Unknownium / Chinesium "304" stainless steel? How to tell.

I've bought a nice selection of (allegedly) stainless steel mandrel bends and u-bends from AliExpress in recent months in preparation for fabricating the mythical S800 exhaust manifold. Hence the related tube bender activity, talk of laser welders etc of late.

There's a lot of suspicion / cynicism about the "accuracy" (honesty) of some of the traders on that platform. So, how would I know if I've been given the 304 grade stainless that I thought I was buying or something else, such as plated mild steel? I've heard of copper busbars turning out to be simply copper coated steel bar stock and copper cables being copper plated loominum.

The usual first step is to check for magnetism, as stainless steels (not least the commonplace 304 and 316 grades) are generally not magnetic. It's a pretty rough and ready test but easy to do.

Firstly, some tubing I bought in the UK from a reputable supplier (metals4u in Wetherby). Interestingly, it's magnetic where I've bent it - but not where it is unmolested.

Magnetic:

Non magnetic:

What about those mandrel bends? Yes, they are magnetic, although only just enough to be able to pick them up with a fairly strong magnet. 

So, after applying the belt sander to the end face and a spot on the curved outer surface, I applied some "gun blue" (selenium dioxide"). If that results in black/blue colouration, we are looking at mild steel. 304 or 316 should remain clear.

Slap it on, boys.

Not a hint of oxide there.

Here's one of the u-bends. Nicely magnetic where it's been bent - enough to pick it up easily. Also, no discolouration.

So (conclusion) - I've been sold genuine 304 parts but they become fairly magnetic when bent. Good to know at this stage.