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!
We assemble 18-8 dowel pins into rolled over / curled 304SS plates (3/16 dia). Currently having trouble with the pins staying put after assembly. Dimensional interference is between .003 and .007. Should we use different material for the plates?
@Jeff – I am sorry, but this is out of our area of expertise. Apologies.
I am looking to buy some stainless steel deck screws. Is there any difference between ss304 and 305 ss. I live in western pa and was wondering if one was more resistance to rust then the other… thanks
@Charlie – 305 stainless has a bit more of the corrosion resisting elements (nickel+chromium) than 304, so theoretically should be slightly more corrosion resistant. That said, I am not a metallurgist, so I cannot be certain that other factors would not affect how it would perform in your application.
We are from liquor manufacturing company in Sri Lanka. We need to know most suitable material to manufacture spirit storage vats, fermentation vats (we use coconut toddy to make alcohol, and it is somewhat acidic PH is about 3.2). Please let me know SS304 steel tanks are ok to storage coconut toddy? And also let me know why don’t use SS304 for food grade applications? Can’t we use SS304 for the food grade applications?
@P.W.D. Sirimewan – Thank you for your question, but we really don’t have the engineers on staff to make those kinds of recommendations.
I want to use ss for my Harley. Is 18-8 strong enough or should I use 316?
@Tick – 18-8 and 316 stainless will have the same mechanical properties (all things being equal). The advantage to 316 is that it is more corrosion resistant.
Can the stainless steel 304 non-magnetic property be converted to magnetic thrucold working
@Berniet – Yes, I believe it can. I am not familiar with the specifics of how it would be done, but cold working can change the grain structure and make the material more magnetic.