Hamon – Manganese - Deep/Shallow Hardening Relationship

H

Heli00

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Oct 19, 2017
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Hi,


Could someone please clarify a few things for me.


So let’s start with the homon. I read on some posts here that the steel has to be up to a maximum of 0.4% Mn to get a hamon, anything higher, no go.


Also, when the Mn is low, the steel is considered shallow hardening and when high, like in the O1 or A2 for example it is considered deep hardening.


Does this mean that steel considered shallow hardening will give a hamon?


I understand that the hamon is formed between areas where the hardness changes. But how is that related with shallow hardening?


And last, when talking about shallow hardening, how shallow is that, say goes no more than 0.8” deep or what is the depth ?


I was recently trying to get some 1095 in 0.250” thick, for a Tanto, but I was told that they don’t carry this thickness since it doesn’t harden deep enough. Is this correct?


Please correct me and I apologize for my confusion.


Thanks in advance,

Constantin
 
Larrin said:
The hamon relies on low hardenability. The spine that does not harden cools only slightly slower than the edge. A high hardenability steel such as O1 or an air hardening steel would still have a hard spine because it would still harden with that slower cooling rate.

As to the effects of different elements on hardenability you can read that here: https://knifesteelnerds.com/2019/02/25/how-fast-do-you-have-to-quench-hardenability-of-steel/
Click to expand...

Thanks Larrin, very comprehensive study, great education.


So let me see if I understand this a bit better now (at least in the context of my questions).

Disclaimer, I read the document pretty fast, so I’m sure I missed some or a lot of the fine points. I’ll need a few more passes to get most out of it.

A hamon forms at grain boundaries. These boundaries are formed where different transformations have occurred.

The different transformations are mandated by hardenability, the time and speed of cooling from the austenitizing temperature.

Mn is one of the elements added to the steel to control and manipulate hardenability. More Mn increases hardenability, meaning increases the quench time and that makes the hardening more homogeneous and therefore no different grain boundaries, no hamon E.g. Air quench steels.

In the case of differential HT to get a hamon, after the HT the spine will transformed to ferrite, carbides or perlite, where the edge that cooled faster will be martensite.

The activity of the hamon we see, is really the intermixing of these grain boundaries ?

Still, not clear how deep does the hardness go in 1095 for examples, but nevertheless, great education so far.

Thanks again,
Constantin
 
Heli00 said:
Still, not clear how deep does the hardness go in 1095 for examples, but nevertheless, great education so far.

Thanks again,
Constantin
Click to expand...

From my limited experience, an object with a thin cross section, such as in a knife blade, the hardness goes all the way through.
 
The transformations of pearlite or carbides occur at the grain boundaries. But on a macro scale you aren't seeing the result of intermixing grain boundaries. The "soft" portion is a lot of pearlite that has formed throughout that portion of the steel.

I can't tell you how deep the hardness goes in 1095, though we could guess based on Jominy end-quench reports such as I have below. If using 1/4" 1095 you may need to grind partial bevels to get full hardness at the edge.
1095 jominy.jpg
 
Larrin said:
The transformations of pearlite or carbides occur at the grain boundaries. But on a macro scale you aren't seeing the result of intermixing grain boundaries. The "soft" portion is a lot of pearlite that has formed throughout that portion of the steel.

I can't tell you how deep the hardness goes in 1095, though we could guess based on Jominy end-quench reports such as I have below. If using 1/4" 1095 you may need to grind partial bevels to get full hardness at the edge.
View attachment 1260565
Click to expand...

Hmm, I've been staring at the graph, but having problems with the x axis. What does it represent? A little confused how to interpret it.
 
It is a Jominy round har hardenability chart. The quench a 1" round bar of the steel and the chart shows how deep it hardens. That is why we call steels deep hardening or shallow hardening.
 
They spray water at the end of the bar and then measure hardness up the bar to see how far the water has hardened it.
 
I believe I was not clear. What I meant to ask, was what is the unit of measure on the x axis?
The 8, 16, 24 etc. measures the Jominy distance as in 8/16”, 16/16”, 24/16”, etc?

Thanks,
Constantin
 
The distance from the end of the bar.
 
Yes the scale is in units of 1/16” so each vertical line is 1/2” from the last. That why you see the hardness drop off in the first 1/8” or so, that would mean 1/4” flat stock would be pushing it to fully through harden. Not that it won’t harden through the entire bar it just won’t be max hardness all the way through.
 
JTknives said:
Yes the scale is in units of 1/16” so each vertical line is 1/2” from the last. That why you see the hardness drop off in the first 1/8” or so, that would mean 1/4” flat stock would be pushing it to fully through harden. Not that it won’t harden through the entire bar it just won’t be max hardness all the way through.
Click to expand...

Excellent, thanks JT. This answers my question and I now understand the graph.
 
So now let me ask this.

Is shallow hardening as in 1095 a good thing when building a tanto or a bigger sword with a thick spine?

Keeping the inside a bit softer than the outside of the blade, would make the blade more resilient to braking?

Now that I think about it the differential HT does exactly that, keeps the spine softer than the edge and the bonus is a hamon. So I bet the Japanese first wanted to construct a solid sword, then discovered the hamon and started to invent processes to modify the look of it for aesthetics. What do you think?

Does anybody know?

Thanks,
Constantin
 
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I believe the Japanese originally accidentally discover how to produce a hamon. And in doing so they found out that differential heating (causes hamons) gave their blade more strength and more flexibility. This was crucial in battle with sword to sword fighting. Once they learned how to perfect the use of differential Harding, they then turned their attention to making the hamons more beautiful and artistically visual, while keeping the function in place. Why if you look at the older Japanese blades with hamon, they tended to be more straight lines and later developed into the more dramatic hamons later on. In modern times. I’d hope that there aren’t anyone swing live blades at each other, so now days the hamons still keep the cutting area hard and the shape of the hamons can be more creative.. just my thought
 
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