What Causes This Discolorization?

MTBob

MTBob

Well-Known Member
Can anyone tell me what causes the discoloration of the steel in this photo? It is heat treated 80CrV2 from AKS, coated with ATP641, 5 min soak at 1575F, quenched in Canola Oil, tempered at 375. Post heat treat, the blade was ground flat (by hand) prior to adding the primary grind. It has been repeatedly media blasted with 70 grit AO (@100 PSI) in advance of doing an acid etch. The blasting failed to remove the discoloration / blotched appearance. It's noticeable that the discoloration shows up only on the spine area, not in the primary grind area. Even after an acid etch (FeCL/vinegar - 1 1/2 min), the dicoloration was still there.
I suspect this discoloration is caused by decarburization, but I've never seen it show up like this before. Is there something in the heat treating process that I could have done different to avoid this? What is surprising is that it showed up after grinding the blade flat, and continued to show up after aggressive media blasting.

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Was this a stock removal knife? Did the steel come with CRA or HRA scale?

I've had this happen with stainless before. My issue was not removing the bark to a sufficient depth. One or the other of the rolling styles is more prone to this but darned if I can remember which one. This issue was partly how I ended up with my surface grinder. Be careful! :)

PS If you get a surface grinder, get flood coolant. I long for flood on my '80s economy ENCO.
 
MTBob said:
No. It was darker, but the blotching was still there. I didn't leave it in a long time in the FeCl, about a minute and a half. Would a longer soak dark serve to cover the marks?
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When you have clean, hard steel you should get an even dark black color. You coated it to prevent decarb, so I wouldn't suspect you had much to deal with, apart what was there from milling. It usually isn't much. But if it was me I'd go at it with a good 120 belt until I got .005"-.007" off of one side and test etch. If that doesn't clean it up, it might be not completely hardened. It does sort of have a "hamon-ish" appearance.

You have a couple of things may be working against you on that. I don't go to 1575F so that would change the needed soak time, but I generally need 15 min at 1525F for as received spheroidized 80CrV2 from AKS. It seems real consistent from heat to heat from there. Also, the canola might not be getting you there.
 
fitzo said:
Was this a stock removal knife? Did the steel come with CRA or HRA scale?

I've had this happen with stainless before. My issue was not removing the bark to a sufficient depth. One or the other of the rolling styles is more prone to this but darned if I can remember which one. This issue was partly how I ended up with my surface grinder. Be careful! :)
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Last HRPA 80CrV2 I got from there was quite barky. Not an issue if you know it's there to grind out, but I do cold rolled if I got the option.
 
Yeah I believe it's decarb... Happened to me with magnacut actually. But if you can surface grind it down like .005" per side it should be removed...this was after I ground the blade aides flat on my platen with a fresh 120 grit belt. But not enough metal was removed

i-Dk3Q3XL-X3.jpg



Here's what one of the blades looks like with a beadblasted finish... I had pre-ground the blades so wasn't able to remove enough metal to remove it on this one

 
fitzo said:
Was this a stock removal knife? Did the steel come with CRA or HRA scale?
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Sorry, I failed to mention this was a stock removal blade, with no initial scale as received from AKS.
As received the steel had no evidence of hot roll scale.
tkroenlein said:
I don't go to 1575F so that would change the needed soak time, but I generally need 15 min at 1525F for as received spheroidized 80CrV2 from AKS.
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I wondered about this. I targeted the mid range of what AKS recommended, but then on further reading (after HT) I saw folks saying 1525 had better results. So, next time I'll drop my temp a bit.

I'm thinking what I should begin doing is more accurately measure the steel thickness before doing a flat grind and then make sure I take off, say, .005-.010" on each side. Then I suppose to check for this color I could do a test etch and see what it looks like - all before doing a primary grind.
 
tkroenlein said:
Do you guys get any warping when you bead blast?
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I haven't seen any signs of distortion. But, I'm not using straight beads. I use 2 blasting materials. For general crud & scale removal I use straight 70 grit Aluminum Oxide (from HF). Then for less aggressive blasting I mix the AO with 50% glass beads. I'll use that mix on G-10 to get a slightly roughened textured surface. I've also used walnut shells on some materials where I don't want to remove any base metal - like removing paint off an object.
 
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