[TheForge] Welding 5160

[Thomas A. Troszak]

> 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