how flexable should a user knife blade be?

A

AJH_Knives

Well-Known Member
I recently broke 2 of my knives on purpose. both made of O1 3/32 steel. heat treated at 1480* and tempered 2 times 375 and 400. first i bent the tang, and it snapped off. I though, maybe it was the extra holes i drilled for the glue, made it week. because that is where it broke.. so i put the blades in the vice and bent them over till they broke. got about 45* bend before they broke. is this bad? should I temper again at a higher temp? this is a skinning knife.

I am ready to put the scales on the others, should i re- temper?

Aaron
 
Maybe one of the people who use O1 will give you a little better advice in heat treating the alloy but, off hand, I would say that your austinizing temperature is fine and the tempering temperature is not bad though, I would have tempered at 400° both times. Also, you didn't say how long you held it at temperature before quenching. You need to heat the steel long enough to dissolve the carbides and release their carbon into solution. Are you hardening and tempering the tang? Another point on the heat treating is that some makers who use O1 like to do a subcritical preheat before taking the steel up to full austinizing temperature.

From what you said, I will assume that we are talking about a full tang knife. Yes, it could be the hole that you drilled for gluing if it was close to the junction of the handle and the blade. Avoid drilling in this high stress area if you can and keep the pin holes small if you can't. Personally, I don't drill anything but holes for the pins in my handles. To increase the gluing surface I only rough up the face of the tang with about 60 grit sand paper.

Knife design and manufacture is a study in compromise and whether or not a blade breaks at a 45° bend is good or bad will depend on what you want to do with the knife. Hardness and wear resistance will fall off by increasing the toughness and ductility needed to let the blade bend without breaking. This could be fine for a filleting knife but maybe not so great for a carving knife. Also being able to bend is only one part of knife design and anything can be taken too far in knife design. I once made a knife that could bend 90° without cracking anywhere. It was too small to try a chopping test on but it seemed to hold an edge reasonably well but the effort needed to bend it was not much at all because I sacrificed too much strength to gain toughness. It would be pretty useless as knifes go. I'm very thankful that I did not sell it or give it away. It's joined my mistake pile.

Doug
 
Thanks Doug.. i'll give you more info on my HT process.

i pre soaked the blades at 1200* for about 10 minutes, adjusted the temp to 1480* and soaked the blades for 10 minutes and quenched in warmed up vegetable oil. I then cleaned up the blades a little and put in the kitchen oven at 375 for 2 hours. I let them cool. this particular batch i did the second temper the next day in the kiln 400* for 2 hours. I prefer to temper in the kiln because I feel the temp is more accurate. the problem is my kiln is so big, it takes 3 hours to get to temp, and it takes longer to cool down.

would i hurt anything by tempering them again at 400* would I gain anything?

Aaron
 
I use O-1 a fair amount and your numbers aren't unreasonable.
I might question your oil choice and temp of oil, if any. A medium speed quench oil at 150*+/- is recomended. The 1200* presoak I have my doubts about but I do it anyway only because I usually treat multiple blades. It's recomended for large cross sections that we usually don't deal with. I use a 20 minute soak, again because of multiple blades and opening and closing the oven to quench. I like to know each blade at least made it through an equal soak before quenching. I upped my temper temp to 425* a couple years ago and still average 60/61 rc but gained maybe a little toughness. At 375* you may still be around 63 rc and at 400* slightly less. I like to make it a continuous process from quench to temper without interuption, I may be wrong but it makes more sense in my mind to follow through with the phases of the steel. All this of course depends on your equipment and maybe ambient conditions .

Rudy
 
the veg oil was at 120*F... I had my earlier knives tested, and they were a bit high in the mid to upper 60s rc.. i have since changed from a charcoal forge to a kiln to get better temp control. also i do 1 blade at a time. so i can keep track of time in the heat better.

Aaron
 
The veg oil may not be giving optimum cooling as in time/temp phase. I don't know, I've never used it. I'm a firm believer of the 'right tool for the job'. Like I treid pointing out earlier, there may be a large disparity between veg oil and actual quenching oil cooling rates. I'd look into the relationship between the quench medium and tempering temps. Also lose the 375* temper unless it's used as a snap temper after quench, go right to your target temp. As far as flexibility, O-1 is an alloy that through hardens homogenously, I wouldn't expect too much flexibility at it's optimum performance hardness of 61 rc unless you're working with longer blades (fillet) in the 1/16"- 3/32" range. I've never seen much flexibility in the 1/8"-3/16" range I use for (4"-6") blades. You could be asking for the wrong thing from the wrong alloy.

Rudy
 
Flexibility is more dependent on geometry than temper. A thin blade will flex more than a thick one, regardless of its hardness. 3/32" is not really that thin in regards to flexing, unless very long.
 
LRB said:
Flexibility is more dependent on geometry than temper. A thin blade will flex more than a thick one, regardless of its hardness. 3/32" is not really that thin in regards to flexing, unless very long.
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That was my point but maybe not as clear as your rendition.
LRB, I know you're pretty savy on O-1 yourself. Flexibility isn't something I've ever looked for from O-1 judging from it's intended uses. There are better choices.

Rudy
 
Rudy Joly said:
That was my point but maybe not as clear as your rendition.
LRB, I know you're pretty savy on O-1 yourself. Flexibility isn't something I've ever looked for from O-1 judging from it's intended uses. There are better choices.

Rudy
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I don't either. I just want to make the best blade I can for it's intended purpose.
 
AJH_Knives said:
I recently broke 2 of my knives on purpose. both made of O1 3/32 steel. heat treated at 1480* and tempered 2 times 375 and 400. first i bent the tang, and it snapped off. I though, maybe it was the extra holes i drilled for the glue, made it week. because that is where it broke.. so i put the blades in the vice and bent them over till they broke. got about 45* bend before they broke. is this bad? should I temper again at a higher temp? this is a skinning knife.

I am ready to put the scales on the others, should i re- temper?

there have been several posts lately about not tempering O1 past 400 as you dont gain anything. why would you want a knife to bend more than 5 or 10 degrees? if you do, start with 1/16" steel.
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I think the 400° temper will be OK. I normally do mine at 425°, but used to do them at 475°. The last one out of the shop was done at 380, by customer request, and it seemed fine.
 
I use a lot of O1 and I also quench in vegetable oil. I heat to critical temp then put back in the forge for about 5 more minutes then quench. I temper three times at 400 degrees for an hour each and cool to room temp in between. I have no way of testing actual rockwell hardness but I always clamp my blades edge up in a vice and hammer a mild steel rod on them to make sure the edges are hard. I try to get half way through a 1/4" rod without damaging the edge. They always pass this test. I learned the hard way about triple heat treating 5160 by doing this. As I only heat treated once and triple tempered and the edge was soft. Now I triple HT and triple temper 5160. O1 does not need this. Also O1 is not intended to be a particularly flexible steel either so as others have said bending past 5-10 degrees may be exceeding the performance level of the steel. The steel rod test may not be overly scientific but it does lot me know the my edges are plenty hard.
 
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First, what do call critical temp? Critical temp is not non-magnetic, and varies for different steels. If one has to triple HT with 5160, one has not done the job right the first time. 01 can be treated to do most anything any other can do. You cannot get the best from 01, with such a treatment as you describe. It simply doesn't work to the real potential 01 offers.
 
LRB said:
First, what do call critical temp? Critical temp is not non-magnetic, and varies for different steels. If one has to triple HT with 5160, one has not done the job right the first time. 01 can be treated to do most anything any other can do. You cannot get the best from 01, with such a treatment as you describe. It simply doesn't work to the real potential 01 offers.
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Perhaps my methods aren't scientific but as I stated I found out that doing only one heat treat on 5160 wasn't working so I tried a triple HT as I have heard of countless others doing so and it works. So yes maybe my first HT isn't correct but my end result is all I am worried about. As for O1 it isn't considered a spring steel and isn't commonly thought of as overly flexible. I realize that is maybe a little bit of a general statement but that is what I have read and experienced. YMMV... I have done soft back draws on O1 and gotten it to be fairly flexible but I don't think I would ever expect it to survive a 90 degree bend. And perhaps I should have stated that I heat to non-magnetic and then soak for a bit longer. I am working with simple steels and I use a forge for heat treat because I don't have an oven. but lets face it with the two steels I am talking about it is not rocket science. Perhaps you could share with us how to get the most out of O1. I think cutting through a 1/4"steel rod is getting quite a lot of the potential out of the steel but if I can do it better or easier I am certainly open to your suggestions. I am always open to learning from others who know more than I do.
 
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Well to start, 01 is not a simple steel. 5160 is barely a simple steel considering the chrome content. You need better equiptment to get the full potential of either.. Oven, salts, or some method of accurately controlled heat. Non-magnetic is not a good quench heat for any knife steel. It is really phase two of three heat ranges. Quench heat varies with different knife steels, with much depending on carbon content, but none in the just non-magnetic range. 01, for the hardening process, should not exceed 1475°, nor be below 1450°, in order to preserve the carbide formations and get the carbon into a full solution. This also requires a soak of 15 to 20 minutes at that temp. Give a little or take. Then quench in an appropriate oil. 5160 needs a quench heat of around 1525°, and does better with a soak at temp of around 10 minutes, give or take. For comparison, non-magnetic is 1414°. As far as cutting steel rod, that has more to do with the geometry of the edge, and is better left to cold chisels. I have never been terribly impressed with bent blades, but more with strength, and edge retention. Those who lack the equipment for HTing the higher alloyed steels need not waste their money on them, when properly HTed 1080/84, can be more impressive than poorly HTed high alloy types. One can make usable blades from 01, or 5160, with a simple HT, but neither will be at best without a good HT with controlled heat. One more thing with these two steels, especially 01, it is kinda rocket science. Whoever told you otherwise is sadly mistaken, and needs to do a little homework before giving advise.
 
LRB said:
Well to start, 01 is not a simple steel. 5160 is barely a simple steel considering the chrome content. You need better equiptment to get the full potential of either.. Oven, salts, or some method of accurately controlled heat. Non-magnetic is not a good quench heat for any knife steel. It is really phase two of three heat ranges. Quench heat varies with different knife steels, with much depending on carbon content, but none in the just non-magnetic range. 01, for the hardening process, should not exceed 1475°, nor be below 1450°, in order to preserve the carbide formations and get the carbon into a full solution. This also requires a soak of 15 to 20 minutes at that temp. Give a little or take. Then quench in an appropriate oil. 5160 needs a quench heat of around 1525°, and does better with a soak at temp of around 10 minutes, give or take. For comparison, non-magnetic is 1414°. As far as cutting steel rod, that has more to do with the geometry of the edge, and is better left to cold chisels. I have never been terribly impressed with bent blades, but more with strength, and edge retention. Those who lack the equipment for HTing the higher alloyed steels need not waste their money on them, when properly HTed 1080/84, can be more impressive than poorly HTed high alloy types. One can make usable blades from 01, or 5160, with a simple HT, but neither will be at best without a good HT with controlled heat. One more thing with these two steels, especially 01, it is kinda rocket science. Whoever told you otherwise is sadly mistaken, and needs to do a little homework before giving advise.
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Thank you...I stand corrected
 
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LBR what is a good temp for the quench oil. I believe Starrett O1 calls for 120*F is there a general rule for this or is it based on the manufacture specs?

I should also point out that I was asking a question in my original post. i wanted to know how flexible it should be. not looking for a knife that bends at 90* just wanted to make sure i was on the right track. I was curious to see how much bend the blades can take before they broke.. i am just sorry i destroyed them...

thanks for all the responses, this is all new stuff to me. I enjoy creating these blades. It is a huge artistic outlet for me and it actually relaxes me.
 
I heat my Parks AAA from 125° to 130°. I'm sure 120° would also be fine. As for breaking, you would just have to bend some samples and see what you get. A thin blade is going to bend/flex much farther than a thick one, and length comes into play also. If you want flex, think long and thin. If strength is a concern, think thick.
 
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Did you differentially heat treat the blade? Put clay on the spine or heat the spine after heat treat? You are not going to get much flex without a differential heat treat I don't think. Maybe some of the experts can comment on this.
 
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