[TheForge] Welding 5160

[gblacksmith]

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.

   Hardening is the transformation of austenite to martensite upon quench,
be it air or liquid.  The air-hardening steels will form martensite in still
air or under air-blast, the others require a liquid medium to cool rapidly
enough to form martensite.  This cooling/hardening actually takes place in
stages denoted by "arrest points" on a graph that compares hardness to
temperature.  Chemically, the carbon is bound to other elements because it
did not have sufficient time to go back out of the austenitic solution
because of rapid cooling

  Elements such as  iron, tungsten, vanadium, molybdenum and others, bond
with carbon (an allotropic element) to form carbides of these elements which
are dispersed in an iron matrix.  The more elements you have by weight, the
more such particles can form upon quench.

Tempering converts some of these particles to alpha and beta carbides of
these elements, a term used to denote a change within a body of material.
These new particles are less "hard" that the as-quenched particles, hence a
"toughening" of the steel.  Note that when you increase the tempering temp,
Rc hardness decreases.  This is because of the transformation of more as
quenched carbides particles into the alpha-beta type.

Heat treating large blocks like you are doing must be interesting.......The
largest I have done are hammer heads.  These take a little cooking.


Grant


----- Original Message ----- 
From: "Thomas A. Troszak" <tom@tomtroszak.com>
To: <theforge@mailman.qth.net>
Sent: Monday, August 25, 2003 3:58 PM
Subject: Re: Re: [TheForge] Welding 5160


> > From: "David E. Smucker" <davesmucker@hotmail.com>
> > Subject: Re: [TheForge] Welding 5160
> > Date: Mon, 25 Aug 2003 08:43:47 -0400
>
> > For most Blacksmithing applications the "hold for one hour per inch of
> > thickness" doesn't make sense.
> > The reason it is required is that to control the temperature in a
industrial
> > heat treat furnace the operator adjusts the set point of the furnace to
the
> > desired critical temperature -- and then expects the work piece to come
up
> > to that temperature over time.  To be sure that the piece fully gets to
the
> > temperature we use that 1 hour per inch of thickness rule of thumb.  "We
let
> > it soak" to be sure that the whole piece reaches critical temperature.
>
> Dave,
>
> There is more to it than that. Even when the entire piece is finally at
> critical temperature, the transformation takes a measurable amount of time
> to spread through a thick section.
>
> Soak times are measured from the point at which the entire piece arrives
at
> critical temp., not from when the piece is tossed in the furnace.
>
> I have used a small volume (about 1 cu ft.), high input (300,000-500,000
> btu.) gas furnace to harden large blocks of tool steel (more than 3 inches
> thick) in 90-120 lb. batches, with manual gas and air controls. In this
> situation, it is possible to set the gas throttle to a fixed heat input.
>
> With fixed heat input, the temperature in the furnace remains relatively
> constant throughout the heating phase, even after the surface of the work
> shows a color matching the color of the furnace interior. This indicates
to
> me that the blocks are still absorbing heat.
>
> 20 minutes or so after the blocks first show color, the furnace temp
> suddenly starts to rise, indicating to me that the blocks are now heated
all
> the way through.  The operator must then throttle back the input, over
> overheat the blocks. I count the "soak time" from the temp spike, not the
> first color.
>
> Rough generalization:
> Think of ice melting in a bucket of water on a hot day. The temp of the
> water will not rise until all of the ice is melted, as all the thermal
input
> is absorbed in melting the ice. With the blocks of steel in the furnace,
the
> temp of the furnace remains remains constant (with a constant heat input)
> until the center of the blocks reach the furnace temp, then the furnace
temp
> suddenly starts to rise.
>
> In actual practice, blocks of H-13, 3 inches thick pulled and quenched as
> soon as color matched furnace temp, rarely got above R 40. Blocks soaked
for
> an hour after the temp spike and quenched get to R 50 or R 55 no problem,
so
> "something important" definitely happens during the soaking, long after
the
> outside of the blocks reached critical temp. Perhaps someone on the list
can
> explain "why", I just know "what happened".
>
> Below one inch of thickness, however, I can find little reason (in my
> experience) to soak any appreciable length of time once the critical temp
is
> reached. I have quenched 1 inch dia. punches as soon as they reached
> critical temp, and I have let them soak for 20 minutes at critical temp,
and
> could not discern a difference in the final product.
>
> Also, according to the tech guys at Uddeholm (whom I hold in high regard),
a
> lot of action takes place between 350-150 degrees F. during the quench, so
> they advise quenching all steels to room temp. before tempering.
>
> I am not an expert, I have just hardened hundreds and hundreds (and
> hundreds) of pieces of tool steel, and I keep good notes, and this is some
> of what I have found out.
>
> Also, the recommended critical temp for a given grade of steel varies
> noticeably from brand to brand, even from batch to batch. The product I
got
> from Crucible was vary variable, and even though it was cheap, it was too
> frustrating to deal with on a production basis. The product I got from
> Uddeholm was incredibly consistent, and never varied from batch to batch.
> If you are buying steel from a reputable manufacturer, ask then for the
heat
> treatment data, and follow it to the letter. If you are using steel from
> scrap, you will have to experiment to find the sweet spot. If you only
have
> one piece, you cross your fingers and hope for the best.
>
> Tom Troszak
>
>
>
>
>
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