The wonders of experimentation...

Re: The wonders of experimentation...

Wiley Horne

Interesting journey David! I think you are, through trial and error, seeking a heat treat regime which is optimal in some sense for clean steel.

It is well-known that a key to forging tougher steel is to keep the grain size small (this is one of the important features of particle steel, CPM). About 20 years ago, a Prof. Verhoeven of Iowa State was (aside from his academic metallurgy) working with blacksmiths on various projects. He decided there needed to be a text which was not dumbed-down, but which could be read and used by educated non-metallurgists. The book is ‘Steel Metallurgy for the Non-Metallurgist’. It is expensive when you can find it. But because Prof. Verhoeven’s work was paid from public dollars, he made a copy of the book available to everyone on-line. And here it is:

https://gooddebate.org/sin/mirror/library/skills/blacksmithing/Metallurgy_of_Steel_for_Bladesmiths___Others_who_Heat_Treat_and_Forge_Steel_-_By_John_D._Verhoeven_(2005).pdf

You might find something that rings a bell in Chapter 8, ‘Control of Grain Size by Heat Treatment and Forging’. Two specific techniques, Phase Transformation and Recrystallization, are described starting p. 66 or so. Essentially they involve triggering new sets of grains to form, by heating and cooling, and by hammering. The point is, that each new set of grains is seeded inside the boundaries of the old grain set, and hence can be reduced to very fine size by repeated heating and cooling, with or without mashing down the cross-section by forging.

Wiley

Re: The wonders of experimentation...

David Weaver

Thanks, Wiley!

Interestingly, I saw a 1973 video from the BBC that showed an animation of new grains forming in 0.8% carbon steel, and they described what seems to be well accepted by knife makers now - that normalization before quench is important (to get a clean set of new grains). Their animation showed the emergence of grains beginning around 750C (emergence starting at the boundaries of old grains).

The interesting thing to me is both that, as well as off-the-charts things people do with a forge to try to make up for the shortcomings of not having perfect heat control.

In the knife world (For anyone with the desire to buy an oven), it appears to be standard to normalize steel in an oven (instead of a forge) and then quench from there - so temperature and duration are well controlled. And then two tempers.

For the forgers, the forge anneal, hammering and triple quench seems to be an alternative.

(Of course, annealing in the forge and then hammering and just heating and quenching also works - it's surprising with the simpler steels how close the grain structure is from one to the next - and what it leaves me wondering is if I'm going to hammer out knives and chisels, am I really approaching anything needing this ideal treatment? I don't think I am, but I'll find out. I wouldn't be surprised to find that annealing a file and then giving it a forge soak and the a pair of quenches and a temper makes just as good of a chisel as hammering and being more careful....

...but the higher end knife makers who push angle and then do things like cut nails with their high hardness knives do show what's possible.

I'm less searching for perfection than guarding against downside. I don't want to come up with a routine and then find out it's less good/more coarse than just annealing and shaping the files and being more careful about temperature control.

I'm not interested in buying a heat treat oven at this point.

I'm headed for your link to read and see what I can glean from it.

Re: The wonders of experimentation...

David Weaver

It appears that document is the origination of the now popular (and controversial) triple quench method.

(for people other than wiley) for steels with complex carbides, triple quench done carefully (first quench doesn't really have a temperature limit, but the last two quenches are done above critical and below the temp where grain growth occurs, and more carbon/other elements are incorporated than a single quench). I guess it does require the skill to get temp right by eye.

More complicated temperature schedules allow control of the grain size better than any of this, but one has to have an oven (and better, an oven with a vacuum environment) to get temperatures correct and normalize prior to quench.

Bottom line, the triple quench lands somewhere between no temperature control and a single quench and the most well done normalizing and then quenching.

The complication for a garage amateur is that some things that benefit 52100 don't benefit 1095 and so on.

I did find out that I can harden 1095 perfectly fine with soy oil, but a proper fast transition oil for quenching (like parks 50) will get the starting point a point or two higher without causing cracking like brine can.

first grain pictures.. going backwards

David Weaver

...so, this is from files and not from 1095. I'm assuming they have more surplus carbon than 1095, but using a triple heat process mentioned in the text that wiley provided (the trouble being that I have no idea what's in the file, but it's probably not 1095, so the process provided may not work).

The first picture is the tang area of a knife (I already broke the knife last night), it was heated and quenched and a file skids off of it - no forging:

The quench was just past non-magnetic (not super high temperature)

I broke a piece off (it came off readily).

The second is three subsequent heats to non-magnetic for about a minute or two (I think these need to be longer to actually improve anything - I have enough control in my can forge to have anything from black heat to yellow as long as I continue to monitor things).

This is broken off of the next length of the file (after the first piece).

Going the wrong way from what I can tell!!

The curiosity here is the dark grains don't appear to be too much larger, but the shiny bits - presumably they are carbides forming from surplus carbon and whatever else is in the steel.

I'm not sure how much this grain size matters on chisels vs. someone pushing the limits on a high hardness knife and cutting nails with it.

Strangely enough, the second bit with the larger carbides also led to file skidding, but as quenched, it took much more force from the ball pein hammer to get the shard to break off.

Considering the size of defects that we typically make in chisel edges (if we make them), it probably makes sense at this point just to forge files, trim to shape and then heat quickly and quench and temper and save experimentation with temperature cycling until down the road if I ever find a used oven locally.

I'm guessing that at the very edge of the cliff here, destructive experiments with processes and the time and effort involved is why someone like Joe Calton or some of the other more specialized knife forgers don't deal with very many steels.

I'll hammer the same file end moderately a little later and then quench once and see what happens.

the structure here is more visible

David Weaver

...than it would be in a honed chisel, etc, as the breaks are clean breaks and no polishing or grinding is done to them. what I gather from reading other technical documents is that the breaks can make things visually difficult to confirm and that it would be preferable to level and polish the surface of a test piece and then etch it.

I'm not going that far.

(I did send steve three subpar chisels, though - two that I allowed to get hot and sit in the forge too long because I was flapping my gums making a video, and a piece of 1095 that will benefit from quench oil that I'll get over the weekend or next week depending on UPS shipping time - parks 50 fast transition quench oil for 1095).

With my skill level, I may find what I found with XHP, and that is, do whatever you're going to do to shape a piece (in simple steels, forging, which does some of the heat cycling itself but leaves desirable deformations before heating to austenite), heat it quickly, a step past critical and then quench quickly.

We'll see if pictures prove that. If I'm right about that, it will more or less show that I need to control grain size or structure by hammering and then minimize high temp time to prevent grain growth.

comparing light forging

David Weaver

I don't really forge the chisels, which would be a high heat setup with significant working. Without the hammering in forging in that type of setup, the grain size in anything would grow large (if one were just to take bar stock and heat it to yellow heat and leave it at that to soak). The whole idea of temperature control in normalizing before heat treatment is to heat the metal but not high enough that grains enlarge. Temperature for this varies by starting grain size, steel type, etc, and the soak times are usually in an hour to hours.

Moderately overheating something that's already fine and heating it (if it's quick) doesn't usually lead to that much damage if the heating isn't done repeatedly (the grain size doesn't increase that much).

So, anyway, this is the same file after the large grain picture from triple heating just heated to somewhere in the ballpark of 1700 (which would be enough for the grain size to grow) and then hammered to about half thickness. This is what I do to shape my chisels and give myself a break hand finishing them. The objective after this, I guess, is not to allow the steel to sit in the forge too long at too high of a temperature.

Sorry, the break is curved, so you can't see too much (the focused area is right down the center). It's not as fine as the first picture (which is a bit puzzling -that was just quick heat of a file that had already been heated several times before), but it does look like some improvement over the triple heat picture. I'll have to repeat this another time or two with another file and see what I can come up with. This broke off much more easily than the large grain picture (which is puzzling, but these haven't been tempered, so it may not amount to much and it could just be the odd shape from hammer marks made the back side unsupported and easy to break).

(and a repost of the triple heated picture for comparison - again, the picture above was taken on the same file, same general spot *after* the piece below was broken off prior. the forged bit is only about 3/16" further down the file length than the prior piece.

metal microscopy

Bill Tindall, E.Tn.

I once worked next to the lab that prepared metal specimens for microscopy. The had various etching techniques for revealing whatever they were wanting to look at. Presumably the etching is necessary.

Re: metal microscopy

David Weaver

That's my understanding - if you really want to be able to tell two specimens apart and won't settle for "well, I think this one looks better than that one", then you have to make a planar surface, make it smooth and level to the microscope's lens and then etch it so that you can see all of the boundaries and shapes.

The field of view is so shallow as a layer that even the smallest of undulations makes for blurry and smooth areas. I don't even know what I'm looking at yet, anyway. Just trying to make things look "finer with smaller bits".