[TheForge] Welding 5160

Tue Aug 26 08:06:00 2003

        Grant,  Thank you for all the information and the suggestion on the
book to look for.  I appreciate all the time you took to write this up for
all of us.
        It makes me realize this is another topic to study so much deeper
into like so many metalworking process we take for granted or have just
scratched the surface on.  What a great explanation, Thanks.

Ralph

----- Original Message -----
From: "gblacksmith" <[email protected]>
To: <[email protected]>
Sent: Monday, August 25, 2003 8:40 PM
Subject: Re: Re: [TheForge] Welding 5160


> 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" <[email protected]>
> To: <[email protected]>
> Sent: Monday, August 25, 2003 3:58 PM
> Subject: Re: Re: [TheForge] Welding 5160
>
>
> > > From: "David E. Smucker" <[email protected]>
> > > 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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