Stainless steel v. mild steel

[Welderpaul]

Maybe another one for the resident clever man.... @TechnicAl

Stainless steel is a 'tougher' material to work than mild steel in that its harder to machine etc, but not as 'strong' as in that less load is carried by a stainless section, compared to the same size section in mild steel.

In cack handed fabricator language, why is this?...Say S275 mild steel with a yield of 275mpa, what is the equivalent yield of say bog standard 304L, a lot less than 275mpa?

I'm not sure if that makes sense.

 

[BrokenBiker]

I can remember hearing somewhere ages ago that stainless is harder to machine because of the chromium content, but doesn't hold the same strength as mild steel because of the alloying in it. obviously this alloying varies for each flavour. as per usual I may be wrong, but the chromium part kind of makes sense as chrome is hard.

 

[TechnicAl]

Its all about the structure. The way the atoms arrange themselves. 304L is austenitic whilst S275 is Ferritic. The result is shown in the density. A 10cm cube of 304L will weigh more than a 10Cm cube of S275.
The austenite is softer but isnt affected as much by sub zero temperature so it doesnt have ductile / brittle transistion temp (generalising)

What makes austenite difficult to work is that it work hardens very easily. Cutting it with even the best blade will result in work hardenening so every pass will increase the hardness (strength). Annealed it can be 500 is tensile but that can go up to 1500 or more after work hardening.

You are also comparing annealed 304L against S275 which is rolled and heat treated to obtain the best results.........anneal some and it becomes really soft.

Does that explain?

 

[TechnicAl]

BB...........The chromium content gives it the corrosion resistance. The Cr forms Chromium Oxide when it contacts the atmosphere. This forms a layer on the surface which is does not rust or stain......so its stainless.
Think of it like galvanised or painted but if its scratched it reforms on its own

Pickling increases the speed and depth of layer which is why pickling a weld stops it discoloring


Work hardened stainless will pull harder against a magnet

 

[Welderpaul]

Its all about the structure. The way the atoms arrange themselves. 304L is austenitic whilst S275 is Ferritic. The result is shown in the density. A 10cm cube of 304L will weigh more than a 10Cm cube of S275.
The austenite is softer but isnt affected as much by sub zero temperature so it doesnt have ductile / brittle transistion temp (generalising)

What makes austenite difficult to work is that it work hardens very easily. Cutting it with even the best blade will result in work hardenening so every pass will increase the hardness (strength). Annealed it can be 500 is tensile but that can go up to 1500 or more after work hardening.

You are also comparing annealed 304L against S275 which is rolled and heat treated to obtain the best results.........anneal some and it becomes really soft.

Does that explain?

Yes it does, thanks Al.

 

[Kent]

BB...........The chromium content gives it the corrosion resistance. The Cr forms Chromium Oxide when it contacts the atmosphere. This forms a layer on the surface which is does not rust or stain......so its stainless.
Think of it like galvanised or painted but if its scratched it reforms on its own

Pickling increases the speed and depth of layer which is why pickling a weld stops it discoloring


Work hardened stainless will pull harder against a magnet

I never knew that about work hardened pulling more against a magnet, I have found it hard to correct an incorrect bend on site compared to MS - which I suppose is the work hardening bit in action? thanks for the informative post

 

[Farside]

Work hardened stainless will pull harder against a magnet

Aha. Bike discs spring to mind.

 

[hotrodder]

http://www.bssa.org.uk/topics.php?article=125 Covers pretty much the same stuff Al already stated and also the difference in stress-strain behaviour