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Changes in mild steel hardness after cutting

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ggeh
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Changes in mild steel hardness after cutting

Postby ggeh » Sat Dec 10, 2016 7:47 pm

I cut some brackets out of 1/2" steel and i need to clean up the holes with a boring bar but the steel is so hard after plasma cutting its next to impossible to machine. Is there away that i can take the hardness back out of the steel so it can be machined?

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Re: Changes in mild steel hardness after cutting

Postby Gamelord » Sat Dec 10, 2016 10:44 pm

You can heat it with a torch, let it cool naturally and it should take the hardeneing out. Normally the hardened area is only about .01 of an inch. I drill it out no problem but you need a good bit and good cutting fluid. Depending on the size of the hole, you could use a deburring bit on them with an air grinder to take the hardened area off and then machine it. something like the image below, I have used these and they work excellent.
https://sc01.alicdn.com/kf/HTB1XbdZJpXX ... ts-PCB.jpg
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Re: Changes in mild steel hardness after cutting

Postby beefy » Sun Dec 11, 2016 3:52 am

Gamelord,

I could be wrong but I don't think you can anneal nitrided steel. The 80% nitrogen in the air causes the surface of the steel to be chemically changed, it's not just a restructuring of the crystals due to heat treatment, which normally requires a high enough carbon content (which mild steel doesn't have).

I picked this up off the internet:

Surface hardening through nitriding is due to the formation of aluminum-chromium¬vanadium and iron azides in the steel and is therefore the result of a chemical reaction and not of a structural transformation of the steels, through heat treatment, as happens in all the other surface hardening processes (casehardening, surface hardening, etc).

Happy to be proved wrong, that will mean I've learned something.

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Re: Changes in mild steel hardness after cutting

Postby Gamelord » Sun Dec 11, 2016 12:41 pm

You may be right Beefy, someone with more knowledge on this may need to chime in. I know that heating it up helps with drilling after it has been cut, not sure wbout machining though.
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Re: Changes in mild steel hardness after cutting

Postby little blue choo » Sun Dec 11, 2016 12:46 pm

Over my head. If I cut a hole that I plan on drilling later I usually just mark it with the plasma and then drill the hole as usual.

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Re: Changes in mild steel hardness after cutting

Postby beefy » Mon Dec 12, 2016 12:09 am

Gamelord wrote:I know that heating it up helps with drilling after it has been cut, not sure wbout machining though.


Then I consider I have learned something.

Prior to hearing that, I would have just assumed it couldn't be done. Now I'll give it a try when the need arises.

Rick,

I do the same. I even made a special microcontroller board to interface between my cnc controller and the plasma cutter for this very purpose. I can get the cutest little plasma torch "centre punch" marks in the top of the steel now. I come along afterwards and put the tip of a real centre punch in them and voila, I have a very accurately located big fat centre punch mark.

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Re: Changes in mild steel hardness after cutting

Postby SeanP » Mon Dec 12, 2016 2:21 am

Good tip Keith, I was just drilling from the mark, but that sounds better still.

beefy wrote: I come along afterwards and put the tip of a real centre punch in them and voila, I have a very accurately located big fat centre punch mark.
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Re: Changes in mild steel hardness after cutting

Postby jimcolt » Mon Dec 12, 2016 12:53 pm

The hardness on the cut edge is nitriding, which is a case hardening caused by the nitrogen content in the air used for the plasma gas. The hardening is in the range of .006" to .010" thick, and can be removed with a couple of passes with a flap disc on a grinder. Annealing the edge will not remove the nitride hardening. Industrial plasma systems are able to use pure oxygen as the plasma gas....which eliminates the nitride hardening effect. Do not attempt to use pure oxygen on a torch designed for air cutting.....it is not safe. Jim Colt Hypertherm

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Re: Changes in mild steel hardness after cutting

Postby ggeh » Tue Dec 13, 2016 4:22 pm

Can you explane how to mark the holes with plasma?

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Re: Changes in mild steel hardness after cutting

Postby BTA Plasma » Tue Dec 13, 2016 4:24 pm

Simple Gary use the drilling operation in sheetcam and a tool with around a .4 pierce delay. You can set min and max hole size in your drilling operation.

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Re: Changes in mild steel hardness after cutting

Postby ggeh » Tue Dec 13, 2016 5:03 pm

I haven't got my pc back yet from being repaired but I'm trying to follow you with a copy I downloaded on a different pc. In the drilling operation it says no tools available in this copy. Are the tools there in my pc? Does this leave just a mark on the steel or does it burn a hole through? or should I just wait till I get my pc back?

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Re: Changes in mild steel hardness after cutting

Postby BTA Plasma » Tue Dec 13, 2016 6:07 pm

You should be able to use any tool. It will leave a dimple or pierce through depending on the actual pierce delay you have in the tool you use.

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Re: Changes in mild steel hardness after cutting

Postby ggeh » Thu Dec 15, 2016 9:47 am

Thanks to every one for your input. I haven't tried anything yet but I will when I get set up again.

Thanks again for the help, Gary

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Re: Changes in mild steel hardness after cutting

Postby RyanS » Tue Sep 12, 2017 6:05 am

Does the edge harden regardless of whether the plasma has a water table or a downdraft setup? I would guess the water has a quenching effect.

What about 1045 and similar? Does the edge harden even deeper, considering those steels can be through hardened due to higher carbon content, whereas mild steel can only be case hardened.

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Re: Changes in mild steel hardness after cutting

Postby jimcolt » Tue Sep 12, 2017 7:26 pm

RyanS wrote:Does the edge harden regardless of whether the plasma has a water table or a downdraft setup? I would guess the water has a quenching effect.

What about 1045 and similar? Does the edge harden even deeper, considering those steels can be through hardened due to higher carbon content, whereas mild steel can only be case hardened.



Most of the harder grade steels (such as Hardox) will actually become softer. Standard mild steels will have a chemically "case" hardening caused by the nitrogen content in air (air plasma). An oxygen plasma (do not use oxygen in any plasma rated for just air or nitrogen.....oxygen plasma torches require a special gas delivery system and a liquid cooled torch).

If you need to bore holes after cutting us solid cobalt or carbide reamers or drills for best results. On outside edges remove the hardening )only .006" to .010" thick) with a few grinder passes.

Water in contact with the edge can cause nitrogen embrittlement (hardening and cracking).

Jim Colt Hypertherm


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