Stainless-Good & Bad

[Randyonr3]

How do you tell the difference between good stainless and the crap stuff comming in from china..
Over the past few years I've replaced a few items and have seen the stainless screws or washers (purchased from west marine) are now rusting. The origional screws, over 20 years old, next to the new ones are still showing bright..
I understand that many of the fasteners from west marine are from china.
How do I tell the difference when purchasing them, wether they're good quality or not......

[David M]

Place a magnet on the metal. The more it sticks, the more iron it has and the lower the quality of the stainless. The best grades of stainless steel wont stick at all to a magnet.

How magnetic the stainless is, is just one of a number of factors though...but at least you can weed out the garbage stainless by using magnet.

For stainless it helps to keep it exposed to oxygen to keep it from going "active".

[GordMay]

A magnet is a good test for MARINE Stainless Steel; but knowing the “Grade”, and understanding it’s characteristics is the best measure for selecting a metal for a particular application.

Stainless Steel is a generic term for steel alloys with a minimum of 10.5% chromium.
There are several families of stainless steels, with different physical properties.

The most common Marine Grades of “MARINE” S/S are the 300 Series (“Austenetic”), and some Duplex Super Alloys (often used in Prop’ Shafts).

A basic stainless steel has a 'ferritic' structure and is magnetic. These are formed from the addition of chromium and can be hardened through the addition of carbon, making them “Martensitic”. The martensitic grades are mainly used where hardness, strength, and wear resistance are required, such as for knives.

The “Austenitic” grades of Stainless Steel are non-magnetic. The most common austenitic alloys are iron-chromium-nickel steels, and are widely known as the 300 series. It's the nickel which modifies the physical structure of the steel and makes it non-magnetic.

The 400 series, which contains steel and chromium, but without the presence of nickel, exhibits magnetic qualities.

“Superalloys” are used when 316 or 317 are inadequate to withstand attack. They contain very large amounts of nickel and/or chrome and molybdenum. They are usually much more expensive than the usual 300 series alloys and can be more difficult to find. These alloys include Alloy 20 and Hastelloy.

[Jim Quibell]

Well - if you are in the food processing / services industry you will want 304 series stainless #4 finish.
How to tell if that's what you got - it will be non magnetic and you can put a slice of tomato on it overnight without it damaging the surface.
Try that with 400 series (standard Chinese these days) and the surface is magnetic and the slice of tomato will pit and mark the surface overnight.
If you are buying a steam kettle to make spaghetti sauce (or any acid filled sauce for that matter), you will want to make sure the kettle has a 316 grade s/s liner.
That has been standard in the food service industry for many years.

[Wotname]

Off topic perhaps (probably?), except kitchens knives, they are usually 400 series - for a whole variety of good reasons but very few are cruising related so I will refrain here.

[MidLandOne]

Is perhaps worthwhile noting that not all "good" ss for marine use is not magnetic.

Common examples of magnetic "good" stainless steels are the high tensile strength, high corrosion and wear resistant (so good in bearings) duplex stainless steels most often used for propeller shafts. My propeller shaft is very magnetic and I hope , very "good".

[David M]

Originally Posted by MidLandOne

Is perhaps worthwhile noting that not all "good" ss for marine use is not magnetic.

Common examples of magnetic "good" stainless steels are the high tensile strength, high corrosion and wear resistant (so good in bearings) duplex stainless steels most often used for propeller shafts. My propeller shaft is very magnetic and I hope , very "good".

Interesting. Did not realize that. So does the iron in the stainless where it is exposed to oxygen eventually oxidize or does it stay as Fe2 or Fe3?

[MidLandOne]

Getting a bit beyond my knowledge for an authorative reply David so I have fished out a few links below.

But, if you are also referring to how some grades of ss go brown from the ferrous part oxidation I can say that the duplex grades used for propeller shafts stay bright and shiny just like 316.

Stainless Steels - Duplex Stainless Steels Properties, Fabrication and Applications, Supplier Data b
Why use duplex stainless steel?

and a link to a typical suppliers information sheet

http://www.clementsmarine.co.uk/PDF/TD001/TD001.pdf

Originally Posted by David M

Interesting. Did not realize that. So does the iron in the stainless where it is exposed to oxygen eventually oxidize or does it stay as Fe2 or Fe3?

Not significantly. The Chromium is the key. Chromium oxide is very hard, and very dense. The surface chemistry of stainless steel is very complex, but in the simple description you can imagine the chromium oxide forming a protective layer that prevents further oxidation of the metal. It is a lot more complex than that at a microscopic level, but that is a good working model.

Bill

[Celestialsailor]

Originally Posted by Randyonr3

How do you tell the difference between good stainless and the crap stuff comming in from china..
Over the past few years I've replaced a few items and have seen the stainless screws or washers (purchased from west marine) are now rusting. The origional screws, over 20 years old, next to the new ones are still showing bright..
I understand that many of the fasteners from west marine are from china.
How do I tell the difference when purchasing them, wether they're good quality or not......

.....Well, I guess a good start is to stop buying from WM and look at lables on products that say made in China. I have had the same problems in the past. Mcmaster-Carr online has a good bulk selection. You can also Goggle SS Fasteners and shop for days.

[Benjamaphone]

Back to the original question; the 316 hardware that we buy is stamped with the '3 1 6' numbers (on the flat washers at least). Other than that, I think that colour might be a little different. The 316 is more silver shiny while the 304 is more grey/dullish looking. Also, price point might be a good way to tell, with the 316 being more $$$.

[Tom Spohn]

As Gord stated above, there are two families of Stainless--the 300 series and the 400 series. The 300 series is normally 18 % chromium and 8% nickel. Carbon is very low-usually limited to .08 or .10% max. This is to make the alloy weld better as carbides are not precipitated to the grain boundaries when heated. These alloys are very corrosion resistant. When Molybdenum is added to 304 it becomes 316. This alloy has all the corrosion resistance of 304 and additional resistance to salt water corrosion. This is the alloy that should be used on most boating applications--including fasteners. The alloy is magnetic.

400 series were developed mostly in Europe where nickel was not as available. These alloys have high chromium levels and carbon levels (up to 1%) to produce very high strength and allow fine edges on cutlery and scissors. They do not contain nickel. The steel corrodes very quickly. Buy it for knives and keep it away from salt water.

Don't know what the Chinese steel is, but I am guessing it is 400 series. Strong when new, but quickly corrodes and fall apart when subjected to salty environments.

[GordMay]

Originally Posted by Tom Spohn

... When Molybdenum is added to 304 it becomes 316. This alloy has all the corrosion resistance of 304 and additional resistance to salt water corrosion. This is the alloy that should be used on most boating applications--including fasteners. The alloy is magnetic...

Although type 316 stainless steel is NOT normally magnetic, it can be made magnetic by working the metal (cold rolling, shot blasting or heavy polishing, etc).
Any austenitic (300 series) stainless steel, which has developed magnetic response due to cold work, can be returned to a non-magnetic condition by stress relieving. In general this can be readily achieved by briefly heating to approximately 1000 - 1150 deg.C.

Cast 316 (CF8M), are likely to crack in the casting process if they aren’t formulated to be somewhat magnetic – the same applies to welds. So castings are usually magnetic (varying from slightly to substantially), unless they have been heat treated.

[Benjamaphone]

I remember that for my high school physics class we had to build a simple motor. I brought the project to my dad's shop and we spent the afternoon designing/constructing a small motor. When I brought it home I discovered that the 316 SS screws that we used for the terminal things were not magnetic, thus rendering the motor useless. I went to Canadian Tire and got the cheapest galvanized screws available. After rewinding everything, I plugged the motor in and voila!

Moral of the story: standard 316 nuts/bolts/screws are not magnetic. Like Gord says, 316 can be made magnetic but for the vast majority of applications it is not.

[Microship]

The best method is to not buy hardware at West Marine... you have absolutely no way of telling what's in those bins (and the prices are too high anyway). The best resource is online at McMaster-Carr... and you can specify the alloy you want. Kind of a pain if you only need two screws and end up with a box of 100, but they make great trade goods, and besides, it's nice to have some decent inventory of known-good stuff on hand!

Incidentally, for inventory storage aboard I use 3-mil zip-close food-grade bags with write-on surfaces for all hardware, and those are grouped in soft packs and zippered 3-ring-binder pages to provide a layer of organization. Not as handy as the wall o' drawers in the lab, but much more likely to survive the rigors of life at sea... and a pile of bags only takes up as much volume as required by the contents.

Cheers,
Steve