How to tell stainless?

[mpierich]

I was given some painted cycle parts to polish. According to my trusty magnet, they are steel. Any quick way to tell if they are stainless? I sanded some bare spots and wet them down but the air here is dry now and I'm afraid it'll take weeks to rust (if it's not stainless).

I don't want to waste time on them if they're plain carbon steel.

[fly]

rub muratic acid on it and then rinse it and dry it.

[mpierich]

Originally Posted by fly

rub muratic acid on it and then rinse it and dry it.

OK...what am I looking for? Will that make regular steel flash rust?

[thesound]

If a magnet will stick to it, the best form of stainless you could hope for it to be would be 409. 409 stainless is the same type of steel used in a good portion of late model OEM automotive exhaust systems. (I know GM switched over to using it as a standard exhaust substrate in all their vehicles in like 94' I believe.) It is far more corrosion resistant than plain carbon steel but it WILL eventually rust if exposed to the elements. My guess would be that the parts are most likely mild steel because from a cost-of-production standpoint it makes little sense for a company to spend the extra money on a higher corrosion resistant steel if they intended to coat it with paint anyway.

[mpierich]

Originally Posted by thesound

If a magnet will stick to it, the best form of stainless you could hope for it to be would be 409. 409 stainless is the same type of steel used in a good portion of late model OEM automotive exhaust systems. (I know GM switched over to using it as a standard exhaust substrate in all their vehicles in like 94' I believe.) It is far more corrosion resistant than plain carbon steel but it WILL eventually rust if exposed to the elements. My guess would be that the parts are most likely mild steel because from a cost-of-production standpoint it makes little sense for a company to spend the extra money on a higher corrosion resistant steel if they intended to coat it with paint anyway.

Hmmm...so you're saying a magnet won't stick to true stainless? Not doubtig you, just want to be sure I understand correctly.

You're right about the costs...this is just a kickstart lever off an ATV. He claims he's seen them polished, but I would bet he saw plated ones.

Anyhow, he wants it polished regardless. The customer is always right, as long as he pays his bill, eh?

[thesound]

Originally Posted by mpierich

Hmmm...so you're saying a magnet won't stick to true stainless? Not doubtig you, just want to be sure I understand correctly.

You're right about the costs...this is just a kickstart lever off an ATV. He claims he's seen them polished, but I would bet he saw plated ones.

If you were to compare 409 stainless with say, 304 stainless, what you'd see is that the 304 stainless is basically "diluted" with a higher concentration of other alloys than the 409. 304 stainless typically contains between 14%-17%
chromium and usually about %4-%5 nickel. Anytime you increase the ratio of non carbon metals to carbon in any substrate, after you get to a certain point the carbon will become "buffered" enough that the substrate looses it's ferritic properties. 400 series stainless is right there on the ragged edge of having juuuust enough chromium and nickel that it's corrosion resistance is improved over standard mild steel, but still has a high enough carbon ratio that it will hold a magnet. Subsequently, as far as carbon density is concerned, this same logic also explains why steel can always be made into a more powerful magnet than say, iron. Hopefully this sheds some light on the "true stainless" question.

Originally Posted by mpierich

Anyhow, he wants it polished regardless. The customer is always right, as long as he pays his bill, eh?

You are the professional. If a customer chooses to ignore your advice or professional opinion, he's going to pay one way or another. May as well be you he pays.

[mpierich]

OK thanks, thesound. Very interesting. One thing I like about learning new skills is that it always leads into other interesting areas, like metalurgy in this case.

Yeah, this thing will make a 1-oz magnet jump at an inch or so distance - it's ferrous for sure. Oh well, he'll pay again to powdercoat it later on. Pity.

[2scoops]

304 stainless is one of the commonly called 18/8 stainless steels (see your kitchen cutlery). This means they contain roughly 18% chromium and 8% nickel and are of an austenitic structure. It is mainly the chromium that gives the corrosion resistance.
When there is no nickel, the stainless grade family is the "400 series". 400 series are not as corrosion resistant as the 300 series and are magnetic (ferritic structure) , where the 300 series are non-magnetic. It's to do with the crystal orientations or lattice within the grains of the metal.
..........
However, under serious coldworking 300 series may become magnetic!!

Regards
2scoops