Small vacuum HT Furnace

[DanF]

T
No Dan , I don t use tube here .This one will work on the same principle but , no ceramic tube .Plan is simple HT oven like all other one BUT little more stronger /reinforced and sealed . But lot of information there If you start to build tube furnace you should think about making in to be capable to work under argon ...it is much easier to do that in that kind of oven

Ah, okay, I will think about the tube oven again later (I priced everything out), but am currently modifying my gas forge to PID HT’ing.

 

[Natlek]

Surrounded from all sides with vacuum , brick would be fine .I just tested one and i can t see any problem . So vacuum will/can only affect steel construction . I don t know ....if i build this oven like on this picture i can finish it in one day say in Sunday .But will be massive and will need lot of gas ...

What if i make inside like this ...........blue is steel ,yellow is Kaowool , red are bricks and maybe ceramic fiber board .But i need to find way so vacuum can easy and fast go behind all that layers . Kaowool would be sucked/pressed from vacuum of course but i don t see problem with that because next second pressure will get it right ? No Kaowool on floor because i want bricks to hold that position for Fan axle , if it is clear what i mean ? Thin layer /say one inch of Kaowool is good because bricks can expand free and will add in insulation . There would be two door or assemble them as one part , one will fit inside oven just from bricks/and maybe ceramic board and other one just from steel so i can easy seal it for vacuum and pressure .That way I will keep heat far from seal for door ? That is most simple end reliable way i think ? Top/roof with HT element would be separated and will go in/out like tray ?I plan to weld steel chamber from all sides only open from front for door so I can insert/pull out if needed whole assembly and outer layer of kaowool will make it easy .

Locking the door .........bolts are out of question , they take time to close/open door .What if i use something like this ...simple to use them . easy to adjust them and they are fast and reliable ? You adjust them once and that is that ....? Red end from bolts would be on chassis .

What do you think ............? I m just loud thinking for now in search for most simple but reliable solution ......

 

[Richard338]

Temperature will be very low inside chamber , heated only from outside of oven temperature ? Actually how will heat pass from oven to chamber if there is vacuum ......no air no convection , no radiation ?? As far I know radiation start only on very high temperature ?

The rate of gas phase heat transfer is very flat with pressure, down to extremely low pressure. The mean free path of the gas atoms or molecules has to be on the order of the container size before it starts to reduce. The textbook example is designing double pane windows to insulate. It is much easier to fill the gap with a heavy monatomic gas like xenon, rather than trying to evacuate it to the degree that would be necessary.

 

[Pinoy Knife]

Natlek
im just curious why not just fill the oven with Argon and have a door/curtain to keep most of the gas inside??
that way you can put things in/out without loosing all the gas held inside..
since the Argon is heavier than air .. a simply small slot in the door ,or a second smaller door might work.
again just thinking out load..
the idea is from using a box to tig weld inside when doing titanium so you completely cover the part with gas while you weld.
the gas only leaves if there is too much movement while welding, acting like a curtain if you will...

 

[Mike Read]

Another interesting idea/project Natlek I wish you lived next door so I could pop in every week and see what you are up to

I have had a little second hand experience with vacuum and argon kilns - both when I outsourced my HT.

The first was a knifemaker who had Argon injection into his kiln. The blades he did for me came back pretty clean but not as clean as when foil wrapped. He soon went back to using foil as it was easier, cheaper and gave a cleaner finish (less decarb) He did not evacuate the kiln before injecting the argon so there must have been some air left inside. Cost was high as he had to have argon flowing throughout the HT cycle but if you evacuate the oxygen and seal the kiln better you might have a cost saving and less decarb. He also said it was a lot of faff messing with the gas!

The second was when I used a commercial HT firm for a large batch of blades. This company air quenched the blades inside inside the kiln and most of my blades came back warped. Ever since I have always done my own HT and plate quenched the blades. I would be concerned at the time it might take to get the blades out of a vac kiln and into a plate quench.

I would suggest that foil wrapping and plate quenching is so easy and the results so good I cannot see what advantage is to be gained from using a vacuum or argon.

It might be fun if you used acetylene - and then I would prefer not to be living next door

 

[Natlek]

Another interesting idea/project Natlek I wish you lived next door so I could pop in every week and see what you are up to

I have had a little second hand experience with vacuum and argon kilns - both when I outsourced my HT.

The first was a knifemaker who had Argon injection into his kiln. The blades he did for me came back pretty clean but not as clean as when foil wrapped. He soon went back to using foil as it was easier, cheaper and gave a cleaner finish (less decarb) He did not evacuate the kiln before injecting the argon so there must have been some air left inside. Cost was high as he had to have argon flowing throughout the HT cycle but if you evacuate the oxygen and seal the kiln better you might have a cost saving and less decarb. He also said it was a lot of faff messing with the gas!

The second was when I used a commercial HT firm for a large batch of blades. This company air quenched the blades inside inside the kiln and most of my blades came back warped. Ever since I have always done my own HT and plate quenched the blades. I would be concerned at the time it might take to get the blades out of a vac kiln and into a plate quench.

I would suggest that foil wrapping and plate quenching is so easy and the results so good I cannot see what advantage is to be gained from using a vacuum or argon.

It might be fun if you used acetylene - and then I would prefer not to be living next door

First one joke about sealing...................

On Motor Show engineers from Zastava / now Serbian Car Factory/ look at a Toyota car, they open , close the door ... inspect windows .........watch it how all seal nicely.... Then they ask Toyota engineers......... how did you manage to make it to seal so nicely? Toyota engineers...........we put cat inside and if we find her dead in the morning,car is good . After a while , Toyota engineers look around Yugo car from Zastava ........they open / close doors, windows...Then they ask Zastava's engineers ....how did you manage to make this car to seal like this ??? Zastava's engineers ...just like you do in Toyota Factory , we put the cat inside and if the cat doesn't find a way to escape from inside by morning, everything is OK................

I will make it Mike and it would work ..........This one need to seal like Toyota not like Yugo I am slowly coming to a final solution, I’ve read a lot of literature around this type of oven these days . Vacuum is a bit problematic, nothing else.Maybe I need to test that the vacuum pump and the argon work at the same time so no vacuum inside chamber was created....Vacuum pump pull air from inside and in same time Argon enter inside ..........at the end just little pressure inside from argon?

It might be fun if you used acetylene - and then I would prefer not to be living next door

Don't worry , I have no acetylene in shop at lest 25 years

 

[Natlek]

Natlek
im just curious why not just fill the oven with Argon and have a door/curtain to keep most of the gas inside??
that way you can put things in/out without loosing all the gas held inside..
since the Argon is heavier than air .. a simply small slot in the door ,or a second smaller door might work.
again just thinking out load..
the idea is from using a box to tig weld inside when doing titanium so you completely cover the part with gas while you weld.
the gas only leaves if there is too much movement while welding, acting like a curtain if you will...

I can try that with one of my tube furnace in vertical position ................All i need to do is to move TC from backside and use it for Argon ...........I can bet that it would work 100 %

 

[JTknives]

I use gas shielding in my shop for my oven. It’s an option we have for customers that want a slightly cheeper price on stainless heat treating. We use nitrogen as it’s around 3x cheeper then argon. It does a decent job but the surface will be discolored and there can be a very thin layer of decarb. Nitrogen does not prevent carbon from off gassing so to speak above 1850°(I think that’s the temp). We looked at vacuum furnaces and trust me I need very little motivation to invest in new equipment. Thy just are not practical unless your batching tons of blades at once. The tricky bit when researching them is getting them hot. So thy suck out the air and then flood with nitrogen. Then start heating the chamber. Once thy get close to temp thy suck out the hot nitrogen and continue to heat under vacuum. Once the soak time has finished thy purge the oven with 5bar nitrogen and cycle it through a heat exchanger quickly to dump the heat from the items.

 

[Stacy E. Apelt - Bladesmith]

As JT is pointing out, it isn't as simple as just sucking the air out of a sealed container with a knife in it and heating up the chamber.

Your vertical argon filled oven idea would be better. Especially if you have a low temp salt pot to quench in.

 

[Natlek]

As JT is pointing out, it isn't as simple as just sucking the air out of a sealed container with a knife in it and heating up the chamber.

Your vertical argon filled oven idea would be better. Especially if you have a low temp salt pot to quench in.

Why you think it isn't as simple as just sucking the air out of a sealed container with a knife in it and heating up the chamber? What you think would not work ? What I m missing here ?
That s why I open this thread Stacy , to hear what other think and maybe help to build this oven .
I really can t understand why it is not that simple .With process/schedule of HT I have no doubt that it would work ? At least i can t see what can go wrong ? I have doubt how to build this oven ...I search for best way to build it .
Watch this video from 1 minute ...........steel is stainless .After soaking steel is on open atmosphere about 6-7 sec. before it is quench between plates .If that works for Solingen why should not work for US ? I plan to HT stainless blank , to grind bevels after HT process. So IF THERE is little discoloration is no problem at all .Real vacuum furnace is more designed for HT complicated machined parts from stainless which are final product before HT , no work on them after HT so everything must be done in protected atmosphere ............Knives are other story , don t you think ? It is vital for us to protect steel when soaking because of longer time on high temperature .A few seconds in the air will not affect steel ....

I found this to , watch how thick is cover ...... seems that I don t need to much thick steel for frame ..........just some reinforcements

 

[JTknives]

The huge issue or hurdle to overcome is heat transfer. This is why the industrial ovens fill with a gas first so the parts can be eavenly heated. Once things are hot thy suck the hot gas out and let radiant heating do the rest. But if you where going to try and start off with radiant heating in a vacuum I believe from reading that it can be quite problematic. Also there is the tricky bit of measuring temp. I don’t know how thy do it but I would think you would have to have a TC in direct contact with the part your heating. The ovens I have seen have silicon carbide heating elaments 360° around the basket that holds parts.

sure stuffing a heat treat oven inside a steel container and sucking the air out would not be hard to do. But will it do what you are thinking it will do.

 

[Natlek]

The huge issue or hurdle to overcome is heat transfer. This is why the industrial ovens fill with a gas first so the parts can be eavenly heated. Once things are hot thy suck the hot gas out and let radiant heating do the rest. But if you where going to try and start off with radiant heating in a vacuum I believe from reading that it can be quite problematic. Also there is the tricky bit of measuring temp. I don’t know how thy do it but I would think you would have to have a TC in direct contact with the part your heating. The ovens I have seen have silicon carbide heating elaments 360° around the basket that holds parts.

sure stuffing a heat treat oven inside a steel container and sucking the air out would not be hard to do. But will it do what you are thinking it will do.

Why should I remove argon from oven, JT ? Is there any particular reason to do that ? There would be fan inside oven to mix argon to help to have little more even temperature inside.Additional /control TC is easy to install from bottom side of oven if needed ?
If i keep argon inside heat transfer is not problem at all ? Or i m missing something again ?

 

[Natlek]

I use gas shielding in my shop for my oven. It’s an option we have for customers that want a slightly cheeper price on stainless heat treating. We use nitrogen as it’s around 3x cheeper then argon. It does a decent job but the surface will be discolored and there can be a very thin layer of decarb. Nitrogen does not prevent carbon from off gassing so to speak above 1850°(I think that’s the temp). We looked at vacuum furnaces and trust me I need very little motivation to invest in new equipment. Thy just are not practical unless your batching tons of blades at once. The tricky bit when researching them is getting them hot. So thy suck out the air and then flood with nitrogen. Then start heating the chamber. Once thy get close to temp thy suck out the hot nitrogen and continue to heat under vacuum. Once the soak time has finished thy purge the oven with 5bar nitrogen and cycle it through a heat exchanger quickly to dump the heat from the items.

Exactly what i want to do with this oven .Vacuum will take almost all air from oven and then replace it with argon so we have almost 100 % inert atmosphere inside ? Since the oven is sealed , I don't see what the problem would be? We have convection and radiant heating plus fan for more even temp. so TC should work as in any ordinary oven?
So , vacuum the chamber ,close vacuum valve ,open argon valve , let gas flow till vacuum gauge shows 0/zero/ . When soaking time passes , equal pressure inside with atmosphere /if it is necessary/ , open the door and that's it ?

 

[Natlek]

The huge issue or hurdle to overcome is heat transfer. This is why the industrial ovens fill with a gas first so the parts can be eavenly heated. Once things are hot thy suck the hot gas out and let radiant heating do the rest. But if you where going to try and start off with radiant heating in a vacuum I believe from reading that it can be quite problematic. Also there is the tricky bit of measuring temp. I don’t know how thy do it but I would think you would have to have a TC in direct contact with the part your heating. The ovens I have seen have silicon carbide heating elaments 360° around the basket that holds parts.

sure stuffing a heat treat oven inside a steel container and sucking the air out would not be hard to do. But will it do what you are thinking it will do.

I think what are you saying is for other type of vacuum furnace oven .The one which use refractory steel for chamber inside .......

 

[Natlek]

This is what i think should work to seal Ac power in this oven , in same principle like this HT element from kitchen oven.What do you think ? It is out of high temperature so i it should work i think .I forget two small ceramic beads on both ends on drawing , to be sure that copper wire is in the middle of that steel tube what ever happened in future ....

 

[Natlek]

About control Thermocouple .........something like this should work I think ? It will take little usable space but ........we will know the exact temperature where the steel stands ? This should work in any oven .......stainless steel part would protect TC from us when we handling steel inside and will heat at same rate as steel so TC will read temperature better ?

 

[Stacy E. Apelt - Bladesmith]

That will work, but response time will be slower. Any increase/decrease in the ]stainless block would have to travel to the TC to be read.

A better method would be to place the TC inside a stainless tube just large enough to fit the TC. You can get a TC already clad in monel very cheap on the internet.

 

[Natlek]

That will work, but response time will be slower. Any increase/decrease in the ]stainless block would have to travel to the TC to be read.

A better method would be to place the TC inside a stainless tube just large enough to fit the TC. You can get a TC already clad in monel very cheap on the internet.

Reading from that Thermocouple would be relevant/important to us only in soaking time ,Stacy ? So there/soaking time/ will be very little +- increase/decrease in temperature ? That TC is there so we can read the exact temperature of steel we HT ? We assume that stainless part and knife steel will have same temperature ? That stainless block have more surface so will get more/faster heat from oven then narrow profile of TC .That s why i was thinking that it is better to use it ?

 

[JTknives]

Honestly if your just flooding with argon then there is no reason to build a vacuum chamber. Just purge the chamber first and close it up. The entire point of a vacuum oven is to be able to use nitrogen which is much cheeper then argon and normally generated on site where the oven is. Thy use nitrogen tell it gets up in temp and then suck it out befor thy cross that 1850° (I think it’s this temp where nitrogen looses effectiveness) threshold abd then continue to heat with radiating heat. Then to quench you pressurize cold nitrogen back inside and circulate it through a heat exchanger.

all your wanting to do is just do gas shielding and that easy. Just use high temp RTV and seal up all the joints and holes in your ovens metal body. Put a fiber door seal on the front and a Argon inlet on the back. The issue with going your style is the blades will have to sit in there the entire time it’s warming up.

 

[Natlek]

Honestly if your just flooding with argon then there is no reason to build a vacuum chamber. Just purge the chamber first and close it up. The entire point of a vacuum oven is to be able to use nitrogen which is much cheeper then argon and normally generated on site where the oven is. Thy use nitrogen tell it gets up in temp and then suck it out befor thy cross that 1850° (I think it’s this temp where nitrogen looses effectiveness) threshold abd then continue to heat with radiating heat. Then to quench you pressurize cold nitrogen back inside and circulate it through a heat exchanger.

all your wanting to do is just do gas shielding and that easy. Just use high temp RTV and seal up all the joints and holes in your ovens metal body. Put a fiber door seal on the front and a Argon inlet on the back. The issue with going your style is the blades will have to sit in there the entire time it’s warming up.

Now we are on same page
Look , I will MAKE this oven from 50mm ceramic fiber board + 25mm kaowool most likely. If i make good calculation and I use max.Watts for that size , that oven will heat fast .... really fast . Inserting blade in cold oven should not be problem .At least I can t see any problem with that .First , blade would be in protected environment so longer time don t matter.When oven get to set temperature , knife is also on same temperature .......start counting time . And steel will heat more evenly , no extra rapid heat like when we insert cold blade in 1100 Celsius . I do that way in my tube furnaces and it works with carbon steel .

The issue with going your style is the blades will have to sit in there the entire time it’s warming up

Not necessary if we don t want that . Start the oven , and when oven get to set temperature , insert knives .Close the door , start vacuum and a few seconds later no air inside .Knives are still cold let the argon inside and that is that ? That s why I think we need vacuum , although I prefer the first way . Heating from zero to set temperature can not make harm only good as far as i understand this matter so far ?

The entire point of a vacuum oven is to be able to use nitrogen which is much cheeper then argon and normally generated on site where the oven is. Thy use nitrogen tell it gets up in temp and then suck it out befor thy cross that 1850° (I think it’s this temp where nitrogen looses effectiveness) threshold abd then continue to heat with radiating heat. Then to quench you pressurize cold nitrogen back inside and circulate it through a heat exchanger.

Well , that kind of oven =do it all= is out of our DIY hands