Why can extra long stainless machine screws have magnetism?

Generally speaking, when we check whether the long screws are made of carbon steel or stainless steel, we usually test them with magnets. Those that can be magnetized are magnetic, usually made of iron. But sometimes is stainless steel material, extra long stainless machine screws will also be a magnet to suck up, this is why?

Stainless steel material is not magnetic, but weak magnetic, generally only when the screw is very small, use a magnet to suck the screw, the screw may be sucked up, but when the screw is relatively large, use the magnet to suck the screw is not reactive. Of course, it is also possible that the following situation will cause the extra long stainless machine screws will be attracted, it appears that the screw is magnetic.

One is that the stainless steel wire used in the production does not meet the standards, leading to the production of extra long stainless machine screws containing magnetic, which can be magnetized to the magnet. Two of them were in production. The surface of the screw was accidentally stained with iron filings, and it was not removed. If the screw was tested with a magnet, it would react. Three are in the cleaning, the stainless steel long screw inside mixed with a piece of the iron screw, this will also lead to the screw surface stained with iron components.