[TheForge] Heat Treating 5160 was Welding 5160

Tue Aug 26 07:35:01 2003

Interested parties:  Transformation time from pearlite and ferrite to
austenite is a function of the mass of the piece, rather than its alloying,
assuming critical temp is reached.  When critical temp is reached, the
transformation begins in earnest.   Why?  the mass is the key, as the more
thickness of material you transform, the longer it takes, just like melting
butter.  Which would take longer at a given melting temp, a pat or a whole
stick?  My observations do not conform to the data attributed to Mr. Bryson.
I have not been able to get even very thin w-1 knife blades to fully harden
in five minutes at critical, as measured by pyrometer, using oil quench.

 However, in a complex steel such as ATS-34, there are refractory elements
that go into solution before the critical temperature is reached.  When I
heat treat ATS, I use a double quench, one for the lower temperature
elements and one at critical temp, which is the point that all of the
elements are in solution.  The second heat/quench allows the piece to be
properly hardened without excessive time at or above critical temp.  How
long does this take?  I use the 1 hour per inch of thickness rule
successfully.  If I'm doing a 1/4" section......15 minutes at critical, as
measured by pyrometer and stop watch.

Why?  Refractory elements (those added vs. naturally occurring) have, in
most cases, incredibly high melting temps.  When combined with iron, carbon
and each other, these temps are drastically reduced.  Another example of
this is the lead/tin solder that melts at 400F or so, even though lead and
tin individually have much higher melting points.  Such steels, like the
solder, are called "eutectic" and because of the high alloy content,
experience large amounts of retained austenite at quench.  These steels
require, in most cases,a double temper as well, to transform the remaining
austenite, which is unstable, into martensite.

The critical temperature should not be exceeded for any length of time, lest
grain coarsening occur.  Elements like vanadium and tungsten tend to retard
this phenomenon, which is why they are added.  These elements also add
abrasion resistance to the finished product as well.

Preheating is a good idea, as it is not good practice to plunk a piece of
room temperature steel in a critical temp environment.  I think in terms of:
time+critical temperature= transformation, rather than "soak"   The preheat
must be followed by ascension to critical temp.

More complex alloys typically have much higher critical temps due to the
high alloying, especially with chromium.  They require higher temps vs.
longer soak times.  If you  do not reach critical temp, you will not have
complete carbide formation, no matter how long you soak.  Transformation to
austenite is a function of both time and temp, neither can be forsaken.

On deep hardening:  Some of the refractory elements are added to promote
deep hardening. Chromium does this, even in small amounts.  That's why O-1
hardens so nicely!   Dr. Bain observed that the highest recorded hardnesses
were reached in plain carbon (1095) steel using a water quench.  He also
noted that such hardening was more often accompanied by cracks and
distortion than the same steel quenched in oil.  Bain concluded that such
steels had a high "intensity of hardening" when measured at the surface.  He
went on to state that steels that hardened fully in oil often produced a
martensite that was "deeper" and "tougher" and he described such steels as
more "hardenable" vs. intensely hardening.

Grant
----- Original Message ----- 
From: "David E. Smucker" <[email protected]>
To: <[email protected]>
Sent: Monday, August 25, 2003 6:46 PM
Subject: [TheForge] Heat Treating 5160 was Welding 5160

> Tom, Grant, Ralph and others -- still trying to learn here, so please take
> my comments in that vain.
>
> The question seems to be "how long does it take for the transformation to
> take place -- once we have reached critical temperature?"  It is clear at
> least to me that the center of a given part -- if it is to be heat treated
> has to reach -- and go somewhat above the critical temperature for the
> transformation to take place in all of the steel.  Once the material at
the
> center has reached critical temperature or slightly above -- how long does
> the transformation take, and why should it take more time (the
> transformation not heating) than a thin section?  I don't believe the
> transformation to be instantiations -- but I do believe it to be somewhat
> fast -- and the higher the temperature is above critical the faster it
would
> take place.  I can find lots of technical information on the rate of
> transformation in cooling (quenching) but no data on the rate direction of
> heating up or the transformation of  pearlite and ferrite to austenite.
>
> I have watch the transformation take place before your very eyes on
cooling
> as product from a hot strip mill is run out on to the run out table (and
> again it is thin at this point).  Because of the volume change the strip
> will form a loop or hump up on the table as it cools through the
> transformation temperature.  This has always been a low carbon or low
alloy
> product.
>
> I am also not suggesting that the transformation on heating, would in fact
> occur at the same rate as on cooling.  I simply don't know.  But wouldn't
it
> be similar?
>
> I still go back to my conversation with Jim Batson -- that the prime
purpose
> of the soak time was to insure that the center of the object reach
critical
> temperature and than the transformation took place throughout the piece.
> Question is how much of this soak time was to insure full heating and how
> much was to insure transformation?  What steel were we talking about ?
1095
> ?  not sure.
>
> Most heat treating manuals I have read, assume that you have a heat treat
> furnace with an automatic controller and suggest you set the operating
> temperature at the prescribed heat treat temperature (and not above as in
> our forges).  They then tell you to heat until the surface color of the
part
> is equal to the color of the furnace and to then start counting soak time.
>
> It also would appear from the soak times suggested that the more "complex"
> the steel i.e. the high the alloy content, requires longer soak times -- 
> hence longer transformation times.
>
> Bill Bryson list the following soak times based on various steels.  I
reread
> that tonight and noted that they were not all 1 hour per inch!!   I missed
> this before -- having often heard the 1 hour per inch rule.
>
> W1 -1095     5 minutes per inch
> 4140 -           5 minutes per inch
> O1-               5 minutes per inch
> H-13             30 minutes per inch
> S-7                 1 hour per inch
> A-2                 1 hour per inch
> D-2                 1 hour per inch
>
> This didn't sink in until rereading Bryson tonight.   Looking at these it
> would seem to me that shorter time alloys 1095, 4140, and O1 show mostly
> heat transfer time with a rather fast transformation time -- while the
more
> "complex" alloys require a much longer transformation time.  (Heat
transfer
> time for all of these will be similar but longer in the high alloy
steels -- 
> i.e. stainless steels as a class have a lower heat transfer rate than
carbon
> steels)
>
> No data on soak time for 5160 -- but it would seem to be more like 4140
than
> D-2   so I would be think in terms of a 5 minute per inch soak time.
>
> As I reread this post before sending I realize I may have answer my own
> question -- those steels designed by alloy additions to have great depth
of
> hardness or hardenablity by their very nature have a much slower
> transformation reactions.  And this slower transformation works in both
> directions.  In the quench direction it gives time to get the cooling deep
> into to the object -- hence the depth of hardening -- or use a slow quench
> medium such as Air to lower the amount of distortion -- in the heating
> direction it means longer "soak" times.
>
> Dave Smucker
>
>
>
>
> ----- Original Message ----- 
> From: "Thomas A. Troszak" <[email protected]>
> To: <[email protected]>
> Sent: Monday, August 25, 2003 6:58 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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