Not all fasteners are created equal, especially when they’re pushed to their limits. In extreme environments, the wrong material choice can result in catastrophic failure.Ěý

From high-heat industrial systems to subzero outdoor applications and high-pressure assemblies, you must consider how temperature and environmental stress affect fastener performance. Material expansion, brittleness, corrosion resistance, and mechanical strength all come into play.Ěý

However, the challenge here is that high-performance materials often come at a premium. Knowing when that added cost is justified versus when it’s not is important. And that’s what this article is meant to help you work through.

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Key Takeaways

• Extreme environments can cause fasteners to weaken, crack, and corrode if improper materials are used
• High-performance alloys (like Inconel or stainless grades) are critical in high heat, cold, and pressure applications
• Industry standards such as ASTM and SAE help guide proper fastener selection

Extreme Environment Fastener Material Comparison

Fastener Materials for Extreme Heat Environments

High-temperature environments, such as exhaust systems, power generation equipment, and industrial furnaces, pose serious challenges for fasteners.Ěý

As temperatures rise, most metals begin to lose strength. This phenomenon is known as thermal softening and can reduce a fastener’s load-bearing capacity. Additionally, oxidation and scaling can degrade material surfaces, leading to corrosion and eventual failure.Ěý

The most common risks for fasteners in high-heat environments are:Ěý

• Loss of tensile strength
• Creep (slow deformation under constant load)
• Oxidation and scaling
• Thread galling in stainless materials

Top High-heat Fastener Material Recommendations

For extreme temperatures, standard carbon steel fasteners often aren’t enough. Some options to turn to are:Ěý

• Alloy steels with heat treatment
• Austenitic stainless steels (like 304 or 316)
• Nickel-based superalloys, such as InconelĚý

Relevant Specifications to Look Into

For more information and recommendations based on your specific needs, take a look into these specifications:Ěý

• (high-temperature alloy and stainless steel bolting)
• (mechanical and material requirements for externally threaded fasteners)

When Is the Extra Cost for a Special Heat-Resistant Fastener Material Worth It?

If fastener failure would lead to safety risks, equipment shutdown, or expensive maintenance, investing in a heat-treated or super alloy material is likely justified. The cost of failure almost always outweighs the expense of upgraded fasteners.Ěý

Just make sure that your application is not something that traditional carbon or alloy steel could handle because high-performance fastener materials do come at a premium — often 5x up to 20x more expensive.Ěý

Fastener Materials for Extreme Cold Environments

Cold temperatures present a different, but equally dangerous, set of challenges. Instead of softening, fastener materials can become brittle and prone to sudden fracture.Ěý

This is especially critical in industries like oil and gas, infrastructure, and outdoor equipment exposed to harsh winter climates. But when temperatures drop even further into cryogenic ranges (typically below -150°F), the risks and material requirements get even more extreme.Ěý

Cryogenic environments are found in applications like liquefied natural gas (LNG) storage and transport, aerospace systems, medical and scientific equipment, and industrial gas processing. At these temperatures, many materials that perform well in “normal” cold conditions will fail without warning. The key issue is ductile-to-brittle transition, where metals lose their ability to deform and instead crack under stress.Ěý

The most common risks for fasteners in low-temperature and cryogenic environments are:Ěý

• Brittle fracture
• Reduced ductility
• Loss of impact resistance
• Thermal contraction affecting preload

Top Low-temperature Fastener Material Recommendations

For standard cold environments, engineers often rely on:Ěý

• Low-temperature carbon steels
• Austenitic stainless steels

For cryogenic applications, material selection is more restrictive. Common choices are:Ěý

• Austenitic stainless steels (like 304 and 316)
• Nickel alloys designed for extreme low-temperature toughness
• Aluminum alloys (in certain applications)

These materials are specified because they do not exhibit a ductile-to-brittle transition at cryogenic temperatures.Ěý

Relevant Specifications to Look Into

For more information and recommendations based on your specific needs, take a look into these specifications:Ěý

• (standard for alloy and stainless steel bolting for low-temperature service)
• requirements (to verify toughness at specified low temps)
• Additional cryogenic material testing standards depending on industry (e.g., LNG or aerospace requirements)

When Is the Extra Cost for a Special Low-temperature Fastener Material Worth It?

Material upgrades are important if your application involves:Ěý

• Temperatures below -20°F
• Cryogenic conditions below -150°F
• Dynamic loads, vibration, or pressure cycling
• Safety-critical or high-cost systems

In cryogenic environments especially, there’s very little margin for error. A fastener that fails at these temperatures won’t gradually degrade; it will fracture suddenly and cause a lot of damage.Ěý

But again, if your application doesn’t meet the requirements listed above, it might be better for your budget to look into less expensive options first. They might do a fine job at a fraction of the cost.

Fastener Materials for Extreme Pressure Environments

Pressure introduces another layer of stress that compounds temperature effects. High-pressure systems like pipelines, hydraulic systems, and pressure vessels require fasteners that can maintain clamping force under intense load.Ěý

Common risks in high-pressure environments are:Ěý

• Bolt yielding or stretching
• Fatigue failure from cyclic loading
• Leakage due to loss of preload
• Stress corrosion cracking

Top High-pressure Fastener Material Recommendations

High-pressure applications demand high tensile strength materials with consistent preload capability and resistance to fatigue and corrosion.Ěý

Fastener materials that fit the bill are:Ěý

• High-strength alloy steels
• Quenched and tempered fasteners
• Corrosion-resistant alloys (or superalloys)Ěý

Relevant Specifications to Look Into

For more information and recommendations based on your specific needs, take a look into these specifications:Ěý

• (commonly used in pressure vessels and piping applications)
• for oil and gas applications

When Is the Extra Cost for a Special High-pressure Fastener Material Worth It?

Similar to the sections above, fastener materials that can handle extreme-pressure environments are going to cost more than traditional alloy or carbon steel materials. It is recommended that you consult the specifications above (or others applicable to your situation) to see what is best suited.Ěý

You don’t want to pay for something you don’t need, but you also don’t want to deal with the consequences of not paying extra for something you should have.Ěý

Don’t Risk Failure — Choose the Right Fastener Material

Extreme environments demand more from your fasteners, and cutting corners on material selection can lead to serious consequences.Ěý

Whether you’re dealing with high heat, freezing temperatures, or intense pressure, choosing the right fastener material is important to maintaining performance, safety, and reliability. Industry standards provide a strong starting point, but real-world applications often require a more tailored approach.Ěý

At şÚÁĎÍř, we specialize in small-batch, custom fasteners built to your exact specifications. We most commonly manufacture fasteners with carbon steel or alloy steel materials, but would be happy to steer you in the right direction if your application requires a more robust material. Give us a shout here — we’re happy to talk you through your options.Ěý

Material compatibility isn’t glamorous. Nobody puts it on a spec sheet header or talks about it in design reviews until something seizes, corrodes, or shears off mid-assembly. Then suddenly it’s very interesting.Ěý

The truth is that pairing the wrong fastener material with the wrong mating surface is one of the most expensive mistakes in mechanical assembly, not because it fails immediately, but because it often fails slowly, invisibly, and at the worst possible time. This post will give you a practical framework for what to pair with what, and what to avoid.Ěý

Key Takeaways

• Galling is a thread-seizure failure caused by metal-to-metal adhesion under friction. It’s most common in stainless steel, titanium, and aluminum fasteners — not in alloy steel or carbon steel.Ěý
• Galvanic corrosion occurs when dissimilar materials are electrically coupled in the presence of moisture. The greater the gap between metals on the galvanic series, the faster the more active metal corrodes.Ěý
• The most dangerous metal pairing is stainless steel fastener + stainless steel tapped hole.Ěý
• The right coating on a steel fastener can solve many compatibility problems without requiring a material change.Ěý

Two Fastener Failure Modes Worth Distinguishing

Before we get into specific material pairings, it’s worth being precise about what we’re actually talking about.Ěý

1. Galling

Galling is a form of adhesive wear that occurs when two threaded surfaces under pressure and motion break down their protective oxide layers, making direct metal-to-metal contact, and begin transferring material to each other. The threads heat up, microscopic high points shear and fuse, friction escalates, and eventually the fastener seizes.

Galling happens during installation, often in seconds, and it doesn’t care what the ambient environment looks like. It is exclusively a concern with materials that form self-healing oxide films, like stainless steel, aluminum, and titanium. Hardened alloy steel and carbon steel, by contrast, rarely gall because they don’t have the same soft, adhesive oxide-then-bare-metal surface behavior.Ěý

2. Galvanic Corrosion

Galvanic corrosion is what happens when two electrically dissimilar metals are in contact in the presence of an electrolyte (moisture, salt). One metal becomes the anode and corrodes preferentially to protect the other. The further apart two metals sit on the , the faster the attack on the anodic (less noble) material. This is an environmental failure mode because it requires moisture to occur and typically develops over weeks, months, or years rather than during installation.Ěý

The practical rule: the fastener should always be the more noble (cathodic) material, or the materials should be close enough on the galvanic series that the potential difference is negligible.Ěý

What Materials şÚÁĎÍř Works With & Why

We primarily manufacture fasteners from alloy steel and carbon steel. These are two materials that cover the vast majority of industrial, automotive, defense, and heavy machinery fastener applications. We do not routinely manufacture stainless steel, titanium, aluminum, Monel, or Inconcel fasteners, though we understand the material science behind them.Ěý

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This matters because alloy steel and carbon steel fasteners sit in a favorable position on both failure axes:Ěý

• Galling risk: Very low. Hardened steel doesn’t gall the way stainless steel does.Ěý
• Galvanic risk: Moderate to manageable. Steel is mid-range on the galvanic series, neither aggressively noble nor aggressively active.Ěý

That said, even alloy steel and carbon steel fasteners can be used incorrectly. Make sure you understand the material compatibility picture clearly before proceeding with any combinations.

Fastener Material Compatibility: What Works, What Doesn’t, and Why

Carbon Steel or Alloy Steel Fastener into Carbon Steel or Alloy Steel

Compatibility: Excellent

This is the default case, and it works well. Same material family, similar hardness, no galvanic potential, no galling risk. The only caution is ensuring the fastener grade is appropriate for the joint load. A Grade 2 screw in a Grade 8 application is a strength problem, not a material compatibility problem. We cover bolt grades in detail here if you need a refresher on that.Ěý

For bare steel assemblies in corrosive environments, a surface finish — zinc electroplate, zinc flake, phosphate and oil — is the appropriate mitigation. The coating changes the environmental durability, not the material compatibility.Ěý

Carbon Steel or Alloy Steel Fastener into Aluminum

Compatibility: Conditional

This pairing is extremely common. Steel fasteners into aluminum castings, housings, and structural members are found in automotive, aerospace, and heavy equipment applications everywhere. It works, with caveats.Ěý

Galvanic risk: Steel is more noble than aluminum, so in the presence of moisture, aluminum becomes the anode and corrodes preferentially around the fastener. The risk is proportional to the electrolyte present; dry indoor environments see little to no effect, while marine or outdoor environments can see significant aluminum degradation at the fastener interface over time.Ěý

Thread stripping risk (not galvanic, but related): Aluminum is considerably softer than steel (roughly 60-80 HV vs. 250-370 HV for structural steel grades). Torque specs written for steel-into-steel will strip aluminum threads. Minimum thread engagement in aluminum should be at least 1.5x the nominal diameter, compared to 1x for steel. See our blog on stripped screws for the full engagement length discussion.Ěý

The fix: In dry or low-exposure environments, a zinc-plated or coated steel fastener will perform well with no further mitigation. In wet or outdoor environments, use an isolating barrier (nylon washer, sealant) or specify a fastener coating that reduces the electrochemical potential gap. Anti-seize compounds also help, but they primarily address friction and galling. For galvanic protection, physical isolation or matched materials is more reliable.Ěý

Stainless Steel Fastener into Stainless Steel

Compatibility: High Galling Risk — Proceed with Caution

This is the most reliably problematic pairing in the fastener world. form thin, self-healing chromium oxide films that provide their corrosion resistance. Under thread engagement and torque, those films are scraped off, exposing bare, soft, highly adhesive metal. When both the fastener and the tapped hole are the same stainless alloy, the exposed surfaces are metallurgically identical and they weld together almost immediately. The seizing can happen before you’ve even reached your target torque.Ěý

This is sometimes called “stainless steel seizing,” and it is the dominant failure mode for stainless-on-stainless assemblies installed without lubrication or material differentiation.Ěý

Mitigations:Ěý

• Use an anti-seize compound. Molybdenum disulfide-based anti-seize is preferred over nickel-based, as nickel can form abrasive nickel oxide particles under heat and actually worsen galling.Ěý
• Specify different stainless grades for fastener and nut/tapped components (e.g., 304 bolt into 316 nut). Different work-hardening rates reduce adhesion tendency.Ěý
• Use duplex stainless or martensitic stainless (400 series) for one component. The hardness differential can reduce galling risk.Ěý
• Slow down installation. Power tools generate frictional heat that accelerates the oxide breakdown cycle.Ěý

Carbon Steel or Alloy Steel Fastener into Stainless Steel

Compatibility: Generally Acceptable

This one surprises people. A carbon steel or alloy steel fastener into a stainless steel tapped hole is actually less problematic than stainless-into-stainless. Hardened steel doesn’t share stainless’s galling behavior, so thread seizure risk is low. Galvanically, stainless is more noble than carbon steel, meaning the steel fastener would be the anode in a galvanic couple, but this is typically manageable because the fastener’s exposed surface area is small relative to the larger stainless structure, and a quality coating on the steel fastener largely seals it from the electrolyte.Ěý

The reverse scenario — a stainless steel fastener in a carbon steel or alloy steel structure — is generally fine galvanically. Carbon steel is the larger anode, but with a large surface area the attack is distributed and slow. However, it introduces stainless’s galling tendency if the tapped hole is also stainless.Ěý

Carbon Steel or Alloy Steel Fastener into Brass or Copper Alloy

Compatibility: Galvanic Risk in Wet Environments

Copper and brass are significantly more noble than carbon steel. In the presence of moisture, steel becomes the anode and corrodes to protect the copper/brass components. The potential difference is large enough to produce a meaningful galvanic attack on the steel over time. In dry environments, this pairing is generally acceptable and widely used in electrical and plumbing applications. In outdoor and wet service, specify a coated or plated steel fastener, or consider whether the application really requires a non-ferrous fastener material or not.Ěý

Material Compatibility Quick Reference

The Role of Fastener Coatings in Material Compatibility

A surface finish isn’t just about corrosion protection on the fastener itself. It’s a compatibility tool. The right coating changes the fastener’s electrochemical behavior and can close a significant galvanic gap between dissimilar materials.Ěý

Zinc electroplate, zinc flake, and phosphate and oil are the most common finishes on carbon steel and alloy steel fasteners. Each changes the corrosion resistance profile meaningfully. A zinc-plated steel fastener into an aluminum casting behaves very differently at the interface than a plain steel fastener; the zinc is sacrificially consumed before the steel substrate, and the aluminum around it sees far less galvanic attack.Ěý

Coatings also affect the torque-tension relationship. A well-lubricated zinc flake coating reduces friction at the thread interface, which shifts the coefficient of friction and changes the effective torque spec. If you’re specifying a coated fastener, your torque values should account for the coating. Using dry-steel torque specs on a lubricated fastener means you’re over-tensioning the joint by more than you think.Ěý

What This Means in Practice

If you’re specifying carbon steel or alloy steel fasteners, you’re already working with the most galling-resistant, broadly compatible fastener material available for structural applications. The primary decisions you need to make are:Ěý

1. What’s the mating material? If it’s aluminum or brass in a wet environment, add a coating or barrier.Ěý
2. What’s the environmental exposure? Dry and indoor is forgiving. Outdoor, marine, or chemically active environments demand a coating selection process.Ěý
3. Is any component stainless steel or titanium? If so, understand whether galling risk applies and specify anti-seize or grade differentiation accordingly.Ěý

Get those three questions answered and you’ve resolved 90% of material compatibility problems before assembly begins. The other 10% is what custom fastener engineers and manufacturers are for.Ěý

Still Sorting Out the Right Fastener Material for Your Application?

Not every fastener problem fits neatly into a table. If you’re working through a design with an unusual material combination, unusual environment, or a history of unexplained failures, our team is happy to think through it with you. We’ve been making custom bolts, screws, and studs for over 70 years, and material compatibility questions come with the territory.Ěý

Start the conversation here.

Advancements in additive manufacturing technology are reshaping industries. With all the buzz about 3D printing, it’s natural to ask if the technology could eventually replace tried-and-true fastener manufacturing methods.

My short answer is no…not yet at least. But depending on your needs, both approaches have their place.

TL;DR: 3D-Printed vs. Cold-Formed Fasteners

Can You Print 3D Fasteners?

Yes, you can 3D print fasteners. Using , , or high-performance polymers, it’s possible to create bolts, studs, and screws.

You can 3D print just about anything these days. The real question is whether you should.

If you only need a few pieces for prototyping and aren’t worried about strength, 3D printing might make sense. But if you need hundreds (or more) of high-strength fasteners, I advise sticking to traditional manufacturing methods. I’ll give more details as to why below.

How 3D-Printed Fasteners Are Made

3D-printed fasteners are designed using computer-aided design and then built layer by layer from powders, resins, or polymers. Depending on the method, lasers, binders, or heat fuse the material together. The most common materials used to 3D print fasteners are stainless steel powders, titanium alloys, and high-performance polymers and composites.

This technology allows for a high level of design flexibility and customization. You can produce shapes and geometries without needing new tooling for traditional methods like hot forging and cold forming. But there are trade-offs.

Pros and Cons of 3D-Printed Fasteners

When 3D-Printed Fasteners Make Sense

These are the use cases where I can see 3D-printed fasteners being successful:

• You’re testing a prototype and only need a few pieces
• You’re working on a non-structural application where strength isn’t important

So maybe they’re a good complement to traditionally made fasteners, but not a replacement.

The Challenges Holding Back 3D-Printed Fasteners

The drawbacks of 3D-printed fasteners are significant for most industrial applications. Because they’re built layer by layer, internal porosity and weaker grain structure make them less reliable under high loads compared to cold-formed fasteners. Production speed is also a major hurdle, as additive manufacturing can only produce one part at a time, which drives up per-part costs and limits scalability. Even when the part is complete, many prints require post-processing such as machining or polishing to achieve the surface finish needed for proper fit and performance.

Finally, industries like aerospace, automotive, and energy often impose strict safety and certification requirements that 3D-printed fasteners can’t consistently meet yet, keeping their adoption limited to niche and experimental projects.

How Cold-Formed Fasteners Are Made

Cold forming, also known as cold heading, is the process of forming a fastener without heating up the material. This is typically done by pressing the material into dies with high-pressure machinery. Instead of cutting material away, the die compresses and displaces it, which strengthens the material in the process.

That’s why cold-formed fasteners are so reliable. If you work in the automotive, aerospace, energy, or any industrial field, chances are your fasteners are cold formed.

You can read more about the cold forming process in this blog.

Pros and Cons of Cold-Formed Fasteners

For most industries and applications, cold-formed fasteners remain the clear choice. They bring the strength, consistency, and cost-effectiveness needed for large-scale manufacturing operations.

Get the Highest Quality Fasteners for Your Project

Manufacturing will continue to evolve as new technologies emerge, but until 3D printing technology advances even further, I believe cold-formed fasteners are the best solution.

At şÚÁĎÍř, my team specializes in custom, cold-formed fasteners manufactured to the highest quality. We manufacture every order at our facility based in Harrison Township, Michigan, and are able to support limited-run quantities. So if you need a small batch of fasteners that will meet your strength and performance requirements, we’ve got you covered.

You can request a quote online or send us a message to learn more.

We’ve talked about how to use head markings to determine the different grades and manufacturers of inch-series bolts, but metric bolt and screw head markings are a whole different beast.

They’re not a beast we can’t tackle though. Let’s walk through how to decipher metric bolt and screw grades, plus their head markings, using one ISO standard and its following property classes as examples:

ISO 898-1 Property Class 8.8 Metric Bolt and Screw Head Marking
ISO 898-1 Property Class 10.9 Metric Bolt and Screw Head Marking
ISO 898-1 Property Class 12.9 Metric Bolt and Screw Head Marking

What Are Metric Fasteners?

Before we discuss the major metric spec we use at şÚÁĎÍř, ISO 898-1, let’s first explain metric fasteners in general. It’s important to be aware that metric fasteners are a part of a completely different system than inch-series fasteners. Metric fasteners have different units of measurement, different threads, and different specs.

Therefore, you can’t necessarily “convert” parts back and forth from metric to inch series, and vice versa. There are rough equivalents and points of overlap —Ěýfor example, an M6 and a ¼” bolt are roughly the same diameters, and an M10X1.50 nut can be threaded onto a â…ś-16 bolt —Ěýbut the systems are separate from one another.

Metric Terminology: Grade vs. Property Class

What’s a property class? Simply put, it’s the metric equivalent of a grade in inch-series terminology. One nice thing about ISO 898-1 is that the property class number designations have a concrete meaning. In SAE J429 grades like Grade 2 and Grade 8, the “2” and “8” don’t actually refer to anything — but with ISO 898-1 property classes, the number designations refer to real numbers.

That said, a property class designation consists of two numbers separated by a dot. Here’s what those numbers mean:

• The first number is 1/100th of the part’s nominal tensile strength in megapascals (MPa).
• The second number is 1/10th of the percentage of the part’s nominal yield strength compared to the part’s nominal tensile strength.

So, in English, a Property Class 8.8 bolt is a bolt with a nominal tensile strength of 800 MPa and a nominal yield strength that is 80% of that nominal tensile strength, so 640 MPa.

*Note: You probably noticed the use of the word “nominal” quite a bit in the last couple of sentences. That’s because the strengths indicated in the numbering system are meant to be ballpark figures and not necessarily the exact required strengths.

Alright, let’s jump into the spec.

ISO 898-1 Metric Bolt and Screw Head Markings

is the only spec we’re going to talk about here because it’s by far the most commonly used metric spec for fasteners. This standard covers bolts, screws, and studs made from carbon steel and alloy steel, and it lays out the requirements for ten different property classes.

Interested in learning about ISO standards for fasteners? Check out this blog.

Of ISO 898-1’s ten property classes, let’s focus on just three of them: Property Classes 8.8, 10.9, and 12.9. We’ll discuss what their metric head markings look like, and what some of their tensile strength requirements are.

ISO 898-1 Property Class 8.8 Metric Bolt and Screw Head Marking

Let’s first look at a Property Class 8.8 head marking:

One thing that’s nice about the ISO 898-1 property class designations is that the approximate tensile strength is right there on the bolt head marking. Property Class 8.8 bolts and screws have a minimum tensile strength of 800 MPa for bolts and screws 16mm in diameter and under, and 830 MPa for bolts and screws with a diameter over 16mm.

To give you an idea of how this compares to the strength of inch-series fasteners, 6.9 MPa are about equal to 1 ksi. 800 MPa are about 155 ksi. So a Property Class 8.8 bolt or screw is a medium-strength fastener that’s roughly the same strength as an SAE J429 Grade 5 fastener.

ISO 898-1 Property Class 10.9 Metric Bolt and Screw Head Marking

The Property Class 10.9 head marking is shown here:

Property Class 10.9 bolts and screws have a minimum tensile strength of 1040 MPa for all sizes. As you can see, they’re slightly stronger than the nominal value in the property class designation. Property Class 10.9 bolts are considered high-strength parts. They’re in the neighborhood of SAE J429 Grade 8 parts.

ISO 898-1 Property Class 12.9 Metric Bolt and Screw Head Marking

Finally, here’s the Property Class 12.9 head marking:

And there’s nothing surprising about it! Property Class 12.9 bolts and screws have a minimum tensile strength of 1220 MPa for all sizes. 1220 MPa are roughly equivalent to 175 ksi. These parts are very high in strength. In fact, they’re the strongest of all the fasteners we’ve covered in our inch-series and metric bolt head marking blogs.

Learn More şÚÁĎÍř Metric Bolt and Screw Grades

with şÚÁĎÍř

Interested in learning a bit more about bolt and screw head markings? While this is currently our only blog on metric grades and head markings, we do have a few others you should check out on inch-series parts:

• What Do Screw and Bolt Head Markings Mean?
• Understanding a Few Common ASTM Bolt Grade Markings

And if you have any questions that these blogs can’t answer for you, feel free to reach out to us. With plenty of experience in the fastener industry, we’ll likely have an answer for you. Give us a call at (800) 656-2658 or contact us online.

In the fastener world, you’ll often hear terms like proof load, yield strength, and tensile strength tossed around when referring to the strength of a given fastener. For those unfamiliar with the precise meanings of these terms, I thought I’d devote a blog post to help define them and their relation to one another.

Proof load, yield strength, and tensile strength are numbers set by a standard that a fastener must meet in order to qualify as a certain grade or property class. All three numbers are set as minimum (and occasionally maximum) values. For example, according to ASTM A354, in order for a ½-13 bolt to qualify as grade BD, it must have a minimum proof load of 17,050 pounds-force (lbf), a minimum yield strength of 18,500 lbf, and a minimum tensile strength of 21,300 lbf. Not all standards specify requirements for all three tests. Yield strength and proof load are similar tests, so yield strength requirements are often omitted in favor of proof load requirements, as in SAE J429.

Before I can talk about individual terms, I should talk a bit about the kind of fastener strength involved here. All three terms involve the load that a threaded fastener can hold when pulled perpendicularly from the head. See Figure 1.

In order to test this force, we use the tensile machine inĚýour lab.

As you can sort of see, the fastener is fed into the slot in the middle. The machine then exerts a vertical force on the part. The machine measures the force as the part holds, distends, or breaks, depending on the test. To get an idea of how each test works, read on.

What Is Proof Load?

Proof load is an amount of force that a fastener must be able to withstand without permanently deforming. So, to use the example above, in order to pass the proof load test set by ASTM A354, a ½-13 bolt must be able to hold a load of at least 17,050 lbf for a minimum of ten seconds without permanently elongating. The length of the part is measured before and after the proof load test to ensure compliance.

What Is Yield Strength?

Yield strength is the load that is carried at the point where a fastener permanently deforms. When subjected to enough force, steel will begin to stretch. If the amount of force is low enough, the steel will elastically return to its original shape when the force is removed. At the yield point, the force becomes strong enough that the steel will stretch and not return to its original shape. This amount of force is the yield strength.

To test yield strength in our example, you would put our ½-13 bolt into the tensile machine, stretch the part until it distends, and calculate the force at the point of yield. In this case, the force would need to be a minimum of 18,500 lbf for the part to pass. The actual process of determining the force at the point of yield is rather engineer-y and involves graphs. If you would like to see it spelled out,Ěý

What Is Tensile Strength?

A fastener’s tensile strength, or ultimate tensile strength, is the force at which the fastener fractures. To test tensile strength, we use a wedge tensile test, where a wedge is placed under the head of the fastener, and force is applied until the fastener breaks.

The wedge is used because it puts extra stress on the junction of the head and the body of the fastener. This ensures the absolute integrity of this junction. If the fastener breaks at a force greater than the minimum tensile requirement, the fastener has passed the tensile test. However, the break must not occur at the junction of the head and the body of the fastener. If the break does occur here, the fastener has failed tensile, regardless of the force at which the break occurred.

So to summarize, proof load is a load that can be held without permanent deformation. It is the lowest force of the three forces that we are discussing. Yield strength is the force exerted at which a fastener permanently deforms. Yield strength is a greater force than proof load. Finally, tensile strength is the force at which a fastener will break. It is the strongest of the three forces.

Before I sign off, I would like to point out that when a properly made fastener is subjected to a force greater than its tensile strength, it will break in a cross-section. In other words, the steel itself will give out across the diameter of the fastener before the threads shear. Threads are strong. Threads are cool. We talk about threads in more detail in our three-article series on threads. Part 1 provides a general introduction to threads. Part 2 talks aboutĚýthe difference between 2A and 3A threads. Finally, we wrap up with part 3, whichĚýdiscusses metric threads.

RELATED: Need a specialty bolt, screw, or stud? şÚÁĎÍř can help.