Tip strength

J

JParisi48

Joined
May 14, 2012
Messages
198
I know this has to do a lot with blade geometry and HT, but I have been working on a bowie, and I know the HT is not perfect as I am using a MAPP gas torch and water to quench in, BUT I test the tip after HT by simply sticking it in a piece of wood, probably not even a 1/4in and prying. The tip bends every time, about an 1/8in down. I don't think this is abuse because its just a piece of wood. If I want to take this camping with me or anything else, I want it to be able to hold up. I did HT 3 times to try and get it right, but nothing seems to be working. The steel is 1084 and I followed the HT intructions Stacy posted in someone else thread recently about HT with a torch minus the canola oil part, but I also read that 1084 can be water quenched. So my question is, for a bowie, or anything similar, how tough should the tip be? What is considered abuse and what is considered use? If there is any other info you need to know just ask, Thanks for any help in advance.
 
1084 as I have read doesn't need a long soak time, I get it to non magenetic, and keep torching for about 20 or 30 sec to make sure then quench. I could be wrong, but I didnt think 1084 needed to be soaked? Thanks for the response
 
I think we would need to see the tip, the objective would be to have the geometry and HT blend to make for a strong tip that would not break or bend. I have tested tips by gouging wood and never had one bend or break, but I may have made mine thicker than yours.
 
What are you using for a quench medium, as that may help get some further insight into it. Also what temp are you tempering at? I have found that ovens vary drastically and it could be possible that you are tempering the blade back too far due to your oven running too hot.
 
I tempered to straw yellow(about 400 degrees F for 2.5 hours straightish) with my 1084 knife I made like last week and I have my tip ground pretty thin and I couldn't bend it nearly as much as you. If I lay my blade on a 90 degree edge, I can barely flex it, only 10-20 degrees maybe.
 
Something the size/shape of a typical bowie shouldn't bend with a little prying in most woods.

1084 doesnt need a long soak time. I suspect you didn't get it fully hardened. I'll use a MAPP torch to ht small knives/parts for personal use but I don't try to ht anything bigger than about 1.5" long because it's near impossible to hold a high enough temp across the entire piece. You have to be careful, once you get part of the blade to non magnetic it only takes a split second to lose that heat and the metal will still be non magnetic at a lower temp.

Heat treating isn't that expensive. I suggest you send that blade out to someone and if you want to heat treat yourself then only do it on a small knife or build a small oven (even then a bowie is probably too big).
 
Nonmagnetic for 1084 is 1414F, but it needs to get well above nonmagnetic, to 1500 before you quench. You're not getting anywhere close to that with an extra 30 seconds using a mapp torch on a big blade.
 
When I used a torch prior to buying a oven I made a small kiln out of a stack of fire bricks, it only takes a few and allows you to get a more even temperature across the entire knife, I also used a digital thermometer to monitor the temperature.
 
Thanks guys, I will get some fire bricks and a good thermometer. I tested the tip of a cheap flea market knife that I got when I was a younger and the tip was pretty damn strong, so it def has to be my HT. Thank you again for the replies.
 
aarongb said:
Nonmagnetic for 1084 is 1414F, but it needs to get well above nonmagnetic, to 1500 before you quench. You're not getting anywhere close to that with an extra 30 seconds using a mapp torch on a big blade.
Click to expand...
1084 is almost a perfect eutectiod steel. It goes into solution shortly after critical(Ac1) which is somewhere around 1350F. Reaching nonmagnetic with 1084 is good enough but I do agree that the inconsistencies of a torch HT could complicate things. Theoretically, you could HT 1084 at 1400F(past Acm) with a proper soak.

I am not recommending that you quench at 1400F
 
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Lots of things.

First - 1084 austenitizes at 1500°F. The eutectiod point is when the ingredients go into solution. The austenitization point is when they are ready for quench....not the same. Some steels have a lower austenitization temp than 1084. What makes 1084 a great steel for minimal HT methods is that it does not require time for excess carbon or alloy ingredients to form carbides. Thus it needs no soak time.

Second - When using a torch, the metal is the only place that gets heated. In a forge, or even a row of firebricks like Pat mentioned, the refractory will get heated and try and retain and/or reflect a bit of the heat back into the blade. With a blade held in the air, and a torch played up and down it, the blade radiates the heat back into the air. In the thick places of the blade, the mass holds some heat, which helps keep that area hot a bit longer. However, the thin sections like the edge have no mass, so they cool down faster. The tip is the extreme, as it has no mass, and no spine ( which has mass) to send some heat to it. The tip will cool off faster than any part of the blade. (It will also overheat faster than any part of the blade for the same reason)

Third - In a small blade, less than 3" , you can play the flame over most of the blade at one time. In a bowie, you can never be heating more than a small percentage of the blade at any one time. Heating from the spine is the best method, but success is unlikely with a large blade unless you are fairly well skilled at torch HT. Most folks make the mistake of using a hot flame, not a large flame. A rosebud tip in a welding torch or any large flame tip is far better than a hot pencil flame. You only want the blade to get to 1500°, so having a flame running at 5000° is a problem, not an asset. The flame should be evenly blue....and as large as possible. When using a forge, the same rule exists. The forge flame/heat should be just high/hot enough to get the blade to 1500°, not hot enough to weld with.

Forth - You are not exactly clear in your description of the tip "bending". Do you mean it bends and stays bent...or does it "flex" and return to straight by itself when it is pulled out of the wood.
A poor ( insufficient) HT will bend and stay bent. No good knife should do this.
A proper HT and good blade geometry will flex a small amount and return to straight. Most hunting knives and general use kitchen knives need this type of tip.
A proper HT with too thin geometry will bend greatly, and also snap off easily. Only thin slicers should have this tip.
A robust tip with proper HT will not flex, or break. Bowies need a robust tip.

It is a bit hard to describe how a robust tip is shaped in words, but I'll try.

1) If you taper the blade from ricasso to tip as a continuous taper ( distal taper), the tip will be thin. This will make it very weak when the edge is added.
If you don't taper the entire blade much or any, the tip will be nearly as thick as the spine is. This will make it strong, but rather blunt for piercing. Machetes are made with this geometry.
If you taper the blade to a degree where the tip end is about 1/2 to 1/3 as thick as the ricasso, you will have a starting point for a robust tip. The final tip will be thin enough to puncture things in a stabbing cut, but strong enough to resist breaking. The distal taper will allow the blade to slide into the hole made by the stab.
On a bowie from .250" stock, the tip should be about .080-.100" thick after the distal taper.

2) After the distal taper is established, add the primary ( main) bevel. This will taper the blade from spine to edge. It also should stop with a bit of meat on the edge for a bowie. I would go to about .030" on a bowie pre-HT. As you approach the tip, you need to change the angle of the bevel grind a bit to maintain the .080-.100" end thickness at the tip. It is a matter of "blending" the two into a smooth transition. Remember that you can always take a bit more off after HT, but can't put any back on.

3) After HT, the bevels are re-sanded, and then the secondary ( edge) bevel is added. This should carry up the main bevel about 1/8-1/4" to allow a good slicing edge. The exact amount of height is determined by how thick the blade edge was before HT and the edge angle. On a bowie, slicing tomatoes and such isn't what you will be doing, so a fairly thick edge is the norm. This is usually referred to as "meat" behind the edge. This is to allow the edge to survive the chopping and hacking a bowie will encounter, but still have a sharp edge. The tip should have more of this "meat" , as it gets far higher degree of stress. The trick is to roll the edge up into the tip , maintaining the width of the secondary bevel as you go. If the edge bevel is 1/4" wide, carry that 1/4" up to the tip. As you go closer, the angle you sharpen at will increase greatly. You may have been sharpening at 20° on the .030" thick edge, but as you go up to the .080" thick tip, the angle will become close to 45°. This will still pierce things, but will resist breaking or bending far greater. If you get this to all blend together, the knife will have perfectly even looking bevels, but will have a thicker tip than the edge. You have now made a Robust Tip.
 
bladsmth said:
Lots of things.

First - 1084 austenitizes at 1500°F. The eutectiod point is when the ingredients go into solution. The austenitization point is when they are ready for quench....not the same. Some steels have a lower austenitization temp than 1084. What makes 1084 a great steel for minimal HT methods is that it does not require time for excess carbon or alloy ingredients to form carbides. Thus it needs no soak time.
Click to expand...

I am terribly vexed about this, Stacy. Every once in a while, I completely fail to grasp a concept until it is handed to me on a silver platter but I thought I understood this. You do know that I wasn't suggesting quenching at 1400F, right? I was just pointing out that austenization can occur before the steel reaches nonmagnetic. Do you have any references I can look up that say the point that austenization and the Ac1/Acm line are not the same? My understanding is that once the steel passes the Ac1 and Acm points(1335F for 1084) that in theory, it becomes a matter of soak time or temperature. Of course, it is more beneficial to bring up the temperature to allow the carbon and whatever other alloying to occupy the gamma iron more readily but nothing else really happens, aside from carbide distribution(which I thought was the point of soaking). Also, what steels have a lower complete austenite(Acm) point than an almost perfect eutectoid?(and if you say "a perfect eutectoid" I'll give you such a pinch!)

Am I misinterpreting the facts?

References
http://www.cashenblades.com/heattreatment.html - see "Hardening"
http://www.ejsong.com/mdme/memmods/MEM30007A/steel/steel_files/FeC_diagram.gif
http://www.cashenblades.com/metallurgy.html - see "Austenite"
http://en.wikipedia.org/wiki/Austenite
 
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Rick, It is pretty complicated, and I don't understand it all, but one thing that makes the difference is that 1084 ( and other steels) aren't just 99.16% iron and .84% carbon. The amount of manganese and things like vanadium, plus the myriad traces of everything that was in the former steel melted down to make this batch all make the steel have a higher austenitization temp for HT than the theoretical point. It also has to do with the lack of available carbon to fill all the holes when the phase changes from BCC to FCC. I don't have a book here at work, but IIRC, if you look at the charts, you will notice that the hardening temps for hypo-eutectiod simple carbon steel is higher than that for hyper-eutectiod.
Another thing is that while the POINT that the changes start happening is Ac1, it happens a bit faster and is complete at a somewhat higher temp. Ask Kevin and the other metal heads in the other forum for a better explanation.

James - Do you think it will help there? I don't mind adding it if you folks want it in the stickies.
 
bladsmth said:
James - Do you think it will help there? I don't mind adding it if you folks want it in the stickies.
Click to expand...

I think it addresses some common and valid questions about HT, and how HT can affect tip strength, yes. The latter part, specifically about tip geometry, may not be absolutely necessary in the HT thread, but I feel they go hand-in-hand.
 
bladsmth said:
Rick, It is pretty complicated, and I don't understand it all, ...
Click to expand...
Whaaaaaaat? The metallurgy of heat treatment is complicated!?:eek:..... that's it... I'm out.:grumpy:

Thanks, Stacy. I do understand that theory and practical application are not always in sync. Nothing occurs perfectly. I was just having fun with charts.

if you look at the charts, you will notice that the hardening temps for hypo-eutectiod simple carbon steel is higher than that for hyper-eutectiod.
Click to expand...
It looks like 1050 would require a higher austenization temperature than 1095.
 
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Thank you guys, this put things into perspective pretty good, Stacy, I am going to save that post you made about geometry of the Bowie and HT, helped a lot!
 
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