machining through HAZ on laser cut 304 stainless?

HI guys,

So I goofed. I sent a bunch of parts off to the laser cutter without remembering to delete the outlines of some of the holes & slots we normally machine in after cutting. This is in .190 304 stainless. We originally designed the part for post laser machining because we figured the laser cut edges would eat tools. Simpler to just machine the slots and c'sinks afterward. (We thought)

Except on this run, where all the cuts are there (lasered) it doesn't seem to be quite as horrible as I would expect.

Anybody have any experience milling or cham-milling through the HAZ edges on a laser cut in 304? I haven't got enough cycles yet to see if it's really effecting my tool life one way or another, but it's certainly not destroying cutters the way I expected it to. (C'mon, it's 304. Every time you look at it funny, it does something nasty to the tooling. You have to believe that a short, tight heat gradient would make it especially ugly. Except it doesn't seem to have.)

So I'm looking for experience and wisdom in doing something I thought was a bad idea. Maybe I was wrong?

Regards,
Brian

No experience machining but do know that the HAZ with a high powered laser can be very thin. Cut with nitrogen and thin enough to not see any discoloration on the top surface, hard to see on the bottom.

The answer is pretty simple. You cannot harden 304 stainless by heat treating. So the laser HAZ is the same hardness as the rest of the piece.

Article: Stress relieving heat treatments for austenitic stainless steels

Larry

L Vanice said:

You cannot harden 304 stainless by heat treating. So the laser HAZ is the same hardness as the rest of the piece.Larry

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Correct. If anything the heat affected zone will stress relieved and softer.

Hi Guys,

Now that I think of it, you're right: you can't harden the stuff via heat. I started out on Titanium, so where most guys liken Ti to grouchy stainless? I sort of think of stainless as slightly less grouchy Ti. And the HAZ on Ti damned well *will* eat tools, (TiN2 anybody?) so I just carried that bit of (wrong) thinking along with me.
This has turned out to be a very instructive screwup.

Many thanks,
Brian

L Vanice said:

The answer is pretty simple. You cannot harden 304 stainless by heat treating.

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That's simply not true. This isn't heat treating. Laser cutting isn't a heat treatment. Its thermic. Molten metal, you take it back to molten, which has to be blasted out of the cut. Depending on the assist gas. Oxy, Air or N2 What do you think happens when some of the gases react with the 18% molten Chromium?

To the O.P, I think your experiencing what's called clean cut in the laser industry. High pressure Nitrogen to blast the cut. You can specify that. Its typically about 30 bar, where Oxy or Air is sub 8 bar for an oxy thermic reaction. Nitrogen does nothing to excite the cutting action, Its all about laser power. N2 only blast the cut clean.

machtool said:

That's simply not true. This isn't heat treating. Laser cutting isn't a heat treatment. Its thermic. Molten metal, you take it back to molten, which has to be blasted out of the cut. Depending on the assist gas. Oxy, Air or N2 What do you think happens when some of the gases react with the 18% molten Chromium?

To the O.P, I think your experiencing what's called clean cut in the laser industry. High pressure Nitrogen to blast the cut. You can specify that. Its typically about 30 bar, where Oxy or Air is sub 8 bar for an oxy thermic reaction. Nitrogen does nothing to excite the cutting action, Its all about laser power. N2 only blast the cut clean.

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If anything the back edge of the cut may be a bit harder. Here the N may react with the molten steel to "nitride" the surface. It's worth it for the operator to get all parameters spot on when laser cutting, as even a slight amount of dross on the back can really suck to remove. It can be very hard and not easy to remove with a file.

Well cut its bloody thin, i had a very routine job for a few years tapping M6 holes in 4mm laser cut 304, use to laser the hole in and with just low cost but decent tin coated tap the tap life was not much worse than on drilled holes in 304.

The nitrogen will very much react with certain alloys in some metals and make a hard to very hard skin, but on laser its incredibly thin, yes the dross is a whole nother animal and is brutal, but to some degree you do get some nitriding action going on, just with good settings very very little.

One trick worth mentioning orientation can greatly effect tool life, you very much want the blanks cut from the same side as your going to post work em, that bottom dross edge is the nasty bit and you want to be cutting - push it off, not go in though it, makes a vast difference to tool life if your opening out laser cut holes!

machtool said:

Its typically about 30 bar,

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30 bar? where do you get that? We cut 1" SST with only 20 on 6kW fiber. For 3/16" like in OP, only 12.

Yeah 30 seams kinda high, we were normally more around 14 bar on circa 3/8" stainless. Mind you thats a fair old bit of gas coming out of a 3mm nozzel!

30 bar of nitrogen and you probably would not even need the beam on to cut :-)