[TheForge] Re: Test mail

[Mike Spencer]

> Any form of cold work breaks up the large crystals into smaller
> ones.


I'm not a metallurgist, either.  Not even a chemist, just a former
one.  So I'll *try* not to say things that I know that ain't so. :-)


There are two major and  different elements of structure in steel: 

     The crystal lattice, which is highly but not perfectly regular.
     The crystal lattice contains iron atoms and may contain other
     kinds of atoms is some tidy, regular arrangement with the iron
     atoms.  This is what's *inside* the grains.

     The grain structure which, although very organized, has a lot of
     random, swirly [1] quality about it.  The spaces between grains
     may contain all kinds of helpful stuff or pernicious crap.

So far as I know, gentle cold working doesn't change the grain
structure.  You have to get it hot or stress it really hard -- put in
a lot of energy -- to change the grain structure.

But cold working *can* change the crystal structure.  The crystals are
highly regular, like 3D matrices of TinkerToys(tm).  But they tend to
have defects, as if you were trying to build a big 3D TinkerToy matrix
with the help of three hyper-active six-year-olds: most of it will be
properly connected but various regions won't match up right.  Unlike the
TinkerToy model, when you stress the steel, those defects can move
within the crystal, a mis-aligned bond changing to a tidy one and
passing the mis-alignment on to its neighbor.

Somebody who knows more than I can tell us just how this affects
toughness, hardness etc.

The stuff between grains makes a big difference to steel properties.
When I visited the Sydney (NS) steel mill, the guys blew out the plug
on the open hearth furnace and started tapping the molten metal into a
big pot hanging in a gantryway.  Then a couple of them went out on a
catwalk and casually tossed in a few 50# or 100# sacks of lumpy stuff.
Our engineer guide said that it was manganese.  If I recall correctly,
he said that there was sulfur dissolved in the iron and as it cooled
it would be incorpoarated in the crystal lattice and make the steel
brittle.  The manganese replaced the sulfur, forcing it to the grain
boundaries, and eliminated that problem. [2]

> This sort of thing happens all the time. Isn't this just semantics?

Yeah, and interesting semantics.  Old Lefty Griswold assured me that a
broken crankshaft broke because it had "crystallized".  Lefty never
finished high school but he had 40 years of working on engines from
the 20s to the late 60s.  He could recognize *something* about the
appearance of the fracture surface that was, in his experience,
consistent with fatigue failure.  No use to explain to him that *all*
steel was crystallized; that wasn't what he *meant*.

So I'm willing to believe that guys who made knives with materials
available before, say 1940 or 1890 or whatever, gained something
(perhaps only on average, some of the time) from the move they called
"packing".   Bert Shaw, Ph.D., the guy who got me interested in all
this stuff, told me about packing a knife edge.  He would have learned
the craft around 1915.

My 2 cents worth.  May be over-valued at $0.02. :-)

- Mike

[1] One them technical terms, dontcha know.

[2] Have I got that right?  It was years ago.  Correction welcome.

-- 
Michael Spencer                  Nova Scotia, Canada       .~. 
                                                           /V\ 
mspencer at tallships.ca                                     /( )\
http://home.tallships.ca/mspencer/                        ^^-^^

--