Low temp quench salts - cause of huge scale problem?

[twistedneck]

Seems my glorious salt pot that was supposed to be scale free is the opposite.

Is it possible that steel samples heated to 1600F in an argon atmosphere kiln, subsequently quenched into the salt pot, is being oxidized by the molten Nitrate / Nitrite mixture?

Quenching 4130 from 1600F into salt at 720F.

I'm doing a brine quench test to compare scale build up, but this might not be a good indicator as the brine nucleate boiling phase is so aggressive it could be just blowing off the scale from the high heat furnace or transfer to quench tank.

Salt is Hubbard Hall 275. Good stuff, great support from Glen over there.

Help needed!

 

[Goahead]

Don't you think that your samples are oxidized in air when removing them from the argon kiln to quench them in the salt bath?

http://www.welding-advisers.com/

 

[twistedneck]

Goahead, i am worried about the transition. To address it I made a special argon wand and a cut out brick with that surrounds the part except for the top access slot. I've got an argon line / wand attached to the holder handle, bathing the part in argon gas in transition. I'm sure a lot gets blown away by moving air. I don't see a lot of dark areas forming.

I'll try the same system tonight with a brine quench instead. If it has less scale then I'll know more.

Its warping i'm most worried about, and polymer might be requried if brine is not even enough.

I did manage to get a non warped part in using the molten salt quench, too bad it was scaled like no tomorrow.

 

[unclesyd]

Is you bath operating in it's temperature range?

Is the 275 the melting point?

How old is your bath?

Have you overheated your bath?

Have you had any contamination?
I've seen all sorts of recommendations to add various materials to salt baths.

 

[twistedneck]

unclecyd, Yes, its operating right on 650F for initial quench and 720F to transform to (austemper) to Bainite.

PID controlled, locally some very small % was overheated when i had the burners on for too long going from solid to liquid, saw some tiny nitrogen bubbling.. but that was a very small amount.

275 is the melting point.

The bath is new, fresh salt, just built, looks clean.

I think its just the nature of nitrite / nitrate salts, they oxidize steel at critical temperatures.

In the heat bath literature they mention that these type of quenching salts are oxidizing.

I find it interesting that every other bit of literature, even the heat bath salt write up never addresses this scale issue.

Tonight i'm going to try an electrolytic cleaning with a dc power supply and some salt water. if the parts don't look pitted then i'll just use this method and suffer the ugly surface.

 

[twistedneck]

Just spoke with the person at Hubbard Hall, Glen. He's been very helpful with this project.

Turns out its normal for that salt to oxidize, usually there is a descaling operation subsequent to heat treat.

I'm going to try an electrolytic cleaning, followed by a 400F soak for 1hr. to get rid of any hydrogen in the surface.

Any thoughts welcome. Thx.

 

[unclesyd]

Looks like your salt bath is OK.

We always see a lite red oxide that is easily removed by tumbling. I never seen any thing that could be called scale.

I see no problem with your transfer time as I've never had any problems moving 4340 from an endo. atmos. furnace to a an oil quench. You just get the normal tight blue oxide.

Is this a kiln in all respects, like top opening?
Opening any furnace or kiln always creates a possibility that you cab get an influx of O2. This is not bad on the part you are transferring but the remainder get a little oxidized.
One other thing is that it is extremely hard to purge O2 from a furnace or kiln.

Are you immediately removing the salt from the part?
Nitrate salts are not as hygroscopic as the Chloride salts but they will grab water very quickly.

What type finish are you looking for on the your part?

Watch your electrolytic cleaning. You need to increase you bake out time to a least 6 hrs and possibly more depending on the use and complexity of your part.

 

[twistedneck]

unclesyd, can you describe the process where you only get a little red oxide? i am getting heavy scale.

Forget the electrolytic cleaning, a wire wheel will do until the descale comes on line. No need to entertain Hydrogen embrittlement.

Prototype kiln is a small furnace for prototypes (kf22.5 by evenheat). Its got a door that opens in the front, not on top. I have argon coming in the rear through a small 1/4" od tube, with the kiln door being the place where i THOUGHT the argon would spill out after filling up the atmosphere. wrong, i need it sealed, then purged, then slow flow of gas.

I will do that brine quench test tomorrow, it will tell me a lot.

Finish? not class A, although it is visual exterior. It will be descaled (sand blasted 80gt), phosphated and coated with an epoxy bake on type - not ecoat.

 

[unclesyd]

Heavy scale is hard to understand unless you are getting the scale in your furnace due to O2 intrusion or the bath is very contaminated. From your description you will get O2 intrusion every time you open the door as O2 has one of the highest diffusion coefficients of the gases.

Most important:
Looked at the information on your furnace and see another glaring possibility. This furnace has the heating elements exposed so you have the potential to expose your parts to extremely high temperature, radiant heat from the elements. Your furnace thermocouple will read one temperature and the parts see a much higher temperature every time the power comes on. Higher temperature and O2 equal heavy scale. Doing what you are trying to do you need a muffle furnace.
You could get by with shielding the parts from radiant heat with a tent made from SS tool wrap.

 

[twistedneck]

Unclecyd, your are a true genius. that radiant heat is zapping my parts to a super high surface temp. i'll talk to the kiln desiner Mike, from evenheat about it too.

I can easly place a high temp ceramic blanket over the top of the channel to block out the radiant heat.

Good thinking out of the box on this.

 

[twistedneck]

All, I shading the parts in kiln from the radiant burners. It worked like a charm. Scale is deminished by 80% or more, the part now looks gray and mottled with some surface scale bubbling - not like shedding snakeskin.

I'm impressed with the improvement shading the parts had, i expect to see grain growth associated with overheating in the samples.

Problem, my 1" thick 'shade' segment of KOwool was too thick, caught on the thermocouple and caused my part to fly out hit the ground! (flying orange hot parts, fun) had to re-do.. then i forgot the salt bath agitation, redo again, by then its 2:00AM and i've been through 10lbs of Argon.

Taking the hack sawed samples to U of M tomorrow for analysis - the old way. knoop hardness and optical etch / microscope.

 

[unclesyd]

OK! You still have too much O2 in the kiln.

You can now work on getting an atmosphere in the kiln.
The tube that you are using for injecting the Argon needs to be sealed on the outside skin to prevent O2 from aspirating or diffusing into the kiln.
See if there is a bottled gas supplier near you that has an endo gas mixture.

Trade secrets: Throw in a naphthalene moth balls as you close the kiln door. If you don't have too many parts smut the parts allover prior to putting them into a furnace.

Check you time at temperature go for the minimum exposure to the austenizing temperature. I am digging for my old handbook.

Use 1018/1020 for testing until you get everything down pat.

Take a look at the Diamond Block Atmosphere System for your kiln. Also checkout the Sen/Pak containers and wrap. The Block is also a liner. If you try the containers or wrap make sure you wrap the parts in plain paper towels prior using either.

http://www.sentryfurnaces.com/

For shielding get hold of some ceramic paper it will do the job. Make sure you get a material for + 1800°F or better.

 

[twistedneck]

unclecyd, sealing up the argon inlet tube tonight.

Considering adding a little C02 to my argon, what is the ideal? Welders like to add up to 75% however i believe that provides more heat than shielding.

Would a 90/10 Argon / CO2 be superior to a pure argon setup for shielding?

 

[unclesyd]

Sealing the inlet tube will help by keeping the O2 out of the kiln and it's insulation. Insulation can hold a tremendous amount of O2 and is easily recharged.

That small amount of CO2 want probably hurt, but will only act as another inert. What you need is small amounts of something that can tie up the O2, like either CO, H2, methanol etc. That's why the previous mention of adding a organic (reducing) material. The weld shielding gases perform several functions besides preventing scale and the primary reason for no scale is time at temperature. Ideally, as you would open the door you see the gas flame up.

How long are you holding at the Austenizing temperature?

Did Everheat put a rigidizer/seal on the inside of the kiln insulation?

Let's don't get in a hurry one doesn't need to get hurt.
Just remember the "Old Bull Philosophy"

 

[twistedneck]

I should be carefull about my Kaowool blanket shading the parts, i'll make it thin not to absorb O2.

Forget that CO2, it will eat my kiln elements. Put that mail into evenheat about the rigidizer. from what i can tell, the bricks are raw and i need some pretty quick.

Holding for just a moment at 1620F - to account for the loss in heat transfering to salt quench. grains looked very fine in the microscope.. will have hardness tomrrow. decarb is bad, obviously - becuase of the scale attack! (not the salt pot).

 

[unclesyd]

Holding for just a moment at 1620F - to account for the loss in heat transferring to salt quench. grains looked very fine in the microscope.. will have hardness tomorrow. decarb is bad, obviously - because of the scale attack! (not the salt pot)

.

Is this some samples of your first tries?

If the Kiln is lined with insulating brick this isn't as bad as fiber insulation. Getting the O2 out still takes a little time. Again I would work with plain CS until I got my atmosphere in the kiln working correct.

 

[rorschach]

Would drawing a partial vacuum before starting the argon flood help with the O2 problem? When we did Nickel brazing where I used to work instead of using Argon we started out with a N2 flood, transisioned to a H2 flood about halfway through the temp ramp up and maintained H2 until about halfway down the cooldown ramp and transisitioned back to N2. I always got nervous as hell when they fired up that furnace though, too dang much H2 for my piece of mind to be honest.

 

[twistedneck]

That is an idea, but how could i make a shop vac setup work, replace w/ argon? I am trying positive pressure.

Reults of micros look good

grain size is very fine off the scale equivalent to 16. to be expected with only 1min at 1600F.

Sample1, (non agitated salt) had very bad decarb, i expected this - took too long transition time btw. kiln and salt.

the other sample, #2 had much less decarb, better furnace door, no ferrite problem from loosing temp before quench. The bad sample (1) above was let cool down too far before quench, hence the ferrite grains sprinkled throughout. Both parts beat pearlite nose, looks like 90% bainite, a few globs of carbides.

Hardenss is good 39Rc, 43Rc respectively. This is working out. thanks for they help everyone. now i just need to seal up that kiln.