns (astm grain size 12). IFF with permision, I post image .

Is there a chart that will convert 1 to 10 fracture grain size to microns? an example, properly HT'd O1 will have a grain size of 9.5, what would that be in microns? ref is http://cintool.com/catalog/Oil_Hardening/O1.pdf
I would guess most tool steel would contain small amounts of aluminum as it is also added to remove free oxygen or "kill" the steel.
most folks don't realize that most of your powder stainless steels were designed for plastic production, and that the blades and knives talked about on manufacturer data sheets are the kind used to turn a bull into bologna or make mechanically separated chicken.

 

[John Stubley]

This video is from VHS and the quality is not the best, but it gets very good reviews, it gets to steel eventually.

John.

 

[Twindog]

I’m quickly coming to the conclusion that the steel type and carbide load, while important, takes a backseat to the heat treat. I’ve been doing an edge-stability test for a while by chopping a piece of bailing wire on a large block of Doug fir.


Carbides certain play a role, but in my tests, the carbide load doesn’t seem to matter nearly as much as the heat treat, which really, really does matter. Grain size may be the key to a good heat treat.


I’ve had high-carbide steels like 3V and Vanadis 4E pass the test (very little damage), although edge acuity plays a big role. And I’ve had low-carbide steels like A2, 1075 and 5160 fail the test.


Here’s the latest: I purchased a cool, no-frills bowie knife with a 10 inch blade and geometry perfect for chopping. I’ve been looking for a bowie like this. It was made by a well-known, experienced knife maker who I’m not identifying so I don’t wreck someone’s family business. But I still want to talk about it.


The steel is 5160 — a super tough, relatively simple steel. The maker says the blade is hardened to 58 Rc. The geometry is not acute. The edge angle is 50 degrees inclusive. The width of the edge at the shoulders is 0.08: pretty much of a brute. It’s advertised as a hard-use bowie.


But the bailing wire beat the heck out of this bully. One chop and it chipped badly. With a 10X loupe, I can see the grain in the chip, and it looks coarse. I also split a piece of firewood with a screw in it. I was splitting the wood to release the screw, but the blade hit the screw a couple times, but not hard. Result: two more chips.

Juha’s 80CrV2 passed the test easily. I’ve had another knife with this same steel that failed really badly. Bluntcut reheat treated a 3V blade for me. Before the reheat treat, the blade was chippy. After the reheat treat, it was good to go.


Heat treat just seems to be the key — although I’m still a steel junky.

 

[me2]

Is there a chart that will convert 1 to 10 fracture grain size to microns? an example, properly HT'd O1 will have a grain size of 9.5, what would that be in microns? ref is http://cintool.com/catalog/Oil_Hardening/O1.pdf
I would guess most tool steel would contain small amounts of aluminum as it is also added to remove free oxygen or "kill" the steel.
most folks don't realize that most of your powder stainless steels were designed for plastic production, and that the blades and knives talked about on manufacturer data sheets are the kind used to turn a bull into bologna or make mechanically separated chicken.

Just an fyi, fracture grain size and astm grain size, like blunt cut listed above, are not the same. Sheppard fracture grain size scale stops at 10, and is more qualitative, where as astm is definitely quantitative. Also, there is no guarantee that what you're seeing on fracture surfaces are actually the grains. Not all fractures follow grain boundaries.

 

[BluntCut MetalWorks]

me2 - thanks for clarified the different between terms.

Just an fyi, fracture grain size and astm grain size, like blunt cut listed above, are not the same. Sheppard fracture grain size scale stops at 10, and is more qualitative, where as astm is definitely quantitative. Also, there is no guarantee that what you're seeing on fracture surfaces are actually the grains. Not all fractures follow grain boundaries.

Twindog - thanks for sharing and plus you are more than just a steel junky... quite informed on metallurgy and accurate failure analysis on your new 5160 bowie poor performance due to coarse grain microstructure. I agree with your assessment however having difficult time to envision a ht recipe to produces a chippy 5160 at hardness below 60rc. Beside coarse grain, it probably has a major case of cementite precipitated in grain boundaries.

I got permission from O.J. to post micrograph of his O1 ht coupon #5. Just look carefully to see gb - those continuous path formed circumference with diameter between 4-6um.

 

[Juha Perttula]

me2 - thanks for clarified the different between terms.



Twindog - thanks for sharing and plus you are more than just a steel junky... quite informed on metallurgy and accurate failure analysis on your new 5160 bowie poor performance due to coarse grain microstructure. I agree with your assessment however having difficult time to envision a ht recipe to produces a chippy 5160 at hardness below 60rc. Beside coarse grain, it probably has a major case of cementite precipitated in grain boundaries.

I got permission from O.J. to post micrograph of his O1 ht coupon #5. Just look carefully to see gb - those continuous path formed circumference with diameter between 4-6um.

Yes, the grain size seems to be around 5 micron, but it is difficult to see because martensite substructures are also visible.

 

[mete]

In higher carbon steels there is a significant difference in fracture between standard steels [large carbides ] and 'powder steels' [with smaller carbides] . Fractures with large carbides normally go from carbide to carbide . Small carbide types the fracture goes through the matrix and only some fractures are carbide to carbide .Thus the powder steels are tougher .
With lower carbon steels another factor becomes significant - the grain boundaries . Especially with poor HT things tend to collect in the grain boundaries .Over heating will increase this problem.
In high carbon steels like 1095 it's also easy to collect high amounts of carbides [even continuous carbides] if very slow cooling is used through the critical higher temperature area. 1095 is often recommended for beginners but if they don't understand the dangers they will get an extremely brittle blade !!
Do it right the first time because it's not always possible to re-HT to correct it!

 

[BluntCut MetalWorks]

In general - there are many benefits from achieving smaller grain diameter, however as grain is getting smaller there are major challenges come with it. If not dealt with them accordingly, these challenges will at some point degrade the result to quality below average standard ht recipe w/o refinement.

Challenges are compounded as grain diameter is reduced. Challenges listed in order of grain diameter and difficulty:

1. 7+um - Dislocation/stress: increase retained-austenite and percentage of plate martensite
2. 3-6um - eldest/oldest grain boundaries polluted with deposit. Cross section and deposited/precip of hard particles, weaken these old gb. oldest gb - refer to first thermal event, whereas subsequent events/cycles will inherit this old-gb while new grains nucleated at old-gb.
3. sub 3um - recalescence and grain merging. too much energy/heat generated so if cooling isn't fast enough, grain merging plus fast diffusion (deposit in gb) taken place. In turn also amplify 1 & 2 above.

In metallurgy field - many decades have passed - grain refinement is more/less stagnant because the 3 challenges above. Many researchs and studies have been done but in the end, there are very limited applications of. As most relied on adding elements/alloys to control/mitigate challenges 1-3. These techniques have their limited utilization but more/less dead-end for general ht advancement.

Um.... ^ sound like a complain/rant... nah, just my conclusion (ok - a very strong opinion) based on my works. Below is a micrograph at 1000 magnifications (oil len) on W2 steel after 2 thermal events. Outlined of big 1st generation (oldest gb) and 2nd generation. Notice the thicker/darker 1st gen gb of a smaller 1st gen grain on the right of outlined grain, that is challenge #2 working against the result. Since this sample peak hrc is 67+, so challenge #1 isn't an issue.

It's very difficult to discern gain/loss w/o better equipments. That said, projection of my refinement where optimal result (smallest possible grain perf gain) between thermal event 2.5-3.5th. Of course, that is not all my ht does. otherwise, without other counter-measures, my optimal ht grain diameter would ceiling around 4-5um.

 

[Mo2]

This guy does a lot of sharpening with the high carbide steel and gets them so sharp he can whittle hair. he uses either diamonds or ceramics, mostly Diamonds for the high carbide stuff. including Diamond strops.

he goes as far as testing them cutting cardboard also showing that they really do keep a very good edge even after 100's or 1000's of cuts.

https://www.youtube.com/channel/UCC-cvUqhuR--z1uUAXqMzaw