5160 questions

[Kette]

So I've got some strange behavior and discoloration that I don't understand.

I'm using known steel, 5160. I forge my knives and then normalize 3 times followed by a nice and even pre-grind. I quench between red and orange (by eye, using the forge), making sure to have a really even temperature throughout the blade. I use parks 50 at room temperature for the oil. Then temper at 400 in the oven for at least an hour, two cycles.

I've had great results overall but have noticed this discoloration on the few breaks I get, in this instance it happened both at the bolster, AND the tip of the blade. Any clue as to what this could be?

Also while you're here, how's my grain structure?

Thanks!

 

[Frazer]

I cant speak to the discoloration. As for the apparent layering, might I direct you to the following image gleaned from the internet.

As far as the grain structure goes that is pretty coarse. Are you using a magnet at all or are you just judging temperature by eye?

[Kette]

I don't normally check with a magnet, thought I knew the color well enough. If anything I was worried I was quenching too hot, but sounds like I'll need to check with the magnet next time. Thanks!

[Frazer]

Judging temperatures by eye is difficult to do. Ambient lighting can make a big difference in what you see.

When I normalize I shoot for three temperatures: slightly past non-magnetic, then non-magnetic, then just barely non-magnetic (in that order).

It's not an exact science, but I have gotten the grain structure down to about the same size as an old hand file that I snapped to serve as a reference.

The tip is the most common to overheat accidentally. Sometimes I use the draw of the chimney to cool the tip slightly while the rest of the blade is coming up to temperature. If you use a gas forge there might be some other tips people can share.

[Kette]

You could certainly be right, I suppose my eyeballs can't be trusted anymore. I also didn't know the temperature reached for normalizing could make a difference, typically I just heat to my normal forging heat and give it time to cool. Thanks for sharing, I'll give your method a try.

[Frazer]

I'm no metallurgist, nor am I really bladesmith, but the normalization temperature makes a big difference, so does the cooling rate. Usually air cooling is fine, but in the winter here in NY air cooling might be too fast when it's ~10F in the shop. So I put the blade on top of my bucket of vermiculite. Not buried, just right on top.

I'm still not sure about the whole discoloration thing, I'd be interested to hear if that's still there after you tinker with your normalization cycles.

 

[Buzzkill]

To my eye the discoloration you're referring to is due to cracks in the steel that you probably couldn't see before breaking the blades.  That type of discoloration is associated with oxidation of steel, which means there had to be a way for oxygen to get to that part of the steel.  That's usually an indication of getting the steel too hot or continuing to forge below the temperature at which you should be putting it back in the forge. I didn't see any of that in the bottom picture, but it was there in the top two.  

I agree that your grain structure appears to be coarse.  Normalization is important for reducing grain size.  For the first cycle it can be quite hot.  This will create large grains, but it's more important to have uniform grain size at that point, so it's ok to go up even into the high end of the forging range for the first one.  The next two should be at successively cooler temperatures with the last one being just past non-magnetic.  This should give you uniform small grain size when you quench.

[ThomasPowers]

+1 on cracks before tempering.  Any chance you accidentally did a contact quench during forging?  I once demonstrated this one winter down here with a piece of spring steel.  Heated to forging temp and then clamped it in the post vise. When I opened the vise it fell in 3 pieces.  Luckily this is what I wanted to happen with my class to drive home the fact that in the winter just laying a blade on a piece of random steel can destroy it.

[Kette]

Buzzkill, you've got it. I tend to cheat and straighten the blades at very low working temperatures, usually near the tip and the bolster! I thought I was getting away with it since it's 5160 and it would get normalized in the end anyways. 

Looks like I've got a dozen knives to heat treat again. Practice makes perfect right?

And Thomas, yes I've seen that before with my big post vice. Though I didn't know exactly why, great info.

Thank you gentlemen!

[ThomasPowers]

Quenching is just the rapid removal of heat and solid contact of cold metal with hot metal does that quite well!