A tale of two knives

[Doug Lester]

I made two test knives from 52100 to test heat treating methods that I have been using.

Knife A was 10.75" OAL with a 6.5 X1.5X3/16" blade. It was heated to just above non-magnetic in a forge running at about 1550-1600° and taken in and out of the fire for a few seconds at a time to keep the austinizing temperature lower. It was then quenched in 350° peanut oil and held long enough for the tang to loose color, which was held out of the quenchant. It was then up quenched to a 430° bath of peanut oil in an electric roaster and held there for 4 hours for austempering/isothermal tempering, depending on what book you go by.

Knife B was 10.5" OAL with a blade 5.5X1.5X3/16". It was austinized as closely as I could to blade A but it was quenched in 520° peanut oil for about 10 seconds and then allowed to cool to ambient temperature. The Ms point for this steel is around 490° at this austinization temperature. It was then tempered in my kitchen oven at 425° for 3 two hour cycles.

Both blades were flat ground with a convex secondary bevel ground with a slack belt.

I clamped up both knives in my leg vice and did the 90° bend test on them. I had to use a breaker bar to get both to bend and it may have taken just a little more force to bend knife B than knife A.

Knife A broke right in front of the ricasso just as I gave it that little extra push to make it to 90°. It broke right through the area where I had hot stamped the letter A.

Knife B was bent to 90° without the knife breaking. As you can see, it bent at the tang right behind the heal of the blade wrecking the Redheart scales. The blade itself bent only slightly and returned to straight. After I photographed the knife I had to clamp it back in the vice and give about 9-10 blows from a 4lb hammer to break the blade.

Continued below.

Doug

 

[Doug Lester]

These show the grain in the broken end. The second one, B, did not break as smoothly as A but both had a smooth gray surface to them.

On other testing that was done, both bit into bailing wire when driven into it with mallet blows with no more than a very slight indentation of the edges. Both chopped through a 2X4 once and were still able to shave hair. I only chopped through once because these were relatively light blades an it took a lot of work to get through the board. I also cut through some 3/8" Manila rope until the blades started to drag. Knife A cut 90 times and knife B cut 80 times. Because this test was rather subjective as to when the blades stopped cutting cleanly, I would call this even. The edges of the blade were touched up just before this test and were shaving sharp but neither blade could shave hair after cutting the rope.

Doug

 

[Diamond G Knives]

Hopeing on of the 52100 Gurus will chime in on this one!

Thanks for sharing your tests with us all. This is good info!

God Bless
Mike

 

[rhinoknives]

Great testing work,
Bob Loveless said that, Thou he use to use a hot stamp for a Maker's Mark he would now only use a etched or Photo mark on any Blade he made, Forged or stock removal due to possible stress risers in the steel introduced by the Hot stamp.

I think we see this in play here?

Laurence

www.rhinoknives.com

 

[Doug Lester]

Reading back over the post I see that I failed to mention that I soaked the blades at temperature for 5 minutes before whatever quenching method that I used. I used a timer to measure how long. I had also normalized the blades three times before I austinized them for hardening.

I also think that I see an etching machine in my future but that may take a little while.

It kind of surprised me how many times that I had to smack on blade B with that 4lb hammer to get the blade to break and I was swinging on it hard.

Doug

 

[Brad Lilly]

Interesting do you think the quench temperature made the difference or the tempering cycles?

Good on you Doug. Testing knives to failure is tough, but you know what your product can handle

 

[Doug Lester]

I marquenched blade B just to test the process to make sure that I was getting good hardening due to the question of whether or not the peanut oil would cool the steel quickly enough with the oil at 520° to miss the nose of the cooling curve.

The method for blade A, the austempering or isothermal quenching, was designed to cool the blade to 350° and produce about 75% martensite and the up quench the blade to 430° degrees for four hours to allow the rest of the austinite convert to lower bainite. I had done an earlier blade by quenching it in the 430° degree bath without the lower initial quench for four hours but the edge seemed too soft. That looked like it should have been about 25% martensite with the remainder lower bainite. I did not test this blade to distruction but went back and rehardened it by marqueching and regular tempering at 425°.

The reason for this was to produce some more toughness in the steel without as much loss of strength as would occur with differential hardening. I'm thinking that I achieved this. Blade A did bend to almost to 90° before it broke, evidently at a stress riser. I wish that I could have watched the edge more closely as I bent it to observe if there was any cracking around the edge though it could have only been where it ultimately broke. The blade sure make a lot of weird noises as it was being bent, like rusty hinges being move slowly.

Blade B was ultimately stronger. There was no permanent distortion to the blade on the initial bend. Only the softer tang took a permanent bend and I really had to hit the blade hard several times after I clamped it back up before I could get it to break. Though I don't think that it bent all that far before it broke. I just have no way of estimating how far it did other than the little bit of a permanent bend in the blade where I clamped it. I doubt that it could have been as far as 45°.

Doug