What causes a blade to bend when quenched?

[bluerooster]

What causes a blade to bend when quenched?  All was going well, it was straight in all directions, heated to just above critical temp, and held there for a minute or two. Then into the quench. It took an immediate hook to the right. 

It's a cable billet that I'd welded up a couple of weeks ago. It was an experimental blade, using clay on the spine to get a "Hamon".  It took a beautiful curve, but also a bend to the right.

Could it be the clay was thicker on one side, causing one side to cool faster than the other?  

[bluerooster]

Pics of the blade.  The second pic shows where the bend was. 

[Buzzkill]

1 hour ago, bluerooster said:

What causes a blade to bend when quenched?

The short answer is unequal forces.

There are a number of things that can cause this.   The spine is usually thicker than the cutting edge, so those will expand and contract at different rates when heated or cooled.  Especially on a long blade that can cause a "saber" effect when quenched.  A blade that is ground slightly more on one side than the other compared to the center line will often warp.  If a blade is a little hotter on one side than the other, that can do it.   Moving the blade in a way that puts pressure on of of the sides more than the other while quenching can cause warping.  Laying a freshly quenched blade on a cold anvil will create unequal forces.

The bottom line is there are lots of scenarios that can cause warping, but if you started with a straight flat blade then something created unequal forces which became evident when the  blade was quenched.

[bluerooster]

Thanx for the information. I suspected something like that. The "saber effect" was what I was after. But the hook to the right (warp) took me by surprise.  I put it back in the heat and got the warp out of it, and left it in the normalized state. I'm afraid to re harden it, as I don't want to curve it any more than it is now.  I guess I'll hang it on the "wall of shame" and start over.

[Irondragon Forge ClayWorks]

I would try re-hardening it. You can always hang it on the wall of shame, or cut it into a shorter knife.

[Frosty]

OR you could talk up how long it took you to figure out how to make the P E R F E C T "Left Handed Mellon Sheller." 

On a more serious note, Cable will retain inconsistent properties unless you reweld it into a mono-steel and the inconsistencies can get you.

Frosty The Lucky.

[velegski]

Try clamping it to a piece of straight 3/16 flat stock.  use c clamps and shims to bring blade back into straight and temper for one or two cycles, 

[JHCC]

There is a short window of time right after the initial quench, when you’ve cooled the blade fast enough to keep the austenite from turning into pearlite, but not all of it has finished turning into martensite. In that window, the blade still has a bit of malleability and can be straightened by being clamped between boards or in a vise or straightening press, or simply being flexed manually. 

This is discussed in more detail in this video:

And there’s song more good information in this video:

(As articulated both by a cranky old British metallurgist and heat treater.)

[anvil]

also you need to move your piece up and down to keep quench medium on your blade. Bubbles can cause problems. I think the most common reason is moving your blade a little sideways. This was already mentioned, but is worth repeating.