Muffle furnace

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Muffle furnace

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  • #766807
    Sonic Escape
    Participant
      @sonicescape38234

      I recently read some books about heat treatment and thought it would be interesting to try it myself. I’m thinking of making a small electric furnace with a usable volume of about 150x150x250mm that can heat up to about 900-1000°C.

      I’ve seen that many types are made of refractory bricks. I’m thinking of making it from vermiculite plates. These are easy to cut and insulate better than refractory concrete. The only disadvantage is that vermiculite is more expensive. But for a small furnace it’s ok.

      I haven’t decided what kind of heating element to use. Probably some khantal wire wrapped on a support. I could size it for a 3kW power, to be on the safe side since I don’t want to dive into heath losses calculations.

      Anybody tried to build an electric furnace or have any suggestions?

       

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      #766813
      bernard towers
      Participant
        @bernardtowers37738

        buy your vermiculite blocks from a fireplace supplier they are much cheaper!!!

        #766821
        Bazyle
        Participant
          @bazyle

          remember there are two types of brick shaped furnace blocks out there (not talking vermiculite). One type is heavy and meant for pottery kilns so has low insulation but very high thermal mass. The type you might use are very lightweight and used in enamelling and similar hobby furnaces. You can aso use loose vermiculite if you can invent some containment.

          #766825
          not done it yet
          Participant
            @notdoneityet

            Dunno the cost of things these days, but it may be more economic (unless you are bent on a self-build) to look for a second hand kiln?

            I have a Paragon (Sc2, I think), which has been fine for most odd bits I’ve wanted to heat treat.  There are several other makes and different types out there.

            Hardening is the easy part for many jobs – heat to red with a gas flame and quench.  Tempering is easy enough, too – up to the maximum temperature of a domestic oven, or colour if you can see it (I cam’t).

            Check out lampworking, metal clay, enamelling and other glass hobby sites (such as slumping) for inspiration, maybe?

            My kiln has a ‘bead door’ (for lampworking) which has, on occasions, been useful for heating the end of longer items without opening the main door.

            #766827
            noel shelley
            Participant
              @noelshelley55608

              Whilst the lightweight furnace bricks used in pottery kilns are excellent and made for this job. Get the right type and they will have grooves for the elements built in. I used them for a smelting furnace where the wall temp was about 1500c, the outside wall was hot but painted, and 25 years old. Plan B is ceramic fibre, this comes in many forms, tube, blanket Etc. This material is very effective in keeping heat in and will easily take the temperatures you mention. DO NOT consider any type of heavy brick, eg those used in night store heaters ! Noel.

              #766835
              not done it yet
              Participant
                @notdoneityet

                DO NOT consider any type of heavy brick, eg those used in night store heaters ! Noel.

                I’ll second that!  Ages to heat up – and likely too slow to cool down on a ramped cooling program.  Little or no thermal insulation, either.

                I’m not sure, but think the paragon (etc) kilns use both bricks and ceramic fibre/wool for insulation.

                #766856
                Roderick Jenkins
                Participant
                  @roderickjenkins93242

                  I built a very small one many years ago from Nichrome wire wound round a fire cement former and loose vermiculite from the garden centre for insulation.  I don’t think I’d do it that way now.  These days, here in the UK, more suitable materials are easily available from ebay.  What I would recommend though is to control it with a PID that you can set a ramp rate on for controlled cooling down – useful for annealing.

                  If you want see someone going over the top at furnace making watch this:

                  Rod

                  #766867
                  Nigel Graham 2
                  Participant
                    @nigelgraham2

                    To add to NDIY’s advice:

                    The CuP Alloys literature on making brazing hearths explains that the materials for storage-heaters etc. are unsuitable because they are intended to absorb heat.

                    You need walls that reflect heat as much as possible, not absorb it, nor of course conduct it away.

                    Hence vermiculite or similarly low-absorbency refractory materials.

                     

                    I’ve sometimes wondered if placing a few pieces of charcoal in the furnace, safely out of contact with the work, would enhance the muffling properties by burning, so absorbing the oxygen from the furnace atmosphere.

                     

                    #766869
                    bernard towers
                    Participant
                      @bernardtowers37738

                      Use boric acid mix to keep away the oxygen

                      #766892
                      SillyOldDuffer
                      Moderator
                        @sillyoldduffer
                        On Nigel Graham 2 Said:


                        I’ve sometimes wondered if placing a few pieces of charcoal in the furnace, safely out of contact with the work, would enhance the muffling properties by burning, so absorbing the oxygen from the furnace atmosphere.

                         

                        Bad idea I fear.  The problem is charcoal absorbs Oxygen by burning, producing mostly Carbon Dioxide, which case hardens steel, and attacks other metals.   12g of Carbon burnt in 32g of Oxygen will produce 44g of CO₂, so more muck not less.

                        If Oxygen is a problem better to remove it first, perhaps by flushing the furnace interior with Argon or Nitrogen after closing the air tight door and then closing the flush taps.

                        Other gases are sometimes deliberately deployed in Muffle furnaces:  for example, the extremely pure soft iron used to make top-notch magnet cores is finished by muffling in 100% pure electrolytic Hydrogen.   My book doesn’t say why – I guess Hydrogen molecules being tiny penetrate into micro-cracks and pores, pushing out impurities and eliminating Oxygen.   The soft iron is 99.99% pure before this done, but a tiny increase in purity greatly improves the Iron’s magnetic properties.   Exotic or what?  I don’t recommend anyone trying Hydrogen at home!

                        Sonic hasn’t said what his furnace is for.   Nothing exciting needed for ordinary work like normalising or enamelling.   Knowing something of Sonic’s interests though, he might be into demanding applications like cooking thermionic valve cathodes or heat treating HSS!  Both need a rather complex automated control system, with temperatures being ramped up and down with several constant soaks at different temperatures in between.  The cycles needed by HSS are particularly complicated and demand accurate temperatures and times.   Difficult muffling shouts ‘microcontroller’ and good temperature sensors, otherwise a thermostat and egg-timer should be good-enough!

                        Dave

                        #766916
                        duncan webster 1
                        Participant
                          @duncanwebster1

                          Tom Walshaw (the UK Tubal Cain) did a design for a muffle in his book about hardening. He also recommended putting a piece of charcoal in to absorb the oxygen.

                          #766930
                          Sonic Escape
                          Participant
                            @sonicescape38234

                            I didn’t knew about ceramic fiber. Indeed they are available in all kind of shapes and are cheap. There is even a liquid that you can spray on them to make them harder. Good to know! But vermiculite has also some mechanical strength. I saw that there is a special glue for it. Maybe I could assemble the furnace simply by gluing together vermiculite plates.

                            About control, I’m thinking to run a PID control loop a laptop. With a USB thermocouple interface. Also I have a 200A/480V SSR that is gathering dust. I’ll use some USB I/O interface to drive the solid state relay. With a laptop is easier to display temperature graphs or to program different heating profiles.

                            I don’t plan to do some very demanding treatments. Just stuff like case hardening of low carbon steel, tempering. Maybe some lathe tools. I read that you could make the steel grains smaller to make finer cuts. I’m curious if it makes a difference. Maybe I’ll make some screwdriver tips, or a cutting blade. Things like this.

                             

                             

                             

                            #766941
                            Nigel Graham 2
                            Participant
                              @nigelgraham2

                              Dave –

                              Thankyou. I’d never thought that small amount of CO2 could case-harden anything, but I suppose the volume of oxygen present is small anyway, certainly far less than would be attacking the work if heated in the open.

                              #766951
                              Bazyle
                              Participant
                                @bazyle

                                Re charcoal and oxygen removal. Case hardening used to be done by enclosing the victim in a box or ‘case’ full of charcoal (and other magic recipe ingredients) to provide the carbon but also remove oxygen hence oxides you would get from open heating. However a side effect was to instigate the formation of CO which then likes to turn into CO2 by releasing some C in an state that is absorbed more readily into the iron (simplified explanation )

                                Anyway Borax as suggested earlier is easier.

                                #766973
                                jaCK Hobson
                                Participant
                                  @jackhobson50760

                                  A lot of knife makers make their own ovens. This bloke has made a lot:

                                  https://www.edgematters.uk/thread/7732-another-oven-build/?thread/7732-another-oven-build/&pageNo=1

                                  Soft refractory bricks are easy to cut/carve – easy to carve channels to hold the khantal wire so you don’t need to wind it around a support (you do need to wind it into a spring).

                                  Tim has tips around how to connect the Khantal wire to maximise life (common fail point in home made ovens) … but I can’t find the tips or remember them.

                                  I made a large ( to get even heat) gas oven (so NOT muffle) from an oil drum lined with fibre but I think it time I made an electric one.

                                  I used the cheapest PID and it works well. Maybe you have to pay a little more to get fancy ramp features. Then doesn’t tie up a laptop.

                                  K type thermocouple should do for most heat treat.

                                  Large grain can be easier to machine – weaker.

                                  I have thought about putting charcoal in the oven – Oxygen is usually a much worse problem than CO2. Purging with nitrogen or argon isn’t difficult if you have access to the gas… you don’t need a high flow rate. Knife makers wrap blades in stainless foil to minimise oxygen for high temp treatments (like stainless) – sometimes more practical than baths of borax. ‘Lo Salt’ available in supermarkets might work as a high temp salt? Big baths of hot salts present risks (don’t spill it, and don’t put anything in it before drying first).

                                  #766992
                                  noel shelley
                                  Participant
                                    @noelshelley55608

                                    Hey Sonic, The thermocouple you need is an Inconel sheathed K type, 3mm dia will be good and of whatever length you think best. They are rated for the temps you want, I use them in dip pyrometers at 1150c or more. Good Luck. Noel.

                                    #767010
                                    SillyOldDuffer
                                    Moderator
                                      @sillyoldduffer
                                      On Nigel Graham 2 Said:

                                      Dave –

                                      Thankyou. I’d never thought that small amount of CO2 could case-harden anything, but I suppose the volume of oxygen present is small anyway, certainly far less than would be attacking the work if heated in the open.

                                      I’m glad you suggested it Nigel, because I’ve learned something.   As Jack says “Oxygen is usually a much worse problem than CO2. ”

                                      My reading on Muffle Furnaces is they were developed specifically to keep combustion products away from the item being heated.  Plenty of circumstances where clean heat is essential, including warming my breakfast croissants!

                                      But having said that, case hardening is a common workshop requirement.  As Carbon is already part of the process there’s no harm in having CO₂ in the furnace and advantage in reducing Oxygen because it attacks steel. Therefore burning Charcoal as you suggested is a good idea.

                                      CO₂ not good in other examples, especially if made by burning Carbon in the oven.   To easy for a  jeweller or potter to ruin an enamelling job by getting Carbon smudges or ash in what has to be a perfect surface.

                                      Borax or other flux may also be good or bad depending on the circumstances.  Whilst flux keeps air away, it’s not good at stopping solids reaching the job.

                                      Muffle heat has to be clean in the context of the job, which means CO₂ and Flux might be either great or terrible!  I now think burning charcoal in the oven will help when Carbon is less damaging than Oxygen, but otherwise setting a fire inside what’s meant to be a clean environment is risky!    When cleanliness really matters, an industrial Muffle Furnace operator is better placed than us because he can get rid of Oxygen, water vapour and dust simply by flushing out air.   Argon, Nitrogen, CO₂ and other clean gases are cheap provided they’re bought in bulk; no problem to industry.   Sadly, most hobbyists have to buy gas in small quantities, and that’s very expensive.

                                      Sonic has since confirmed he’s not planning anything demanding more than ordinary cleanliness.  Case hardening rather than Jewellery.   I don’t think he can go far wrong whatever he does!

                                      Most educational this forum, thanks again,

                                      Dave

                                      #767019
                                      duncan webster 1
                                      Participant
                                        @duncanwebster1

                                        I’m not convinced that even red hot iron will react with CO2. Would the carbon rather be attached to oxygen or iron?. You don’t get iron carbide out of a blast furnace, and you have molten iron and CO2 in that. Needs a chemist’s knowledge, out of my pay grade.

                                        #767048
                                        Bazyle
                                        Participant
                                          @bazyle

                                          I think you are right Duncan which is why I mentioned the involvement of CO above. You can perhaps make nitrogen for flushing by having an external container to have the fire in and carbonate to absorb the CO2. ‘Deflagisticated air’ as Count Frankenstein would have called it.

                                          #767056
                                          Fulmen
                                          Participant
                                            @fulmen

                                            Iron does not react with CO2, at least not at any significant rate. The case hardening reaction involves CO2 first reacting with carbon to form CO, which in turn reacts back to form CO2 and carbon:

                                            CO2 + C <=> 2CO

                                            These are equilibrium reactions, meaning that you need high concentrations of C and CO2 to form CO. This CO then diffuses into the steel which is low in C, shifting the equilibrium back. For most steels the heat/soak time should be too short to add any significant amounts of carbon, and unless it’s a sealed retort there will always be an excess of oxygen that will decarburize the steel somewhat. So any carburizing effect should serve to offset this more than anything else.

                                            #767069
                                            jaCK Hobson
                                            Participant
                                              @jackhobson50760

                                              Scale is the main thing to avoid when heat treating steel i.e. oxidisation. My experience is there is always a risk of scale even when running a rich gas forge or electric furnace without a purge. For forge welding, the best damascus makers I know heat the forge up maximum with a stociometric flame (perfect blend of oxygen and gas for hottest burn) by checking temp with a platinum thermocouple, then change the mix to rich/reducing (more gas) when the billet to be welded goes in. You can then weld without borax (or any other flux). Maybe dip billet in kerosine first just to get a protective layer of carbon initially. So for steel blades, no one usually cares much about other chemicals in the atmosphere – just Oxygen. Charcoal forges can run oxidising or reducing depending on air flow and position in charcoal. So a charcoal forge can case harden.

                                              Some people I know have purged with ammonia for nitriding. I guess that must stink.

                                              One reason I want to make an electric furnace is so I also can try purging. I’ll be trying a hot salt bath first though… probably an easy way to heat treat really fine files without decarb on the teeth. I guess bathing in borax is ‘hot salt’  … not sure why it isn’t more commonly used as the hot salt dip – maybe the Lo salt type mix melts a little lower.

                                               

                                              I’ve been experimenting with low temp salts for the quench. For small parts, you can fill up a carbon crucible in one of those cheap electric furnaces as a salt pot. Gives a really fast quench (deep hardening) but gentle as you keep the salt temp above the martensite start temp (~220c)… so good to stop cracking or warping. High temp salt with low temp salt quench can result in parts needing minimal cleanup.

                                              #767086
                                              Grindstone Cowboy
                                              Participant
                                                @grindstonecowboy

                                                I think This Old Tony has/had a video on YouTube about a purging modification to an electric furnace.

                                                Yes he did, and here it is

                                                Rob

                                                Links to https://www.youtube.com/watch?v=O_Fo7mfZg7k&list=PLW66x6oEKSSJoZ05exs9VA7vmKpxGNEb5&index=188

                                                #767118
                                                Mark Rand
                                                Participant
                                                  @markrand96270

                                                  Table salt melts at 801°C, which is perfect for hardening EN24 (823-850°C). Downside is that everything else in the vicinity tends to go rusty 🙁

                                                  #767138
                                                  SillyOldDuffer
                                                  Moderator
                                                    @sillyoldduffer
                                                    On Fulmen Said:

                                                    Iron does not react with CO2, at least not at any significant rate. …

                                                    True, but it’s not how I understand the mechanism by which Carbon and Iron interact.  My book is pre-war though, so might be wrong.  My source says there is no chemical reaction.  Instead, Carbon dissolves in the Iron in the same way Sugar dissolves in tea.  Sugar molecules fit unchanged into the gaps between water molecules and if the water is boiled off, the sugar is recovered unchanged.  Any number of salts and other liquids make solutions, making it possible to distil spirits such as Brandy from a low alcohol source, or to spoil the bosses tea with a little salt.

                                                    Rather small quantities of dissolved Carbon alter the properties of Iron considerably.  It dissolves in Iron and the alien atoms alter the grain structure of the alloy by altering the way crystals interlock. Diamond wheels aren’t recommended for grinding steel because although diamond is far harder than steel, it dissolves in it.  Physics rather than chemistry!

                                                    The reaction described by Fulmen might well speed up the dissolve process, but I don’t believe it’s essential.   Items being case hardened are usually packed solid with Carbon, often sealed in a box to keep air out.  There is no Oxygen available in a box to make CO or CO2.

                                                    When steel is made hundreds of tons of Iron are melted and Carbon added. Like sugar in coffee it’s given a good stir, not with a spoon though!  And unlike beverages the melt is sampled and analysed to confirm the steel is to specification, and adjusted using heat, oxygen and flux to burn out impurities, and by adding coke, Vanadium or whatever else is needed to get the mix right.

                                                    Case hardening only diffuses Carbon close to the surface. Whilst creating a deep layer takes time, the process starts immediately.  I don’t know how quickly, but obviously more Carbon and heat will speed it up, whilst low carbon and less heat take longer.

                                                    Oxygen rusting steel is a chemical reaction.

                                                    Dave

                                                     

                                                    #767157
                                                    duncan webster 1
                                                    Participant
                                                      @duncanwebster1

                                                      Well it just shows how wrong I can be, see link below https://www.designnews.com/metals/understanding-case-hardening-of-steel

                                                      If you heat charcoal in air it will initially form CO2 until all the oxygen is absorbed, but then the CO2 will react with more carbon to give CO,

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