What caused this?

[adamriley]

Hi all.
I have hardened plenty of silver steel in my kiln, but today I experienced something I've not come across before (and wouldn't you know it was for a customer, and no, I don't have any more 'in stock' )

The part was heated in the kiln (set for 800c as it runs a little lower), and when ready quenched in water.
As usual it was a little 'scaly', so I stuck it in a cordless drill to spin it with some scotchbrite pad.
However, there is some SERIOUS pitting on the surface. I mean this thing looks like a leopard!

What went wrong, do you think?

The only thing I can think is that it was some kind of reaction to something on the surface, but I can't see how that would cause deep pitting.
The part, by the way was SMOOTH, almost polished before being heated.

Adam.

[awemawson]

Did you make it yourself for the customer or did he bring it in for hardening. ie do you know for sure what it was made from?

Obviously you've had oxidisation in the kiln, so either the material was more susceptible or the kiln atmosphere was more corrosive than you've previously experienced. What was the kiln used for last - any residues of anything left in there. (I've seen certain enamels cause odd effects if someones been using the kiln for jewelry previously.

Critical stuff I wrap tightly in stainless steel foil (with a sliver of wood to absorb the oxygen within the parcel) 

[adamriley]

Thanks for the reply. I made it myself from silver steel (not sure what grade though).
The kiln is only used for heat treatment. I'm going to have a go with some offcuts to see if A. it does it again and B. if I can figure out how to stop it.
Where do you get stainless steel foil from?

Thanks again.
Adam.

[Arbalist]

Critical stuff I wrap tightly in stainless steel foil (with a sliver of wood to absorb the oxygen within the parcel)

I've heard of this before. What grade/thickness SS is best and where can you get it in UK?

[awemawson]

I have some stock that I bought literally 10 plus years ago and am still using - forget where I got it - it's maybe a thou thick perhaps less. I know it cost a kings ransom when I bought it, but I don't use much. I also know it cuts your hand 'orribly if you're not careful 

[jonesie]

HI ADAM.i have done a lot of heat treating in moldwork and where i now work, and  i never wrap anything that i am going to oil or water quench,it is to hard to get out of the foil and , into the oil or water,the quicker into the oil or water the better. i am sure some will disagree.you can get the foil from msc or travers.it comes in two thicknesses but get the thinner as it is easier to tear to get parts out. we always leave a little on the  over all to remove the decarb.on the o1 or 4140. we seldom ever use w1, like said before using wood i use a piece of paper.be careful as the foil is like a new razor, wear gloves.doing another test piece is the best idea to see if it does again, your  steel might have been miss marked or is not w1. just the way i do it and i am not a metallurgist, good luck jonesie

[awemawson]

I fold up a parcel of the foil in such a shape I can hold one one with pliers and cut the other end off with snips. So the action is - withdraw from kiln onto firebrick surface, grab one end with pliers, snip off the other end, pick up brick, tilt over oil vat still holding one end with pliers. Item slips out into vat. Not so easy for big items, but works a treat for lathe tool sized things.

[adamriley]

Thanks for the responses. I tried heating some more silver steel offcuts (from bar that I KNOW DIDN'T DO THIS BEFORE), and it's happened again, albeit to a lesser extent this time.
Also, it doesn't seem to be as hard. It's definitely not hard enough to worry the file.
As an aside, what is the best way to minimise/prevent scaling?
The parts I'm making are for 'fancy' woodwork tools and they need a polished surface.

I'll try to get a picture of the failed attempt.

Adam.

[adamriley]

Just to update this thread.
I ordered more ss, and remachined the parts.
This time they hardened properly, with no changes.
Must have been a duff batch.

Thanks for all the help.
Adam.

[vtsteam]

Thanks for the responses. I tried heating some more silver steel offcuts (from bar that I KNOW DIDN'T DO THIS BEFORE), and it's happened again, albeit to a lesser extent this time.
Also, it doesn't seem to be as hard. It's definitely not hard enough to worry the file.
Adam.

Doesn't explain the above.

Another explanation which would include the above would be kiln contamination that gradually burned off after a few heating cycles.