Heat treating the blades

A big shout of thanks to friend Stuart, who has invested many hours over the past weeks designing and building the most fabulous quench tank in all England!  Because of its cylindrical design (bottom half of an old gas cylinder) with 3 supporting legs, it has been affectionately named "Thunderbird 3", with a nod back to the TV series "Thunderbirds" that I loved watching as a youngster.  (aka "Thunderbirds Are Go", aka "Redding Internationaal" dubbed into Afrikaans in South Africa.)  For any readers who were deprived of that childhood pleasure, this is a picture of the original.  The notable difference with my quench tank is that any flames will be coming out the other end!

This is the finished tank.  The removable lid prevents the ingress of dust and bugs while not in use and can also be used as a flame suppressor if the oil should catch fire uncontrollably (highly unlikely, but one has to be safe!)  The tank is about 100mm (4") in diameter and 450mm (18") tall, and it weighs about 140 tons!  OK, maybe that's a slight exaggeration.

Yesterday saw the inaugural launch of Thunderbird 3 for the two blades I had ready and waiting for hardening.  Decided to do this in the garage as the weather was a bit rainy and windy.

From many YouTube videos I have learned that an easy way to tell whether the metal is hot enough to harden is to test it with a magnet.  If it is still attracted to a magnet, it is not hot enough yet.  If it does not respond to a magnet, it is hot enough for quenching.  A few weeks ago I took a small magnet and mounted it onto the end of a wooden block as shown in the next photo.  The circular magnet was from an old speaker and the circular disc you see is the central iron core that protrudes about 0.5mm (0.02") proud of the stainless steel casing.  How big is this? ... just big enough to hold comfortably in your hand.

I lit the forge and tuned it to give a nice hot flame ...

"What's the W-shaped piece of wire for?" I hear you ask.  It is threaded through a hole at the back of the tang so that I can drop the blade into the oil, where it will hang suspended without me needing to hold it with the tongs.  You can see this in action in the video at the end of this post.

As the blade heated up in the forge, I removed it occasionally to do the magnet test, watching the colour change to give me a visual indicator for future reference.  Here you see the magnet test ...

When hot enough throughout, it was time to transfer the blade from the furnace ...

... into the tank ...

... avoiding the flames!

A minute or two later it was all done.  The heating and quenching process does discolour the steel, but this is to be expected.  The blades will clean up nicely after several hours of sanding and polishing.

I have attached a short video (21 seconds) showing the quenching process.  Many thanks to my long-suffering wife, Ann, for her photography skills, and for not complaining about being engulfed in the smoke cloud from the oil!

Now to get on with the polishing ...