Jesse, I am not sure I understand. Are you asking about testing for hardness using a hardness tester such as in a Rockwell "C" scale test, or are you asking about impact toughness as in what would be measured with a Charpy/Izod type impact test? These are two completely different properties, tests and equipment. Many knifemakers will have a penetrative hardness tester, mostly for the Rockwell "C" scale test, it is the most common standard by measuring higher hardness in steel. Almost no knifemaker would have the means to do actual Charpy/Izod impact testing to determine impact toughness, and it is a very touchy test in knife blade levels of hardness. There are scleroscope hardness tests that work off from rebounded impact, but they only measure hardness rather than toughness, and I don't know of any knifemaker that has worked with one, myself included.

If it is Rockwell type hardness testing we are discussing, I utilize that test very much, as do many others, to dial in my heat treatment procedures. Every blade, or piece of metal, that sees a heat treatment in my shop gets Rockwelled and recorded. One can get by without those numbers, and many do, but it needs to be replaced with other practical testing to insure the quality of the heat treatment. Rather than eating up time using every knife I make to death, the Rockwell numbers, combined with other standard tests and controllable equipment that allows very repeatable results allows me to be much more productive and have a higher degree of confidence in my heat treatments.

I am the very oddball exception, in that I do also own my own Charpy/Izod impact tester as well, but it is of little use in direct measurement of blade toughness, and can give you nothing about hardness. It is used more to tell the heat treatment that will achieve optimal impact toughness (a property that can often be at odds with hardness) in a given alloy, rather than a knife blade. For the test to give meaningful numbers a standard sample of steel 10mm X 10mm is used rather than blade shapes.

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Jesse, I am not sure I understand. Are you asking about testing for hardness using a hardness tester such as in a Rockwell "C" scale test, or are you asking about impact toughness as in what would be measured with a Charpy/Izod type impact test? These are two completely different properties, tests and equipment. Many knifemakers will have a penetrative hardness tester, mostly for the Rockwell "C" scale test, it is the most common standard by measuring higher hardness in steel. Almost no knifemaker would have the means to do actual Charpy/Izod impact testing to determine impact toughness, and it is a very touchy test in knife blade levels of hardness. There are scleroscope hardness tests that work off from rebounded impact, but they only measure hardness rather than toughness, and I don't know of any knifemaker that has worked with one, myself included.

If it is Rockwell type hardness testing we are discussing, I utilize that test very much, as do many others, to dial in my heat treatment procedures. Every blade, or piece of metal, that sees a heat treatment in my shop gets Rockwelled and recorded. One can get by without those numbers, and many do, but it needs to be replaced with other practical testing to insure the quality of the heat treatment. Rather than eating up time using every knife I make to death, the Rockwell numbers, combined with other standard tests and controllable equipment that allows very repeatable results allows me to be much more productive and have a higher degree of confidence in my heat treatments.

I am the very oddball exception, in that I do also own my own Charpy/Izod impact tester as well, but it is of little use in direct measurement of blade toughness, and can give you nothing about hardness. It is used more to tell the heat treatment that will achieve optimal impact toughness (a property that can often be at odds with hardness) in a given alloy, rather than a knife blade. For the test to give meaningful numbers a standard sample of steel 10mm X 10mm is used rather than blade shapes.

It is Rockwell C that I am asking about. Specifically, when in the heat treatment process do you take the Rc test(s) and how do you deal with the ensuing divot? Thanks.

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It is Rockwell C that I am asking about. Specifically, when in the heat treatment process do you take the Rc test(s) and how do you deal with the ensuing divot? Thanks.

The "C" scale works best above 30 so annealed stock is best dealt with on the "B" scale. The major mistake that is very common is to only take a final reading after tempering. This single reading at the end is really just a selling point for uneducated customers. To actually monitor the success of heat treatment you need a reading at full hardness, immediately after the quench, as a comparison to those final numbers, after tempering. Yes the test will leave a divot, some makers incorporate it into their logo, but to me that always seemed like a lot of work when all I had to do was put it somewhere where it will not show on the finished knife. Your choice of locations are limited as an accurate Rockwell reading can only be taken on a well finished, flat, and parallel surface, this leaves mostly the ricasso area. I take all of my readings just behind the ricasso where the guard will rest. This all works great for 5160, 52100, 80CrV2, L6, O-1 and other alloyed steels, but it is quite problematic with 10XX series and W-1 or W-2, as those steels, unless water quenched, will not harden enough at the ricasso area to give an accurate reading unless the ricasso in less than about .187".

The first readings are necessary, because a final reading alone only tells me that the O-1 blade I am holding is now 59 HRC, it doesn't tell me if it was only hardened to 61 and saw very light tempering to arrive that that number. This would mean that I have a number that sounds good despite the fact that the quench was unsuccessful and long term edge holing of that blade will suffer. In other words, Rockwell is much more relevant when it is used to check every step of the heat treatment. On that O-1 blade I want 65 HRC out of the quench, or that heat treatment needs to be redone, I then want to get slightly higher than desired tempering readings to start out and then walk in the exact number I want with increasingly higher cycles. This will typically be something like 63 HRC after 30 minutes at 350°F, then 61.5 to 62 after 30 minutes at 375°F, and 60 to 61 at 400°F. If the numbers plummet very quickly below 60 HRC, my soak for hardening was insufficient and I need to pay more attention to that. Some steels, like properly soaked 1095, will readily give up those first two points and then get very stubborn in surrendering any more, forcing you to surprisingly high tempering temperatures to wrench those points from its grip.

Thank you, Kevin. Do you have any heat treat QA suggestions for 10XX > 3/16", quenched in parks 50?

Kevin,

Great information as always. I thought I would throw in some of the discussions on problems that I have had. I know you heat treat in salt baths, which I believe limits access to oxygen. I thought I would throw out the issue of the dreaded decarb layer, which gave me fits when I first started testing hardness. I couldn't figure out why I was getting such low hardness values immediately after heat treat. Needed to grind away the decarb layer to expose the hardened steel.

Interesting information regarding the area of the ricasso on water quenching steels. Is this because the thickness retains enough heat to prevent the steel from cooling at the proper rate to achieve optimal hardness?