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Cold forging of ingots


#1

Was: Melting gold

Hi James,

As cast grain structure is very large. The slower it cools the
larger the grains. So forging followed by annealing produces a
small equiaxed grain structure that is best for rolling. Also due
to the way alloys solidify the chemistry varies within the grain so
the annealing helps to homogenize the distribution of the elements
in the alloy. 

Wouldn’t you achieve the same thing by a sufficient amount of
rolling? I don’t mean just one or two passes.

Hammering is inconsistently, and randomly, impacting the the metal
that reduces the metal at the point of impact. Rolling is (usually)
consistently reducing the metal in a uniform manner.

The reason I’m thinking this, is that I used to work in the
aluminium industry, and was fortunate enough to have metallurgists
conveniently located so that I could ask questions. Ingots would come
out of the furnace and be rolled, or extruded, and of course due to
the mechanical forces involved the aluminium alloys were work
hardened. There was no hammering prior to rolling. BTW it was very
cool to watch machines of that size in action :slight_smile:

I’m open to changing my opinion, it’s just have to work out all the
questions that come up due to my personal experiences. I’m not being
rude it’s just they way my brain works.

If forging an ingot is desirable to do before rolling, what’s to
stop me getting a handful of large ball bearings, and a leather bag.
Putting the ingot into the bag, and manipulating the bag with my feet
whilst I do something else with my hands. I’m thinking it would
achieve the same results as the preliminary cold forge before
rolling. It would also make the ingot nice and shiny. This technique
isn’t something I’ve cooked up in the last five minutes, it’s a very
old technique that has been around for about 12 Centuries (although
the ball bearings are my addition, the technique is the same).

Initially I thought that using a tumbler would be the same process,
but it isn’t.

Regards Charles A.


#2

Hi Pedro,

You're still dealing with the big grainsize in delft clay casting.
Forging is still desirable. Grainsize is more concistent with
delfts clay but still to large. 

It seems the consensus is that the large grain size is the issue.
That’s cool I can see that.

I’m thinking that if you refine the grain size by rolling alone, it
could still work, as long as you go for the extra roll. I feel like
an organ grinder playing the music for the monkey :smiley:

I’ve noticed that when I’m rolling my sheet, stock gauge or wire, I
can feel the metal getting harder to manipulate, and that tells me
when to anneal (although reducing by a third and annealing is good
too).

Cold forging a hammer and rolling in a rolling mill are both cold
forging techniques.

Just bare with me I’m putting the pieces together.

Regards Charles A.


#3

Hi Charles,

Bear with me, I’m winging this based on a theory. Jim probably knows
the real answer.

My thought about why rolling is an issue is based on some diagrams
in Brepohl, and a couple of other books that I can’t remember at the
moment. (I can remember the diagram, but where I saw it is an open
question.)

Point one: when an ingot cools in a mold, you get larger grains at
the outside faces of the ingot, oriented generally such that they
’point’ into the body of the ingot. The central body of the ingot
gets smaller, random orientations.

Point two: rolling puts a lot of shear stress on the top and bottom
faces of the ingot. If you look in Brepohl, there’s a diagram showing
metal motion in various parts of the billet when run through a
roller. Note that while the whole thing compresses some, most of the
compression (and thus lateral expansion) is coming in the top and
bottom 25% of the billet. Sets up a lot of shear stress. (Look at
the end of a sheet that’s been rolled down a lot. Notice that the top
and bottom faces stick out on the rear edge, which makes the center
look sucked in.)

Conclusion: with a fresh ingot, you have a natural failure plane,
(because of the larger grains due to mold chilling) right in line
with the boundary plane of the area of maximum shear stress when
rolling. This would seem to be sub-optimal. Not that it never works,
but that it might not. Why risk it when the answer’s pretty
simple?

Regards,
Brian


#4
Initially I thought that using a tumbler would be the same
process, but it isn't. 

If you had a tumbler that actually tumbled like a ball mill rather
than those smooth (inside) rock tumblers that most everyone uses, it
might.


#5
If you had a tumbler that actually tumbled like a ball mill rather
than those smooth (inside) rock tumblers that most everyone uses,
it might. 

A ball mill would work, and it would leave my feet free to do other
jobs :smiley: CIA


#6
Wouldn't you achieve the same thing by a sufficient amount of
rolling? I don't mean just one or two passes. 

No, the distribution of stress is what makes rolling problem, you
cannot get away from it.

Hammering is inconsistently, and randomly, impacting the the metal
that reduces the metal at the point of impact. Rolling is
(usually) consistently reducing the metal in a uniform manner. 

but it is not uniform and on the small rolls we use the penetration
of force is not though out the whole billet. So the top and bottom
surfaces are getting worked and the interior is just getting dragged
along for the ride this is one of the causes of high stress in the
rolled billet.

If forging an ingot is desirable to do before rolling, what's to
stop me getting a handful of large ball bearings, and a leather
bag. Putting the ingot into the bag, and manipulating the bag with
my feet whilst I do something else with my hands. I'm thinking it
would achieve the same results as the preliminary cold forge before
rolling. It would also make the ingot nice and shiny. This
technique isn't something I've cooked up in the last five minutes,
it's a very old technique that has been around for about 12
Centuries (although the ball bearings are my addition, the
technique is the same). 

Nope not going to do anything for you, you need to heavily forge the
material, 50% reduction in thickness type of forging.

Seriously get the Brephol book and read it.

James Binnion
James Binnion Metal Arts


#7
but it is not uniform and on the small rolls we use the
penetration of force is not though out the whole billet. So the top
and bottom surfaces are getting worked and the interior is just
getting dragged along for the ride this is one of the causes of
high stress in the rolled billet. 

Ah, that pretty obvious when you spell it out :slight_smile:

Seriously get the Brephol book and read it. 

Yep I’m going to get Brephol’s book, seems to be what I need to tie
up loose ends.

Regards Charles A.


#8

Ok so today I poured a bunch of ingots and decided to forge them
before I went to the mill just to see if this made any difference. I
would say that my ingots rolled out better than they ever have and
the draw plates produced a cleaner brighter wire than they ever have.
A very visible difference. I am forging from now on. Try it yourself
and you will be able to tell the difference it is quite drastic.


#9

So…

I fairly often convert “bought as scrap” gold bands into sizing
material, wire, etc. Should I straighten, flatten, and forge them
first, before running through the mill?

-BK in BWA