Paul Borian on fri 5 oct 07
I have always wondered why the tool-steel cams on the breather valve for
my Davis style pugmill wear out faster than they should - Vince told me
that he was getting like 7 years out of his cams where as I have
replaced mine at least once per year, sometimes twice, even though I
followed the directions for quenching them after welding on his plans. I
just got off the phone with someone from Speedy Metals in WI who was
amazed at how clueless I was about tool steels when I was asking about
pricing and various grades - one of these guys who kind of laughs at you
but also helps answer your questions at the same time - and by the end
of the conversation I realized I have been doing it all wrong. He said
for A-2 tool steel (the most common grade) you need to heat it up with a
torch to around 1750 F and then let it air cool instead of quenching it
because it was designed to be an air-hardened steel. I guess Vince was
using a totally different type of tool steel that worked best with
water-quenching.=20
For anyone out there familiar with tool steels, does this sound like the
best way to go for the application or should I consider a different type
of tool steel that was designed to be water-hardened? I am sure they guy
I spoke with knew what he was talking about, but I would still like to
hear it from someone else just to be sure.
Thanks,
Paul
Ben Shelton on fri 5 oct 07
Paul,
Depends on what the original steel type is.
The most common old world tool steel type is high carbon steel. This steel
is hardened by heating to the correct temp (usually when it looses its
magnetism and a magnet wont stick to it) and quenching is some liquid. The
O-1 type like to be quenched in oil heated between 130f and 200f. The W-1
types like quenching in water or salt water, not sure about temp.
The A- types of steel are air hardening and when heated to the right temp
are usually cooled in air.
If you have heated and cooled your steel and it isn't wearing well I suspect
it isn't tool steel at all but plain steel. If so you can case harden it so
that it develops a hard skin but the whole piece of steel isn't hard through
out. This is advantageous when you don't want the piece to be brittle. It
depends on the size of the part though because the procedure for case
hardening envolves packing the part in some sort of air tight, heat proof
container with the case hardening agent that is teh source of carbon and
heating for several hours at high heat. The part is the removed from the
container and quenched.
For you, the easiest thing might be to buy some hard surfacing welding rods.
These rods can be welded on the existing piece of steel in a layer and will
be HARD HARD HARD when done. Layers can be built up as needed. Ask your
welding supplier for a recommendation as to type.
I'm not sure which pugmill part you are referring to since I don't have my
book here with me but I would bet that the parts are made of mild steel or
plain steel. It doesn't have enough carbon in it to harden.
Does this part have a designation on Harry's plans? I'll look it up and make
further recommendations.
Hope this helps,
Ben Shelton
T. Toler on fri 5 oct 07
Greetings
A2 tool steel is air hardening - hence the "A" designation
(Reference - Jim Hrisoulas "The Complete Bladesmith")
Breakdown of A2 -
Carbon 1%
Chromium 5%
Molybdenum 5%
Wear resistance - high
Toughness - medium
Distortion in heat treating - low
Red hardness - high
Forging Temp 1850 - 2000 F
Austenite forging - Yes
Hardening 1700-1800 F
Quench - Air
Tempering 350-1000 F
RC hardness 62-57 Rockwell Degrees
Air hardening tool steels
How to harden Carbon tool steels - good rule of thumb -
A torch will heat small thin pieces of steel fairly evenly but it would be
difficult to heat a large metal mass evenly such as a cam.
It would be better to form a small brick kiln and use a forge burner to heat
the item. After all - forges are just small kilns.
Do not over heat or heat unevenly as you can and will get different hardness
in the steel or could burn the carbon out of the alloy.
A 2 steel only has 1% carbon content. The carbon is what will make it hard -
burn out that 1% or even just .25% and your steel is no longer going to
harden properly.
To determine correct heat at the steel itself -
take a magnet and touch to steel - it sticks pretty well when cool.
Slowly and evenly heat the steel until it becomes non magnetic - the magnet
no longer sticks.
At this point the steel is in "transition phase" - and loses its magnetism -
and is ready to be pulled and cooled.
After the hardening process, the steel will be totally hard, you may want to
draw a temper depending on the use,
tempering will reduce the Rockwell hardness of the steel but improve its
toughness, ability to withstand shock and torsion.
A2 steel tempers at 350 -1000 F the higher temperature gives softer and
lower Rockwell hardness.
T. Toler
Texas
Ivor and Olive Lewis on sat 6 oct 07
Dear Paul Borian ,
The air hardening metal you are asking about will be one of a family of =
alloys known as Carbon Alloy Steel. The composition I have is Carbon =
1.0%, Chromium 5.0%, Vanadium 0.5%. Molybdenum 1.0%. remainder Iron.
Anneal at 970 deg C, air cool and temper in the range 380-400 deg C. =
Fully hardened steel can be brittle and needs to be tempered if it is =
subject to impact or bending stress..
The lower the carbon content the faster a steel has to be cooled.
If you wish to learn more about these processes, a good introduction is =
W. Alexander and A. Street. "Metals in the Service of Man". Penguin =
books.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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