When the Best Bolt Is Not the One to Use

May 2022 

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Definition: A fastener, typically metal, having a cylindrical shank with a helical thread, and topped with a head with geometry developed for driving or rotating so as to securely join multiple components.  

 


In this article I’m hoping to give you a new perspective on choosing screw fasteners and hopefully keep you out of what can be in some cases, a heap of trouble.

Repeatedly in my engineering consulting practice I have seen 12.9 strength level fasteners fail from hydrogen embrittlement. Over and over again, companies have engaged Spectrum Materials Engineering’s services because they have had such fasteners fail at static stress levels well below where they are expected to fail.

Before I go any further, I should give you a just a little background first. When selecting popular steel metric socket head cap screws, engineers can choose between three different strength levels:

  1. 8.8, which is required to have a minimum tensile strength of 800 MPa,

  2. 10.9 which has a minimum tensile strength of 1000 MPa, and finally,

  3. 12.9 which has a minimum tensile strength of 1200 MPa.

These strength levels are specified by ISO 4762.

Many of the engineers I work with must look at these different strength levels and naturally reason that the 12.9 strength level fasteners are the strongest and therefore the “best.” So why not use them? They don’t cost much of a premium and they want nothing but the “best” for their projects.

In some cases, they also specify an electrolytic zinc coating to keep the fasteners from rusting.

If 12.9 fasteners are used without being electroplating, they can indeed be an excellent choice. However, when they are electroplated, the risks of an unexpected failure increase depending on whether or not your favorite plating shop has been careful with their post-processing.

In broad strokes, the electrolytic zinc coating process involves submerging the fasteners in an aqueous bath in which there are zinc ions floating around. A voltage bias is created that attracts and deposits these ions on the fastener. Indeed, this process works great. However, the applied electric bias also causes the water in the bath to break-down into oxygen and hydrogen.

With free hydrogen atoms on the loose, they readily diffuse into the base steel of the fastener. If the steel has a tensile strength less than or equal to 1000 MPa, this isn’t much of problem. However, a 12.9 fastener has a tensile strength above this threshold and when hydrogen accumulates within these fasteners, gas pressure begins to build up at certain grain boundaries until the material is severely weakened and embrittled.

This is when the unexpected failures occur.

It is possible to bake-out the hydrogen directly after electroplating and avoid any problems. Not surprisingly, there are published specifications that describe the necessary bake-outs, such as ASTM B850.  Some plating shops will only do a bake-out for 3-4 hours thinking that they are meeting the spec and eliminating the risk for hydrogen embrittlement.  However, hydrogen embrittlement is statistical in nature and sometimes 3-4 hours (at 220˚C) is simply not enough. Consequently, when using plated 12.9 fasteners I always insist that the plating shops I use do no less than a 12-hour bake-out at 200˚C. Sometimes I even require a 16-hour bake out depending on the thickness of the coating. I do this to ensure I never have to deal with hydrogen embrittlement because it can be very dangerous.

When you buy zinc plated 12.9 fasteners from a catalog (online or not), not only do you typically not know the time and temperature of the bake-out used, but you don’t know if the plating shop was diligent and responsible.

My recommendation to my clients when buying plated fasteners from a catalog is to do a careful stress analysis and then downgrade to 10.9 or even 8.8 fasteners if their design allows for it.  This greatly reduces the risk of suffering an embrittlement related bolt failure.

If they truly need the strength of a 12.9 fastener, I advise them to specify zinc flake coatings which  consists of zinc flake (and sometimes aluminum flake too) in an organic binder. Such coatings do not involve electrolytic processes and therefore have far low chance of exposing your fastener to loose hydrogen atoms.

Finally, if the 12.9 fasteners are required to live at elevated temperatures (> 400˚C) and consequently zinc flake coatings are not option, I advise them to avoid buying the final fastener from a catalog. Rather, I advise that they buy non-coated fastener from the catalog, then have them coated at a plating shop they trust and define a process plan with a proper minimum 12-hour bake out.

So, when it comes to buying screw fasteners the “best” may not always be the best for your application. Take the time to do a thorough stress calculation and determine if you really need to use 12.9 socket head cap screws. If your stress levels are low enough, you can avoid a lot of risk by using 8.8 or 10.9 strength level fasteners and save yourself or your company a few dollars at the same time.

Beryllium - The True Unobtanium?

October 2020

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Long ago in a beautiful Southern California town located at the base of the Santa Monica Mountains, I was a half nerd, half jock high school student. For some strange reason, I had caught a bug to build a scale remote- control car, from scratch. The yearning introduced me to carbon fiber and the Southern California carbon fiber industry. I built a rather primitive chassis out the stuff and learned the hard way the excruciating pain of getting carbon fiber stuck in your skin.

The chassis was done, and I needed chassis components – tiny A-arms, tiny sway bar mounts, tiny suspension knuckles, etc. What to do? It turned out that one of my best friends had a neighbor down the street who was a professional machinist and had a rather well-equipped shop in his garage.

So, off I went down the street to meet Mr. Marx. I found him high up in a palm tree holding a pretty serious looking chainsaw. He was only about 75 years old. He climbed down to greet me. I explained my predicament and showed him my hand-drafted component drawings. He invited me into his machine shop and showed me around.

His specialty, as a machinist, was machining beryllium. He had billets of beryllium and a few machined components in process on one of his lathes. Apparently, he had several active contracts with, well, some branch of the military (he wouldn’t tell me which one).

Mr. Marx was an extremely fortunate man indeed. It turned out that he had some rare genetic disposition that allowed him to machine beryllium without getting the dreaded sensitization immune response and chronic beryllium disease (CBD) most people get from machining the material. These are most often the result of inhaling tiny air-born particles generating during machining the metal. If you get it, expect severe pneumonia-like symptoms, scared lungs, cancer and potentially even death. But Mr. Marx had been machining it for somewhere between three and four decades with no issues. He told me he built beryllium cameras for NASA that went to the moon, and to this day they are still up there.

Mr. Marx probably wasn’t happy about it, but the stupid kid (me) picked up one of the round finished components sitting next to the lathe. It was about three inches in diameter and about an inch and a half thick. It was a beautiful flat white-silver color, kind of like sand-blasted aluminum.

To this day, I can say with confidence, that it is the weirdest piece of metal I have ever held in my hand. It was bizarre lightweight to the extreme. Think balsa wood. Think cold balsa wood. It was completely different than any piece of titanium or aluminum or any other metal I have held in my hand ever since.

The numbers support my impressions. The density of beryllium is 1.85 g/cm^3 compared to aluminum which is 2.7 g/cm^3, titanium at 4.5 g/cm^3 and heavy-old steel at 7.8 g/cm^3. The only common engineering metal with a lower density is magnesium at 1.7 g/cm^3.

But here is the real unique and valuable part about beryllium; it is stiff, really stiff. Its Young’s modulus is about 303 GPa. That is nearly 48% higher than that of steel and nearly 439% higher than that of aluminum. Magnesium might have a slightly lower density, but the Young’s modulus of beryllium is nearly seven times that of magnesium’s.

Far more impressive are the density normalized (aka specific) values. Because beryllium has such a low density, its specific stiffness is just shy of six and a half times higher than that of both steel, aluminum, and magnesium.

Holy structural optimization!

This means that if you were designing a beam in bending for minimum deflection and needed it to be as light as possible, without sacrificing any stiffness, a beam made from beryllium could be 5 times lighter than if it were made from steel. It would be two and a half times lighter than if it were made of aluminum.

Before the governing body of Formula 1 outlawed the use of beryllium, Italian racing brake manufacture Brembo made brake calipers from beryllium. This is perhaps the most ideal application of beryllium that I’ve ever heard of. Brake calipers must be lightweight because they not only add to the overall weight of the vehicle, but their weight affects unsprung weight which in turn negatively affects vehicle handling. So lighter is acutely better. Stiffness is also critical because a stiff brake caliper adds confidence to the way the brake pedal feels. Finally, brake calipers get so hot that an ultra-stiff carbon fiber reinforced plastic would never survive the heat without severe oxidation and mechanical degradation.

Specific stiffness is not the only reason to use beryllium. Its thermal expansion is nearly identical to that of steel, and far less than half that of aluminum. Consequently, it is considered to be very thermally stable. This is why it is often used in space applications because for its low density, components made from beryllium show little dimensional change as they orbit the earth and change temperature as the component goes from the dark to the light. NASA’s James Webb Space Telescope has multiple beryllium mirrors.

Beryllium is also, interestingly, transparent to x-rays. As a student, I remember using X-ray diffraction equipment with beryllium windows. But its main use in this capacity is in medical X-ray machines.  

There are indeed challenges with applying pure beryllium as a structural material. Its strength is only about 240 MPa (yield strength). Nonetheless, its density is so low that if you were designing a beam in bending for strength and minimum weight, you could produce a beam from beryllium that would be more than half the weight of a steel (assuming the steel had a moderately high yield strength of 880 MPa) beam without sacrificing load handling capability.

Another big hurdle in using beryllium for structural applications is its low fracture toughness. In many cases it is only on par with a moderately tough technical ceramic. This means the fracture toughness is about a tenth that of a good steel and about a third of that of a 6XXX series aluminum.

But the biggest issue with beryllium is that it is stratospherically expensive. The amount of money you would need to pony up to buy a pound of beryllium would get you about 500 pounds of low-alloy steel.

The main contributor to the high cost of beryllium is that elemental beryllium is rare. In the Earth’s crust, atoms of beryllium always come attached to oxygen and silicon. They are mainly in rare minerals such as beryl and bertrandite. These minerals are also known as emeralds and anyone who has bought a ring with one of those stones knows how expensive they are! The energy required to separate these beryllium atoms from the oxygen and silicon in these minerals is formidable. Combining these factors together goes a long way to explaining the high cost of pure beryllium metal.

Making matters worse, in most cases (except for Mr. Marx’s) machine shops require special equipment to keep the beryllium dust out of the air to mitigate the risks of the aforementioned health risks.

Finally, the US Military considers beryllium metal to be a critical strategic metal and maintains a 45-metric ton stockpile of it to support their current and projected future needs. Given their budgets and demand for the material, it is easy to visualize the supply-demand curve that gives further explanation to beryllium’s high price.

Of course, there are alloys where beryllium is mixed with either copper or aluminum. These alloys do succeed in lowering the cost, addressing the health issues and getting some improvement in specific stiffness compared to the base aluminum or copper. But the thermal expansion is significantly higher, and the stiffness is still low compared to pure beryllium.

In the US, the two leading companies in beryllium and beryllium alloy production are Materion and IBC Advanced Alloys. Both companies are developing new alloys that offer as much of beryllium’s advantages as possible at the best value proposition. In addition, Materion is known to be hard at work minimizing the cost of beryllium by managing the entire value chain and by developing recycling methods.

Regardless, at this point in time, pure beryllium metal is unfortunately the true unobtanium.