Advice on spot on blade

TLR

TLR

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Oct 5, 1998
Messages
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When I got this blade back from HT I noticed a weird "bloom" spot on the top of the blade but just got to cleaning this one up last night. When I ground down one side with an 80 grit belt I noticed that at the center of the "bloom was a dark spot that seems to be deep. It is right at the top edge of the blade and when I cleaned the spine up I can find no corresponding mark on top. Is this a HT issue. Anything to be worried about or do I just need to keep grinding deeper?

Sorry I just don't have any mentoring up here so don't have anyone else to ask. Pics are below of both sides of the blade. One side is ground the other has not been.

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We need to know a bit about the HT. Was the spine drawn by a torch? How was the blade HTed? Was the blade forged or stock removal?

The colors showing look like the steel got a bit hot in that area. Since it is in the spine, it isn't much more than a cosmetic issue, and may well grind out in the finer grits.
BTW, Go easy using an 80 grit belt at high speed on a HTed blade...or you can get hot spots.
 
Sent out to Peter's. Blade is cpm 3v and was stock removal.

I appreciate the advice about the hot spots. Being very careful and dunking a lot.

I guess part of the question is concerning the cosmetic issue. Is this likely to be colored all the way through or just at the surface?
 
The colors shown are surface effects of heating. They will sand out.
The dark spot is a bit harder to say about without closer examination than a photo will allow. I would expect it to sand out, but, only taking it to a claen 400 grit will determine if my guess is correct.
 
The dark spot is what I was concerned about. The coloration sanded out with 120 grit but the spot being more stubborn is why I dropped down to 80.

On a separate note your warning about overheating the blade is noted and understood. One question in that regard. If a blade is overheated after HT at what point is the HT compromised. At any color or depth of color or is there a specific coloration or depth of coloration that is indicative that it has gone to far and the edge is trashed. I did have some very very light discoloration (very light tan) on my first thin blade. This 2x42 is horrible for post HT and it's limitations are really showing up now more then during the initial grinding.

Once again thank you for your willingness to teach on here.
 
Hard to tell from the photos but it looks like a Low spot on the spine. I imagine you would see if that were the case, sorry not much help.
 
Peter's tempers 3v at 980F for 59-60 hardness. The blade should be well into the blue-black color range by then. It still isn't a great idea to let it get hot during post-HT finising but this steel's elevated tempering range makes it forgiving in this regard.
 
It looks as though they had the blade resting in a stand when it was heat treated and the blade got burnt in that spot.

Richard
 
What Justin said was what I was going to say. 3V has a comparatively high tempering range, so it's unlikely you blew the HT unless you got it red hot. I actually asked specifically about if it goes black like that when I was chatting with Brad, and I understood him to say that I didn't really have to worry about ruining the HT until it got red (which is quite hot, something like 900 degrees or something like that? I'd have to review my notes to be sure). The black happens a good bit before that, and generally sands right out, since it's just on the surface. There's another thread I started about 3V in here somewhere that has a fair amount of the information I got from talking to Dan Keffeler and Brad at Peter's, among others. You might also find it useful.

If you're using a Craftsman 2x42, dip after every pass, and don't press too hard. Both of those will help you mitigate the heat issue with the high speed. I also find that heat doesn't seem to build up quite as fast with higher grits (or maybe I just don't press as hard or something). I don't think I ever dropped down below 220 grit post HT when I was using my Craftsman as my primary grinder. It's slow, but don't rush things and you won't have a problem, especially not with the 3V. It's good stuff.
 
^ Actually, heat tends to build up more quickly with higher grits, because of the increase in friction, where lower grit belts will be removing more material with less fuss.
 
I'm less sure about that. With higher grits, there's a lot less surface area on an given length of belt, because the grit size is smaller. In contrast, the lower grit belts, such as the 60 grit, you've got larger chunks of abrasive material, thereby creating more vertical surface area, and hence, more friction. When I grind with a 60 grit belt vs with a 400 grit belt, at the same speed, the 60 grit belt will throw sparks, while the 400 grit won't. I can also do more passes before it gets too hot to touch with my bare hands using the higher grit belt. How exactly do you explain these phenomena if, as you claim, the higher grit belts have more friction? Or are you thinking the same thing that I am, but meaning 60 grit etc for higher grit, and 400 grit etc for lower grit?
 
Tinbasher said:
It looks as though they had the blade resting in a stand when it was heat treated and the blade got burnt in that spot.
Click to expand...

Very doubtful. As far as I know, the folks at Peters hang all the blades from hooks or wires, specifically to avoid that problem. I only know this because I once sent them a hidden tang blade with no holes in it, and Brad called me for permission to drill a small hole in the tang. He explained that hanging the blades in the furnace, rather than standing them in a rack, helps prevent hot/cold spots.
 
It did in fact sand out although it took a bit. I have been swamped with my real job and so haven't started on the other side yet but at this point I'm not to concerned.

I appreciate the advice and help.
 
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