DIY Kiln element

S

Schultan

Joined
Apr 18, 2019
Messages
7
Hi guys,

So, I decided to build my own kiln for heat treating. I cheaped out, and bought a chinese no name PID controller. I had a lot of issues dialing in my temps and figured I need a proper PID. After installing it I did a auto tune on 1080 degrees C and it was stable at ± 3-4 C.
I was quite happy.
Next day comes around and I make a couple of pieces of 12c27 steel to test what kind of hardness my protocol would get me. I get the kiln up to temp, put the pieces in and set a timer for 5 mins. At the 3.50 mark, the temp start to drop drasticaly. I turn everthing off and look inside. My heating element is destroyed.
Today I pulled out the element, and it's fallen apart completely. At least 13 different pieces.

The wire is suppsed to be Kanthal A-1 1,2mm, but I'm starting to think I've been ripped off.

Can anyone offer any insight as to what has happened here?

kad7B8F.jpg
 
There are lots of variables that you have not told us about.

What were your Volts, Amps and target Wattage? How hot did the oven get during your testing? how were the elements fixed/supported? What is the controller? Are you switching an SSR or a relay/contactor? What is the output cycle time?

1.2mm seems pretty thin for an element in an HT oven. I built my first few with 16 AWG Kanthal A1 (1.29 mm diameter), and had a couple of element failures, before going to 1.6 mm for my later ovens. So far, the 1.6mm seems to be holding up much better. Your elements look to me like Kanthal A1 looks when it has failed. Apart from not being in many pieces, my elements look pretty similar when they are still working. The Aluminium and Chromium in the alloy are there to form a strong Oxide layer at high temperature and to keep it attached when it cools.

If you search online for "Kanthal handbook pdf", you will find the book that tells you everything you need to know about elements. Surface loading is a big factor when it comes to element longevity and it is something you should be aware of when rebuilding.
 
I had a spreadsheet with all my calculations, but I've edited it to calculate a new element.

I live in the EU and have a 230v/16a outlet in my shop. If I remember correctly it was pulling about 3000w / 13 amps.
The oven got to 1095°C for a brief period but nothing more. My kiln is built from white firebricks, and the element is suspended in grooves filed into the kiln walls. The element would fall out the groove, so I made small clips from left over wire and used those to keep the element in place.

The controller is an Inkbird ITC and I've got two SSR's switching the element on and off. I'm not sure what you mean with "Output cycle time".

1.2mm was the biggest wire I could find and I know my surface load was not good. I those to ignore it.
I bought the wire on Ebay, but maybe it was fake?

I called Kanthal last week, and they directed me to a company close by where i bough 1,4mm wire with about 0,9114 Ω/m
My new element is at around 18-19Ω, but I can't remember all the specs. I'll have to look them up when I get back from work.
 
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