Some more Heat Treatment Oven time.

The little Heat Treatment Oven has been in use for a while and I have been thinking about the process cycle time for a while so yesterday we wound a new element with a calculated 2.5kWatts which should speed things up a bit.
First job was crunch some numbers and I use a spreadsheet for this, in fact I use spreadsheets for everything, even our RFD register! The advantage of the sheet is I can see what changes to the element length (Resistance) make to things like the current draw and power. I can also experiment with different gauges of the Kanthal A-1 wire I use which was handy as when I went to order the gauge used before, nobody stocked it in the length I needed.
A couple of entries on the sheet and I could instantly see if the alternative works and how much was needed. In the end I opted for 19AWG and ordered the wire.
The thing about the wire is it is just that, wire and not a heating coil so first job is clean it with acetone and wind it onto a mandrel on the small lathe. This is a bit of a two person job as having someone to hold and feed the wire to me saves potential kinks in the wire. Luckily the Viking was to hand as she usually is.
With the coil wound it was a simple matter of trimming it to final length and putting it in place, connecting in the terminal box and starting to test.
My target temperature for this test was 925-950 degrees centigrade (1700/1750F) This sort of temperature allows me to heat treat spring steels which I work with, a lot. Hence the target and I wanted to get to it in under half an hour so testing is important if only to prove my maths are about right.

Starting at 25C the oven hit 100C in 42 seconds, 500C/932F in just under 4 minutes and I stopped the test at 925C/1697F at a bit under half an hour which was perfect.

.The oven should be good for another 100-150C so I can have a play at knife making one day.

Right now I am watching the cool down temperature time.

The outside of the oven was cold to the touch at the 30 minute point however it is quite warm to the touch now at 90 minutes, however it is not hot which is excellent.
On the subject of spreadsheets, I created a plot of time vs temperature which I can use as a reference. The highlighted areas are the spring hardening and tempering temperatures I tend to work with, however these are material dependant.
Finally, I know, I really should get around to putting some decent labels on the control cabinet. Maybe next year
As a final note, if you fancy having a bash at making your on HT oven, I am happy to share whatever information may be of use to you.
Take care all.

Heat Treatment continued, the build starts.

Finding time for such projects is always problematical, we have customer jobs to get out, both gun work and shiny stuff. It is also the game season so I am out a couple days a month as a minimum and we have the two pups to train, plus the old Labrador who is out working, in fact he is out 4 out of 7 days this week so finding time for my HT is difficult so today I am going to work on the frame plus machine some strikers for a couple of shotguns, one of which needs a good service.

Anyway, this is where I am with the project right now, the design has been updated slightly and will now have a removable top so I can re-build it and/or replace the element if needed. The angle iron for the frame is in the workshop ready to be cut and the aluminium in-fill plates to protect the bricks have been cut to size and are ready to go. This is my first oven build so I really hope the insulation properties of the oven bricks are such that the external surfaces do not get too hot.

The bricks are remarkably easy to cut, however the dust is incredibly hazardous so I am taking care to keep dust to a minimum and a face mask and vacuum cleaner keep things reasonably safe (Fingers crossed)

I am gluing the bricks together with a thin bond of refractory mortar which makes assembly and cutting a bit easier as I can form the four sides and them move them as individual sections. The element slots still need to be cut however I have not yet cut the element so that is something for an evening next week.

I sat down and designed the electrical circuit during the week. There are many examples on the internet however they are not ideal for my purposes so I developed my own control circuit for the PID controller which is housed in the control cabinet to the right hand side of the oven.

More to follow on this…

Heat Treatment

I have been meaning to design and build myself a dedicated heat treatment oven for small items for ages and recently kicked the project off by ordering a PID controller and cabinet and a couple of other electrical parts. Today I ordered the insulating fire bricks and mortar which will be used for the construction. The physical capacity of the oven is small at 154 x154x228mm or 6″x6″x9″ however that is more than enough for a handful of strikers (Firing pins) or replacement hammers or for hardening dies and similar.

The image shows the HT Oven concept with the open door, I have omitted the liner plates and frame as this image was the original I used to determine the physical size of the internal space and the layout of the insulating bricks.

The box on the right hand side houses the controls and I have also omitted the screw catch that holds the door closed and the hinge system.

Frame construction will probably be 25,0mm angle iron and the infill will be either stainless of aluminium sheet depending on what I can source locally.

Maximum operating temperature is a theoretical 1050 Celsius which is  1922 Fahrenheit, or more than I will ever need for most applications.

The addition of the HT Oven is something we have recognised a need for as it means I can more accurately control the temperatures for both hardening and annealing of materials in house instead of resorting to a gas torch with the lights out, or shipping things off and waiting days for them to be returned.

The bar at the top is to enable it to be picked up and moved and will probably be shifted towards the right to bring the balance to a central point.

I will use stainless foil sheet to wrap items prior to heat treatment to minimise scaling as it works surprisingly well if you flatten the envelope you have formed to hold the part to be treated. I guess some pictures at some point would better explain this.

More to follow on this project.