Hi Jesse!

  First thing I cringed at in you post was "After a 15 minute soak".  I can't think of any reason to soak any plain carbon steel that long.  You're dealing with Iron, carbon, and Manganese.  In my expereince there is absolutely no need to soak that long. 

  Part of the problem is soak time (too long), and the other part is such a thin edge prior to soaking it that long/quenching.  If you insist on soaking like that, you need to leave things THICK (and even then I'd not soak for anything longer than about 2-3 mins). 

  You didn't mention the thickness of the blade, but based on the profile, I am suspecting 1/8" or less?  Personally, any blade that is 3/16" or less gets profiled ONLY, and then heat treated in my shop.  Doing so, and grinding the majority of the blade POST heat treat, prevents the issues you encountered.

  As far as tempering temp... that is something that each Bladesmith must find for themselves, for the particular steel being used, and the particular grind geometry the individual bladesmith applies.  There's just no way you can look up a "formula" for heat treating and have all the numbers apply absolutely you your blades.  Use advertised heat treat formulas as a STARTING POINT....then they will require adjustment based on the unique circumstance that exist in your shop.  

well I am going to have to disagree with Ed on this, thickness should be fine and that soak time in a oven at that temp should work fine, not what I would use, but nothing that should cause issues.  

 I forge my chefs down to 0.030 at the edge and right at or sub 1/8" at spine, pretty regularly I profile then heat treat and rarely have any issues. in fact have had three cracks in the last ten years, (one from a pattern flaw the other two being stupid and straightening when I knew it was to cold)  you don't mention you normalizing schedule, however as you seem to have hardened fully and didn't get any warpage in the quench indicates that was not the issue.  You mentioned that tempering was done the same day but not how long between quench and temper. sitting untempered for more than a few mins can be problematic at times. if I cant immediately temper (IE letting the oven cool ) I will do a snap temper just to prevent any possible cracking. one technique i have used was to let the blade cool to room temp (or at least cool enough to touch bare handed) Ill wet the blade and hold it in the hot oven just until it boils off the water. I normally will do that twice just to make sure. that short low temp temper is just enough to relax the stress of quenching with out effecting overall hardness.   

I would suspect if it didn't sit around much waiting to temper that any cracking is from the forging. 1084 is pretty forgiving, but repeated cold forging will induce cracking.  likewise with "wet forging" if done wrong, and repeated over heating can also cause issues. (all things I have run across in classes. )  i would suggest breaking the blade at the crack. look at the crack is it gray and show signs of high heat? (indicating it was prehardening )  how is the grain? is the grain around the break the same as in the break ? (if the break is courser it is at least prehardening , possibly pre normalizing ) 

Hope that is helpfull 

MP

Continued thanks. The latest knife failure dimensions are attached. Each of the previous failures, shown, also were clayed in a wave pattern to deferentially heat treat the blade primarily to enhance the hamon as a decorative feature. Is this an achievable goal? Obviously not for me at this time, but does the drastic wave pattern create uneven and unwelcome stresses in the steel? If the artistic goal can be met, will the utility of the blade be compromised from internal stresses? See the next post for pictures of the grain structure.

The attached photo shows the grain at three areas along the break. Leftmost is the break I made to take the photos. The center photo shows the grain at the upper extent of the original crack. Rightmost is the grain near the edge of the original crack.

I have had the pleasure and good fortune to be able to take a number of ABS classes, but most of the techniques I use are not second-nature to me and so I must follow the recipes and interpret when I can't find an exact fit. My normal soak is 10 minutes from my notes from my original ABS class, but this blade had a extra five minutes of soak. Maybe dertimental? I followed the normalizing temps followed NJSB' tech sheet, air cooled to ambient-ish temps in still shop atmosphere between three descending temp heats in a calibrated electric oven. After HT, I put the blade in my easy bake oven in a pan of sand while the blade was ~120F for the two 2-hour temper cycles. I let the blade cool to ambient in shop air between the two hour cycles.

To my feel, the break happened as I was grinding with a A45 TriZact belt against a steel platen. But, the blade could have been cracked anywhere along the process and I just did not detect it.

 From the shape of the crack Jesse, I am thinking it happened after the  quench. The temps sound good for quenching, so It comes down in my mind to stress cracks from forging or the antiscale coating or too much time between quench and tempering. The antiscale coating is just meant to be a very thin coat over the whole blade, not being used for hamons. I do not use it on 1084 as I finish grind anyhow after heat treat, leaving my blade at 120 for heat treat. If you are trying for a hamon 1084 is not the best choice. Too much manganese which promotes deep hardening and you want a shallow hardening steel for hamons. Also for hamons just use satanite, rutlands, or something similar. I also do not use much soak time for 1084. It really does not need it. I am really leaning towards not getting it in the tempering oven quick enough. You really need to get it in the oven asap. 

Brion

Each reply has given me fodder to chew on. Thanks for your collective time in supporting the mission of the ABS. Brion- Is there an ideal temperature to shoot for as the blade cools from the oil quench, immediately prior to tempering?

When I finish the next blade of similar style, I will post the improved results.

Thanks to all.

Obviously, I do not have as much experience as any of the gentlemen here.  My question would be in regards to your forging and normalization.  I would be concerned that either you may be initiating a crack from forging way too cold, forging way too hot, or you are dealing with some massive grain growth that is not getting fixed with your normalization. 

What you have mentioned should not be causing the problem you are having.  You should not get any grain growth below 1600 degrees in your heat treat process, so I don't think the 15 minutes soak is your problem.  You do want to get your knife into temper as soon as it cools to room temperature, but 1084 should be much more resistant to cracks forming from the internal stresses than some other higher carbon content steels.

Thanks, Casey. I don't know if you saw the photo a few posts back titled,

? Left to right, the image is of the grain at the spine, at the crack terminus, and at the edge. Are the photos sufficient to suggest good normalization? To my limited eye, the spine grain looks fine and uniform. At the crack terminus (middle shot), the grain is larger and at the edge relatively small and uniform in size again. Do you agree with that assessment of the grain structure? I've only recently been able to take the magnified photos, so I'm learning about the digital magnifier, as well.

I will continue to watch and improve as needed: my forging temps and time from forge to oil quench, as well as the time from quench to temper. Knock wood, circle three times, salt over my shoulder... This is the only style knife (with the clay applied) so far that has cracked on me. Distilling the process descriptions to describe the issue is helpful to me. Re my thanks to all for the thoughts and time.

The photos of the crack show me a few things of interest , first the blue color indicates the crack happened after the quench before the temper or it would not have the blue, if it was cracked in forging it would be gray or black. There is a section just bellow that that is gray/ rusty. that may have happened earlier and the blue section extended from that stress point. 

I would guess this crack happened between hardening and temper. It is possible it was in the quench, but normally you see oil in the crack if that was the case and don't get that blue color. 

The grain looks reasonably fine, but there is a roughness to it that I have seen in steels that were improperly normalized. 

I am using the Industrial term here not what we call normalizing. It is important to understand the purpose of normalizing, it is not as is so often mentioned to refining grain, in fact it is intended to grow grain. The purpose is to even out the grain structure, that includes braking up carbides in some cases that is done in a single higher temp step.  Generally about 150-250 deg above critical temp with a short hold. Once that is accomplished the grain can then be evenly shrunk through thermo cycling (the rest of what we call normalizing) these cycles only need to reach at or just bellow critical temp depending on of the steel. we really only want to reduce the grain two to three times at most or the steels hardenability will be lowered to much (smaller the grain the more difficult it is to harden. 

The look your steel has is what i have seen when that first higher temp equalizing step is skipped but the slightly cooler temps were used to refine the grain size. sort of an over all fine grain with random spots of thicker coarseness.  good clean grain should look like chipped ceramics. 

MP 

I would agree with Matt.  Based on the photos, you probably picked up a crack during quenching.  The oil got inside the crack, and turned blue during tempering.  I've seen people leave oil on their blades which will turn purple or blue during tempering, and they believe that they over heated their blades.

I would also agree on the microstructure.  It looks a little rough.  For 1084, I generally normalize at 160 degrees.  I do three cycles at 160.  This could start a fight on here, which I am not trying to do, so I won't go into details about why I do this.  However, Matt's explanation was very good on normalizing.

I would recommend working on your normalization profile and see if that helps.  That first cycle at around 150-250 above critical is very important in fixing a lot of issues that are created during forging.

Also, what is your final grit you are grinding with before quenching?  I am concerned you may have some stress risers that are causing you issues during the quench.

I finished the blade to 240 grit prior to the quench to avoid stress risers. Is there a typo in your normalization schedule, Casey? 160? I followed the spec sheet from Aldo supplied with my purchase of 1084. 1,650 F, 1500 F, and 1,350 F, with cooling below black heat prior to the next cycle. If there is another opinion about how to do this step or additional steps I am missing, please be specific. I have no innate sense for heat treating steel, so I am limited to following instructions and recipes. Thanks.

Yes, that was a typo. 1600 was what I meant to type.

I don't see anything inherently wrong with your process that stands out to me. My initial guess would be cold forging, but that would be mere speculation.

Sorry I can't be of more help.