Trouble with S7 heat treatment, won't air harden.

[Adam R.]

I purchased some S7 from Mcmaster Carr to make some punches and hot chisels.  The trouble I ran into is that it will not air harden.  I tried in my electronic controlled oven first, held at about 1725 and took it out to air cool, but never got hard (a file bit pretty easily).  I though maybe it was just decarb so i tempered it and finished up the grind.  I tried the hot chisel to cut some material and the edge flatten out like mild steel.

I tried to re-heat treat it in the forge, brought it just to about orange heat and let it cool but it is still soft.  At this point I got irritated and reheated it and water quenched it.  Hard as glass and during my testing I was able to very easily break off the edge of the chisel.

Am I doing something wrong?  Any suggestions to try something different?  Or would you also assume it's a mix up from the vendor (or the vendor's vendor)?

I am actually hoping I am just doing something wrong, but I suspect I got the wrong alloy.  My very first test with A2 worked great and without issue.  Mcmaster is going to be checking with their vendor to see if there was a mix up, I doubt that will bear fruit.

Should I be gun shy from ordering s7 from Mcmaster in the future?

[Frosty]

Maybe there's an intermediate quenchant between air and water? Water absorbs heat 80x as fast as air,  yes I believe you're doing it wrong.

Frosty The Lucky. 

[Adam R.]

I know I can oil quench steel, I do with other alloys.  S7 is supposed to be air hardening steel.

Thanks for the information though.

[David Kahn]

You would think material purchased through McMaster would be what it says it is, but I guess mistakes are possible.  If it is really S7, it should air harden.
 

 If it were me, I’d cut a little piece off and send it in for elemental analysis. It will cost you $150 or so, but it would be very satisfying to find out what the stuff really is. And it’s not worth trying to puzzle out why your S7 isn’t air hardening until you know it’s actually S7. 
 

Fwiw, I always, always have material tested when I buy tool steel at a tailgate or from another source that is less than perfectly reliable.  I once put 50 hours into making a tanto out of “S2” that turned out actually to be S5. The edge cracks from quenching oil hardening steel in water weren’t apparent until I got to the polishing stage.  Made me realize that my most valuable commodity is my time, and you can waste gobs of that working with material that you think is one thing but is really something else. 

[Adam R.]

Good afternoon David,

$150 is a good chunk of money, and while I agree it would be satisfying, it could turn a $30 loss into a $180 loss.

Mcmaster did confirm their color coding on their end is correct and is going to check with their vendor on Monday.  I've had a very good track record with them so far on purchasing alloy steels, so I am certainly not accusing them of anything willful.

Your point is a good one though, I really should have; as a minimum good practice, taken a piece of the bar and test hardened it before putting any work into it.

I think I will spend some money on another bar of it and spark test in comparison to my current bar.  If a new piece works as anticipated I think I can just chalk it up to bad luck.  If not than I am just doing something wrong and will need to figure it out from there.

Thank you for your response.  It is appreciated.

 

[Irondragon Forge ClayWorks]

I would order from another supplier, like Hudson Tool Steel.  Might check their site for quenching & tempering specs.

 

[Adam R.]

So as it turns out, I am now much more confident this is in fact S7.  During my initial heat treatment I think it was in the oven too long and I ended up getting a fairly deep decarb layer that i didn't grind sufficently through.  I tried taking a hacksaw to the 2nd tool I made and it stopped dead about 1/16" deep.  This would also make sense, I think, why I couldn't get it to air harden on a 2nd attempt.

I suppose when I got mad and tried to water quench it there was still enough carbon to harden.

So good news all around, I am pretty sure I got what I paid for and learned a valuable lesson about heat treating.  Not bad for the price of one chisel.

Thanks for the response guys, it helped me noodle through this one.

 

[Robert J]

I run a tool shop and harden all kinds of steel on a daily basis.

McMaster-Carr has a problem they won’t admit to. I just sent a piece of S7 out to a metrology shop to get tested. We only buy small amounts from McMaster and have wasted a lot of time and money on S7. No problem with 420, H13, A2. 
And yes the S7 came with certs.

[Frosty]

Welcome aboard Robert. If you put your general location in the header you'll have a better chance of meeting up with members living within visiting distance. 

Who are you affiliated with, company name, products, etc.? And in what way would a "metrology shop" determine the chemistry of steel sample?

Lastly why take this up with the middleman when your gripe is with the manufacturer? You don't think McMaster-Carr tests everything that they sell do you?

Maybe change suppliers to one that specializes in metals instead of one that resells, like Amazon but with brick and mortar stores?

Frosty The Lucky.

[arkie]

Robert, you mentioned that you sent the "S7" piece out to be tested...what did the testing show?  I have never used S7 and would like to know if it may be a problem with getting truly air-hardening steel named "S7"