Stainless Steel Nominal Thicknesses 1

[JohnnyEnglish]

I've been doing some research on stainless gauges (archaic) recently to try to find what the actual standard nominal thickness is for each one, and which ones are commonly available from suppliers. It has been quite frustrating and a little surprising/enlightening. Below is what I've found - they are in imperial (inch) measurements in North America. CR Coil typically goes to 7 Ga, which is why there is also a 3/16 which is where HRAP Coil / Plate begins and CR Coil ends (usually). Formal tolerances (not shown) are per ASTM A480 but these are generally much wider than a mill can actually produce.

Note there is no formal definition (as far as I'm aware) of these nominals in any standard. They are just known. Some of you might know these like the back of your hand; others may be be surprised for example to find that 11ga stainless is NOT 0.125in as is commonly reported on various websites (I was, a bit). Also, depending on what a distributor calls "nominal" may mean these values vary (maybe a little, maybe a lot).

The reason for this exercise is because I want to ensure designers and engineers can select material thicknesses with confidence, knowing that next week they won't be obsolete or at least difficult to source. As an example, we have often historically called for 0.03125 stainless. As you can see, although 1/32 feels like it should be a standard gauge, it isn't. But now that I know 0.0293 is "common" I can confidently design with 0.0293 ±0.003 (stress / mass) and always find a good selection of distributors for the stainless sheet.

Or so I thought, until I started looking at 321. Does anyone know if these nominals apply in any way to 321? It's standard thicknesses seem to be almost entirely different in the CR Coil range. This seems like madness, but I'm sure there is a good reason. 321 nominal thicknesses typically progress as follows : 0.016, 0.020, 0.025, 0.032, 0.035, 0.040 etc.

What gives? Any experience?

--- CR COIL ---

26 0.0178
24 0.0235
22 0.0293
20 0.0355
18 0.0480
16 0.0595
14 0.0751
12 0.1054
11 0.1200
10 0.1350
7 0.1874

--- HRAP COIL / PLATE ---

3/16 0.1875
1/4 0.2500
5/16 0.3125
3/8 0.3750
1/2 0.5000

 

[3DDave]

In the US metal gauge thickness for steel is established by weight per square foot.

https://uscode.house.gov/view.xhtml?req=granuleid:USC-prelim-title15-section206&num=0&edition=prelim

https://www.ryerson.com/resource/the-gauge/steel-gauge-the-why-and-how

Edit: I will add that it appears that the Stainless Steel industry seems to have taken the carbon steel gauge chart and rounded to the nearest fraction thickness with similar tweaks from the galvanized steel group.

 

[JohnnyEnglish]

Yep understood - so is that why 321 is typically offered in different decimal thicknesses to 316?

 

[3DDave]

Never heard of 321. 312, 314, 316 - Yeah. 321? Nope. 321 stainless - oh a specialty grade -

"321 stainless is a titanium stabilized grade commonly used for service in the 1000-1600°F temperature range."

That's a material I would expect is mostly ordered to decimal thickness. Ryerson doesn't even list 321 by "gauge." The top Google search was a company that re-rolls it to whatever thickness you like.

 

[EdStainless]

321 is hardly special, it is very common. Meant for cases where you need corrosion resistance in the as welded condition or after exposure to higher temperatures. It resists sensitization since it is 304Ti. In Europe they also make a 316Ti.
312 is not standard alloy designation and 314 is really 310H+Mo. Never figured out how it ended up being called that.

In thinner SS sheet products the standard gauges do follow traditional gage numbers, but we don't use them.
We always refer to thickness using decimal. This is largely because there are different gage designations for steel, stainless, sheet, and wire. And while they are close, they are different.
It gets more involved when we were trying to make tubing from the strip.
We had to order exact thickness, usually as minimum with one sided tolerance.
And yes, we expected half ASTM tolerance. In this case we are buying pounds and shipping feet, so we want it as light as possible.
So if a tubing order was for 0.083" average wall (14 ga) we would be allowed +/-10% on the wall.
While this is now 0.075" min we don't want the strip that light as there is risk of dropping below.
So we would order 0.0765" or 0.077 min thickness strip.
Whereas through distribution you never know what actual thickness you will get. Some mills roll light and some heavy, and the sheet may have been bought to meet the requirements of a large customer and everyone else has to live with those tolerances.
In our world SS sheet ranged from 0.120"nom (11 ga) down to 0.020" nom (25 ga).

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P.E. Metallurgy, consulting work welcomed

 

[EdStainless]

For SS sheet, they all have to be 3 decimal places because of how ASTM specs are written.
11 - 0.120"
12 - 0.109"
13 - 0.095"
14 - 0.083'
15 - 0.072"
16 - 0.065"
18 - 0.049"
20 - 0.035"
22 - 0.028"
23 - 0.025"
24 - 0.022"
25 - 0.020"

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P.E. Metallurgy, consulting work welcomed

 

[MintJulep]

In the US metal gauge thickness for steel is established by weight per square foot.

Not entirely clear to me how that works.

Larger gauge number --> thinner --> lighter per square foot.

No obvious formula.

 

[3DDave]

True - the gauges are enumerations of the different weight-per-square foot values. In ounces per square foot, the ounces are whole numbers. I linked to that chart in the US code URL.

 

[btrueblood]

from Machinery's Handbook, "the thickness of steel sheets now are based upon a weight of 41.82 pounds per square foot per inch thick", i.e. how many 12"x12" sheets are needed to get the equivalent weight of the 1" thick 12"x12" plate.

 

[EdStainless]

Many gage designations have their origin in wt/sg ft, or ft/oz of wire, or number of draw passes and such, however the modern definitions have been codified and defined in ways that don't always agree with the original values. Such as modern pipe schedules.


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P.E. Metallurgy, consulting work welcomed