SS 304 Material Became magnetic after Formning?

[bmoorthy]

Hello All

After forming Dish Head the SS 304 material became magnetic. I know Austenitic SS are not magnetic.

We doubted on the source of material and conucted re chemical analysis. The chemistry perfectly agrees with the material Spec SA 240- 304 not only that it perfectly matches the certificate.

Now why should the material which was non magnetic turn magnetic.

Had i performed Micro analysis before forming (Now also possible to perform since balance cut pieces of dish blank is availale) i would have seen Austenitic. What would i see in micro examination, Would i see Ferrite or martensite. If so how does this happen?

Would my ferrite scope show different reading in unformed plate and formed dish.

The problem with ferrite scope is that it shows various readings on the same plate.

 

[unclesyd]

How did you form the head?

Cold working 304 will cause to become magnetic.

 

[bmoorthy]

Yes Cold forming. By Pressing. The Plate used was 16 mm in thickness prior to forming and after forming in some location it has 14mm. The ferrite scope give different readings at different places. (The calibration of the scope was verified and it is fine and consistent). Ferrite check is being performed for academic purpose (To understand the magnetism phenomenon)

 

[TVP]

bmoorthy,

Many austenitic stainless steels like Type 302, 304, etc. are only metastable, that is, upon deformation some of the austenite transforms into martensite. The martensite makes the stainless steel somewhat magnetic. This topic has been covered on Eng-Tips before, so try a search for more information.

 

[metengr]

Make sure that you carefully review the allowable forming limits for this material in accordance with Code of Construction requirements. There have been instances where SCC (stress corrosion cracking) has occurred in cold formed austenitic stainless steel heads under certain operating conditions.

 

[rorschach]

If you bright anneal the part after forming, then both SCC and magnetism issues will be eliminated, but the strain hardening will go away too.

The reason your magnetometer gives different readings in different areas is because you have differing levels of strain hardening in different areas. If you mapped it, you'd find it will conform quite closely to the amount of deformation in the given area I'd bet.

 

[bmoorthy]

TVP

If i put a portable microscope on the dish head would i see martensite at least in some locations?

Does it mean that if an austenitic material is formed or hammered severely and then we put it under micoscope we will see phase change to martensite?

 

[Flesh]

bmoorthy

A metallographic sample will show the phase change via the use of one of many etchants:
Ralph's Reagent
Vilella's Reagent
... to name a few.

 

[TVP]

bmoorthy,

As Flesh mentioned, you will need to create a polished metallographic section and then etch this in order to see the martensite. The microscope will need to have a magnification of at least 100x in order to resolve the martensite, which will have a needle-like appearance.

 

[bmoorthy]

Since i am not a metallurgist, i take it is perfectly possible to observed phase change even with out heat treatment.

Further with cold working one can make even Austenitic material magnetic as the material would transform into martensitic after cold forming.

 

[TVP]

Yes, it is possible to observe the microstructural difference, but since only some of the austenite transforms to martensite, it is not a complete change. Also, higher alloyed grades like Type 316 do not transform as much as grades like 301, 302, and 304.