[paul cotter (OP)]

If one wants to cut regular grades of stainless steel options are limited, in comparison with carbon steels. A grinder will eventually do the job by abrading rather than burning. An oxy/acetylene torch is virtually useless. I don't know if the same applies to ferritic and martensitic stainless steels.

[SeanB]

Plasma cutter works on all grades of stainless steels, using the available oxygen in the compressed air, and the electric arc, to cut it with ease. Lot faster than the grinder, and cheaper to run than the gas axe as well, just needs a lot of compressed air, and power for the arc. even works on other metals, including things like titanium and aluminium, and even copper, though the cut and finish can vary, it depends a lot on flow rates, cutting speeds and arc power.  Laser cutting simply provides the energy input using a laser of some form, generally IR to provide the energy to melt the material, and the air jet does the rest.

[paul cotter (OP)]

That's interesting, i'm a bit behind the times and have never used a plasma cutter. What I wanted to understand is why it's not possible to "burn", ie oxidise the iron content of stainless in an exothermic process. Most stainless steels that I have experience of contain ~75% iron-how do the nickel and chromium prevent an otherwise exothermic process from happening?

[Bored chemist]

It's an interesting question and I don't know the answer.
The melting ranges of stainless- and mild- steel pretty much overlap.

Nickel is easier to melt than iron, and harder to oxidise, but the melting point of the oxide is lower than for iron.
Chromium is harder to melt than iron and it is easier to oxidise than iron, but the melting point of the oxide is higher.

(with some hand waving added because the oxides aren't well defined and they will be mixtures anyway)

[SeanB]

The oxide layer is the protection, it adheres well, and prevents the further oxidising of the iron in the alloy, so providing protection. Just like aluminium, which is incredibly reactive ( the good old gallium trick, or for more show the mercury blob and a scratch to penetrate the oxide, growing oxide flowers), the oxide keeps the metal non reactive, at least till you remove the oxide by some method. Simplest is to wear it off, which then allows a short period for oxide regrowth, or expose to a chloride atmosphere, which slowly reacts with the oxides to form metal chlorides, which are soluble in water, and thus the metal slowly erodes away as it reacts.

Or you can create an electrochemical cell, most common when welding stainless steel, which changes the composition of the alloy, allowing electrochemical cells to form. You prevent that in welding by both the choice for the filler material, using a rod that has higher amounts of nickel and chromium, amongst other additives, to allow the cooled puddle to be the same electrochemically as the parent metal, and also by annealing the whole item, so that the stress from the rapid cooling that takes place, as the puddle leaves the joined area, is relieved, as the tress is also a source of electrochemical cell.

Take a piece of annealed stainless steel, and either drill a hole in it, or simply crease it, and expose in an environmental chamber, and you will see the preferential corrosion taking place slowly at the hole and the bend, plus at the cut edges, if they were not there at annealing time.

[paul cotter (OP)]

Oxide layers are important in corrosion scenarios but I doubt their relevance to molten metals. Heat some stainless till molten with a cutting torch- when you pull the oxygen paddle to burn it just cools the molten puddle as though it were an inert gas.

[jeson]

With sufficient quantities of nickel, stainless steel remains austenite at room temperature, creating the austenitic steels. They are nonmagnetic and cannot be heat treated for through hardening like carbon steels because the phase transformation to martensite does not occur in these alloys.

[paul cotter (OP)]

Yes quite correct(if I remember correctly!). What I want to know is why the resulting alloy fails oxidise exothermically at elevated temperatures in either air or oxygen eg a torch or grinder are ineffective in cutting such materials.