Will Plasma Screw up Knife Steel?

T

The Mighty Ginsu

Joined
Aug 11, 2016
Messages
251
I was thinking about using a plasma cutter to do major trimming before taking jobs to the belt grinder. It's really fast, and it would save wear on belts. But I wondered: will it mess up the steel? Would it leave me with hard areas that would negate whatever advantage I got from slicing steel off quickly?
 
yes, plasma will screw up knife steel.

I use plasma to break sheet down into wide strips that will fit a big horizontal bandsaw to cut into blanks. The HAZ is up to 1/8". So long at that HAZ doesn't get near the actual knife in the blank you're fine. But, the skin on that HAZ is very hard and will wear belts and tooling. I grind the worse of it off with an angle grinder before further processing to preserve things like bandsaw blades.
 
I was told by a very experienced knife maker that plasma is OK for S35VN and S30V. It is NOT OK for M390, although he did not say why.

Tim
 
So it sounds like I can blast the knife's basic shape out, run an angle grinder over it briefly, and go to the belt grinder!
 
tim37a said:
I was told by a very experienced knife maker that plasma is OK for S35VN and S30V. It is NOT OK for M390, although he did not say why.

Tim
Click to expand...

That is the kind of story that gets repeated and becomes knifelore. Just because someone well known said it doesn't make it so ( including me). There is no difference in steel types as far as HAZ zone goes. The higher the alloying, the more it may air harden, but the decarburized zone is there for every steel. The thinner the sheet, the faster the cut, and the less HAZ, but the knife blank perimeter should always be ground back 1/8".
 
I run a plasma and am surprised how little it's used or an excepted tool. I have not done any official tests but would wager a bet that the steel right on the other side or close to the skin is fine. Now befor we go off let's talk about it. I cut a lot of stuff with my plasma and if it's running good and I'm running it at the proper speed there is very little haz. But all HAZ is is a heat affected zone. But just becaus the steal is heated does not mean it's bad. It's amazing how little the steel is heated actually. It's no where melting or even red, the plasma arc is very focused and it removes anything it heats to a melting temp. The skin is the boundary between where the melted steel was and the cold steel is. This thin skin is very hard because it's been oxidized. I would think with a fancy steel like stainless the steel would need to be heated hot enough to change it to be considered a bad area. How hot is this, I don't know but the steel in the haz is heated into the hundred degs not thousands of degs. The closer you get to the actual cut surface/skin the hotter that steel got. And as long as you run the plasma right very little heat is transferred into the steel. If you run slow or pause for a sec that area will heat up and over heat the area and defently need to be removed. Now with all this being said is it worth chancing it NO. How I do it is use a template. If your free hand cutting and fallowing a drawn profile then leave a good amount. But when I use a template I leave a little extra meat. It's easy to grind off with the grinder and ensures fresh clean steel.
 
Plasma is pretty cool; no doubt about it. Another nice tool for cutting stuff without getting it too hot: the dry cut saw.
 
My gas sheald on my plasma cuts .160 from edge of cut to sheald. I just run my plasma around the template and it automatically cuts the blade bigger. I was going to make new templates to leave less to remove but honestly it takes less time to grind away the extra then to make a new template(s). But my gole in the future is to transfer my templates to 1/4" thick stock that's negatively cut. This way I just lay the template on the stock and clamp it down. Then run the plasma around the inside and done.
 
In the early days of EDM we found the best thing we could do was to teach the customers to anneal after plasma. The sub-critical anneal would soften the plasma hardened HAZ. Avoids possible cracking and eliminates grinding to remove HAZ, and stops the waste !
1200 F for 2 hours
 
I'm not sure how plasma "ruins" blade steel. It does create a decarburized edge, and there is certainly a heat affected zone, and regardless of the materials ability to air harden that edge is hard on tools and abrasives (we cut and machine literally tons, of A36, even in that steel, the edges cause significant tool wear) but what exactly is believed to be going on in that 1/8" profile zone that is thought to "ruin" it?

All but the very edge - .010-.030" should be able to be annealed and re-heat treated.

I don't use it very often because I can use a laser. If I had access to a water jet I wouldn't use the laser. But I've never seen evidence of an 1/8" deep decarb, even in 1" or 1.25" cutting with a 260 amp head. A hand plasma torch and templates, or if you're really ambitious, a pattern tracing jig, is probably the least expensive (in consumable cost, startup cost, vs time spent per blank cut) method available to the typical home shop for blanking out knives. As well as being incredibly useful for other projects.
 
kuraki said:
I'm not sure how plasma "ruins" blade steel. It does create a decarburized edge, and there is certainly a heat affected zone, and regardless of the materials ability to air harden that edge is hard on tools and abrasives (we cut and machine literally tons, of A36, even in that steel, the edges cause significant tool wear) but what exactly is believed to be going on in that 1/8" profile zone that is thought to "ruin" it?

All but the very edge - .010-.030" should be able to be annealed and re-heat treated.

I don't use it very often because I can use a laser. If I had access to a water jet I wouldn't use the laser. But I've never seen evidence of an 1/8" deep decarb, even in 1" or 1.25" cutting with a 260 amp head. A hand plasma torch and templates, or if you're really ambitious, a pattern tracing jig, is probably the least expensive (in consumable cost, startup cost, vs time spent per blank cut) method available to the typical home shop for blanking out knives. As well as being incredibly useful for other projects.
Click to expand...

The concern is areas where the steel got hot enough to dissolve carbides. These areas can become coarse grain. While it is common sense to remove any burnt steel, it might be less obvious that the heat condition of the steel can be effected some distance from the cut. Probably not 1/8", but I think 1/16" wouldn't be hard to do. Folks can use plasma all they want, but they should be careful to stay away from the finished knife inside the blank.

Anyone who thinks all the heat remains in the material blast out of the cut and a very thin area directly adjacent should cut some material with water on it and see how far back it boils off. I think you're probably getting it red hot at least .020" away from the cut. And that's on a well adjusted setup. Just my opinion. (edit: based upon using a Hypertherm Max 45 by hand in materials up to 1/4")

Red hot steel is not going to look red hot through a #5 lens and against a very bright plasma stream.

If you're using a smaller plasma cutter and cutting by hand, I think there is opportunity for steel to be damaged perhaps even 1/8" in spots. By damaged I don't mean burnt or decarb, just a change to the heat condition that can effect grain. I wouldn't take any chances with it, so I leave a 1/8" from my part to be sure.
 
Last edited:
With the EDM we were dealing mostly with tool steels . If the plasma ,or other , cutting will form martensite ,then it must be tempered or it will most likely crack with high carbon steel !!
 
We ran some tests with a laser cutter and the HAZ was bad. Even on 1084, the cold mass of the rest of the steel sucked heat out of the hot edge fast enough to harden it. 1084, A2, and 154CM were all affected. Water jet or bandsaw for me.
 
Nathan the Machinist said:
The concern is areas where the steel got hot enough to dissolve carbides. These areas can become coarse grain. While it is common sense to remove any burnt steel, it might be less obvious that the heat condition of the steel can be effected some distance from the cut. Probably not 1/8", but I think 1/16" wouldn't be hard to do. Folks can use plasma all they want, but they should be careful to stay away from the finished knife inside the blank.

Anyone who thinks all the heat remains in the material blast out of the cut and a very thin area directly adjacent should cut some material with water on it and see how far back it boils off. I think you're probably getting it red hot at least .020" away from the cut. And that's on a well adjusted setup. Just my opinion. (edit: based upon using a Hypertherm Max 45 by hand in materials up to 1/4")

Red hot steel is not going to look red hot through a #5 lens and against a very bright plasma stream.

If you're using a smaller plasma cutter and cutting by hand, I think there is opportunity for steel to be damaged perhaps even 1/8" in spots. By damaged I don't mean burnt or decarb, just a change to the heat condition that can effect grain. I wouldn't take any chances with it, so I leave a 1/8" from my part to be sure.
Click to expand...

I don't disagree with any of that. A change in the heat condition and grain size can be corrected through normalization and heat treating was my only point. Unless you don't think it can? I would defer to your knowledge on that subject.

All I'm saying is whether you correct the issue through normalization or by grinding the affected area away, is simply two ways to skin a cat.
 
daizee said:
We ran some tests with a laser cutter and the HAZ was bad. Even on 1084, the cold mass of the rest of the steel sucked heat out of the hot edge fast enough to harden it. 1084, A2, and 154CM were all affected. Water jet or bandsaw for me.
Click to expand...

No question about it. That edge will be hard. Whether it's hard enough, deep enough, to make a decision like choosing bandsaw vs laser is my question. I would much prefer to deal with removing a hard edge and a couple normalization cycles than by sawing out profiles if given the choice.
 
make sure you have the right tip on your cutter and having the right settings. the settings for cutting 3/8" AR plate probably are not the best for cutting 1/8" 1084 or O1.
scott
 
You must log in or register to reply here.
Back
Top