Does Laser Cutting Harden Mild Steel?

This might help:

http://www.technologystudent.com/equip1/heat2.htm

Hi Mikerho,

Thanks for the speedy reply. I had a look at this site before I posted my question. My understanding is that the laser-cutter would not quench the parts after cutting. If air-cooled, would any case-hardening take place ?

Regards

DesEng

This is not my area of expertise. Why do you want to know? Are you hoping for hardening on the edges?

As with the explanation in the link carbon is added from a torch set to burn rich. The edge cut with a laser can acquire carbon from the air, oils or preservatives on the surface of the steel.

Tony - That's good info. You'd never be able to get a reading on something that tiny with any kind of indenter type hardness tester.

A couple more thoughts. A little work under a 10X microscope with a #6 cut swiss jeweler's file or maybe a used dental rasp like they us to clean out root canals might show the difference to the trained eye between hard and not hard. Cheaper than the metallurgical lab.

Another thought that might take some experimenting. Change the power and/or focus on the laser and have it temper or even anneal the hardened edge on a second pass. This is where the file test on the hole can come in handy in determining the right laser setup.

Ed Weldon

Anyone involved with surface chemistry will tell you that within minutes of the most fastidious cleaning, any surface exposed to air will become coated with a thin film of carbon. I would suggest that laser cutting might alloy this carbon layer and form a very thin layer of "case hardened" material.

I am not a surface chemistry professional.But i am curious, is it due to the diffusion of Oxygen atoms from atmospheric air to the surface of molten steel and subsequent formation of oxytrides (another form of nitrides)?

No, it is because Iron has an affinity for Carbon and the iron/carbon alloy system has two distinct crystal configurations depending on temperature. at high temps, steel is Face Centered Cubic, which means that you have 8 iron atoms configured as the corners of a cube, and there is a carbon atom centered on each face of the cube. but as steel cools, it wants to switch to Body Centered Cubic and has just one carbon atom, centered in the middle of the volume of the cube. But if you cool steel quickly, it doesn't have time to get rid of the extra carbon atoms and that strains the atomic links. That strain is what makes Steel harder/stronger.

Any time you use enough heat to cut you will get a chemical and mechanical change locally. If just trying to satisfy your curiosity then take a fine file and stroke the burned edge and compare it to a raw edge. Sound should tell if hardened. Plasma and laser are much gentler on the steel but waterjet is the way to cut without charring the edges.

most laser cutters use a jet of high pressure gas, many use nitrogen (n2), some use CO2, to help blow the molten metal away from the kerf. in the plasma stream, N2, becomes ionized and becomes atomic Nitrogen. Atomic nitrogen hardens steel as well, not as much as carbon does or as deep. and of course if it is CO2, then that just adds carbon to the molten surface. a micro-hardness survey (either vickers, or my perference for something this small which is Knoop, the indenter for knoop is wider but not as tall so you can stack the indentions closer together vertically which makes surveying something this thin much easier.) of a polished section across a HAZ will show how hard and how deep it is.