Problems shop-hardening 416 SS, need wisdom

[Spencer in NH]

I am making some small pins from 0.25-in diam. 416 SS. These have a 0.06-in. long head, a roughly 0.125-in. diameter body, and a groove for an e-clip. Overall length about 0.56-in.

I want to harden these by torch, but they were not coming out too nicely. I had originally made these out of O-1, and they worked very well, but the real thing needs to be 416SS. The hardening temperature for 416 is 1800F. I cross-referenced that to an approximate color (a red-orange), held that with a MAPP-air torch and the part supported by Inconel wire and oil quenched.

I was not too thrilled with the resulting hardness, so I decided to soak it longer at red-orange heat. A one-minute soak resulted in decarb (or so it seemed to me), and the part was very hard to clean up.

For the next one, I tried using the anti-scale compound from Brownells. This seemed to be the hardest part, but the crust from the compound is impossible to remove.

I do have a muffle furnace with a top end of 1000C (1850F), and stainless foil. Have not tried that approach, mostly because it is a 2-3 hour project to get the small furnace up to temp.

How can I achieve my goal and produce decently hardened, small prototype parts in 416SS in a similar manner to how I do it in O-1 (torch)? Do I have unreasonable expectations?

(And, yes, I will temper them once I get this process figured out.)

Thanks in advance for the wisdom.

-Spencer

 

[Tom Lg]

I think you're going to have to bite the bullet and fire up your little furnace and use the wrap/bag. That's been my experience with 416 if you want it to come out looking good/easy to clean up.

 

[BrianH]

And treat it like air-hardening. You don't need to quench it.

What hardness are you looking for?

 

[precisionworks]

Let me summarize the information in Metals Handbook Desk Edition:

Martensitic steels, like 416SS, are heat treated much like plain carbon or low alloy steels. The principle metallurgical difference is that the high alloy content of the stainless grades causes the transformation to be sluggish ... Most every reference on hardening Type 416 suggests a 30 minute soak at transformation temp - even on small diameter parts, a one minute heat soak will not cause complete transformation, and the resulting hardness will be lower than desired.

The long heat soak will cause surface problems, unless you use an atmosphere-controlled furnace or unless you wrap with stainless foil. Brownells (and others) sell the stainless foil, which is effective at preventing surface contamination & heat oxidation.

Do I have unreasonable expectations?

Yes

O1 and W1, in small sections, respond well to a quick warm up with a torch, followed by an oil (or water) quench. Typical recommendation is a 10 minute heat soak for small diameter work ... roughly 1/3 the time needed for Type 416.

----------------
Barry Milton

 

[Spencer in NH]

Thanks, All!!

I think I am smarter now. A couple of things are clear:

1) Transformation in 416 happens slowly, and the 30 min. soak is not optional. (Thanks, Barry!)

2) Though most sources call for oil quench, at least one says that air is almost as good (within one point of Rc). (Thanks, Brian)

Today I am firing up the furnace. May do two small packets, and oil quench one and air quench the other.

Do you put a small bit of brown paper in the stainless envelope to scavenge oxygen?

S

 

[precisionworks]

Do you put a small bit of brown paper in the stainless envelope to scavenge oxygen?

Spencer,

I've seen recommendations for just that, theory being that the burning paper will eliminate all O2 within the sealed foil envelope. In fact, Brownells says To use: Clean part, wrap sharp corners with a layer of brown wrapping paper to protect the foil. Wrap part with foil, fold edges double and seal with wallpaper steam roller. Heat treat as usual.

It would seem that the most important step is elimination of as much air as possible from the foil enclosure. These instructions seem to emphasize this point, click here for info.

---------------
Barry Milton

 

[Spencer in NH]

Barry,

Great reference, thank you!

I ran a batch of four parts with the brown paper, and am running one now with none.

Will let you know how it goes. My little furnace takes about two hours to stabilize at 1800F, but will keep it +/- 2F all day long.

S

 

[Spencer in NH]

The batch with no brown paper worked great. I opened the pouch while the pins were still warm (though after I ran cold water over it). I actually saw the stainless turn a very light yellow-red before my eyes. I think the secret is to quench in the pouch all the way to room temp. If everything is squeaky clean (including the foil) the parts should be bright.

I really liked the idea of using a piece of titanium as an oxygen scavenger; I have ordered a small piece to try that. The brown paper left a dark grey color on the parts, which was not unappealing, but not what I wanted.

Thanks, again, Barry for the excellent references.

 

[precisionworks]

Glad that worked for you, Spencer. Sometimes the smallest projects require the greatest amount of time/energy/thought to reach a good conclusion.

I think the secret is to quench in the pouch all the way to room temp.

That makes sense. The stainless is reactive at elevated temps, and will combine with O2 to form those nasty surface oxides. Quenching in the sealed pouch prevents oxidation.

I actually saw the stainless turn a very light yellow-red before my eyes.

I've seen similar color changes when TIG welding stainless. As long as the shielding gas covers the red hot weldment, there's no color change. But as soon as the post-flow gas stops, the weldment immediately changes color. I normally increase post-flow by double or triple when welding stainless.

-------------
Barry Milton

 

[gwilson]

Don't use much brown paper.It will burst the foil open if you put too much in.For a medium size knife blade,a piece the size of 2 postage stamps will do,if you wrap the part so that a minimal amount of air is left inside.

 

[boslab]

i dont do any hardening 'yet' but could you purge the bag with argon b4 sealing up?
mark

 

[Spencer in NH]

gwilson: I used a piece of brown paper for the first experiment that was about 2/3 the size of a stamp. I may be able to post some pics.

boslab: I have been thinking the same way. Four feet away from my oven is my argon tank. Will try it on the next batch.

S

 

[precisionworks]

If you want to be sure that nothing but Argon gets inside the pouch, a simple 'glove box' works well. Quick & easy to build with mostly scrap materials, except for sandblasting gloves that you'll need to buy.

The photo above shows a sandblast cabinet made from a laundry tub (although a plywood cabinet would also work), instructions here. It would work well for an inert atmosphere glove box by sealing all openings except for one to introduce the Argon (low on the side), one as a vent (high on the opposite side).

Place the unsealed pouches in the glove box, close the lid, fill with Argon until you've pumped in enough cubic feet to displace the air, seal the pouches.

------------------
Barry Milton

 

[Lumpsmith]

Since heat treated 416SS is only in the low 40's RC you can do the heat treating first and then machine it to plan sizes,it machines very nicely with either carbide or HSS.The finish is very nice,smooth and shiny.
The heat treated material is fairly difficult to tap but if you slightly oversize the tap drill hole you can tap it ok,and since it is heat treated the threads are strong enough for most things.