Inconsistent Hardness...80CrV2

S

Shane Csepp

Joined
Oct 21, 2019
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84
First of all, thanks to everyone for the earlier grinding suggestions to fix grinding into the spine on this knife.
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Turned out great cosmetically with only a few minor blemishes, however I am concerned about the HT.
Blade is 80CrV2 and it is one of the first blades I’ve HT’d in my DIY oven. I followed the suggested HT recipe of 1525* soak for 10min, quench 130* canola, two temper cycles at 400* for around 60 HRC. I did several hardness tests throughout the blade and the tang is dead on at 60 HRC, however the blade ranges anywhere from 50-60. Often in the low 50s. There is substantial decarb in the center of the blade on the opposite side as pictured. Blade had a substantial warp out of quench that I was able to get out as well. I did not thermal cycle before quench since this steel came from AKS.

Any ideas as to why these inconsistencies may have happened? Perhaps I ground too thin pre-HT while chasing my overgrind?
 
Grinding too thin before HT should have no effect on hardness unless it was so thin that you still have a layer or decarb along the edge. My first guess would be that you overheated the edge chasing the grind. The grind came out good BTW.

Now in looking, I see some decarb by the tip. Is that where you had the low readings?
 
scott kozub said:
Grinding too thin before HT should have no effect on hardness unless it was so thin that you still have a layer or decarb along the edge. My first guess would be that you overheated the edge chasing the grind. The grind came out good BTW.

Now in looking, I see some decarb by the tip. Is that where you had the low readings?
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If I recall the tip readings were mid 50s. The low 50s were in the center of the blade where most of my decarb was. I was unable to grind it all away without potentially grinding too thin or being off geometrically.

Most of my grinding chasing the mistake was done Pre-HT. The only post-HT grinding was cleaning up the surface from as much decarb as possible. The backside of the blade has a deep decarb spot approximately in the center of the blade and roughly 1" x 1.5" long.
 
Try doing the brass rod test on the cutting edge. I have learnt that that it a really good indicator of whats happening at the edge. Hardness testing helps to dial in the HT but the rod test will tell you if the the HT is optimal for the steel choice. I recently had an AEB-L kitchen knife that wasn't holding an edge well (one of my first knives). I tested and got 60,60,58,60,58. Soft but not terrible numbers. Then I did the rod test and sure enough the edge was soft and would not spring back.
 
I always heat treat in a dimly lit room so I can determine the heat color of the blade. Soak time can often be unreliable. I think it could be that. Decarb too...
 
This is exactly why is is a bad idea to grind much bevel before HT. The decarb may be deep, and it will be deeper in the thinner sections. Your observations are what I would expect ... softer toward the tip. As Larrin said, the decarb will make the hardness reading inaccurate.

The good news is the edge should be harder, because the decarb is ground away there in shaping and sharpening.

In the future, grind the profile and on any carbon steel blade less that .100" thick, don't grind the bevel. On thicker blades only grind a pre-bevel with an edge about .050" thick.
 
Stacy E. Apelt - Bladesmith said:
This is exactly why is is a bad idea to grind much bevel before HT. The decarb may be deep, and it will be deeper in the thinner sections. Your observations are what I would expect ... softer toward the tip. As Larrin said, the decarb will make the hardness reading inaccurate.

The good news is the edge should be harder, because the decarb is ground away there in shaping and sharpening.

In the future, grind the profile and on any carbon steel blade less that .100" thick, don't grind the bevel. On thicker blades only grind a pre-bevel with an edge about .050" thick.
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Thanks for the suggestion. I am definitely finding I much prefer to grind Post-HT on almost everything so far.
 
I’ve found decarb after two normalizing cycles, two grain refinement cycles, then heat treat can be 0.010” per side. That has to be taken into consideration when planning final geometry.
 
That blade looks to be a FFG, and it's hard to get an accurate Rc reading when trying to test on an angle. As said before, on anything less than .100", profile only leaving the sides flat. Top and bottom really needs to be parallel for decent Rc testing. Also use an anti-scale coating on carbon steel to help prevent scale during HT. Anything from commercial anti-scale to roach powder (boric acid) works just fine.
 
There are some things to consider about using straight borax (roach powder) as an anti-scale.
One is that borax gets corrosive around 1600F. That is dangerously close to the HT temps for carbon steel. Usually not a problem for short soaks, but something to consider when HTing in an open forge and trying to keep the temp at 1500F a 10 minute hold.
The second is that if you use borax in a HT oven it will likely get on the floor and erode the firebrick. It isn't a big deal to repair or reline a forge floor, but a HT oven floor is not as simple to fix.
 
How did you measure the hardness?

I would leave some flat on the blade or grind after heat treat and testing if you want to directly measure the blade (vs handle) hardness. Surfaces are supposed to be parallel for hardness testing.

Indenting an angled surface might explain the low reading on the blade vs handles. I assume that the angled surface would allow the indenter to move further, giving the lower reading, but that is just from figuring, not from experience. Could also be the decarb, or overheating, but I’d confirm the test first.
 
TILLER said:
How did you measure the hardness?

I would leave some flat on the blade or grind after heat treat and testing if you want to directly measure the blade (vs handle) hardness. Surfaces are supposed to be parallel for hardness testing.

Indenting an angled surface might explain the low reading on the blade vs handles. I assume that the angled surface would allow the indenter to move further, giving the lower reading, but that is just from figuring, not from experience. Could also be the decarb, or overheating, but I’d confirm the test first.
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That's a good point. I have a Rockwell Tester and did try to make it as parallel as possible, however there is still the opportunity for the indenter to move more than it should. I also, don't know 100% if that's a real thing, but it makes sense to me. I know things need to be parallel for the most accurate readings.
 
Stacy E. Apelt - Bladesmith said:
There are some things to consider about using straight borax (roach powder) as an anti-scale.
One is that borax gets corrosive around 1600F. That is dangerously close to the HT temps for carbon steel. Usually not a problem for short soaks, but something to consider when HTing in an open forge and trying to keep the temp at 1500F a 10 minute hold.
The second is that if you use borax in a HT oven it will likely get on the floor and erode the firebrick. It isn't a big deal to repair or reline a forge floor, but a HT oven floor is not as simple to fix.
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Stacy, Good points all. I'd not considered the issue of corrosive or oven damage. I've used Roach powder in the past with no issues. When using roach powder I heat blade just enough to melt the powder so it forms a clear "glass" coating over the metal. I've never seen any of the "glass" coating melt off during HT in the oven.

Better than boric acid I think is the dry graphite spray on coating I've been using for carbon steel, and even for AEB-L a few times. I've got SS foil for SS HT'ing, but the dry graphite does seem to work ok.
 
Shane Csepp said:
That's a good point. I have a Rockwell Tester and did try to make it as parallel as possible, however there is still the opportunity for the indenter to move more than it should. I also, don't know 100% if that's a real thing, but it makes sense to me. I know things need to be parallel for the most accurate readings.
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They MUST be parallel for accurate reading and reasonably smooth surface finish .Think about dent which tester make in steel ..........it measure how deep it is and that transfer in rockwell on scale .....
 
I would guess decarb based on the pics and how thin you ground pre-HT. Turned out great btw! Recovered from that mistake beautifully.
 
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