Explanation of "overbuffed" dies

[messydesk]

I took a little time at the ANA show to talk to Chris Pilliod, who knows a thing or two about die steel and manufacturing of dies, about these coins. My assumption has always been that the die sunk or collapsed a little in the affected areas, contrary to the "overbuffed" moniker that has been given to these.

Chris explained one of the differences between die hardening now and in the 19th century. In order for the die steel to be hard enough to strike coins, it needed to have a carbon content of 0.5-1.0%. This had long been known and wasn't a new concept to the mint. Part of the process of hardening the dies was heating the dies in a furnace. The dies were packed in carbon to protect them from gases in the furnace, particularly oxygen. If the dies weren't sufficiently protected, the oxygen could react with and leech out the carbon in the dies, reducing the carbon content close to the die's surface below what it needed to be to strike coins. These dies would have a soft surface and quickly wear, developing the ripples we see. Today the heating of dies for hardening is done in a vacuum, all but eliminating the problem.

[Unc90o]

Very interesting. Thank you for the news.

[Jodemyer]

Thanyou! @messydesk
Very interesting

[vamnuke]

[vampicker]

That's interesting, thanks John

[iwillbenice]

Carbon powder?

[RogerB]

Charcoal.

An additional difficulty was accurate control of temperature for both hardening and then tempering the die. The Royal mint did extensive steel testing and had a metallurgist and chemist on staff. The U.S. Mint had the seats of several people's pants.

[MarkyB]

And here I thought a Blacksmith just dipped the die in some coals until red-hot and then dipped it in water. JK but good stuff John. Keep 'em coming plz.

[keilg1]

Chatted with some coin folks in the UK and they agreed 'overbuffed' was not the correct term, too. They liked the fun varieties I've shown them: 1880-O VAM-45A, my 1880-O VAM-81 DC, 1881-O VAM-16, 1883-O VAM-1C1 and 1C2, and the 1890-O VAM-13A amongst others...

Trying my best to generate excitement across the pond and this information helps! Thanks JB.

[Albannach]

I’m curious was the treatment applied during all softening stages? Both before and after setting the impression in the working die?

The leeching of carbon would also help explain cracking “patterns” in both Morgan and peace series…seemingly occurring in a cyclical pattern millimeters fro the edge, basically creating a weakness where carbon “stopped” leeching from the exterior surfaces.
I would also submit the die would need prolonged striking to achieve sufficient “softening” for the inner steel to become viscus enough to flow.

Good info, a lot more questions now too
Thanks JB

[RogerB]

I’m curious was the treatment applied during all softening stages? Both before and after setting the impression in the working die?

The leeching of carbon would also help explain cracking “patterns” in both Morgan and peace series…seemingly occurring in a cyclical pattern millimeters fro the edge, basically creating a weakness where carbon “stopped” leeching from the exterior surfaces.
I would also submit the die would need prolonged striking to achieve sufficient “softening” for the inner steel to become viscus enough to flow.

Good info, a lot more questions now too
Thanks JB

Hmmm...where to start?

Annealing in an oxygen free atmosphere was the goal each time a working die was heated...either between multiple blows to transfer from the hub or for final tempering. The US Mint folks were not metallurgists unlike Roberts-Austin at the Royal Mint -- our Mint mostly fumbled around and tried their best....Kind of like TPGs trying to decide what a "Specimen" is. Our mint used the same basic face hardening method in 1890 as in 1797. (See From Mine to Mint for illustrations, details, etc. -- although I omitted the carbon material mentioned by Chris P.

Most of the "hardening" took place on the die face and to about 10mm below. The Mint's standard process was to make only one hub blow and annealing per work day for each die. (Morgan said in 1877 that 2 could be done in a day.) Almost all "die cracks" were superficial -- that is, on the surface of the die face -- and were shallow enough to be polished (abraded) off during routine die maintenance.

Failure to correctly, and thoroughly harden and temper dies could cause collapse, deep cracks, early failure, or catastrophic failure (die explodes like a grenade).

[Albannach]

I will re-visit that chapter thanks Roger

So I guess we’re back to the quench…

Remember, the last thing you really want to ever have to do is quench harden more than once it weakens everything and causes stress fractures

There are precious few “scarface” type breaks…but plenty cyclical rim cracks and breaks. There must be a manufacture reason for that.

Good talk!

[RogerB]

From JNR, Spring 2013:

The turned die is then passed to the hands of the die hardener to be hardened and tempered for use in the press. The hardener is provided with a square cast iron box which has side loops attached and a close fitting cover. These loops are used with metal rods to lift the box and place it in the furnace and are similar to lifting bars found on common tubs. These boxes are made in different sizes according to the number and size of dies to be hardened.

To prepare for hardening, a charge of equal quantities of finely powdered char-coal and common soot is placed on the bottom of the box. Several dies, but always six or fewer, are laid face down on the charcoal. They are then covered with charcoal powder and placed in a furnace fired with charcoal. The Paris Mint hardener, who has been doing this work for many years, is under the impression that animal charcoal does not make the dies as hard as charcoal made from wood.

The heat of the furnace is regulated by steam and dampers as described by M. A. Puymaurin in his modifications to M. Bonnemain’s furnace regulator. When the dies are brought to a clear cherry red, which is ascertained by lifting the lid and examining one of the dies, an iron rod is passed into the loops on the box and it is lifted out safely and expeditiously. The assistant lifts off the cover and one of the dies is taken out, slightly brushed in order to remove the charcoal, and laid on its face. It is then seized by a pair of tongs and put into the water.
[Based on Peale Report, Note 5, pp.208 and contemporary French materials, 1835.]

[rodmeader]

Thanks for the information, JB.

[UNCLE BINGO]

I'll bet the guys performing this work had no idea how we today find their efforts fascinating . Thank you Roger