Portland Bolt Logo Part
Home > Technical Information > FAQs > Differences Between 18-8, 304, and 316 Stainless Steel


Differences Between 18-8, 304, and 316 Stainless Steel

Question: What are the differences between 18-8, type 304, and type 316 stainless steels?

Answer: 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!

Posted by Derek Marquez

Phone: (800) 599-0538 Email: derek@portlandbolt.com
View other posts by

Tags:

The below content is submitted by readers and has not been researched or checked for accuracy. It is not endorsed in any way by Portland Bolt.

28 Responses to “Differences Between 18-8, 304, and 316 Stainless Steel”

  1. mike massey says:

    what is the difference between 17-4 and 316 stainless steel?

    • Dane McKinnon says:

      @Mike – 316 stainless is classified as an austenitic alloy, whereas 17-4(630) is a precipitation hardening alloy. Stainless 316 has as it’s primary alloying elements Chromium and Nickel, whereas 17-4 has, in addition to Chromium and Nickel, Copper and trace amounts of Columbium and Titanium. 17-4 can be heat treated to increase the mechanical properties, where 316 cannot. I cannot speak to any corrosion or performance differences, as we have limited experience with 17-4 stainless and do not have any engineers on staff.

  2. AMIR says:

    dear Mr. Derek
    can we use A304 IN FABRICATION SEWAGE PLANT GATES
    OR WE HAVE TO USE 316 INSTEAED.

    — THANKS FOR YOUR ADVISE IN ADVANCE ….

    AMIR ALSAYED

    • Dane McKinnon says:

      @Amir – Portland Bolt does not have engineers on staff and cannot make recommendations as to the suitability of materials for specific applications.

  3. Faqir says:

    Dear Sir,

    I need your kind advise on below.

    I want to fabricate a a small adaptor (5 inch) which is to be connected to shaft to hold my shaft speed sensor. The adaptor was made origionally from SS 303 but its not available in UAE.

    My question is ” can I use SS 304 instead of 303? as I read on internet the corrosion properties of SS 304 are even better and SS 303 is usually used for its easier machinablity”

    Please guide

    Best Regards

    Engr. Faqir Nasir

    • Dane McKinnon says:

      @Faqir – Portland Bolt does not have engineers on staff and cannot make material recommendations for specific applications.

  4. lambos says:

    I need 30 stainless threaded inserts.
    pluss 2 screws

    • Dane McKinnon says:

      @lambos – I would encourage you to contact a distributor in your area for those items, they are not anything that we normally handle.

  5. Robert Fields (Fields Electric Co., Inc.) says:

    January 3, 2013

    To Whom It May Concern:

    Can Type 304 SS be used together with Type 316 SS or should they not be mixed. Example Type 304 SS pipe W/ Type 316 SS fittings used around lake water, where they could all go under water at various times?

    Sincerely Yours,
    Robert Fields

    • Dane McKinnon says:

      @Robert Fields – Using mismatched alloy types is somewhat common in some applications in order to combat the effects of galvanic corrosion, but whether this is appropriate in yours or any specific application is really a question for an engineer familiar with the project. We are not able to make those kinds of recommendations.

  6. Steve Kalmbach says:

    When assembling SS bolts and nuts it is recommended that the nut and bolt be of different grades. Is there enough difference between 18-8 SS nuts and 316 bolt for proper assembly?

    • Dane McKinnon says:

      @Steve – Occasionally engineers will specify mismatched alloys either for galvanic corrosion or anti-galling reasons. There are slight chemical differences between SS 18-8 and SS 316, so those may be acceptable, but you will need to contact an engineer in order to get any specific alloy recommendations.

  7. Joe Hamilton says:

    I am asst. to Metal Scuplturer and he want to use Stainless steel 20 gauge and he didn’t want any “black Rust” on it as it happened to his Stanless steel scuplture for Interior so We bought T3042b Stainless steel sheet and he said itis different in weight . So please help us understand if T3042B is wrong or right kind to bend and stay bend and no corrosion ?????

    • Dane McKinnon says:

      @Joe Hamilton – Your question is a bit out of the realm of our expertise. Apologies, but we will be unable to help.

  8. shoaib says:

    questions on material 18-8 for screw lock. Why not use 304 SS?

    • Dane McKinnon says:

      @Shoaib – stainless 304 falls under the umbrella of 18-8, so there should be no reason that 304 could be used when 18-8 is specified.

  9. Siyue says:

    Is there any magnetic permeability difference between SS303 ans SS304?

    • Dane McKinnon says:

      @Siyue – Apologies, but we do not have any information regarding magnetic permeability.

  10. chuck coats says:

    Why are teck screws labeled 18-8 as so magnetic. And are they corotion resitant?

    • Dane McKinnon says:

      @Chuck Coats – Yes, the 18-8 screws would be corrosion resistant. As for the magnetic question, the screws may or may not be magnetic, it depends on how they were processed. Magnetism has more to do with the steel’s grain microstructure than with its chemical make-up, so whether or not your stainless will be magnetic will depend on not only its microstructure, but also how it was manufactured or processed. The most common stainless steels, 304 (18-8) and 316, have an austenitic microstructure and start out life non-magnetic. However as they are manufactured from raw steel to fasteners, they go through several cold forming processes (drawing, roll threading, heading) which changes the microstructure from austenitic to martensitic, thereby making the material more magnetic. The more cold forming, the more its microstructure is changed to martensite, and subsequently the more magnetic the fastener is likely to be. The screws can be annealed after forming to rid them of their magnetic properties.

  11. berniet says:

    Can the stainless steel 304 non-magnetic property be converted to magnetic thrucold working

    • Dane McKinnon says:

      @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.

  12. Tick says:

    I want to use ss for my Harley. Is 18-8 strong enough or should I use 316?

    • Dane McKinnon says:

      @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.

  13. P.W.D.Sirimewan says:

    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?

    • Dane McKinnon says:

      @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.

  14. charlie says:

    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

    • Dane McKinnon says:

      @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.

Leave a Reply

*

*Please note: Your comment will not appear on the site until it has been moderated. We will approve and posts comments as soon as we are able.



Ask the Expert is powered by WordPress | Entries (RSS) | Comments (RSS).