Please help me, I'm going nuts over this problem. I am trying to heat
treat O1 oil hardening tool steel but I keep getting a soft layer on the
outside of the part. The layer is only about .010 to .020 deep and
ranges from 47 to 60 RC. Below this layer the part is at least 70RC
before tempering. The part is about 1" dia by 1" long. My problem is I
need to be 62RC on the outside after tempering. I'm using a vacuum oven
and have tried temps between 1450 up to 1600 degrees F. Naturally I'm
quenching in oil. I thought I had the 3 basic requirements- high carbon
steel, heated above the critical temp, and cooled faster than the
critical rate but obviously I'm missing something. The outside is not
loosing carbon as I'm in a vacuum, I'm quenching quickly, and agitating
thoroughly. Does anyone know what I'm doing wrong? Thanks.
Tom
I'm a little confused at your process, how does the work item get from the
vacuum furnace to the quench oil? If it is removed from the vacuum furnace
and then plunged into the quench oil this is where the decarburisation takes
place, hence the soft outer layer. My advice would be to wrap the work in a
tool steel foil and then harden. For the best results with O1 tool steel at
work we harden at 800 degres C in a neutral salt bath and quench into a
martemper salt bath at 200 degrees C, thus the work peice is protected by a
layer of salt at all times thus stopping any decarb and giving a surface
hardness of 64RC.
Ian
Thanks for the advice, I'll try the wrap idea. I can't ever recall
problems in the past even when using a torch. I thought the decarb
problem took quite a while. For reference I'm removing the part from the
vacuum oven with a large set of tongs. The total time between releasing
the oven vacuum and quenching take less than a couple of seconds.
Ian Humphrey wrote:
Tom
When using a torch the thing to remember is that both acetylene or propane
both contain carbon in there chemical composition and therefore will stop
any decarb or oxidization when the flame is set correctly. As to the decarb
I should also really have mentioned oxidization, which would occur instantly
when you remove it form the protective vacuum, I am assuming scale is
forming on the work piece before it gets to the quench, also if the vacuum
is not high enough this would be detrimental. The vacuum furnaces I work
with go down to about 3 x 10 -4 and use nitrogen quench straight into the
main chamber in conjunction with a circulating fan so I am not au fait with
the concept of heating in a vacuum and then removing to quench in oil. My
furnaces would go into self-destruct mode if I tried to open them at high
temperatures. Pumps would explode and elements disintegrate, but I have
treated more O1 in a salt bath than you can shake a stick at.
Ian
You do not say whether the tool steel you are trying to harden has been
ground prior to heat-treatment. All the "black" carbon tool steels have a
layer of about the dimensions you quote that has lost carbon during
manufacture. It has to be machined away prior to ht if you are to achieve
full hardness at the surface.
Bob
Sorry, I should have pointed out that it's the "decarb layer removed"
type. Also, after hardening the part is soft all around including
surfaces that have machined down from the raw bar.
Bob Redfern wrote:
This is probably a decarbonization problem, the carbon in the outer layer is
oxidized therefore the alloy in the outer layer is no longer a high carbon
alloy. You need to use an inert gas atmosphere or a carbon rich atmosphere.
Thanks to all that posted responses. I tried the wrap without any luck.
For the hell of it I went to 1800 degrees F and it worked in the vacuum
oven. I haven't got the foggiest idea why but I'm pleased just the same.
Thanks again.
Tom
Actcom News wrote:
No, I just assumed that a this was not possible in a vacuum oven
(approx. 10 micron). Are you saying that it's possible to loose carbon
in a high vac furnace?
Michael Dahms wrote:
How do I check the carbon level in the outer layer?
BTW - my assumption is based on several sources of reference materials
which suggest that vacuum ovens do not deplete the carbon levels.
You should look to the microstructure at first. Maybe, you can show us a
picture. A decarburized zone can be seen as ferritic surface layer.
Michael Dahms
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