5160 chipping

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PMQ

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Feb 17, 2020
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I'm making a chopper from 5160, technically it's SUP9, Japanese 5160 with a bit less Carbon, but basically the same.

I have a tiny diy forge, which I used to grain refine the edge. I heat the edge to non-magnetic and slightly higher. Since it's a long chopper, I could only heat it section by section. I let it cool in still air. Did this 4 times.

Then I send it off to a heat treating service, the recipe is: 1500F for 20 minutes, dry ice for 1 hour, 350F for 2 hours twice. I got a hardness of 60-61 HRC.

After finishing the blade, I went to sharpen it. This is when I realize that it's super chippy, lots of micro chips on the edge.

I thought maybe there's something wrong with my coarse stone, so I took quite some time with a 1000 grit stone, it's decent still somewhat chippy.

Anything I might did wrong? Thanks.
 
Looking at heat treat data in Larrin's book and it shows toughness diving drastically with 350 F temper. If you want the best out of 5160 looks like you should be aiming for 58-59 Rc.
 
Seedy Lot said:
Looking at heat treat data in Larrin's book and it shows toughness diving drastically with 350 F temper. If you want the best out of 5160 looks like you should be aiming for 58-59 Rc.
Click to expand...
Yes, I have Dr Larrin's book, I know that 5160 when 350F temper will drop it's toughness drastically, but at 60 HRC, the toughness should still be at 10-15 ft-lbs, roughly the same toughness as D2 and many other steels, so I don't think that's dangerously fragile.
 
I would re-temper it at 400F for an hour. Also, don't make the edge too acute on a chopper. Try a 20-25DPS edge,
 
Thanks for the advice guys, here's an update, and it's not looking good:

I re-temper the blade ar 400F in my home oven for 1 hour. I re-sharpened it at a higher angle. Something I didn't mention above was that the chipping wasn't happening when I'm chopping, it happens when I sharpen. Which is really strange.

After that, I tried chopping into a brass rod, and I got a massive chip. Looking into the cross section, it was the coarsest grain I have ever seen.

From the same bar, I cut out a small piece, grain refine 3 times (heat to non-magnetic and slightly higher, then air cool) then austenize using the same method (heat to non-magnetic and slightly higher), quench in water (I was in a hurry so no oil) . Break it in half, and the grain in very fine, not visible grain growth.

I remember someone told me that chipping when sharpening is due to grain growth. And grain growth is caused by austenizing too hot or too long. So I called my heat-treating guy, asking if he had accidentally over heat my blade, he assured me that he did it correctly, 1500F for 20 minutes.

I have no idea what's the cause of this. If anybody have any theory or suggestion, I'm all ears.
 
Steels are a lot more sensitive to temperature than they are to time. It would take hours to affect grain size.

The blade was either over heated or the prior condition was bad for the results you are getting.

It needs normalizing and annealing before the austenitizing, quench and temper.

If the heat treater is using the right temperature, then the steel has excessive grain growth to begin with.

Hoss
 
1500°F for 20 minutes would not make extreme grain growth in 5160.
I suspect there was some error in the HT, or in grain refinement. Are you sure you did the grain refinemet on the blade right?
 
Come to think of it, the chipping with sharpening was happening near the tip, the section near the plunge was fine.

I did grain refine the edge (heat to non magnetic and slightly higher, air cool, 4 times), maybe the tip has less steel so I might have over heated it. Does it has to get much hotter to cause grain growth?
 
Read Larrin's book page 202 to explain what the issue is. Do a full normalization and test again

To refine the grain you normalize to re-set the grain size. Start with a temperature about 150°F above the target temp and drop in two more 50°steps. Hold for 10 minutes each step before cooling. Cool to black on the first two and quench in oil on the last one (around 50° above the target temp). Then do the HT and quench at the target temp.

If you have a really badly overheated blade due to extensive forging, the normalization may need to start as high as 1850°F to get the massive grain size and cementite reduced.
 
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I only stock removal. I did notice that when cutting out the blank with an angle grinder, some spots seems to be harder. Could it be that the grain size is already so huge and so bad that my grain refine cycles didn't do anything?
 
It could be. Depending on your steel source it could have arrived in a very course state that needs to be reduced before you can harden reliably.

For instance, I have some W2 from new jersey steel baron. I know that lots of their steel comes in a course spheroidized state. So I'm going to have to break that down with high temps, then process it further, so I end up with a state that is reliably hardenable.

If you don't know the condition of the steel when you get it, just assume that it's crap.
 
PMQ said:
I have a tiny diy forge, which I used to grain refine the edge. I heat the edge to non-magnetic and slightly higher.
Click to expand...

PMQ said:
Could it be that the grain size is already so huge and so bad that my grain refine cycles didn't do anything?
Click to expand...


I'm no expert but I'd bet 10 rail spikes and a John Deere mower blade that the the grain refinement treatment to the edge was the problem. 😁
 
Just to cap this. As mentioned, the blade was heated in separate parts due to a small forge. When the tip was heated, due to tapering size, it overheated a lot. Usually tip is left out of the flame almost an ". It would then get heated by conduction efficiently. Overheating that much makes grains go nuts.
So, making a bigger forge would help a lot.:) Maybe a pipe inside as a container to balance the temp so no problems with the tip arises.
 
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