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!
As a Certified Welding Inspector and welder and fabricator in oil refineries and chemical plants, I have worked with many varieties of exotic metals including all types of stainless steels. In my 30 years of construction l have learned some rules about stainless steels and corrosive environments.
#1. Any time you thread two pieces of stainless steel together (ie, bolts and nuts or pipe threads, etc.), always apply anti-seize compound to one or both pieces. ALWAYS! Or you can be guaranteed that they will gall (friction weld together) at some point. Use anti-seize made for stainless, not the copper-based anti-siege.
#2. Always use 316 or 316L stainless everywhere on your boat. Even 304 or 304L will rust when constantly in a saltwater environment.
#3. Stainless steel items that are forged may still have a slight magnetism to them, but generally, 300 series stainless ( 304, 308, 316, 321) should be non-magnetic.
Happy sailing.
I encountered quite a bit of issues with stainless steel nuts and bolts. I’ve noticed a high tendency for both 304 and 316 grade stainless bolt and nut combinations to gall upon installation and removal after being exposed to the environment for a period of time even if installed properly. 316 seems to happen more often. it happens so much that when we are performing work we have to have extra bolts and cutters on hand to be able to remove them and subsequently replace them. i have noticed that many overhead line products for transit industries utilize a combination of stainless steel bolt and a silicone bronze nut which seems to remove the galling issue even on 30 year old components. how does using this combination of fasteners affect torque and or clamping strength of the assembly?
@John- The use of dissimilar materials with stainless in order to mitigate galling is somewhat common. The strength of silicon bronze is not all that different from that of stainless, so we don’t think the clamping strength would be any different. As for the torque needed, we don’t have any reliable torque data for stainless of silicon bronze, so we are unable to help with that portion of your question.
Hey Dave, I am having problems with galling of stainless steel B8 class 2 bolts when grade 8 stainless steel nut is used. However, an interesting observation I made is that when using plain carbon A563 grade DH nut, it was fine and the SS bolt managed to achieve the minimum required pretension. My question is do hardened stainless steel heavy hex nuts exist and are there any hardness values I can use to compare to regular carbon alloy nuts?
@John- Galling is a common issue with stainless steel parts. Strain hardened nuts are sometimes available, but they are not as strong as their heat treated, alloy brethren. Stainless nuts do not have hardness requirements, so we will compare their proof load values instead. A563DH nuts are proof loaded at 175ksi, whereas strain hardened stainless nuts will vary from 90ksi to 125ksi depending on the alloy type and the diameter. However, hardness or strength is not the only factor to consider. Strain hardened stainless nuts, although they are harder, will still have a tendency to gall when assembled under pressure.