[TheForge] Welding 5160

[gblacksmith]

Thomas:  You have hit on a fundamental concept here.  The phenomenon you
describe is real and in tool steels, is known as coring and segregation.
Think of the process of steel going from molten to solid as a form of
freezing in which the material is solidifying from the outside in.

The outer edges, cooling faster, will produce a finer grain structure than
the slower cooling center, which will reveal larger grain structure.  This
is because the presence of heat in the center allows slower cooling which
makes larger grains possible.  This is not problematic unless the grains are
allowed to grow too large (coarsening).  The solidification takes place over
a range of temperatures as the material cools, producing differing
structures.  The crystals grow and grains form unless they are prevented
form doing so by collision with other grains and the faster cooled, smaller
grains at the margins act as a physical barrier.  This is why the grains
appear to be larger at the center....their growth is arrested later than
those at the margin.

In the fully molten state, metal is homogenous in crystal size.  Once the
solidification process begins, the crystals form according to the rate of
cooling and the piece ceases to be homogenous, forming finer crystals at the
margins and larger ones toward the center.  This is especially true of high
alloy steels containing elements that form large amounts of segregate
structures.

This is the condition of the steel as it comes from the mill.  Even though
the steel has likely been annealed and is soft because of a lack of formed
martensite, the segregates are still present in the form of dendrites
(tree-like crystal structures).  It is the forging that initially breaks up
these large structures into smaller, more homogenous structures.

The best explanation of these phenomena are found in an article entitled
"The Blade:  Why Forge?"  by Dr. Charles Bear and Ronald Koeberer.
Published in the January 1990 edition of Knife World, this article has a
large bibliography


Grant
----- Original Message ----- 
From: "Thomas A. Troszak" <tom@tomtroszak.com>
To: <theforge@mailman.qth.net>
Sent: Monday, August 25, 2003 8:05 PM
Subject: Re: Re: Re: Re: Re: Re:[TheForge] Welding 5160


> > From: "gblacksmith" <gblacksmith@alamedanet.net>
> > Subject: Re: Re: [TheForge] Welding 5160
> > Date: Mon, 25 Aug 2003 17:40:37 -0700
>
> > Thomas:  Ditto....the "something happening" you refer to is the
formation of
> > austenite over time, at the critical temperature.  This critical
temperature
> > must be held long enough for this transformation to occur.  Your
> > observations support what is known to  be physical law.  Upon reaching
the
> > austenitic state, elements such as iron, tungsten vanadium and
molybdenum
> > join carbon in solution.
>
>
> Dear Grant,
>
> I have a real question this time. I once poured a casting from zinc, and
> when cut and etched, it was apparent that the grains near the "chilled"
> outer edges were quite small, and the grains near the center were quite
> large, where they had time to grow before (or during) solidification.
>
> Is there a corresponding effect occurring in the tool steel? Will tool
steel
> "as quenched" have smaller grains (and thus greater hardness and
> brittleness), and then do they (the grains) grow again while tempering
(thus
> helping making the steel tougher)?
>
> Am I even close?  Or is grain size not a factor?
>
> Also, It seems to me that the fracturing of larger, tougher grains into
> smaller grains (by mechanical stress) is essentially the mechanism of
"work
> hardening"  Yes, No?  Is a light about to go on in my head, or am I
barking
> up the wrong track here?
>
> I am actually really curious to know, even if I have put my foot in my
> keyboard again...
>
> Tom Troszak
>
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