I am looking for input from those of you who have more experience in what I am planning to try.
I have been wanting a heat treating oven for a while now. Inspired by the recent postings here, specifically in regard to PID controllers, and the need to anneal, machine, then heat treat, some large hardened steel heavy equipment pins, I need to get moving on this project.
Last week, in search of a quick solution to anneal the pins, with input from ideas on this forum, I put together an atmospheric propane forge burner. Then built a chamber out of hard fire brick held in place by random clay brick I had on hand. It worked great to heat the steel. I measured the temp at over 1700˚f. But it is nearly impossible to control the cooling as required for proper annealing. So, now I have a forge, but still need a good, reliable way to heat treat, specifically anneal hardened steel.
I have a section of ceramic kiln from a school where I used to teach. It was a nice Skutt kiln https://skutt.com/ceramic-kilns/ . These kilns are basically round, 10 sides, and built in sections so you can add or remove sections to make the kiln larger or smaller. Each section consists of 2 rows of 10 bricks 4 1/2” high so each section is 9” tall and about 24" inside. I have one of these sections, with the elements, along with the stand and the top and bottom from the original kiln.
I am thinking I can cut the bricks, to make a smaller oven. I have a plan to cut the bricks so I can get two from each so I will end up with 4 rows of 10.
This would allow me to make two sections about 9” in diameter and 9” tall. I would be able to use just one section, or if necessary, put them together and have a 9” by 18” if necessary for longer parts. The bricks are held together by large stainless steel bands, much like big “hose clamps” I should be able to reuse these bands also.
Originally I was thinking of laying the oven on the side. Now I am leaning toward standing it up, like the kiln was originally built. It already have the original kiln stand I can just make smaller.
I will also need a PID that has ramping and soaking capability. Currently I am considering one used on a build here about a week ago. Auber SYL 23X2P Ramp and Soak. They have 3 models where X = 4, 5, or 7. I am not certain which s best for my situation. I have eliminated #7. So the choice is between 4 and 5.
#4 would work with the many 24vac and a few 208-240 vac contactors that I have on hand, removed from various equipment over the years.
#5 requires Solid State Relays, I have no experience with SSR. I am not afraid to learn, but wonder if it is worth the money since I already have old school contactors I can use.
All opinions are welcome.
I am trained as a metallurgist, with development experience both on university and in industry. There I learned thoroughly that the critical property of a heat treatment furnace is the temperature variation across the furnace, and over time. For industrial processes, an acceptable variation used to be +20F, for university level research +1F would be barely good enough, and we went to a lot of effort to get it to a tenth of that value.I have been wanting a heat treating oven for a while now.
To get temperature variation over time to within 1F with a PID is piece of cake - if you know where to place your controlling thermocouple. To decide on where to place it, you need to consider thermal inertia, and thermal conduction over the extent of your furnace. Be assured that wherever your bricks change direction (bottom to side, side to furnace door!, etc) there will be substantial variation in temperature. On my research furnace, that was dealt with by enclosing the test sample (a cylinder about 15mm diameter and 20mm high) in a steel vessel having nearly 20mm thick walls, and allowing 12 hours for the furnace to heat up and reach equilibrium. For your much larger furnace, and I presume, pins, you don't have that option, and thus you are stuck with industrial tolerances. When one of my technicians measured the temperature variation across a furnace of about the size you are considering, he measured some 80F variations from the hottest, to the coldest spot inside that furnace - door closed, of course. I still shudder to think what the same approach would have produced if measured across the large furnace, 20 meters long and 3 meters wide which had to be used to heat treat our steel plates. And the temperature inside was measured with pyrometers, not thermocouples! Luckily, it was a continuous furnace where the plates traveled on rollers, thus (pseudo) thermal equilibrium could be assumed after some weeks of continuous operation. We did get it right, eventually, but only after substantial arm wringing, since the production people, who were not metallurgists, had a hard time to understand that an exceptional steel required rather different operation procedures from what they were used to. We managed by tweaking tolerances, but we managed and successfully produced several thousand tons of the stuff . . .Inspired by the recent postings here, specifically in regard to PID controllers, and the need to anneal, machine, then heat treat, some large hardened steel heavy equipment pins, I need to get moving on this project.
When I, now that I am retired, need a heat treated piece of steel, my preferred option is to buy a bolt of the appropriate strength class and machine it to whatever I need, using carbide tools if necessary. If I cannot get that bolt and have to heat treat, I use a piece of mild steel, heat it with a torch orange red, quench in water, and skip any tempering. Such rude treatment is possible because of the high temperature tolerances permissible with carbon steel, tolerances that are not permissible with alloy steels.Last week, in search of a quick solution to anneal the pins, with input from ideas on this forum, I put together an atmospheric propane forge burner. Then built a chamber out of hard fire brick held in place by random clay brick I had on hand. It worked great to heat the steel. I measured the temp at over 1700˚f. But it is nearly impossible to control the cooling as required for proper annealing. So, now I have a forge, but still need a good, reliable way to heat treat, specifically anneal hardened steel.
And alloy steel is what you appear to want heat treat, because why else would you want ramp and soak capabilities? For carbon steel you only soak for 1 minute/millimeter thickness, more is not needed. That assumes, of course, that the furnace is on temperature already and does not cool down due to inserting your piece of steel.
To repeat: don't heat treat yourself, leave that to someone with the appropriate facilities and competence, or limit yourself to small pieces of plain carbon steel. And then you don't need a sophisticated furnace. Then a forge will do. If you do need to anneal, you can allow your piece of steel to cool in the furnace, or you can immerse it in a thick layer of some stuff with poor thermal conductivity like ash or perlite. That is how thick turbine shafts are cooled.
All I can hope for here is that I have not used too much metallurgy knowledge and lost you in the process. Here is the graph on which I base my heat treatment decisions:
What I have not discussed at all is mechanical properties, and toughness in particular. And if you want good toughness together with high strength, tolerances you are allowed to use shrink quite dramatically. Full Stop
Thank you for your valuable input. I am familiar with heat treating processes but not the specific temperatures as those are available in reference materials. My first introduction to the processes came through my uncle who was a metallurgical engineer. I still have his collection of technical reference materials, BUT, I have do not know specific types of steel I have collected. My actual working experience comes from 35 years of teaching Industrial Technology. Some of those years were spent introducing students to industrial processes, among those processes; machining, forging, casting, annealing, hardening, tempering. We did not have any sophisticated equipment. As you mentioned we annealed by heating in the gas forge to a certain color, turning off the forge, and covering the vent hole with a firebrick.
Our heat treating was simply, heat in the forge, watch the color, quench in water, emery off the scale, and temper over a hot hunk of iron while watching the colors move down the shaft of the screw driver or chisel. I realize it needs to be a much more precise process for most manufactured products that need heat treatment.
In my situation, I have done repairs on heavy equipment for years, I have a considerable pile (100's of pounds) of worn or broken pins, large broken axle shafts, leaf springs, worn cutting edges from earth working equipment, worn or broken hydraulic cylinder shafts, etc. All of this is good steel, but too hard to machine with my capabilities.
I should have been more specific, the main purpose for oven is to be able to anneal this steel. I may in the future be able to do hardening and tempering, but the first step in this is getting the steel annealed.
From your comments on temperature variations in the furnace due to corners, etc, I am thinking that the more rounded shape of the chamber will be a good thing. The only 90˚ corners will be at the top and bottom. Do you feel there will be a much more significant difference in temperature with the vertical vs horizontal? The completed chamber will be about 6" dia, X 9" long. I have enough materials to make 2 chambers so I can stack them if necessary to make the chamber 18" long.
Now you sound no longer as someone acting out of an industrial / commercial environment, and that permits me to relax standards of accuracy: I give you tips that in my experience will work, but I do not accept responsibility if something goes wrong. With that firmly understood, get yourself a copy of Japanese Industrial Standard JIS G 0566: Method of Spark Test for Steels. This standard provides a means to roughly sort steel by its chemical composition, by just observing the color and shape of the sparks produced with a grinding wheel. With that in hand, you can group your steel supply into what can be quenched and tempered successfully with primitive means, and those that cannot.
Restrict yourself to carbon steel, i.e. steel that produces bright orange sparks and plenty of bursts. Steels producing dark red sparks are high alloy steels and not suitable for heat treatment with primitive means.
Then, build your furnace with two or even three heating zones, zones that can be controlled individually so that you get a better temperature profile. If I read the picture of your furnace correctly, it is heated by open, uninsulated electric wires. Because they are uninsulated, safe operation mandates they are present in the walls, not in the floor or ceiling / door. With this design, ceiling and floor will be colder than the heated walls - how much requires measurements. Scale falling off your steel will otherwise cause short circuits.
There was recently on this forum a suggestion to use the heating elements from domestic ovens to build a heat treatment furnace, over a central core made from a stainless steel pipe, which in turn is insulated using aluminum silicate fiber blankets. That would be my preference, since it could be build for 100-200 dollar , plus the cost of the pipe.
But since you already have the bricks, in your place I would explore the idea of dispensing entirely with a PID controller, heat with gas, and control forge temperature entirely by controlling gas and air flow. The big advantage of a gas powered furnace is it heats up much faster than an electrically powered one, but leave it alone unattended for an extended time?
Lastly, I would take a K-type thermocouple (chromel-alumel), attach its measuring tip to a small block of steel, and measure the temperature distribution over the length and height of the furnace. Then I would know where to place my steel piece.
I have to wonder why you gave up on gas fired ovens for a healing profiles. You will likely need a different arrangement of nozzles and plumbing but it should be cheaper to operate.
Ive worked on a number of different ovens and furnaces over the years and frankly tight tolerances on temperature require air circulation via a fan or other method. Frankly I wouldn’t put something like a pin in an oven that is round and expect it to maintain an even temperature profile. Without a fan you would want a rectangular oven that would keep the pins evenly away from the elements.
as the other responder mentioned temperature profiles in industrial ovens can be huge. Even in the case of much lower temperatures in ovens used for specific processes operating at much lower temperatures getting a flat profile in the oven is a chore. The higher the operating temperature the bigger the potential errors. In an oven with static air you would need to keep the element distance from the workpiece as constant as possible. Note we are talking simply about issues of temperature control here. This has little to do with knowing the optimal annealing profile.
here in is the next issue, I really doubt that those pins and such are plain carbon steels. Maybe maybe not. In some cases they may not be through hardened as toughness might be the goal so a “case” or outer hard layer might be in place. Some of these steels May machine well once through the outer hard shell.
by the way you wouldn’t be the first person to throw a piece of steel into a fire to soften it. Totally uncontrolled but it does work for some simple steels. By fire I mean a substantial coal or wood fire that can be fed with air. The steel is brought up to temp and the fire allowed to cool and burn out.
I am looking into to the electric oven after trying to anneal with gas. I have also tried it in a large wood fire. I was easily able to get a pin up to temperature in the gas fired furnace as well as the big wood fire. I was not able to cool it down properly. My gas "forge" was crude at best. If is difficult to achieve only 70˚f of temperature loss per hour. If did not help that is was cold outside when I tried it. But that is another reason to have it be electric.
I switched my focus to electric as I have the old kiln. I have heavy duty contactors, I have most of the parts. The most expensive things I need to buy is the PID controller, and ceramic blanket insulation. Also, I can keep and use an electric one in the work shop in the basement. I have an exhaust hood, but it is not adequate for a gas fired forge or oven.
In regard to uneven heating, I do realize that is an issue, but not insurmountable. If you think about it, a round oven, with the elements totally surrounding the workspace make a much more even heating device, especially for round shapes. That it the main reason that Skutt Kilns are built that way, there are no corners to prevent even heating of ceramic pieces. According to my preliminary sketches, I can get cut the existing kiln into enough smaller bricks to make a chamber about 6" diameter, and 18" long. I have not decided yet of the orientation will be vertical or horizontal, I am leaning toward vertical because of the ease of positioning parts centered between the elements.
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