HT idea will it work?

After this perculated through the gray matter a bit I realized that Josh did have a good point even if it was for the wrong reason. If the bar of steel is heat treated before grinding it might very well not harden, except at the corners. If the bar is over twice the thickness of the depth of hardening then the bar will cool too slowly throughout to form martensite. It will form pearlite and the hard corners will be ground away on the edge. Hardening before grinding requires a deep hardening steel.

One thing that might get past this problem of heat differential during the quench would be to marquench. Heat the oil to about 440-450 degrees, just above the Ms point, and quench the blade for a few seconds to bring the outside and inside of the bar down to the same temperature and then allow the blade to cool in air. This will give the blade about a minute and a half to go below the Mf point. This should allow the thinner parts of the blade to form martensite and, hopefully, to prevent some of the temperature shock as the blade passes through the Ms-Mf range.

Doug
 
Doug,

First- Thank you for helping me understand ! If not for you and Kevin I really don't think I'd really understand the properties of shallow hardening steel.

I do have to admit I'm a little skeptical of Vorhoovens take on this aspect.

He's probably correct of course but how does Vorhooven explain the mixed structure that creates the activity in hamon ? If it's cut and dry that you form either martinsite or pearlite then you would end up with a distinct "temper line" with no activity. Martinsite on one side pearlite on the other just like a black block sitting on top of a grey block.

Mixed structure is clearly evident even on deep hardening steels that are differentially HT-ed so I'm thinking it's clearly not a one or the other scenario reguarless of the steel type.

I think it's quite possible that really fine grained steel 1/4" thick will display the same result as a thick section where you end up with a shell of martinsite and a core of pearlite. To get a definitive answer It would take some serious testing that I just can't devote the time to right now but it's an interesting theory. It stands to reason that if a thick section allows the possibility of a shell of martinsite around a core of pearlite it's possible that as your raise the needed quench speed by refining the grain you can get a thin section (1/4) to behave just like a thick section.

I'm fairly sure I've seen this happen in my own blades by breaking them and looking at the grain. I've definitely been guilty of over-refining the grain but also do understand that looks can be decieving and my eyes are not a scienific instrument.

I'll be very interested in hearing your thoughts - Josh

P.S. Yet another great and civil heat treating conversation here. Ya gotta love Knifedogs !!!
 
Sometimes I feel the need to encourage people to not be shy, so Josh feel free to e-mail me, I have examined a lot of hamon strucutres under the microscope, with a keen focus on the "transition area" and would be happy to chat about it with you. kevin@cashenblades.com (I far prefer e-mail over P.M.)
 
Actually, what Verhoeven said about about shallow hardening steels in thin sections works fine with hamons. One of the problem is that it sounds like there is a sharp line where on one side there is pearletic steel and on the other side there is martensetic steel but there's not. You will have an area where there is a mixture. Also the makup of the pearletic steel is strongly effected by the carbon content.

The issue of film heat-tranfer control that Verhoeven pointed out is not just his position. He was pointing out an exception to the rule that has an effect strong effect on what bladesmiths encounter in heat treating. This effect is what causes the "natural" or "automatic" hamon (more properly, heat treating line) that will occure in shallow hardening steel when no clay coating or edge hardening was used.

It's the same with the curve on an IT diagram. One thing that I've noticed is that not all show seperate ferrite and pearlite lines for hypoeutectic steel. Even if both ranges are represented by one line, the regions are there. Also you can't say to down to the level of the nose the steel forms pearlite by slow cooling. Then from the nose dow to about half way to the Ms line upper bainite forms and from there to the Ms line (actually a little below) lower bainite forms. There is overlap between all these regions where both structures can be formed. The exact conditions that the of the testing also effects the results and the limits of the lines and areas are generated by an average of points generated by multiple samples.

I also can't think of anything better than to correspond with Kevin. He helped me out a lot in understanding what I was reading (and not reading) in the text books that I had. He is one of the few bladesmiths out there who has the equiptment to do his own testing of microscopic structures in the steel and the knowledge to interpret what those structures mean.

Doug
 
Doug,

Thanks once again for the explanation ! It is greatly apperciated.

I've read VerHoeven's book 3 times but it has been awhile ago. I lost my copy when our old computer crashed though. Never gotten into the whole e-book thing either but I'm gonna start scanning for a deal on the actual book. I think as my understanding grows I'll be able grasp some of the more advanced things like film heat-transefer control.

I did just score a copy of Metallurgy Fundamentals on e-bay for $10.73 so I'll be looking forward to reading it and having it on hand for reference.

Not that I'm concerned about it right now but curious- I'm under the impression that getting bainite pretty much requires some level of marquenching and that only some steels will realistically form bainite even with a marquench. I guess I'm curious if without marquenching if it's possible to get bainite mixed in with martinsite ?

No need to derail the thread from the properties of 1095 just thinking out loud.

I'll be collecting my thoughts on all this before dropping Kevin an e-mail.

Dang Ain't Steel Fun ! It's more complex than women :)

-Josh
 
The difference between marquenching is slightly different than austempering, which is what you use for forming bainite. If you are talking about holding the steel at just above the Ms point the main factor is time. With marquenching, which is sometimes refered to as martempering, involves holding the steel at just above the Ms point to allow the steel to stabilize at the same temperature throughout and allow all parts of the steel structure to convert to martensite at about the same time and reducing the stress in the steel during conversion. The steel is then down quenched in air or regular warm oil.

With austempering at just above the Ms point the steel is held for a longer period of time. Anywhere from 3/4 hour to a several hours, depending on the alloy. The final HRc achieved will also depend on the alloy being used. However, there are also other austempering methods.

One of those ways is to quench the steel at a point between Ms and Mf temperatures to form a desired percent of martensite to achieve a higher HRc. This happens pretty rapidly. After a few seconds to assure conversion the steel is then quickly switched to a bath above the Ms point to allow the formation of bainite. You will have to be able to measure hardness to check the process for final HRc level.

A third method is to hold the steel at a point just a little below the Ms point until conversion is complete. This will take longer than upquencing as above, like 3-4 hours as opposed to about 45 minutes for the 52100, but the IT diagram will give you an idea of what HRc you should be able to achieve.

You are right that some steel lend themselves to the process of austempering for knife blades better than others. I like to use 52100 and it lends itself to the third case of austempering that I mentioned for a probable HRc of around 59-60 when I go about 80 degrees below the Ms point. This is due to the limits of the electric roaster that I use as a quenching tank more than by design. One of my projects that I have waiting is to see if I can over ride the controls on another roaster that I have so that I can get up to about 500-510 degrees and austemper at just above the Ms point for 52100 and get an HRc of around 56-58. Another steel that I use 9260 just doesn't lend itself to austempering, at least without upquenching. Austempering it at just above the Ms point only gives an HRc of around 55, which is a little low except for big knives ment for heavy chopping, Austempering at 450 degress, which is about 40 degrees below the Ms point and pretty much the limit for creating bainite, will only give an HRc of about 56-57 and would require maybe 6-8 hours. Even up quenching may not be a gooc option for a harder blade because I might have to creat so much martensite for hardness that there will be little left over for conversion to bainite.

One more comment on bainite, it may also form as a byproduct of regular tempering depending on the alloy and the temperatures used.

Steel is complicated but it does have one advantage over women. It won't change the rules if it appears that a man is starting to understand them.:9:

Doug
 
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