To answer this question, you first need to know some basic information about stainless steel. “Stainless Steel” is the general name for a large family of alloy steels that contain at least 10.5% chromium as part of their composition. At and above this level of chromium, a complex chrome-oxide surface layer forms that prevents further oxygen atoms from penetrating into the steel and thus protects the iron in the matrix from rusting. This layer is what makes the steel “stainless.” Higher levels of chromium and the addition of other alloying elements such as molybdenum and nickel enhance this protective barrier and further improve the corrosion resistance of the stainless steel. There are many different types of stainless, but by far the most popular and widely used are the 300 series stainless steels, also known as the austenitic stainless steels.
The 300 series designation contains many different compositions of alloy steel (303, 304, 305, 316, 321, 347, etc.) but the common factors among them are:
- Their carbon content is generally held to a maximum of 0.08%
- They (generally) have 18% chromium
- They (generally) have 8% nickel
- They are non-magnetic
- They cannot be hardened by heat treatment
- They can be hardened by cold working the material (“work hardening.”)
The term “18-8” is often used to designate products made from 300 series stainless. This “18-8” call out is referring to the 18% chromium/8% nickel alloy mixture of the steel. “18-8” is not an actual specification, as it only refers to two different alloys in the steel. While all 300 series stainless steels share this 18/8 mix, slight differences in chemical composition between the different grades of the 300 series do make certain grades more resistant than others against particular types of corrosion. In the fastener industry the term “18-8” is often used as a designation for a bolt, nut, or washer manufactured from 300 series stainless steel material that has the 18% chromium/8% nickel alloy mixture. However, a fastener manufactured from stainless material that meets the 18/8 alloy mix does not necessarily meet the other slight differences in chemistry required to certify it as Type 304 stainless. Type 304 is by far the most popular of the 300 series stainless steels.
The second most popular type of stainless, after Type 304, is Type 316. In Type 316 stainless, the chromium content is lowered from 18% to 16%, however, the nickel content is raised to 10% and 2% molybdenum is added to the mixture. This change in the chromium/nickel ratio and the addition of the molybdenum increases the resistance to chlorides. This is why Type 316 stainless is often used in more corrosive environments where the material will be exposed to chemical, solvent, or salt water corrosion and makes it the preferred material for marine construction.
Although fasteners can (and often are) ordered as simply Type 304 or Type 316 stainless, the actual ASTM specifications that cover stainless steel fasteners are A193, A320, and F593. A discussion of the differences between these ASTM specifications can be found in another FAQ.
Portland Bolt can manufacture stainless steel bolts, rods, and bent bolts from both Type 304 and Type 316. If you would like a quote on stainless product, you can submit a quote request through our website right now!
It was really an interesting article.
we have a request for 3/4″ grade 8 stainless steel bolts to be used in an assembly. what is the equivalent grade of a SS bolt in this requirement.
@Mike- ASTM A193 B8 is stainless 304, perhaps that is what they meant? If instead you need a stainless steel bolt with the equivalent strength of an alloy SAE J429 grade 8, you are out of luck. A193 B8 class 2 bolts are a bit stronger than their class 1 counterparts, but they are the strongest stainless we know about and they are not close to the strength of alloy grade 8 bolts.
Do your 316 stainless bolts meet the specification for 316L (low carbon)?
316 SST = 0.08% Carbon max.
316L SST = 0.03% Carbon max.
Almost all 316 sheet metal purchased these days is dual certified 316/316L (Carbon and tensile strength properties are met. I need to know if this also holds true for bolts / fasteners.
Thanks
@Bex- All of the bolts that we manufacture will be dual certified 316/316L since all of the available raw steel is dual certified. However, many of the off the shelf imported stainless fasteners are not, so it is hit and miss. We would need to check the availability of 316L for specific sizes upon request.
Should ASTM F593 be referenced/used if the SS316 Bolts are not for High Temperature Service ? The same question for SS316 Nuts and the requirements of A194 High Pressure – High Temperature vs. F594 ?
@Randy- ASTM F593 was written for general purpose service, not high temperature. ASTM A193/A194 are specifically for high temperature service, although because those grades are widely available, they are commonly used for general purpose applications. Our recommendation is to use the simplest callout that gives you what you need. If commercially available 316 is all you need, then specify it that way. If you instead need the tighter chemical and mechanical requirements of F593, use that. If you need the high temperature properties of A193, use that. The more specific you are in your call outs, the more expensive and less available your options.
Are 316L bolts/nuts available in heavy-hex for structural applications, similar to A325 carbon steel specification?
@Ryan- Heavy hex stainless bolts are available through the ASTM A193 specification. You are not able to exactly match the mechanical properties of A325 structural bolts, but if you use class 2 bolts, you can get a lot closer than the normal class 1 bolts.