Heat Treat or Test Issue?

I

IrishBladeworks

Joined
Jan 2, 2023
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3
Hey folks, I recently attempted my first Heat Treat on some AEB-L. It was 1/8" stock from Jantz, placed in foil, held at 1560 for 5 minutes then ramp to 1940 and held for 15 minutes before plate quenching between aluminum quench plates and blowing air between the plates. Blade was removed and placed in a freezer while the oven cooled down enough to temper. Temper was two rounds of 300 degrees for two hours, letting the knife cool down to room temperature between tempers. Blade was cleaned up using a 120 grit belt. Using a set of HRC files, the 65 bites but the 60 does not. I also ground in the bevel and hit the knife edge-on-edge against another blade I have that was tested at 57HRC and it took damage but this AEB-L did not. Both blades have similar edge thickness.

For piece of mind, I took the blade to a machine shop and had them test the hardness. This is where my confusion comes in. I ground into a small area of the tang (maybe .020 deep) using a coarse small wheel on a Dremel. This is how previous blades had been tested when they came back from being heat treated. The shop tested hardness on the flat of the tang, and tested hardness in the ground area. The flat came back at 60 HRC, but the ground area came back at 55. The guy said it could be due to the ground area not being flat, but I'm super confused. My testing would indicate the edge is at least harder than 57, but should it be that much softer below the surface? My concern is that if the blade is really that much softer in the middle, once I get my bevel and final profile, will I be into the "soft" part of the steel. I had them test two other AEB-L knives and they both 5-8 points softer in the ground area than on the flat.

Thanks,

Guy
 
IrishBladeworks said:
My concern is that if the blade is really that much softer in the middle, once I get my bevel and final profile, will I be into the "soft" part of the steel. I had them test two other AEB-L knives and they both 5-8 points softer in the ground area than on the flat.
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You need to rule out that you are not unintentionally tempering the steel further with the heat and friction from grinding.

It's not clearly explained how you are using the Dremel and there are no pictures to see your test area empirically to help you.

The test area needs to be dead flat on top AND bottom. The finish is equally important; needs to be finished to at least 120 grit and higher to get an accurate enough average of 3 tests. Higher finish will increase accuracy (repeatability) but not if it's causing heat and convexity.

It's not clear if your hardness results are an average of results or single test. To be clear, a single hardness test is useless, needs to be an average of at least 3 in these circumstances.
 
Thanks for the responses, lets see if the pic works. My knife is on top and a knife I had professionally heat treated is on bottom.
https://imgur.com/MvSw4pW

Dremel process is to simply use a sanding drum, although it is coarser than 120 grit. Sanding the area did not generate enough heat to discolor the steel, nor did it make the knife too hot to hold.

Machine shop only took one test from each spot and did not do an average. Overall, sounds like the trip was not as beneficial as I'd hoped it would be but I know what to do differently next time.
 
IrishBladeworks said:
Thanks for the responses, lets see if the pic works. My knife is on top and a knife I had professionally heat treated is on bottom.
https://imgur.com/MvSw4pW

Dremel process is to simply use a sanding drum, although it is coarser than 120 grit. Sanding the area did not generate enough heat to discolor the steel, nor did it make the knife too hot to hold.

Machine shop only took one test from each spot and did not do an average. Overall, sounds like the trip was not as beneficial as I'd hoped it would be but I know what to do differently next time.
Click to expand...

1. The diameter of that grinding drum is too narrow for that method to work and will cause error due to differences in plastic flow if we compared to a perfectly flat specimen. Just like how there are correction factors for hardness testing on a steel rod versus a flat sheet.

2. It's also going to be impossible to make that little, narrow "trench" perfectly flat and perpendicular to the diamond penetrator which will also cause error in results.

3. You also may have problems with heat and friction inadvertently causing some local micro tempering in that spot. It is very important to use fresh abrasive and water cooling to avoid inadvertently changing the hardness, This is also important for knife grinding.

I would say that your results are currently inconclusive until proper testing is done.

Non warped, flat surface on top and bottom at 120grit finish or greater, don't forget to slightly chamfer the edges. Only test one side with an average of 3 test and make sure the tests are not too close together. They have to be at least three test indentation diameters apart. Also if you are tactful in conversation politely have a peak at the hardness testing calibration and confirm it's within ASTM E18-20.


The good news is I don't see anything wrong with your heat treatment although you may want to explore a slightly higher austenitizing temperature and tempering temp.

300°f temper is easy to overshoot without careful grinding practices.
 
Wow, thanks for the great info!

Unfortunately there isn't a local shop with the equipment to test hardness, so I visited this shop while out of town on business. I think my next move will be to clean up the other knives I did, maybe heat treat some test coupons, and send them off for proper testing.
 
IrishBladeworks said:
Thanks for the responses, lets see if the pic works. My knife is on top and a knife I had professionally heat treated is on bottom.
https://imgur.com/MvSw4pW

Dremel process is to simply use a sanding drum, although it is coarser than 120 grit. Sanding the area did not generate enough heat to discolor the steel, nor did it make the knife too hot to hold.

Machine shop only took one test from each spot and did not do an average. Overall, sounds like the trip was not as beneficial as I'd hoped it would be but I know what to do differently next time.
Click to expand...
Rockwell tester read hardness based on HOW deep diamond penetrate steel .Let say hole from diamond tip is deep 0.12mm on 1000 grit sanded surface and hardness reading is 60 HRC .Now , if surface is 60 grit ..............imagine how inaccurate result you will get because you have a deep / let say they are 0.05 deep / serration/scratches on surface from 60 grit ...............
 
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