What did I do wrong?

A

AlaskanHunter

Joined
Nov 23, 2013
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I thought I had ground this one too thin, so I decided to test it under normal use, then work/abuse it until it broke to see what it could take and if I had don my HT right. It survived a wide variety of normal cutting jobs, and cut 3/4" twisted nylon rope with ease. I think that I got my edge geometry right, but a bit thin. No chipping or rolling when I cut/whittled/chopped spruce and birch, both seasoned and green. The rope cut was after the wood. Blade seemed to be fine. Further testing was postponed till I had more time, but it broke before in the sheath before I had time to do so. My blade broke roughly .1" forward of the bolster. I thought my weakest point was exactly at the front of the bolster, where the tang was the smallest and had the most leverage?
My blade was Aldo's 1084. HT was as follows: heat to approximately two shades above non-magnetic using a 2" sch 80 pipe as a muffle in a charcoal forge. Quenched in 2 gallons of canola oil, heated to very warm. I did not have a thermometer, so I was guessing on the temp, roughly 120F give or take 5-10 degrees. Immediately after quenching to ambient temp (45F that day), they went into the kitchen oven (preheated to 425) for one hour for temper #1. Temper # was 450F for another hour. I had a minor warp in the tang that I attempted to correct immediately after it came out of the oven. It broke half way up the tang, so per you guys recommendation, I submerged the blade in water, and welded the tang back with plenty of pre and post heat.
Did I mess up my HT, or just grind it too thin? Why did it brake forward of the bolster where there was more metal to resist braking rather than at the bolster where it should have been weakest?
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Wait...I'm kinda confused...you only tempered at 450 for one hour or two.
On USA knife makers website, they have a comprehensive list of HT recipies, I follow thier 1084 one which says to temper at two 2 hour cycles at 500.
What were you doing with it when it broke?
 
Sorry for the confusion. First temper @ 425F for one hour. Second temper @ 450 for one hour. I didn't know that the oven could go that high on my first temper, just before the second my wife showed me how to crank it up to 450. This was measured with the oven thermometer, so it's only as accurate as you trust a Sears oven.
Was driving my truck when it broke. It was in a small of the back sheath. When I hopped out, I realized it had broken inside the sheath.
It sounds like my temper may have been too low?


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Also, this was forged to shape, not sure if that makes a notable difference. All forging was done at a bright orange heat.


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The temper was not too low. That isn't your issue. 450 F. is getting on the high side of necessary for 1084 and 500 is too high, IMO.

I don't know what the issue WAS, but you can probably rule out the tempering temp being too low.

From the pics at least, the grain looks a little large, which would suggest overheating. Without a good way to monitor temps accurately, 'About two shades past nonmagnetic' as a very ambiguous term, the definition of which can vary widely from person to person. You may have had a stress crack prior to or resulting from the quench.

You might also try tempering for two hours per cycle. The steel may not have had enough time at the actual tempering temp to do what it needs to do.
 
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The grain looks pretty coarse. Also, the curved break indicates a line of weakness being followed.

If forging was done at too low a temperature ( below 1600F) internal fault lines may form. These can not be repaired in subsequent forging, normalization, or HT.

You never mentioned normalizing the blade before HT. All forged blades need to be brought back to fine grain or this type problem will be likely.

I also suspect you may have overheated the blade in HT.
 
Looks like a combination of perhaps overheating and a quench crack at one end or the other of the break. I'd guess it started just in front of the bolster and ran back to the spine. Quench cracks happen sometimes. I've cracked 4140 in round bar with a lab grade oven to control heat. Crack was in the center and wasn't visible until the bar end was polished.
 
it looks like you had a right angle instead of a curve where the ricasso meets the tang and goes into the handle ? that would crete a weak point. that combined with the large grain would make it very weak.
 
Sounds like the knife was still under stress from the heat treating. The failure could have occurred anyplace, it just more likely to happen in the thinner areas or near contours. Heat treated 1084 can shatter like glass prior to tempering, or so I've been told. I think the problem likely had to do with the tempering process. 1084 thats been forged should receive 2 cycles of tempering at a temperature of 500 degrees F for 2 hours per cycle. All you can do is try it on the next knife and see if it does the trick. Addendum.... Aldo 1084 comes already normalized, but has to be renormalized if forged. Like Stacy said, this may very likely have been the issue.
 
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Good eye, Woodster.

Yes, there was a 90° angle at the tang/shoulder junction. In the flex test, stress is concentrated at the bolster. This caused a stress riser to form at the angular junction which ran wild through the coarse grain. Result - SNAP.

The transition from tang to shoulder should be a small radius, not a right angle. This transfers the energy/stress back into the tang without creating a riser.
 
I'm with the others. You likely didn't normalize after forging, and/or your austentizing temp was too hot. Your grain is too large, which led to brittleness, thus failure.
 
Ok, so it looks like my problems are: no normalization, possibly too hot or cold of forging temp, and too tight of a corner at the base of the tang?
What is the proper procedure for normalizing? Other than the obvious of a thermocouple and PID, how can I get a better idea of my forging temps? And how hot is too hot vs not hot enough for 1084? What would be a good radius for the tang/blade junction? I had a very slight radius there, probably way too small.
I still suspect that I may have ground too thin, but it's apparent that there are other problems as well. What would you consider to be a good blade and tang thickness for this type of knife? From what I understand, thin cuts better, but also breaks easier. Is there a good rule of thumb for the balance of the two?
Thank you all for your help!


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TonyRV2 said:
Heat treated 1084 can shatter like glass prior to tempering, or so I've been told.
Click to expand...

Does this ^^^ mean that you don't have any personal experience with 1084 at all, or just not with it shattering untempered?


TonyRV2 said:
I think the problem likely had to do with the tempering process.
Click to expand...

Just curious, based on what?


TonyRV2 said:
1084 thats been forged should receive 2 cycles of tempering at a temperature of 500 degrees F for 2 hours per cycle.
Click to expand...

Again, curious as to the parts in bold......based on what? I'm not sure any knife made of 1084, even a big chopper, needs to be tempered at 500 F., let alone SHOULD. A small thin knife of 1084 tempered at 500 F. is leaving a lot edge retention on the table.

If the steps prior to tempering were all done correctly, a temper of 500 F. would put the blade at aprox. 55-56 Rc. That's more like the hardness of blades in the early frontier days of America. We've come a long way since then with better steels and better processes. A big chopper of 1084 with proper edge geometry should perform well in the 59-60 Rc. range. Some guys push the number up there even higher and do well.

As I said earlier....probably not a tempering problem and I'm curious why people think they need to go to 500 F. with 1084?
 
so for the radius at the gaurd/riccaso junction i use an 1/8" chain saw file for mine and round the gaurd mating surface to match. for me i do my post forging normallizing cycles in the even heat oven first cycle 1550 hold for 5min let cool to room temp, heat to 1525 let cool to room temp, heat to 1500 let cool to room temp.
 
Evan Miner said:
so for the radius at the gaurd/riccaso junction i use an 1/8" chain saw file for mine and round the gaurd mating surface to match. for me i do my post forging normallizing cycles in the even heat oven first cycle 1550 hold for 5min let cool to room temp, heat to 1525 let cool to room temp, heat to 1500 let cool to room temp.
Click to expand...

Thanks! I did my radius with a 1/16 needle file, which should have given me a 1/32 radius. Looks like I have two of my possible four flaws figured out- and what to do next time.
 
Evan Miner said:
so for the radius at the gaurd/riccaso junction i use an 1/8" chain saw file for mine and round the gaurd mating surface to match. for me i do my post forging normallizing cycles in the even heat oven first cycle 1550 hold for 5min let cool to room temp, heat to 1525 let cool to room temp, heat to 1500 let cool to room temp.
Click to expand...

Hey Doyle, nice to hear from a fellow Michigander. I've never shattered a knife after heat treat, so I can only go by what others have said in that regard. As far as tempering at 500 degrees, thats from information supplied from USA Knifemaking website who's heat treat recipes I've always relied on with problems...well of course except for the times I've screwed up.
 
A question here about the forging temps - would forging on the hot side be better than allowing to be a bit cool while forging? Any grain growth from too hot while forging could be handled during post-forging normalization process. Forging too cold could allow internal cracks to form that are never seen - do I have this correct?

Ken H.
 
Ken, Yes you are correct.

Tony, I found the reference on the USA knifemakers supply. The 500F for a 59-62RC in 1084 and also the 1095 seems a bit high. Since I do not have an RC tester I cannot verify data but I do not temper over 450 for any of the simple carbon steels. Never have a chipping issue and holds an edge quite well. I normally only go to 500f when tempering O1 for a big chopper or I am making an Axe. In fact my Damascus recipe for 1084/15n20 on my MS test knife did a 380F for 2 hours x 2. Did the test with no problem even the 90 degree bend. That was a full quench back draw type HT.

Breaking like glass is kind of a misnomer. Yes it can snap easily but will not shatter if you drop it. It can crack or break but not really shatter.
 
My metallurgical info says that a 500F temper on 1084 will get Rc 59.

I find most people recommend a temper far too low. I feel that any temper on a knife blade below 400F is too low for a good blade. I use 450F for most carbon steels.

The notion that "A lot of edge retention is left on the table with a 450-500F temper" is common . Actually, the toughness goes up considerably. Additionally, an edge usually degrades from micro-chipping long before it degrades from abrasion.

So where is this difference of opinion coming from? Probably from the fact that the vast majority of HT is done with less than optimal heat sources and in uncontrolled conditions. People from Ed Fowler to newbie You-Tubers have suggested using a torch, a forge, a fire pit, motor oil, canola, dish soap concoctions, and many other cobbled together heat treatments. I have suggested some of these to people with no equipment. BUT, the metallurgical charts are made from repeated tests done in good equipment with the exact proper temperatures.

The biggest thing that affects the final hardness/toughness/brittleness/edge life of the final blade is not the temper ... it is the austenitization temperature and condition of the steel at the time of quench. Proper normalization, grain refinement, and dead exact temperature control is how one person gets Rc59 from a 500F temper, and another gets Rc55.

What isn't always understood is that a difference of five degrees in austenitization temp can make a significant change in the outcome, while a difference of 25 degrees may barely shift the final temper hardness.


The places to look for getting more out of a knife is usually not the temper oven.
 
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