'Splain stainless steel to me

[Chik]

The thread about convection oven and induction hobs reminded me of something I've been wondering since the spring, which is when induction hobs entered my life. Someone please edumacate me about stainless steel.

We have All-Clad cookware that we've had since the early 90s. All of them work on the induction hob except for one piece: the pasta pot. I confirmed that it's not ferromagnetic because a magnet doesn't stick to it.

I realise that stainless is an alloy and comes in a variety of mixes, but doesn't it always include a fair bit of iron? I can prolly Google this, but where's the fun in that?

Also, what I find striking is that All-Clad used different alloys for different products or simply at different production lots. Despite their claim about their construction being very efficient in terms of heat conductivity, the pasta pot in question was always a bit shit in that regard -- it took ages for the water to boil whether over gas or electric hobs.

Anyway, I'm just mystified by this non-ferromagnetic stainless steel piece.

[Scott G.]

[snip]
Anyway, I'm just mystified by this non-ferromagnetic stainless steel piece.

Made from steel salvaged from a Soviet SSN, have you checked for radiation ?

[Kev inc.]

Is your pasta pot aluminum cored?

[thollandpe]

Two flavors of stainless steel are Ferritic and Austenitic.

Ferritic has a body-centered cubic (BCC) microstructure, like iron. So it's magnetic. No nickel in the alloy so they're usually cheaper.


Austenitic has a face-centered cubic (FCC) microstructure, like aluminum. Non-magnetic, pretty much. Most stainless is this: 304 or 18-8 (18% Cr, 8% Ni), 316 (more Ni).

[Chik]

Is your pasta pot aluminum cored?

They are all 3-layered construction, but I can't remember what the middle is. Prolly aluminium.

Two flavors of stainless steel are Ferritic and Austenitic.

Ferritic has a body-centered cubic (BCC) microstructure, like iron. So it's magnetic. No nickel in the alloy so they're usually cheaper.


Austenitic has a face-centered cubic (FCC) microstructure, like aluminum. Non-magnetic, pretty much. Most stainless is this: 304 or 18-8 (18% Cr, 8% Ni), 316 (more Ni).

Interesting. Thank you.

As an aside, my Reynolds 931 tubing is ferromagnetic. Reynolds say that it's 17% Cr, 4% Ni, and a bit of Cu: https://www.reynoldstechnology.biz/m...s/steel/s-931/ Do you reckon it's ferritic?

[thollandpe]

As an aside, my Reynolds 931 tubing is ferromagnetic. Reynolds say that it's 17% Cr, 4% Ni, and a bit of Cu: https://www.reynoldstechnology.biz/m...s/steel/s-931/ Do you reckon it's ferritic?

Austenite can be made slightly magnetic by cold working. You're stretching my decades-old metallurgy memory (yes I should work on my flexibility) but I think you can heat treat austenite into martensite.

That link says, "931 has a 17% Chromium, 4% Nickel, and Copper content, and is designated a stainless steel. When cold-worked and then aged/welded, this alloy cools to form a fine-grained, tough martensitic structure (precipitate) which offers builders ductile but strong fabrications."

Martensite is a body-centered tetragonal (BCT) microstructure which is a distorted BCC so I think that's why it's magnetic.

PS Will your bike heat up if you put it on your induction cooktop?

[Mark Kelly]

Austenite can be made slightly magnetic by cold working. You're stretching my decades-old metallurgy memory (yes I should work on my flexibility) but I think you can heat treat austenite into martensite.

That link says, "931 has a 17% Chromium, 4% Nickel, and Copper content, and is designated a stainless steel. When cold-worked and then aged/welded, this alloy cools to form a fine-grained, tough martensitic structure (precipitate) which offers builders ductile but strong fabrications."

Martensite is a body-centered tetragonal (BCT) microstructure which is a distorted BCC so I think that's why it's magnetic.

PS Will your bike heat up if you put it on your induction cooktop?

I think it easiest to explain it in terms of Curie temperature.

All ferromagnetic materials have a temperature above which they lose their ferromagnetism and become paramagnetic, discovered by Pierre Curie and named in his honour*. For example in iron this is about 770 oC so if you get ordinary iron red hot it loses its ferromagnetism. It also changes crystal structure to austenite: the two are related, the crystal structure affects the way the electron spins can interact which determines the Curie point. When the iron cools the crystal structure reverts back and it regains its magnetism.

Room temperature austenitic alloys have components (Nickel) that stabilise the austenite so it remains in that state when cooled. Usually you can't heat treat an austenitic alloy because there isn't any phase change on cooling.

If you work harden them the crystal structure changes again.


Caveat: metallurgy has probably changed since I was an aeronautical engineering student in the 70s.





* It has to do with the way electron spins couple to create magnetic moment. Heat disrupts this; I think that's due to electron phonon interaction.

[echappist]

Two flavors of stainless steel are Ferritic and Austenitic.

Ferritic has a body-centered cubic (BCC) microstructure, like iron. So it's magnetic. No nickel in the alloy so they're usually cheaper.


Austenitic has a face-centered cubic (FCC) microstructure, like aluminum. Non-magnetic, pretty much. Most stainless is this: 304 or 18-8 (18% Cr, 8% Ni), 316 (more Ni).

Thanks for the post.

Were it not for your post, I wouldn't even begin to suspect that austenitic steel is largely non-ferromagnetic. Blind spots and assumptions. I may very well have bought stainless thinking all of it are compatible with induction.

[hampco]

A former employer once told me: "There are two things to know about stainless steel: it aint stainless and it aint steel."

[e-RICHIE]

A former employer once told me: "There are two things to know about stainless steel: it aint stainless and it aint steel."

and, according to my opinion, the most overrated and misunderstood material to have ever been added to the menu.

[htwoopup]

A former employer once told me: "There are two things to know about stainless steel: it aint stainless and it aint steel."

Ha, when I was working summers at a boat builder that built 12 meter America’s Cup racing sailboats the grumpy shop foreman made us polish and polish and polish stainless fittings….”They call it stain-less not stain-free, boy”. I wish I could type the accent and anger with which he said it.