In the previous articles in our Bolts 101 series, we spoke briefly about Stainless Steel Bolts, but we did not discuss specifics. With typical alloy steel bolts, there are grades (like Grade 5, Grade 8, etc.), but with Stainless Steel, Brass, Plastics and other materials, it is not that simple.
We’ll call this Page 3, continuing in our Bolts 101 article series. In this section we will look at non-graded bolts, and especially Stainless Steel bolts, because Stainless is really attractive for things like trailers that see harsher environments.
Perhaps this page is a touch beyond the 101 level, but we will discuss it anyway. I like the entry concept of “101” because it fits. We do not need a full understanding of all there is to know, but we do need enough information to make appropriate choices. That is the level of these articles.
Bolt Grades Review
On Page 1 of Bolts 101 there is a fairly long section about bolt Grades for both English and Metric. If you are not familiar, we recommend reading that first article. This chart (from page 1) is a quick summary of Grades for standard alloy steel bolts – with the approximate strength for each category.
The various strengths for the bolt grades is achieved by a combination of both material and processing. Steel Alloys of various types are part of it. Of course, different alloys have different strength characteristics so they use appropriate steel for each bolt grade.
Processing is a combination of how they form the metal. Cold working, Hot working, forging, and other processes all effect strength. Then, heat treating (both temperature and quenching) come into play. The final bolt strength is a combination of all these factors. And, probably some I don’t know. Do a Google search for a lot more information, if this is of interest.
Where Do Stainless Steel Bolts Fit?
The markings on alloy steel bolts for each grade are super convenient. Just look at the head and you know what to expect for strength. Stainless steel does not have this nice convenience. Instead, we have to know a little more.
Just like the discussion above about bolt materials, the various Stainless Steel alloys have different strength characteristics. The most common Stainless Steel alloy for bolts 18-8. The next most common is probably 316.
Let me interject. The numbers, like 316 or 304, etc. have reference to the type of stainless steel alloy. For instance, 18-8 is a 300 series stainless steel with approx 18% chromium and 8% nickel. But, we don’t really need to know all that. I just use the numbers as a name and leave the rest to the experts. (I know a little, but I am not a materials expert.)
With Stainless Steel, there are 2 big differentiators. 1) How corrosion resistant each alloy is. 2) Material strength.
Stainless Steel is known for its resistance to corrosion. This comes, if I understand correctly, with the high nickel and chromium content. Anyway, Stainless Steel 316 is one of the least reactive (best for resisting corrosion), and is actually a preferred material for surgical instruments. Screws and pins used for surgical bone repair are often 316 Stainless Steel. A few others also hit this “exceptionally corrosion resistant”, but they are much less common for bolts.
Bolts made from 18-8, 17-4 PH and Stainless 450 all are very corrosion resistant to chemicals and wet conditions, but maybe not perfect for continuous sea water (or Great Salt Lake) environments. * Please don’t take that statement out of context. All of these Stainless Steels are far superior in corrosion resistance than any of the alloy steel bolts.
For the sake of time, I mention only the common bolt materials.
If we look at the chart of bolt strength above for comparison, bolts made from 18-8 and 316 Stainless Steel are both a little stronger than Grade 2. Grade 2 is ~60,000 psi, and 18-8 is often specified as in the range of 60,000 psi to 80,000 psi – sometimes up to 100,000 psi. 316 Stainless Steel is typically a little more like 80,000 psi to 100,000 psi.
In general, if you don’t know otherwise, it is usually safe to think of Stainless Steel bolts as a little above Grade 2 as a comparative measure.
Higher Strength Stainless Steel
There are, of course, many more alloys in the Stainless Steel family. Some of these are pretty high strength, and while they don’t make bolts from all the alloys, there are some.
Stainless Steel 17-4 PH is a very strong material. It is not so common for bolts, and it is quite expensive, but it does produce some pretty strong bolts. Think of 17-4 as fitting between Grades 5 and 8 comparatively. Grade 5 is ~120,000 psi, Grade 8 is ~150,000 psi. 17-4 PH Stainless Steel bolts often come in around 130,000 psi.
The next step up in strength is 450 Stainless Steel. Again, this is not very common, and quite expensive. Bolts from this material are in the range just above Grade 8, at something like 160,000 psi to 170,000 psi.
So, if you have an application where corrosion resistance is super important, and strength is also critical, these 2 higher strength Stainless alloys can get you there. However, it may take some searching, and a high price for them. Even places like McMaster-Carr don’t have a very large selection of bolts in these materials.
Identifying Stainless Steel Bolts
One of the most common myths about Stainless Steel is about magnetism. They say Stainless is non-magnetic. While this is mostly true for some alloys, it is not so true for others. Since the base of Stainless Steel is iron, all have some level of magnetism, but some, like 316, don’t have much.
In general, Stainless Steels like 304, 316 and 18-8 are considered non-magnetic. Others, like 17-4 and 450 are magnetic, or become magnetic when they are hardened. This is an area I don’t understand well, so I don’t want to go into much detail. The point? A magnet is not a sure fire way of identifying a bolt as being Stainless Steel or not.
Most Stainless Steel bolts do not usually have identification. However, some do, like the code on the head in the image, but that is not so common. That makes it a little dangerous if we think the bolt is a high strength alloy, but it is not.
(The B7 or B8 code has to do with strength, kind of like the Bolt grades above, but different. The numbers don’t correlate, like B8 is not Grade 8.)
My Recommendation: If you work with Stainless Steel bolts, keep them in a marked package so you know the strength. Otherwise, if in doubt, treat them like a Grade 2 bolt.
One of the less desirable characteristics of Stainless Steel has to do with galling. Wikipedia describes galling as “adhesive wear caused by the microscopic transfer of material between (sliding) metallic surfaces” “especially if there is a large amount of force compressing the surfaces together.”
Large forces? Sliding surfaces? Yup, that’s what happens when we tighten a nut and bolt together.
Stainless Steel seems especially susceptible to Galling, and it will mess up the threads of a bolt and nut. In my experience, this is especially true when the materials are the same, and even more true when they are NOT hard. The classic symptoms are: 1) it’s hard to get the bolts to tighten properly; or 2) you can’t get them back apart.
So, beware when tightening a 18-8 Stainless Steel bolt with a 18-8 nut. I recommend using a little super tough grease or anti-seize when you put them together. Perhaps even better, use a 316 nut with the 18-8 bolt (and use the grease).
Hardened bolts like 17-4 are somewhat less susceptible, but after one experience with stainless bolts where I ruined them trying to get them undone, I treat all stainless steel bolts with caution and always use a little lubrication when putting them together. Anti-seize is messy, but it works, so that is my agent of choice.
So Much More To Learn
I know this article just scratches the surface with respect to Stainless Steel and bolts. There is just so much to learn and so much I don’t know. If you want more, you can read for days on searches from Google. I certainly appreciate all the good information others have put out for us to learn. Thank you.