S7 heat treatment

[kenre]

I have been told i need to heat treat S7 tool steel in a stainless bag to avoid de-carburisation. If the bag is sealed, wouldnt it pop from pressure buildup? of is this not an issue. I need to harden, then, temper to 58/60 Rc.

Also, where can i locate bags in Australia?



Ken

 

[swall]

What are your options for heat treat--furnace, torch, salt bath? How big is this item?

 

[unclesyd]

Here is two sites that will give you what's available. Your supplier of the S7 should be able to point you in right direction. Also a supplier like Precision in Au should have the material. This material only works in a furnace.

If you use the material make sure you wear gloves, cut proof, as this material is thin and the edges will cut quick and fast.

Though it is not stated in the instructions for use of this material, if you will give the part being treated a light wrap with something like a plain paper towel prior to enclosing it will help prevent any oxidation.
As far as heating up the package isn't absolutely air tight so the air slowly vents during the heating process.

http://www.precisionbrand.com/products/default.asp?p_catid=29

http://www.sentryfurnaces.com/heattreating.htm

 

[kenre]

Thanks Guys!

I will be using a furnace. Items are only small, 70mm long, 6mm width.

Ken

 

[tripleZ]

Yes, the bags work. I used to make them for S7 punches. There are two types of foil they normally sell. For S7 and other grades like A2 or D2, I'd recommend the 0.002"-0.003" thick stuff. Thicker will work and goes to higher temperatures, but it's a witch to fold. The negative to this is that the thin stuff will develop pin holes more easily.

My old method for making bags is as follows (note the stuff should come on a roll...and is fairly expensive):

1) Roll out the foil. Measure out the length of your part and then add 4"-6" to it. Make a mark with a Sharpie marker (or your finger...the grease will show up).
2) Use metal scissors to cut the width at this point.
3) Take the cut sheet and fold it in half longways (so you would have a piece that is as wide as the roll and as tall as your part + 2")
4) Using your part as a gage, mark a line from the edge of your foil that is about 3"-5" wider than your part (this grows as part shape/size/complexity grows). Again, make a mark and cut.
5) Now you'll need a straight edge and a roller, preferably one with a heavier metal end. The bottom of the bag where the fold is should be facing you. You should have a cut edge to the right, left, and open at the top. Take the right edge and extend it over the side of a table...eyeball it for a 1/4" or so. Put the straight edge inline with the table and fold the foil up over the straight edge.
6) Get your straight out of the way and press the fold down with your fingers. Once complete, tap the fold in a few places with the small roller to set the fold. Then roll up and down the fold line with the roller until it's flat.
7) You want to repeat step 6 until you have a minimum of (3) folds, then do the same to the left side.
8) After your folds are in, use something like a broomstick handle to open up the top end of the bag. Slide your parts in until they're maybe 1" or so from the bottom. Using a metal crimper, once the part is in place you'll crimp the bottom 3/4" below the part.
9) Now seal the top just like you sealed the sides. 3 folds are a minimum. Don't try and get your folds all the way to the top of the part. You'd like at least 1" of gap. Also, the heavier roller helps here to beat the folds down. If you don't have that, the crimper will help you develop arm strength.
10) Now take your bag and extend the bottom 1" of open space over the edge of the table. Fold up towards you. Fold the top over in the opposite direction. It will look like a "boxy" letter S. You now have a bag that will stand upright in your furnace, but 1" or so away from the elements on the walls. As your parts sound thin, this vertical method will give you a much better heat through than setting them on the ceramic hearth plate (which acts like a heat sink).
11) Visually check your exposed bag areas for pin holes. if you see one, cut your bag open, pull the part out, and do it again.
12) When you heat treat these, make sure you get them quenched quickly. Check your hardness out of heat. If you're not at the peak out of heat hardness on these, drop down on your tempering temperatures. It's better to temper the stuff a few times than drop it under as I've never had much luck re-heating S7 to peak hardness. I always seemed to end up 1-2 points off the second time around.

If you're new to this, get some surgical anti-cut latex gloves. They're thin so you won't lose dexterity. When I learned, I didn't use gloves. I did manage to cut each finger at least twice in the first week. This stuff is like a pliable razor blade. Luckily, with experience, you learn how to do it extremely quickly w/o gloves.

If you do try it barehanded, you'll note the cuts are like nasty papercuts (they also dry out quickly and like to crack). At night, before you go to bed, goop up the cut areas with vaseline/neosporin and throw socks over your hands to keep the goop off of your sheets. That'll help 'em heal nice and quick.

 

[EdStainless]

I used to "Z" fold the tops of the bags. I didn't fold the top edge over, but made a fold/crimp down a couple of inches. That way I could spread the top edges and when we pulled parts out you could grab them with pliers and rip the bag apart. We were doing stuff (not S7) that was very quench sensitive.

If keeping parts bright is critical you could look for some Ti or Zr or Ta foil. This stuff is very thin (0.0005") and even sharper than the stainless. It is also very reactive. A small amount in one end of the bag, not touching your part, will scavenge oxygen and leave parts very bright.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[kenre]

Thanks for the info!

I seem to be reasonably immune to metal cuts on my hands, so im told! removing swarf, any kind bare handed is a daily ritual. I have heard of using a match head will also remove any oxygen to keep parts bright, i dont need to worry bout this as parts will be finished ground.

Next prob is to find the CORRECT S7 alloy.....

Ken

 

[tripleZ]

Interesting method Ed. I'll need to try it, although in my current they don't like me to play on the grinders and in the furnace room anymore. I miss getting grimy, although I like the current paycheck. Maybe someday.

In case you try my bag style Ken, the method I used requires a small cut in the lower right hand near the fold seam, followed by a jerk/pull. (You're holding the left side of the bag with tongs) That would open up the entire right side of the bag and allow the part to roll out. No picking or pulling. It's the only way I could ever get small core rods into the oil fast enough.

And if you're heat treating with foil, you WILL get cut with bare hands. I guarantee it. If you make it a year without getting nailed once, post up and I'll send you a congratulatory cake. That or you aren't moving fast enough...=)

 

[kenre]

I will require fast part removal for quenching!

And thanks for the warning on using gloves to avoid cuts, i have taken you all seriously.

Also to look at other options, what else will avoid decarb?

besides a vacuum furnace?

Ken

 

[Maui]

Salt bath heat treatment will provide you with the best hardening response from S7. If you choose to utilize this type of process you won't have to use stainless foil to protect your parts - they won't decarburize.

You might consider reading faq330-837 for a general idea of factors to consider in heat treating your parts.

Maui

 

[twistedneck]

The only way to avoid scale w/ salts is to use a Neutral salt. Nitrate / Nitrite low temp salts do add a layer of scale (ask me how i know).. Decarb is the same story, but i'd watch the austenetizing atmosphere more than the salt..

Also, unless you have a big part, the foil will retart the quenching rate.

 

[kenre]

Thanks guys!

I am looking into doing the heat treatment myself, hence the stainless bags. I work from home. Salts, Vacuum furnaces etc, are out of the question. Scale isnt an issue as i will need to finish grind the parts to size. What sort of reductions in strength/ hardness etc will decarb cause?

Ken

 

[unclesyd]

If you use the bags/wrap as described there should be no decarb. Wrapping the part with paper will take care of any O2 that remains in the wrap.

I have never seen decarb on any S7 heat treated by properly using the SS wrap. I used this method making single parts for impact wrenches where heating up the big furnace wasn't warranted.

 

[Maui]

Yes, neutral salts will be required for this application. For more information on the subject, consult the following link:

http://www.hubbardhall.com/files/2282003.pdf#search='salt bath carburize'

A salt bath process can be used for a variety of purposes other than simply heat treating parts. For example, liquid nitrocarburizing can be performed in a salt bath if the appropriate salts and set up are used. Kolene is one source for this type of surface treatment:

http://www.kolene.com/applications/sbn.asp

There are a variety of other surface treatments that can be achieved by using a salt bath process as well.

Maui

 

[unclesyd]

Don't know if this material which works without the wrap is sold down under but you might want to look around.

If you still have any worries about decarb you can always use as heat treating stop-off prior to installing the part in the wrap. The paper has always worked for me. If you can't locate a stop=0ff you could use a saturated solution of Borax or Plain Milk of Magnesia, the kind you take.
To use either the Milk of Magnesia or Borax you essentially follow the procedure for the RBC product in the pdf below. You want you part warm where it will drag out a good coating. Stop-offs still require the warp.

http://www.rosemill.com/html/msds/pbc_how-to.pdf

 

[tripleZ]

Decarb on S7, A2, D2 or other mid-range tool steels will probably drop your hardness by 2-3 points on the Rockwell "C" scale. This will be in the vicinity of the decarbed area of course.

If you wrap the bag correctly, the only likely decarb potential you'll get are from pin prick holes. If you have a good eye you'll catch these. Small pin pricks can leave decarb spots about the size of a US dime. They can grow bigger, but that all depends on your soak time.

One little trick that sometimes works is to make sure you leave some air in your bag (not a lot mind you). When you put your bag in the furnace, it should puff up between the folds and hold the puffed form. If it puffs and then deflates quickly, you've likely got a pin prick (or your folds were poorly formed). Usually you can see a small puff of flame or a stream of outgassing smoke from the puncture area.

Also, when you oil quench, just don't drop the part in. Holding with your tongs, get it as close to vertical as you can eye up. Dip it fully into the oil and hold it for about a minute. Then begin to swish it around in the oil to assist the cooling process. If oil quenching, you may have a hard time telling the difference between oil scale adn decarb. Bead blast the part after tempering and you should be able to discern the difference.

 

[kenre]

Help!! again, i have tried to locate a source for the SS tool wrap in Australia and only come up with dead ends. Even Precisionbrand Aus doesnt stock it. many phone calls and emails later im starting to wonder if this product is even used here.

Ken

 

[unclesyd]

Check with some of the knife material suppliers.

 

[tripleZ]

http://www.thermafoil.com/

If worse comes to worse and you need to import, that URL will take you to the company I used to purchase my foil from. Or, contact them and see if they have a distribution agreement with someone in Australia or Asia. It's possible they might.

 

[twistedneck]

If you plan to use a standard electric heat treat oven (radiant heat with the coils), watch the atmospheres - degassing can cause elements to break quickly.

Also, as i learned here from swall, the part gets much hotter than surrounding air in one of these kilns. this is due to radiant energy hitting the part more intensely than the surrounding air. Since the thermocouple is shielded, it has no issues with radiant heat.. answer? use Kaowool as a shade blanket!