Stainless steel metallurgy question

[saltyrob]

Hi,
I have been checking to establish if a length of exhaust pipe is dull stainless steel or mild steel using magnet . I thought that a magnet was not attracted to stainless steel , however as a check noted that it was not attracted to stainless propshaft but could pick up cutlery stamped as stainless steel . The exhaust pipe attracted the magnet.

Would be grateful if you could explain, as thinking this may be down to grade of stainless steel etc.

Many thanks

Rob

 

[coopec]

"All stainless steel is magnetic except austenitic stainless steel which is actually 300 series stainless such as 304 and 316. However, 300 series stainless is non-magnetic only after it is freshly formed. 304 is almost for sure to become magnetic after cold work such as pressing, blasting, cutting, etc".Nov 7, 2019

The Magnet Test for Stainless Steel is Not Accurate

 

[rogerthebodger]

As Clive says its generally austenitic stainless steel that is non magnetic to the same extent a mild steel but

The stainless steel that cutlery is generally made from martensitic stainless that is magnetic as is duplex (a combination of austenitic and martensitic).

Also stainless steel is as it says stainless not stainfree of totally rustless at it can rust and stain but not to the extent as other steels or ferrous metals.

Its the oxide layer of chrome that protects the under lying metal from corrosion in a oxygen environment but it still can corrode from other means mainly galvanic corrosion.

Vyv_cox is the real expert on the subject.

 

[sgr143]

Fully austenitic stainless steel (typically 18%Cr,8%Ni, balance Fe) has the face-centred cubic (FCC) crystal structure and is non-magnetic. The high chromium content is what makes it stainless, as it provides a thin, non-conducting film of chromium oxide on the surface. The nickel is added as it strongly encourages the FCC structure, which pure iron transforms to only above 910C; chromium encourages the low temperature body-centred cubic (BCC) structure. The FCC structure is more readily deformable to high strains than the BCC structure - i.e. it's ductile. But it has a relatively low yield stress.

If you want a hardened steel, the simplest way is to heat it up high enough for it to be FCC, dissolve a little carbon in it, and cool rapidly ("quench") to a temperature at which the BCC structure is favoured. The BCC structure can't hold as much C as the FCC structure, but at low (room) temperature the C can't get out to form carbides. It is stuck in place and puts lots of local distortions into the crystal structure, which makes it very hard and brittle. You can then gently re-heat ("tempering" ) to ease the carbon out controllably, to get a good mixture of strength and ductility.

So to get a hardenable stainless steel, you have to back off on the nickel content, and add carbon, so that it is possible to heat the steel into the FCC region and then quench it to BCC. If it's FCC at all temperatures you can't do that. A typical composition is around 12%Cr, which is only just enough to make it "stainless". It is BCC, and thus magnetic.

There are steels that are a mixture of FCC and BCC ("duplex") - if done that way intentionally they can processed have very good properties and will be a bit magnetic as there's some BCC in there.

Steve (retired - hooray! - materials scientist)

 

[pcatterall]

Hi,
I have been checking to establish if a length of exhaust pipe is dull stainless steel or mild steel using magnet . I thought that a magnet was not attracted to stainless steel , however as a check noted that it was not attracted to stainless propshaft but could pick up cutlery stamped as stainless steel . The exhaust pipe attracted the magnet.

Would be grateful if you could explain, as thinking this may be down to grade of stainless steel etc.

Many thanks

Rob

I have found that some 'stainless' bits on the boat have a slight magnetic attraction, and some of course non. The attraction of 'ordinary' steel is usually much stronger.

 

[saltyrob]

Many thanks for info , always appreciate the depth of knowledge in this forum.

Rob

 

[vyv_cox]

Cutlery and various other items are usually made from a 5% chromium stainless steel, 400 series, maybe 413. This is attracted to a magnet.

Austenitic 300 series is nominally non-magnetic but there can be some retained ferrite that gives slight magnetic attraction. Plastic deformation makes this effect stronger, so for example drillings are often quite strongly attracted.

 

[earlybird]

If the OPs exhaust pipe is stainless and from a road vehicle, then it's most likely to be ferritic, 409, hence magnetic. This grade is cheaper than 300 series and more easily cold worked as it doesn't harden so much. AFAIK, exhausts are also often aluminised mild steel
Re cutlery, some knives can be, ( used to be?) austenitic handles welded to martensitic blades to give a better edge. After time a band of corrosion pits can appear in the heat affected zone of the joint