Recent problem with knives cracking post heat treat.

[Danneskjold]

Hi there! I'm new to this forum, but I've been forging knives for about 2 years, and blacksmithing about 6 months longer than that. I've been selling my knives for just over a year.


But recently I've been having a lot of problems with my knives cracking. Not all of them, but enough that it's really causing me a problem. It usually happens during or after finish grinding. But not always. It usually happens in the ricasso area, but not always. And I've been adjusting what I've been doing to try to stop it.

First, all of my knives are forged and heat treated in my forge. I use a pipe or square to attenuate the heat for the quench. And all of this cracking steel has been 80CrV2.

Since the problem started I've taken to annealing the steel (mostly to make drilling pins easier). I put all my forged knives in the forge at once, get them up to temperature, and then turn off the propane.

It's not a cold shut. The last crack happened on a forged bevel. And one of the first things I learned was how to avoid making cold shuts.

I've gone from rough grinding to ~100 grit, up to ~120 grit. And I've started dressing up all edges with 240+ grit to make them smoother and avoid any cracks there.

I've gone from 1 normalizing cycle post grind pre-heat treat to 2 (or more if I think I have the propane).

I temper my knives in my oven on half sized firebricks. 2 x 2 hours. I went from just putting them in there, to sanding a little bit to make sure it's hitting the right tempering colors. And I went from finishing all my quenches in a batch, then putting them in the oven, to getting the oven already going, and putting the knives in individually immediately after each is quenched.

I've got 3 sets of quench tanks. One .50 cal ammo can with canola oil. One mortar can with canola (for larger items). One .30 cal ammo can with Parks 50. I always preheat the canola. But all the cans are outside in all of the Pennsylvania weather. The tops seem tight. Has the canola gone bad? Has it been corrupted by condensation, a water leak, or freezing / separating?


I'll try replacing the Canola oil next. Maybe even upgrade to Parks AAA. But the only other conclusion I've reached by going from never cracking a single knife, to cracking about a third of the knives I've done, is that I've got bad steel. As best as I can remember, all of the cracked steel came from one place, and the same batch of quarter inch bar stock.

Or am I missing something?

 

[tkroenlein]

Lots of good info.

If I'm following your process you've described correctly, this is the order:

1) Forge
2) Slow cool anneal in forge.
3) Grind/drill
4) Normalize/thermal cycle
5) Quench
6) Temper
7) Finish grind
>insert cracks here<

If I have that order right, I expect your cracking is actually occurring right after step 2, and it's opening up or becoming visible later. Forging will blow the grain up. The time to normalize and do a couple lower temp cycles is right when you're done forging. Even do a couple of dull red heat and cool to black while forging will keep your grain in check. But if your normalizing and cycling is done at the end of your forging session, you really shouldn't need to do the anneal step to drill or grind. That's my guess.

 

[KenH]

, to sanding a little bit to make sure it's hitting the right tempering colors.

One thing to note, perhaps not to do with cracking, but colors mean nothing during tempering. DO NOT use color to determine tempering, use temperature and time.

 

[Danneskjold]

Lots of good info.

If I'm following your process you've described correctly, this is the order:

1) Forge
2) Slow cool anneal in forge.
3) Grind/drill
4) Normalize/thermal cycle
5) Quench
6) Temper
7) Finish grind
>insert cracks here<

If I have that order right, I expect your cracking is actually occurring right after step 2, and it's opening up or becoming visible later. Forging will blow the grain up. The time to normalize and do a couple lower temp cycles is right when you're done forging. Even do a couple of dull red heat and cool to black while forging will keep your grain in check. But if your normalizing and cycling is done at the end of your forging session, you really shouldn't need to do the anneal step to drill or grind. That's my guess.

Ok, I sometimes (now) do one normalizing cycle immediately after forging. But I can't remember when I really stopped doing that and started the annealing, vs. when the cracking began. This has been happening over several months and I'm only trying to add it up in retrospect.

I knew that cracking sometimes happened, and thought I was just lucky that it hadn't ever happened to me before. But when it kept happening, then I knew something was wrong.

One thing to note, perhaps not to do with cracking, but colors mean nothing during tempering. DO NOT use color to determine tempering, use temperature and time.

It was initially done with temperature, time and an IR laser thermometer just to check the accuracy of my oven thermometer. The colors were done just to verify everything on top of all the stuff I was previously doing. I.E. Trying to eliminate possible sources of problems.