During MPI test of steel part (probable AISI 4340 ) , inspector found indications everywhere. After careful observation, it was revealed that they are the grain boundaries. I am not sure what could be the reason behind it. The part was unetched and it was not polished. Only little grinding was done for MPI test.
Hi Dhruvit,
False indications in MPI testing is quite common when you have two phases that have differing magnetic permeability. For example in duplex stainless steel you see false indications at the grain boundaries.
Although you may expect only one phase in your steel, martensitic microstructure. I think you may have two phases, for example retained austenite, which could form on slow cooling rate or due to segregation of austenite stabilising elements.
The following discussion is about the same problem you have and there are a number of suggestions for the causes, which you may find useful.
https://www.eng-tips.com/viewthread.cfm?qid=498096
Hope this helps.
Kind regards
Ameer
Thank you for your answer Ameer K Ibraheem, but I guess there are very slim chances for this reason, as I have two similar parts for the same use. They are casted from the same material . One part gives these kind of indications ( looks like grain boundaries) ,whereas the other doesn't have any indications like the previous one.
I can see the columnar grains and equiaxed grains which are the characteristics of the castings after solidification. I checked the microstructure , it is tempered martensite.
In this case, the indications could be due to surface irregularities or inhomogeneities in the material. The grinding process might have also left rough surfaces. As you know, MPI is sensitive to magnetic flux disruptions, and these disruptions can occur at material interfaces like grain boundaries, making them appear as indications. Try to conduct fine polishing on the sample and see if it is still showing the grain boundaries indications.
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