What are the differences between 18-8, type 304, and type 316 stainless steels?
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.
It was really an interesting article.
we have a request for 3/4″ grade 8 stainless steel bolts to be used in an assembly. what is the equivalent grade of a SS bolt in this requirement.
@Mike- ASTM A193 B8 is stainless 304, perhaps that is what they meant? If instead you need a stainless steel bolt with the equivalent strength of an alloy SAE J429 grade 8, you are out of luck. A193 B8 class 2 bolts are a bit stronger than their class 1 counterparts, but they are the strongest stainless we know about and they are not close to the strength of alloy grade 8 bolts.
Do your 316 stainless bolts meet the specification for 316L (low carbon)?
316 SST = 0.08% Carbon max.
316L SST = 0.03% Carbon max.
Almost all 316 sheet metal purchased these days is dual certified 316/316L (Carbon and tensile strength properties are met. I need to know if this also holds true for bolts / fasteners.
Thanks
@Bex- All of the bolts that we manufacture will be dual certified 316/316L since all of the available raw steel is dual certified. However, many of the off the shelf imported stainless fasteners are not, so it is hit and miss. We would need to check the availability of 316L for specific sizes upon request.
Should ASTM F593 be referenced/used if the SS316 Bolts are not for High Temperature Service ? The same question for SS316 Nuts and the requirements of A194 High Pressure – High Temperature vs. F594 ?
@Randy- ASTM F593 was written for general purpose service, not high temperature. ASTM A193/A194 are specifically for high temperature service, although because those grades are widely available, they are commonly used for general purpose applications. Our recommendation is to use the simplest callout that gives you what you need. If commercially available 316 is all you need, then specify it that way. If you instead need the tighter chemical and mechanical requirements of F593, use that. If you need the high temperature properties of A193, use that. The more specific you are in your call outs, the more expensive and less available your options.
Are 316L bolts/nuts available in heavy-hex for structural applications, similar to A325 carbon steel specification?
@Ryan- Heavy hex stainless bolts are available through the ASTM A193 specification. You are not able to exactly match the mechanical properties of A325 structural bolts, but if you use class 2 bolts, you can get a lot closer than the normal class 1 bolts.
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.
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.
What is the temperature rating for 304 stainless hex caps?
@Danny- I am sorry, but we do not have any information on temperature ratings for fasteners.
I have been unable to find 8 each 316 stainless steel tap bolts, 5/16″ x 1.75″ long.
Do you have any suggestions on where I could purchase them ?
@Steve- My suggestion would be to call a couple of your local fastener distributors. If it is available, they’d have it. It isn’t an item that we carry.
what tonn will an 18mm &a 20mm & a 25mm ss bolt lift on a straight lift,not a shear.thankyou
@Chris- Apologies, but we do not have any information on metric sized fasteners.
As far as tensile strength which is superior 18-8 or 316?
@Ron- In their normal cold drawn condition, they are identical. Both have a 75ksi minimum tensile and 30ksi minimum yield.
Dane/Derek:
My main question regards the % of Cu in each: 18-8, 316, 316L, 304L, and 304. Is there a max/min tolerance for each that is standardized? What is the average?
Thank you in advance,
–Jason
@Jason – there is no requirement for copper in those stainless grades at all. A quick look at a couple dozen recent heats from our inventory shows that the actual copper content range was 0.33 – 0.53%.
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.
Why are teck screws labeled 18-8 as so magnetic. And are they corotion resitant?
@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.
Is there any magnetic permeability difference between SS303 ans SS304?
@Siyue – Apologies, but we do not have any information regarding magnetic permeability.
questions on material 18-8 for screw lock. Why not use 304 SS?
@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.
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 ?????
@Joe Hamilton – Your question is a bit out of the realm of our expertise. Apologies, but we will be unable to help.
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?
@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.
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
@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.
I need 30 stainless threaded inserts.
pluss 2 screws
@lambos – I would encourage you to contact a distributor in your area for those items, they are not anything that we normally handle.
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
@Faqir – Portland Bolt does not have engineers on staff and cannot make material recommendations for specific applications.
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
@Amir – Portland Bolt does not have engineers on staff and cannot make recommendations as to the suitability of materials for specific applications.
what is the difference between 17-4 and 316 stainless steel?
@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.
estimado trabajo en el área de válvulas industriales
y me piden
acero 18-8 sMo 3HF
que tan especial es ¿
saludos cordiales
Alexis Ayala
@Alexis- We are not familiar with sMo 3HF material, and how it is different from standard 18-8. Apologies.