Hardened tool steel has soft outside layer

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

spend a little more money and get some A-2 and wrap it tight. never fails me
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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
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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:
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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
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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:
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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
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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.
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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:
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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:
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tomcas wrote:
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I am saying that it is useful to check assumptions that are checkable.
Michael Dahms
BTW: Please do not full-quote and top-post any more! http://www.xs4all.nl/~wijnands/nnq/nquote.html

Michael Dahms wrote:
> I am saying that it is useful to check assumptions that are > checkable.
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.

tomcas wrote:
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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