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!
ARE NUTS AVAILABLE IN DUAL CERTIFIDED 304L?
@Jim- Occasionally we are able to source stainless nuts that are certifiable to 304L. It just depends on your specific needs.
IS 410 STAINLESS AND 18-8 STAINLES COMPARABLE IN STRENGTH
@R Hill- It depends on the condition of the respective bars. 18-8 steel can be as low as 30ksi yield/75ksi tensile in its weakest condition, but can be significantly stronger than that if it has been cold drawn or strain hardened. Similarly, 410 stainless in its weakest condition has a minimum 40ksi yield/70ksi tensile, but 410 can be heat treated to much higher strengths. So apples to apples they are similar, but without more information we can’t draw any concrete conclusions.
I am designing a 19″ x 5.25 x 0.25 inch rack mount panel that will support a 24 lb cage. I want to use black oxide counter sunk screws to hold the two units together. These may be carried around in racks in trucks, vans or large trailers at times. I see other similar products using #6-32 counter sunk SS screws for this application. I believe these are 18-8 or 304, but I don’t think these people bother to ask. This stress will be distributed across 6 to 10 screws. I am considering #6, #8 or #10 SS screws. To do this correctly I need to have access to shear strength specs for these metals. Although available for steel, I have not found such specs for stainless. Can you help?
@JMoss- We do not have any published information for the shear strength of stainless steel, but the rule of thumb is that shear strength is about 60% of tensile. Grades within the 18-8 family of stainless alloys have a minimum tensile of 75,000psi, so the shear strength would be roughly 45,000psi. If you multiply that by the stress area of the screws, that will give you the shear strength of the screws.
Do 316L bolts need passivation for salt water environments and if so can your company do this inhouse?
Thank you
@Don- It depends on how the bolts were manufactured. Cold formed bolts, which are virtually all mass produced bolts, shouldn’t need additional passivation after manufacture. Hot formed bolts would benefit from passivation, but we couldn’t say if it is absolutely necessary. The protective ‘film’ that stainless has will generate by itself, the passivation process speeds that up. No, we do not offer that in-house, but can send the bolts to a local company to have it performed.
HI, CAN WE USED SUD BOLT A193 B8MA CLASS .1 INSTEAD A193 B8MA CLASS .2
@Bahman- A193 B8MA class 1 will be significantly less strong than its class 2 counterpart. Any substitutions like this would require the project engineer’s approval.
Hello Dane, do yo have any information/recommendation regarding torque value for various stud bolts (A193 GR.B7, A193 GR.B7M, A193 GR.B8M CL.2, and A320 GR.L7M)?
Thanks in advance.
@Hendra- We have some general purpose torque values on our website, but they are mostly for headed bolts in steel to steel connections. We do not have any torque values for studs nor stainless steel since we have yet to find reliable technical information due to the tricky nature of torque.