The reason why stainless steel cannot be inspected by magnetic particle testing is that we cannot induce a magnetic field on the material as this is non-magnetic. We need a flux leakage so a magnetic field must be present. I hope this will help in your query.
Its only the austenitic stainless steels which are incapable of magnetisation. The ferritic stainless steels ARE ferromagnetic. Apply a small permanent magnet to the test-piece if you are uncertain whether a material is austenitic or ferritic stainless steel.
For a good expalanation of which stainless steels are paramagnetic (can't be subjected to MPT) and which ones are ferromagnetic (can be subjected to MPT) go to this link: Magnetic Stainless Steel or Not
It is true that you can perform mag particle testing on ferritic stainless steels. The easiest method of determining if the material can be mag particle inspected is to see if a permanent magnet is attracted to it. The three elements which are ferromagnetic are iron, nickel and cobalt.
All stainless steel materials are nonferrous hence not possible for magnetise the same. For magnetic particle testing, flux leakage is must which is not possible in this case. Hence Magnetic particle testing is not possible. To find out the defects, we may carry out RT / UT which will suffice the requirements.
Pl. correct your understanding of the subject of stainless steels. All stainless steels are not non-magnetic - ferritic stainless steels and martensitic stainless steels etc. are magnetic. Only austenitic stainless steels in fully annealed condition are non-magnetic (paramagnetic).
S V Swamy
Metallurgist and Quality / NDT professional (now retired)
Ferritic Austenitic Stainless Steels also called duplex steel could be magnetic. As already explained earlier, check whether the material is magnetic or not using a permanent magnet. If the magnet is attracted to the material, MPT is feasible.