Building tempering Oven

[Stacy E. Apelt - Bladesmith]

Yes, the muffle is what protects the blades from direct exposure to the hot elements. The metal would replace what you had as glass.

 

[Natlek]

Well , don t get angry on me I have not started building yet . . . I was thinking about marquenching and on melted lead ...melted tin , and I get some idea ... again What if I make tempering oven like on this drawing ?? Square shape and one heater on every side of oven , thermocouple in the middle and filled with AO sand which I already have a lot ?
It is more simple to build and have some great advantage......I think ? I don t care how much time it will take to get to set temp. Heaters I have can go as far as 750 C degrees . . .so marquenching is possible , too ? What do you think guys about this ? I can install even air to mix sand , just like in sand pot I work on ?

 

[timgunn1962]

IIRC, lead-tin alloys are not good for this, as the tin forms intermetallic compounds with the steel, leading to embrittlement. Lead-Bismuth Eutectic is effective, but Bismuth is pretty spendy stuff. Hence the widespread use of salts, which are much cheaper.

I think Verhoeven covers it in detail.

AO sand is probably not a good enough conductor to give the initial quench unless fluidised. It might be viable to fluidise for the first minute or two, then stop the air.

 

[Natlek]

IIRC, lead-tin alloys are not good for this, as the tin forms intermetallic compounds with the steel, leading to embrittlement. Lead-Bismuth Eutectic is effective, but Bismuth is pretty spendy stuff. Hence the widespread use of salts, which are much cheaper.

I think Verhoeven covers it in detail.

AO sand is probably not a good enough conductor to give the initial quench unless fluidised. It might be viable to fluidise for the first minute or two, then stop the air.

Thanks for the reply ,@timgunn1962 Yes , no lead no tin but AO sand is good choice ? And safe . I read a lot pass week about mar-quench.... Plan is to quench blade in oil to black color and immediately in tempering oven already preheated to set temperature . . . I don t see why this would not work ? Air will serve for even temp inside and easy to put blade in sand .I still have to much work on my daily job ,summer still don t want to go away And I work on too much project in same time ,this is my biggest flaw

 

[Natlek]

Finally I start with this project ....I decide on this dimension 35 x 25 x 25 cm .I need to drill some more hole ...for TC , for metal shields for heat elements , for fan and to cut tile on dimension , then glue everything with high temp. silicone and make angle iron frame and to make door , for which I have excellent idea .I will use 5 cm . stone wool for insulation .

This picture is up side down ...

 

[Natlek]

I can t enter in my shop anymore .....to much project unfinished I promise to myself to use this quarantine time to finish every one ! I will start with this tempering oven .This is stone/mineral or whatever name wool insulation , rated to 800 Celsius .It is not pressed/that one will smoke some period/ thickness is two inch .So what do you think ? One layer or two ? Or maybe second one from pressed wool ???? I have it enough , it is so cheap that I do not have the spare ? Plan is to use this oven up to somewhere 550 Celsius ...HT element are rated to 750 Celsius ...
Also i have no problem with larger outlying dimensions, when I clean the shop there will be room for dancing

PS . That light aluminum frame is just example for dimension .Real one will be from stainless and big enough to cover edges of insulation......

 

[Natlek]

The only machine capable to bend sheet steel in a way i want is not in proper condition and will not work for the next two months...No way I would weld frame from angle iron , so if you read in a newspaper that one town in Macedonia was burned to the ground ............. you know the culprit
I can t wait , I need to see if this will work so .........
Who knows ....maybe at the end this is not bad idea ?
I plan two door , one from ceramic tile will slide on side and over that real door with insulation .....

This days i will weld inner chamber/mufle/ from stainless and work on fan..........

 

[Natlek]

I have other plans for fan but today I found original new one for electric stove.... So here it is , I make some stand from 2.5mm stainless ,there is enough place for one layer of insulation between fans.. ...One step closer

 

[Natlek]

Well , i m ready to put HT element ,TC , fan and wire them .Fan is also ready , maybe shim here and there to align it perfect with hole in back side but it is 99% done ..And of course to align that wool in right way .On inner chamber and door i can work later .I have question before I do that .This is thin mirror polished aluminium plate /from old Solarium/ .I know that will not make harm if I use it between wood and wool , but did will make any good with heat reflection ? What do you think ,worth effort ?

 

[Natlek]

I was playing with this thing almost all day last Sunday. HT elements are connected parallel and it take 15 min. to reach 200 Celsius with fan ON from start , with door side not insulated yet. Without fan it is way more slower............. I mount two thermocouple ,one ordinary and contact one inside chamber between two piece of steel . I try lot of things with both TC and fan on/off and all I have to say is ..................FAN ...FAN ...and FAN is mandatory in any tempering oven .
I think that ceramic tile are right decision for this build .With help of thick isolation it take about one minute to drop temperature 1 Celsius ,fan running all time . Next is to connect that MF ..........PID . I found one smart young men which probably know more for PID then Chinese who build it ... I read in several place that when SSR fall , most time they fall in close position which is no good in tempering oven so I will use thermal cutoff switch on 250 Celsius for protection .
I have not used muffle inside because I still don't have it open from back side so fan can suck air , but even without muffle air circulated perfectly .....probe with one layer toilet paper .

Now ................ I got three tubes of different diameters , biggest one / 38cm Dia./ is aluminum rest two are stainless ....IF i don t build from them tempering oven /Up to 550 Celsius/ in two hour I will quit this hobby !! I do not know what I was thinking ! How I hadn't thought of that before And i already have build two HT oven like that ????
It can t be simple then this .............................

 

[Natlek]

Ok , some update ...
I make some change to this thing...............I needed PID /only one i have/ for other HT oven so I buy kitchen oven thermostat ...spare part for oven in my kitchen, Made in Germany.
For test I insert tube from thermostat in place of Thermocouple .I used my other temperature control for reference , tip of probe clamped between two pieces of steel and also I used this infrared /expensive one/ for control temperature...........
Well this thing work surprisingly accurate , of course with fan always ON ..........accurate data when i finish rack and make more tests .
Now i want to make new rack for knives ..................What do you think ? I think that IF probe from thermostat is there/look drawing/ oven would be even more accurate ?It will measure temperature of that steel ...That mean that temperature of tempering steel would be same .
I will get another device for measuring the temperature and I will set it up in the same way as the oven thermostat. It will be used to control and fine-tune the thermostat knob to the right temperature.
Picture from internet of one thermostat , just to get idea how it look , it is not same like mine ........

I already start to work on new rack for knives , in this way it will not interfere with the air flow in oven

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[Natlek]

The other day I used this tempering oven a lot / 52100 bainite / and I have time to observe how it works .........
It take about 20 min. to reach 230 Celsius . After that the cycles are like this....
230 Celsius ................................................5 min. stay on that temperature and in next 3min. temperature drop to 226 Celsius .
ON for about one minute and temperature raise to 230 celsius .......... Fan always ON . More then enough precise for me .
temperature sensor is inserted in that 6 mm thick steel .Almost exactly the same behavior has on 190 Celisius
Without fan always ON ...... simple answer is I don t LIKE IT !! Complicated behavior ! HT element get red hot till oven reach set temperature, changes in temperature are greater ,cycling is erratic ..................
If temperature sensor is in air , cycles ON/OFF are shorter but / Interesting / STEEL stay on same temperature all time , not even 1 Celsius change ..... but I choose 226/230 C for energy saving . +/- 2 C degree is enough for me .Sayonara PID controller

I think that mete / R.I.P./ was right , ceramic tile were the right choice for this build ...........