What Are Screws Made Of? A Complete Guide to Fastener Materials and How to Use Them
Published by Mutual Screw & Supply — Serving Manufacturers and Industry Since 1947
Screws are the backbone of virtually every assembly, structure, and piece of equipment in modern manufacturing. Whether you're sourcing fasteners for aerospace applications, construction, or industrial machinery, understanding what screws are made of — and why it matters — is the difference between a joint that lasts decades and one that fails prematurely.
At Mutual Screw & Supply, we've been helping manufacturers and industry supply houses find the right fastener since 1947. This guide covers everything you need to know about screw materials, coatings, thread types, and best practices for selection and installation.
What Are Screws Made Of?
Screws are manufactured from a variety of base metals, alloys, and polymers — each chosen for its specific performance characteristics. The most common screw materials are:
- Carbon Steel — the workhorse of the fastener world
- Stainless Steel — for corrosion-resistant applications
- Alloy Steel — for high-strength, heavy-duty use
- Brass — for decorative or electrical applications
- Aluminum — when weight savings are critical
- Titanium — for extreme strength-to-weight performance
- Nylon/Plastic — for non-conductive, chemical-resistant applications
Let's break each one down.
Carbon Steel Screws
Carbon steel is the most widely used screw material in industrial settings. It is an iron and carbon alloy, and its mechanical properties — tensile strength, hardness, machinability — are determined by its carbon content.
Low-carbon (mild) steel is easy to form and weld but relatively soft. Medium-carbon steel offers a better balance of strength and toughness. High-carbon steel is harder and stronger but more brittle, making it common in specialized cutting or high-load applications.
Most standard fasteners you'll find in the Mutual Screw catalog are made from medium-carbon steel and heat-treated to meet specific grade requirements.
Best for: Indoor structural applications, machinery, woodworking, automotive, and general construction where corrosion is not a primary concern.
Grade markings: SAE Grade 2, Grade 5, and Grade 8 are the most common carbon steel grades. Grade 8 screws and bolts are heat-treated alloy steel rated for the most demanding applications. Browse our Grade 8 fasteners to see available sizes.
Stainless Steel Screws
Stainless steel is the go-to material for any application where corrosion resistance is required. What makes stainless steel "stainless" is its chromium content — a minimum of 10.5% chromium forms a passive oxide layer on the surface that prevents rust from forming, even in wet or chemically active environments.
The two most common grades for screws are Type 304 and Type 316.
- 304 Stainless Steel is the standard choice for most outdoor and mildly corrosive environments. It resists oxidation, fresh water exposure, and many chemicals.
- 316 Stainless Steel adds molybdenum, which dramatically improves resistance to chlorides — making it the preferred choice for marine environments, coastal construction, chemical processing, and food-grade applications.
Shop our full selection of stainless steel screws in both 304 and 316, available in standard and metric sizes.
Best for: Outdoor fastening, marine and coastal applications, food processing, chemical equipment, medical devices, and any environment where moisture or salt exposure is expected.
Pro Tip: Mixing stainless steel fasteners with carbon steel components can cause galvanic corrosion. Always match your fastener material to the materials being joined, or use an appropriate isolating washer.
Alloy Steel Screws
Alloy steel screws are carbon steel with added alloying elements — typically chromium, molybdenum, vanadium, or nickel — that boost strength, hardness, or toughness beyond what plain carbon steel can achieve. These fasteners are almost always heat-treated to meet precise mechanical specifications.
Alloy steel is the material behind the highest-grade commercial fasteners: Grade 8 bolts, socket head cap screws (typically made from 4137 or 4140 alloy steel), and specialty fasteners used in aerospace and military applications.
For customers with AS 9100 or ISO 9001 quality requirements, Mutual Screw's sister company stocks certified, traceable alloy fasteners for aerospace and defense programs. Contact us to discuss your specification needs.
Best for: Heavy-duty machinery, high-load structural connections, aerospace, automotive, and military applications requiring certified mechanical properties.
Brass Screws
Brass is an alloy of copper and zinc. The ratio of these two metals is adjusted to achieve different properties — higher zinc content increases strength, while higher copper content improves conductivity and corrosion resistance.
Brass screws have a distinctive golden appearance that makes them popular in decorative applications, but their real strength is their excellent corrosion resistance without the need for coatings, along with their non-magnetic, non-sparking properties.
Best for: Electrical enclosures, plumbing fittings, decorative millwork, marine hardware (non-structural), and any application where a non-magnetic or spark-free fastener is needed.
Aluminum Screws
Aluminum screws are a natural fit for any application where minimizing weight is a priority. Aluminum naturally forms a protective oxide layer that provides reasonable corrosion resistance, though it is softer than steel and not suitable for high-clamping-force applications.
Common aluminum alloys used in fasteners include 2024-T4, 6061-T6, and 7075-T6. The 7075 series offers significantly higher strength and is used in aerospace applications where the weight savings of aluminum must be paired with structural performance.
Best for: Aerospace and aviation assemblies, electronics enclosures, automotive trim, and applications where weight reduction is a key design requirement.
Titanium Screws
Titanium is in a class of its own. It offers an exceptional strength-to-weight ratio — stronger than most steels at roughly half the density — combined with outstanding corrosion resistance in seawater, chlorine, and many acids where even 316 stainless steel would fail.
Titanium is also fully biocompatible, which is why it is the standard material for orthopedic implants and surgical devices. In aerospace and high-performance motorsports, titanium fasteners are specified wherever every gram counts and structural integrity cannot be compromised.
The tradeoff is cost — titanium fasteners are significantly more expensive than steel or stainless alternatives, and they require careful installation to avoid galling.
Best for: Aerospace, medical implants, marine, chemical processing, and high-performance motorsport applications where cost is secondary to performance.
Nylon and Plastic Screws
Polymer screws — made from nylon, polypropylene, PTFE, or polycarbonate — are the right choice when metal fasteners simply won't work. They are lightweight, electrically non-conductive, non-magnetic, and highly resistant to many acids and solvents that would corrode metal.
Best for: Electronic assemblies requiring electrical isolation, food processing equipment (FDA-compliant materials available), chemical handling equipment, and applications where metal contamination is not acceptable.
Screw Coatings and Finishes
Even the best base metal can benefit from a surface treatment. Here's a quick overview of the most common coatings applied to screws:
Zinc Plating (Electroplating): A thin zinc layer deposited electrically. Provides moderate corrosion protection and a bright, clean appearance. Common on standard hardware and fasteners used in non-harsh indoor environments.
Hot-Dip Galvanizing (HDG): The fastener is dipped in molten zinc, creating a thick, metallurgically bonded coating. Significantly more corrosion-resistant than electroplating. Standard for exterior structural fasteners, utility poles, and infrastructure applications.
Zinc-Nickel Plating: An alloy coating that outperforms standard zinc plating in corrosion resistance and is often used in automotive and aerospace fasteners where tight dimensional tolerances must be maintained.
Black Oxide: A chemical conversion coating that adds mild corrosion resistance (with oil) and a matte black appearance. Common on socket head cap screws and tooling fasteners.
Ceramic and Mechanical Zinc Coatings: Provide excellent corrosion resistance without the hydrogen embrittlement risk associated with electroplating, making them popular on high-strength fasteners. These are often specified for offshore, marine, and heavy industrial environments.
Dacromet / Geomet: Zinc flake coatings that provide outstanding corrosion resistance with very thin film thickness — often specified for automotive, wind energy, and infrastructure applications. No hydrogen embrittlement risk.
Understanding Screw Thread Types
Thread selection is just as important as material selection. The two primary thread systems used in the United States are Unified National Coarse (UNC) and Unified National Fine (UNF).
Coarse Threads (UNC): Wider thread spacing means faster assembly, better resistance to cross-threading, and a stronger grip in softer materials like wood, plastic, and cast iron. This is the default choice for most industrial fastening applications. Coarse threads are also more tolerant of minor surface contamination and are easier to strip cleanly.
Fine Threads (UNF): Tighter thread spacing produces a higher clamping force for a given torque and greater resistance to loosening from vibration. Fine threads are preferred for precision assemblies, thin-walled materials, and applications subject to dynamic loading — common in aerospace, automotive engines, and precision machinery.
For metric applications, we carry a full line of metric fasteners in both coarse and fine pitch.
If you're working from a tap drill chart to match thread size to hole diameter, bookmark our Tap Drill Size Reference Chart — one of the most-used reference tools on our site.
Choosing the Right Screw for Your Application
Selecting the correct screw comes down to answering a few key questions:
1. What materials are being joined?
Metal-to-metal joints typically call for machine screws or bolts with pre-tapped or clearance holes. Wood-to-wood or wood-to-metal applications call for wood screws or self-drilling screws with coarse threads. Thin sheet metal is best served by self-tapping or self-drilling screws.
2. What environment will the fastener live in?
Indoor, dry environments: carbon steel with a zinc finish is usually sufficient. Outdoor or moist environments: hot-dip galvanized or 304 stainless steel. Marine, coastal, or chemical environments: 316 stainless steel or a premium coated alloy fastener.
3. What are the load requirements?
High clamping force and structural loads require high-grade fasteners — Grade 8 for carbon steel, or socket head cap screws in alloy steel for precision assemblies. Lower-load applications can use Grade 2 or Grade 5.
4. Are there any special requirements?
Military and aerospace specifications (NAS, MS, AN standards), food-grade compliance, RoHS requirements, or certified material traceability? Reach out to our team — this is exactly the kind of specialized sourcing we've been doing since 1947.
Proper Installation Tips
Choosing the right screw is only half the job. Proper installation ensures you get the full performance the fastener was designed to deliver.
- Match your driver to the head style. Using a worn or wrong-size bit is the single most common cause of cam-out and stripped heads. Phillips, Torx, hex socket, slotted, and square drives each require the correct tool.
- Pre-drill when working in dense materials. Hard woods, composites, and thin metal sections should be pre-drilled with the appropriate clearance or tap drill to prevent splitting, cracking, or snapping the screw.
- Apply the correct torque. Overtightening stretches or yields the fastener shank and can strip threads in the mating material. Undertightening leads to joint loosening under vibration or load. When precise torque is critical, use a calibrated torque wrench.
- Use thread-locking compound when vibration is a factor. Products like Loctite 243 (medium-strength) or 271 (high-strength) prevent fastener loosening in dynamic applications without permanent bonding.
- Avoid galvanic incompatibility. When fasteners and base materials are dissimilar metals in contact with a conductive fluid (like moisture), galvanic corrosion can occur rapidly. Use insulating washers or choose compatible metals.
- Lubricate stainless steel threads. Stainless steel fasteners are prone to galling — a cold-welding phenomenon where the threads seize during installation. Apply a thin coat of anti-seize compound to prevent this, especially on larger diameter or fine-thread stainless fasteners.
Common Mistakes to Avoid
Using the wrong material for the environment. Specifying zinc-plated carbon steel screws in a coastal or outdoor setting is one of the most common — and costly — fastener failures. When in doubt, step up to stainless steel. The upfront cost difference is minimal compared to replacement and rework.
Mixing metals without considering galvanic compatibility. Even 304 stainless steel in contact with carbon steel components in a wet environment can initiate galvanic corrosion on the carbon steel. Know your material pairings.
Ignoring torque specifications. Especially on high-grade alloy fasteners, specified torque values are not suggestions — they define the preload the joint is engineered to carry. Over or undertorquing both reduce clamping efficiency.
Choosing grade based on cost alone. Grade 8 bolts cost a bit more than Grade 5. But in a joint that sees real structural load, specifying the lower grade to save pennies per fastener can cost significantly more if the joint fails.
Shop Screws and Fasteners at Mutual Screw & Supply
Mutual Screw & Supply stocks one of the most comprehensive fastener inventories in the Northeast, spanning standard commercial fasteners, military and aerospace specifications, and metric sizes. Whether you need a box of stainless pan head machine screws or a certified lot of aerospace-grade alloy fasteners with full material traceability, we have you covered.
Have a specialized requirement? Our team has been solving unusual fastener specifications for manufacturers, aerospace contractors, and industrial supply houses for over 75 years. Contact us today or call 1-800-222-0324 — we'd be glad to help.
It's Better When It's Mutual.
Mutual Screw & Supply | Est. 1947 | 1-800-222-0324
Frequently Asked Questions
What is the most common screw material?
Carbon steel is the most commonly used screw material due to its strength, machinability, and low cost. For applications where corrosion resistance is needed, 304 or 316 stainless steel is the most common choice.
What's the difference between 304 and 316 stainless steel screws?
316 stainless steel contains molybdenum, which significantly improves chloride resistance compared to 304. 316 is the preferred choice for marine, coastal, and chemical processing applications. For most general outdoor or mildly corrosive environments, 304 is sufficient and more cost-effective.
Can I use stainless steel screws with aluminum?
Yes, in most cases stainless steel and aluminum are compatible, especially in dry environments. In wet or marine environments, a slight galvanic potential exists and it's worth using an isolating washer or applying a corrosion inhibiting compound.
What screw grade do I need for structural applications?
For most structural steel applications, Grade 5 or Grade 8 is appropriate. Grade 8 (or metric property class 10.9 equivalent) should be used for joints carrying significant shear or tensile loads. Always consult the applicable structural code or your engineer of record for specific requirements.
Do you carry military and aerospace fasteners?
Yes. Mutual Screw stocks and sources fasteners to NAS, MS, AN, and other military and aerospace standards. Our sister company holds AS 9100 and ISO 9001 certifications for customers requiring certified supply chain traceability. Contact us for specifications.
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