Question regarding a coins Rockwell hardness

CaptainRon · December 9, 2008 8:39AM

Will the Rockwell hardness of a coins field, be greater then that of a blank planchet?

Thanks
Ron

AU58WALKERS · December 9, 2008 8:41AM

yes

dan8402 · December 9, 2008 8:43AM

I'm intrigued both by the question and the answer? What would cause someone to ask this and why is the hardness greater after striking? Is it because of the pressure exerted while striking the planchet?

adamlaneus · December 9, 2008 8:44AM

It is due to 'work hardening'. Not all metals do this, but most do.

What would cause someone to ask this? The Rockwell test leaves a mark!

Bayard1908 · December 9, 2008 8:49AM

<< <i>It is due to 'work hardening'. Not all metals do this, but most do.

What would cause someone to ask this? The Rockwell test leaves a mark!

>>

I'm not even sure what Rockwell scale a silver coin would register on. It's too soft for the C scale.

ArizonaJack · December 9, 2008 9:43AM

Any displaced metal will have a higher density and more resistance to the ball used on the hardness tester. So, yes......
The C scale may be too hard for the soft metal used in coins, perhaps a Brinnel ( spelling) would be better?

RWB · December 9, 2008 10:19AM

Does this help? Sterling silver.

Annealing Temp. Hardness:

700°F (371°C) 23 Scleroscope 47 Rockwell
1000°F (538°C) 16 Scleroscope 42 Rockwell
1200°F (649°C) 14 Scleroscope 21 Rockwell

CaptainRon · December 9, 2008 12:33PM

Thanks all,

This actually came about as a curiosity while we were testing a coil 304L stainless. We had a batch come in low on nickel, and it was giving our tube endforms fits on the weld seams. Last year at this time we had a batch come in with a higher nickel content, and had many problems with irregular bend springback and it seemed to chew up our perishable tooling for our endforms quicker as well. Both times the wall thicknes (coil stock) also fluxuated to the high and low sides. Anyways... Back to why the question came up.

The humor and amusement of razzing our poor Quality Engineer while we was fumbling around with our Rockwell tester, which was not even remotely giving him the results that he was expecting, shortly turned to boredom. That is when we got sidetracked to a completly irrevelant converstion about nickels, and to which was actually harder, the two nickels types or 304l stainless, and as to whether or not nickels and coins in general got work hardened after striking.

Well when we asked the poor quality engineer, that we just got done razzing the heck out of for 20 minutes then completed ignoring for another 10 minutes, to throw a nickel onto the machine and test it, the ungratefull chump told us to .... Oh wait this is a family forum ..... anyways I don't think the Rockwell tester was designed to test what he had in mind.

So here we were without an answer to our nonwork related question, and one real pissed off quality engineer that will not let us back in his Lab. (Man they are allmost as cranky as IT geeks)

So I meekly ask -- Do any of you know the answer?

Thanks
Ron

Sunnywood · December 9, 2008 12:48PM

1) In general, the hardness of any coin (after striking) will be greater than the hardness of the planchet (immediately prior to striking). This is due to work hardening, which is a phenomenon that occurs in virtually all metals and alloys. Different metals and alloys may have different hardnesses (for example, tungsten is harder than silver), and different metals may work harden differently (for example, highly ductile and malleable metals like gold exhibit very low work hardening, while high-carbon steels may work harden easily). But generally speaking, working the metal at room temperature by forcing it into different shapes and forms will induce work hardening. Striking a coin is precisely such an operation. Therefore, the answer to your question is that the coin is harder than the planchet. Even the field areas are compressed and worked during striking.

2) Planchets are blanked from rolled and/or clad metal coil or strip. The cladding, rolling and blanking induces work hardening in the planchets, making striking difficult. This is why planchets are "annealed" (i.e. softened by heat treatment) prior to striking the coin. An annealed planchet may be noticeably softer than the struck coin.

3) There are many hardness test scales. They all utilize an indenter such as a diamond pyramid, conical carbide tip, or hardened steel ball to make an indentation in the surface of the metal being tested. The depth of the indentation correlates inversely to the hardness. Hardness test scales include not only Rockwell, but also Knoop, Vickers, superficial, and others. The Rockwell "C" scale is the most familiar, as it measures the hardness of hardenable tool steels. Most of the other scales refer to softer metals.

I hope this helps to answer your question.

Best,
Sunnywood

Nysoto · December 9, 2008 1:05PM

As RWB showed, planchets become much harder when struck into a coin. While cold working (as in a coin strike) the hardness increases greatly from an annealed planchet, the density changes very little, this can be proven by a specific gravity test. Metal displaces, but does not compress.

Question regarding a coins Rockwell hardness

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