Why polish a blade before heat treating?

SpacemanSpiff23

SpacemanSpiff23

Joined
Feb 22, 2009
Messages
160
I've read that a lot of people finish their blades up to about 500 grit before they heat treat. This sounds like a waste of time and energy as you will have to continue to grind the blade down after the heat treating is finished to get the edge slimmer than the 1/16th it was when you quenched it.

Or maybe polishing first just doesn't apply to full flat grinds like I'm doing.

Or am I totally missing something?
 
Instead, tring to get rid of coarse scratches after HT is some waste of time. You can easily go up on an unhardened steel and after HT you'll sand a lot less and spend a lot less time. I do full flat grind a lot and the time saved by polishing before HT is hours...
 
I feel like after I heat treat it, I will have to put new coarse scratches in it to thin the blade out. But I guess these wont be as deep sinse the blade will be much harder. That makes sense. How polished do you get it before heat treating?
 
SpacemanSpiff23 said:
I feel like after I heat treat it, I will have to put new coarse scratches in it to thin the blade out. But I guess these wont be as deep sinse the blade will be much harder. That makes sense. How polished do you get it before heat treating?
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A typical knife in my shop shaped with 80 and 120X on belt grinder, then remove the coarse scratches with 240 and then 400. After that on a palm sander with micarta backing going back to 240 to remove grinding irregularities then finished 400 before HT. I use only D2 and RWL34, therefore there is no scale after HT. I go back to 240 after HT and go up to 1000 or 2000 sanding perpendicular to previous grit.
 
It would, my air hardening steels have covered with HT foil therefore no scale forms on blades. There can be a thin laver of decarb but it is easily gone with 240 grit. On carbon steels you will have a hard scale after HT if you are not using a scale protection like PCB. This scale can be difficult to remove and you might want to start from 120 or 80 after HT. Therefore before HT it will be enough to go with 120 of 240 I guess... I have done some carbon blades and it was pretty easy to sand them compared to the D2 beast, so even with scale the sanding process will take less time with those steels you have mentioned...
 
There is a metallurgical reason for having a smooth surface before HT. Any deep scratch, or any sharp inner angle, can be the source point of a stress riser. Sanding to a smooth finish and stress relief pre-HT will help avoid these things.

Stacy
 
bladsmth said:
There is a metallurgical reason for having a smooth surface before HT. Any deep scratch, or any sharp inner angle, can be the source point of a stress riser. Sanding to a smooth finish and stress relief pre-HT will help avoid these things.

Stacy
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I had to learn this the hard way.
I use mostly 10xx steel, since I started going to 180 before HT I havent had any more trouble.
 
Stress risers for one but trying to get scale out of a scratch left from a 60g belt is a pain in the everything. Not to mention it saves a lot of time when you do your hand sanding. Non ht'd steels are alot easier to get the finish you want.
 
My carbon steel blades are ground to an edge thickness of .040"+/-, so they have to go back to the grinder after HTing. For this reason, I see no point in hand sanding or polishing my blades before HT. I get a nice even 120 grit belt finish before HT, then take .015" +/- off each side after HT, with no probs.
 
There's also salt bath (I think that's correct terminology) where the blade is heated in melted salt, and is temp controlled, but when you quench it, it has little to no decarb or scale, because it is in an oxygen free environment. Phillip Patton showed me his. Maybe someone who has one can give more information on how they work.
 
I go as far as a 240 grit prior to HT. At that point the knife has a basic finish plunge lines are crisp, clean and deep scratches that could cause stress risers are removed. After HT then I clean up any residual scale with the 240 and then go onto finishing with 400 grit for satin finishing, or higher to prepare for buffing up a mirror polish.
 
I like to go all the way to 600 grit and buff before heat treat.I also grind to .020 lines and I don't have warping issues. Maybe it's just a personal preferance. Some scratches don't show up until they hit the buffer, I would rather take care of it before HT. Saves belts, time and stress both on the blade and your mind. I mostly use D-2 and A-2 so I can't speak for 10xx steels or any other steel for that matter. By the way I 'm trying my first hamon on 1095. Good or bad I will post some pics.
 
It depends on the finish I want on the finished knife. If I'm going for a modern mirror or satin finish, I just go to 120 grit before heat treat since I'm going to be sanding and polishing off any of the effects from heat treating. If I'm going for an antique look on a knife that looks like it came from the 1800's, I'll intentionally pound a little scale into the blade while forging and chemically remove it before sanding - gives it an 'olde' pitted look. Then, I'll sand up to 400 grit and polish with emery before heat treating. After heat treat, all it needs is a little soap and water before adding that 'old time patina' to the finish.
 
Don Hanson III said:
My carbon steel blades are ground to an edge thickness of .040"+/-, so they have to go back to the grinder after HTing. For this reason, I see no point in hand sanding or polishing my blades before HT. I get a nice even 120 grit belt finish before HT, then take .015" +/- off each side after HT, with no probs.
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Don,

Do you go back to the 120-grit belt to take the 0.015" off each side after HT? What belt brand/type do you use for this? How high in the grits do you go with belts after HT?

Mike
 
Don Hanson III said:
My carbon steel blades are ground to an edge thickness of .040"+/-, so they have to go back to the grinder after HTing. For this reason, I see no point in hand sanding or polishing my blades before HT. I get a nice even 120 grit belt finish before HT, then take .015" +/- off each side after HT, with no probs.
Click to expand...

Same here.
 
I've been doing a lot of silpjoints lately and they are so much easier for me to hand sand after I get them out of the oven if they have been taken to say 600g before HT. Especially D-2 for some reason. But there isn't as much scale when you HT in Foil.
 
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