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!
When comparing 440A and 17-4 (630) stainless steels, which has the greater magnetic response? Thank-you.
@Radicaltangent- Apologies, but that is slightly above our level of expertise.
what little mountains and scar signs mean on 304 and 18-8 hex nut ?
@David- Many times the nut manufacturer will put their manufacturer’s mark on their product, even though it is not required for that grade. I would suspect that is what you are seeing.
I purchased 1/2″ diameter thumbscrews labeled 18-8 stainless steel. They are magnetic and rust when left outside for a month or so. Are these not made from stainless steel as claimed?
@Richard- to be 100% certain you’d need to have them analyzed. A common misconception is that stainless steel is not magnetic. Magnetism has more to do with the grain structure than the chemical make up, so it is certainly possible that your thumbscrews could be magnetic if they were manufactured in a way that aligned the grain structure just so. As for the rusting, you are correct that after just one month it is surprising to see surface rust. However, if you live in a humid, salty area (like the beach), that will effect the screws more than if you live in a dry environment. All stainless steels will eventually corrode, just at a slower rate than their steel counterparts. It is also possible that your screws have some residual steel dust from the manufacturing or storing/shipping process, so that could contribute as well. At the end of the day, a chemical analysis is the only 100% certain way to tell.
I service nut and bolt bins in manufacturing facilities and I have a customer who is having problems with SS bolts and SHCSs “galling” when using with SS nylocks. They think the problem might be because of mixing 316 with other grades. Do you have an opinion on this issue?
@Roger- Galling is very common with stainless regardless of the alloys used. We have heard of people mixing stainless grades to help minimize galling, but I am not sure how well that works. A better solution might be to look at a never-seize type of lubrication.
@Roger, I work for a fastener company and we have a customer who had the same issue with the stainless nylocks. We have all of their nylocks sent to a plating company to be “waxed” which stops the galling.
Hi There,I am looking for High tensile grade 8.8 socket head cap screws but they are only available in 316 SS. are they compatible with grade 8.8.
Thanks in advance
@Mohsin- High strength, alloy socket head caps screws should be available in the marketplace in addition to stainless ones. I would call around to your local fastener distributors – someone is bound to have what you are looking for.