# Question for blade smiths and metallurgists about honyakis and retained Austenite



## Nemo (Nov 1, 2016)

I've been reading John D Verhoeven's 2005 treatise on metallurgy for bladesmiths (I'm not planning to make blades but I'm fascinated by the process). It's a fascinating read that's fairly easy to understand given the complexity of the subject matter.

There is one thing that I can't quite get my head around:

Please tell me if & where I've got it wrong:

My understanding is that when austenitic steel is quenched rapidly to room temp (I assume that traditionally this was done with water?), it will rapidly reach the Ms (martensite start) point (temp), and will begin to transform into Martensite. Because the transformation to martensite is so rapid (in comparison to pearlite and bainite), once Ms is reached, the formation of pearlite and bainite are circumvented.

The Mf point (temp at which austenite is fully converted to martensite) is lower the higher the C content. If the carbon content is under 0.3-0.4%, it will reach Mf (will be fully converted to martensite) at room temp but if the carbon content is more, then the Mf point will be sub-zero and a cryo quench is needed to fully martensise (if that's even a word?) the steel.

So after a room temp quench , steels over 0.4% C or so will have retained austenite (which will soften the steel compared to a fully martensitic steel). 

The hardness of the martensite is dependant on the amount of carbon in it (more carbon 'stretches' the Fe lattice more which puts the bonds in the lattice under more stress, giving them less leeway to move- I kind of think of it like being a taught rope)

In steels up to 0.8% C, the harder martensite (created by the increased carbon content of the martensite) is more important than the retained austenite and the (as quenched) steel will harder than a 0.4% C steel. However, beyond 0.8% C, the retained austenite becomes more important and the overall hardness drops.

This can be improved with a cryo quench (bringing the higher %C steels closer to their (sub-zero) Mf temp, giving less retained austenite and more hardened martensite). So a higher C steel would be harder than a 0.8% steel with a cryo quench.

I'm assuming that honyakis have been around long before we had cryo quenching?

A simple 0.8% carbon steel like 1080 should be able to get to about HRC 65 _before_ tempering with a room temp quench. This would be reduced after tempering. Because of retained austenite, higher carbon steels should have a lower HRC with a room temp quench.

So how do honyakis made from a steel like shirogami 1 (~1.3% C) and quenched at room temp get to HRC 64-65 _after_ tempering?

Is there any benefit of a 1.3% steel over a 0.8% steel with a room temp quench? If not, why do shirogami and similar knife steels have so much carbon?

Thanks for explaining.


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## natto (Nov 3, 2016)

Increasing the amount of carbides increase hardness, the carbon content above 0.8% is intended to become iron carbide. But I don't know why razors and similar thin blades aren't made at 0.8% carbon, the eutectic point should offer the toughest lattice.


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## Jacob_x (Nov 3, 2016)

Great content, I remember looking at his essay years ago. Hope you get the answer you're looking for.


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## Nemo (Nov 3, 2016)

natto said:


> Increasing the amount of carbides increase hardness, the carbon content above 0.8% is intended to become iron carbide. But I don't know why razors and similar thin blades aren't made at 0.8% carbon, the eutectic point should offer the toughest lattice.



Thanks natto. When you say iron carbide, do you mean cementite? My understanding is that cementite has a hrc of 70 and will form in thr old austenite grain boundries or as a part of pealite (which has a much lower hrc).

Does that mean that the trick with a honyaki is to get the cementite to form in the grain boundries (and in so doing, soak up most of the carbon in excess of 0.8%) but to trigger the formation of martensite before there is significant formation of pearlite?


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## Nemo (Nov 3, 2016)

Jacob_x said:


> Great content, I remember looking at his essay years ago. Hope you get the answer you're looking for.



Thanks Jacob. I'm fascinated by the complexity that is possible just by combining two elements (Fe abd C) in different ways.


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## natto (Nov 3, 2016)

Hi Nemo, you remind me to read my copy of Verhoeven, and I hope some more knowledgeable people chime in.

If I got it right Fe3C means iron carbide and cementite. As carbide they are smaller than tungsten carbides, but I can't remember the structure.


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## Nemo (Nov 3, 2016)

natto said:


> Hi Nemo, you remind me to read my copy of Verhoeven, and I hope some more knowledgeable people chime in.
> 
> If I got it right Fe3C means iron carbide and cementite. As carbide they are smaller than tungsten carbides, but I can't remember the structure.



Hi natto. I think on page 154 he's saying that cementite has the formula (Fe2Mg)3.C, but it could be interpreted as (FeMg)3.C.

My understanding is that it can be present in 4 forms:
1) Small granules throughout the grain (often done deliberately to improve the machinability of hypereutectiod steels prior to heat treat).
2) Cementite on the grain boundry (formed when the temp of austenite falls below the Cm line). I'm not sure what effect this has on the overall hardness and toughness of the steel.
3) In pearlite (a complex of cementite and ferrite) which significantly reduces the hardness of the steel.
4) In bainite (a different microstructure of cementite and ferrite that can get almost as hard as tempered austenite and is a little tougher).


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## Nemo (Nov 3, 2016)

Nemo said:


> Hi natto. I think on page 154 he's saying that cementite has the formula (Fe2Mg)3.C, but it could be interpreted as (FeMg)3.C



Sorry, I meant Mn rather than Mg, so it should read (FeMn)3.C


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## Nemo (Nov 13, 2016)

Any one else have some knowledge to share?


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## Kippington (Nov 15, 2016)

Nemo said:


> So how do honyakis made from a steel like shirogami 1 (~1.3% C) and quenched at room temp get to HRC 64-65 _after_ tempering?



One of the important parts of heat treating is controlling where the carbon goes.

Carbon is locked up in steel and doesn't really move at room temperature.
As you've probably heard, you can ruin a knife's heat treatment if you overheat it. What this really means is that you don't want to let the temperature get high enough where the carbon can start moving. As we increase the heat, more and more carbon begins to move.
You can take a 1.3% carbon tool steel and, with the correct time and temperature, effectively have 0.8% carbon moving around inside the iron lattice ready for the martensitic transformation (the quench). This would result in a standard 65-66HRC untempered martensite.
Where are the other 0.5% carbon atoms you ask? They're still locked up in the carbides and can be released with the addition of more heat. This extra heat is actually something we do before-hand, during normalising. By the time we're ready to quench, we should already have the extra carbon in the places where we want it to be. These carbides effect the properties of the steel all the way to the end product, and can add extra hardness among other things.

Let's say I have accidentally overheated the steel before the quench. Now I have more then 0.8% carbon moving around in the iron lattice, which is more then it can hold during the martensitic transformation. I quench the overheated blade and a large amount of the austenite transforms into martensite, which spits out the extra carbon that it cannot hold on to. This carbon quickly bunches up in and around the austenite that hasn't yet transformed and grid-locks it in it's current state. This is one way of causing retained austinite (RA).

There is always _some_ RA and we deal with it as best we can. You'll notice on TTT graphs theres a Ms (martensite start point), then a M50 (50% conversion), M90 and so on. Some graphs don't show a M100/Mf (martensite finish point) because it's not realistic. The ones that _do_ show Mf place it where the calculations work it out to be.
95% conversion is great for bladesmiths. Other industries can take advantage of RA and keep it for it's toughness (e.g. 52100 and the crazy world of ball bearing steels).

I hope this helps.


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## Nemo (Nov 15, 2016)

Thanks for your insights Klippington. That all makes sense.

Am I correct in thinking that in a fairly pure carbon steel like Shiroko 1, The extra 0.5% carbon is tied up in the iron carbide called cementite? My understanding is that cementite can exist as fairly pure cementite in the grain boundary or in pearlite (A cementite-ferrite mixture) or in bainite (a differently structured cementite-ferrite mixture). Am I correct in thinking that pearlite makes the steel softer? Does pure cementite in the grain boundary make the steel harder? Is that what you are trying to achieve with a honyaki?

To what extent are honyakis tempered?

Does anybody deliberately make kitchen knives out of lower bainite (which apparently can be as hard and maybe a little tougher than tempered martensite)?


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## Kippington (Nov 15, 2016)

Glad to be of assistance.



Nemo said:


> Am I correct in thinking that in a fairly pure carbon steel like Shiroko 1, The extra 0.5% carbon is tied up in the iron carbide called cementite? My understanding is that cementite can exist as fairly pure cementite in the grain boundary or in pearlite (A cementite-ferrite mixture) or in bainite (a differently structured cementite-ferrite mixture). Does pure cementite in the grain boundary make the steel harder?



This is correct. Cementite can also form grains of its own.



Nemo said:


> Am I correct in thinking that pearlite makes the steel softer?



Pearlite is a combination of cementite and ferrite - the ferrite is what's soft. The cementite strengthens it, normally in the form of reinforcing sheets throughout the pearlite grain...







...but we can control this to a degree during heat treating operations such as annealing. For example we can move the carbon and get the cementite to group up into little balls that make the steel easier to grind and machine.






There is only _so much_ carbon that a pearlite grain can hold (the eutectoid amount, roughly 0.8% by weight), and the rest forms another kind of grain of pure cementite; iron carbide. If there are other elements involved (e.g chromium, tungsten, vanadium), they can also take up the extra carbon to form their own really hard carbide grains because carbon really likes to get around.
:bliss:
This brings up another point: Carbon LOVES oxygen! Bladesmiths will often start with steel that has a higher carbon content then required with the expectation that oxygen in the air will steal a bunch of it during hot working. Remember how carbon moves faster at higher heats? The swordsmiths of ye ol' Japan that made katanas needed to heat the metal up to bright orange over and over again for many high temp forge welding/layering operations. The steel they start with would had a much higher carbon content than the finished product.

Bladesmiths _should_ consider all of this and choose the appropriate steel prior to making the knife, with the intentions of heat-treating it accordingly mid way through its creation.



Nemo said:


> To what extent are honyakis tempered?



That's up to the maker. I'm not trained in traditional Japanese bladesmithing so I can't really say much on the topic, mostly due to the term 'honyaki' and the meaning it carries.
As an end user, it's possible for you to temper a knife more yourself (in say a normal oven) if you'd like to sacrifice a little hardness for extra toughness. In doing so basically says you would like something different to what the bladesmith had in mind from the start. This sounds insulting but it's no worse then re-profiling or changing the bevel angles. 



Nemo said:


> Does anybody deliberately make kitchen knives out of lower bainite (which apparently can be as hard and maybe a little tougher than tempered martensite)?



People have made bainite swords.
Banite is tougher but less hard then martensite. This isn't practical for kitchen knives which don't get put through much stress and where edge retention is preferred over shock resistance.
Tempered martensite is easier to achieve _and_ more suitable for our needs.


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## Nemo (Nov 16, 2016)

There's some fantastic info in there Kippington (sorry for the incorrect spelling in previous post) and it does give me an idea of the answer to my question. I say "give me an idea" because your answer also gives an idea of the complexity of the subject and the multiple variables that affect the answer.

It astounds me that such a beautifully complex system is created by different interactions of just 2 elements. Not to mention what happens when you add alloying elements.


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## Kippington (Nov 16, 2016)

I'm happy to help! :biggrin:

There are two old videos from America in the 1940-50s that are a great watch if you're interested in this topic:

On the formation of Crystals, Grains and Polycrystals:
[video=youtube;kw84ZH_kXr8]https://www.youtube.com/watch?v=kw84ZH_kXr8[/video]

Steel Heat Treatment: Elements of Tempering, Normalizing, and Annealing:
[video=youtube;xXOtdTa2ypU]https://www.youtube.com/watch?v=xXOtdTa2ypU[/video]


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## Kippington (Nov 16, 2016)

Oh and I forgot to post this one, which works directly off the 'Crystals, Grains and Polycrystals' video:
[video=youtube;uG35D_euM-0]https://www.youtube.com/watch?v=uG35D_euM-0[/video]


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## Matus (Nov 16, 2016)

Thank you very much for all the information


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## BloodrootLS (Nov 16, 2016)

Lots of good stuff here. Wanted to add that extra carbon over 0.8% that is brought into solution forms jackets of cementite on the grain boundaries as was mentioned. However, this is a bad thing (creates brittle, easily chipped blades) and is typically considered overheating a blade. The extra cementite is ideally spread evenly throughout the interior and exterior of the grains in finely dispersed spheroids embedded in the nearly fully martensite matrix. Very high carbon simple steel blades such as White #1 etc. and almost all of the steels chosen for traditional honyaki construction are very sensitive to any mistakes in temperature control (over or underheating and holding it at the appropriate temperature for the right amount of time (which depends on the structure the steel had been in previously!)) which I think is likely why there's the mystique and price tag and why they're only made by the "expert" smith. With temperature control like most makers have it is not difficult to acheive and acheive consistently, but by traditional by-eye methods it is very difficult. Bad heat treatments by under or over heating would result in poor edge strength/ prone to rolling, or chippyness. Properly heat treated, good carbon steel is not particularly fragile even at 64-65 HRC. 

You may want to read what Verhoeven says about lathe vs. plate martensite as I can't remember all the details, but here's my off-the-cuff. Martensite forms in different structures and lathe (up to 0.6% carbon forms completely lathe martensite with some ferrite, and 1% carbon makes full plate martensite, and in between those amounts of carbon brought into solution creates a mixture of plate and lathe). Lathe is tougher than plate, but plate if I'm remembering correctly provides more strength. Sometimes smiths "overheat" the steel some to bring in more than 0.8% carbon in order to increase the percentage of plate martensite to some incremental loss of toughness through grain boundary cementite formation in order to boost edge strength. The more carbon in solution also creates more retained austenite, but this can be remedied by cryogenic treatment to bring the steel to or closer to a true martensite finish for the steel. 

~Luke


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## Nemo (Nov 17, 2016)

Thanks Bloodroot and Kippington for giving some great info and even more importantly some real perpective to this question.


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