Sword heat treat

[Jordan Hunt 5]

Hey guys, I've made a bunch of knives, finally got set up for a sword and swung for the fences with a monster double edged blade about 38 inches long, made from a leaf spring, my question is I quenched and tempered the same way I do Bowies, and I know the sword should flex and it does with moderate force, but then the bend stays til I flip it over and straighten it. Is it too soft? Not springy enough?

Thanks in advance guys.

[Joël Mercier 505]

What makes a blade springy is it's geometry. If you want it to be flexible, grind thinner cross sections. 

Now if your blade stays bent after a light bend, it's probably not properly hardened. 

[Alan Longmire 2,634]

What Joel said.  If it stays bent, it didn't harden properly.  What is your setup?  I used to use a long, charcoal-filled trench beside the shop with a perforated pipe in the bottom for swords, and if it was too sunny out I never got a good heat treat.  Plus, swords need to be springier than bowies.  Shoot for a temper of around 550-575 F on 5160.  You should be able to bend it almost into a circle and have it spring back straight.  Geometry (thickness, distal taper) is your friend here.  Think fishing rod.  

[Joël Mercier 505]

5 minutes ago, Alan Longmire said:

550-575 F

I've been wondering for a bit, isn't 550°f in the brittle zone? Impact toughness must be important for swords so why do that? Or perhaps my tempering chart is wrong...

[Jerrod Miller 422]

3 minutes ago, Joël Mercier said:

I've been wondering for a bit, isn't 550°f in the brittle zone?

Depends on who you ask.  There is a bit of debate on where it actually starts and ends.  I don't think I would worry too much with 550F, but I would certainly avoid 600F.  Generally speaking, 900F is where I start feeling pretty comfortable on the other end of the range, and some people say as low as 800F (there are those that say 500F is the start though, too).  

[Joël Mercier 505]

Interesting, so there is no evidence?

[Jerrod Miller 422]

48 minutes ago, Joël Mercier said:

Interesting, so there is no evidence?

There is a ton.  It is very much a real thing.  It is just that it is alloy dependent, and generally speaking, not very important for the vast majority of commercial needs. 

There is both one step and two step embrittlement.  Here is some info.  

[Joël Mercier 505]

Very interesting read, tyvm!

[Alan Longmire 2,634]

I shall have to read that and convert temperatures, but at a glance it does appear that 5160 at 550-575 F has no issues.  The last blade I did that way was 9260, and it was niftily springy.