Automotive Fastener Material Selection: Carbon Steel, Alloy Steel, Boron Steel and Stainless Steel

Automotive Fastener Material Selection: Carbon Steel, Alloy Steel, Boron Steel and Stainless Steel

Automotive fastener material selection is a critical engineering decision because the selected steel or alloy influences mechanical properties, 

manufacturing feasibility, corrosion behavior and the cost of the finished component.

A bolt used in a chassis connection may require a different material strategy from a screw used in an electrical enclosure or a threaded insert installed in an EV battery structure.

The correct material must be evaluated as part of the complete fastening system, not simply selected from a list of steel grades.

For automotive OEMs and Tier-1/Tier-2 suppliers, material selection must balance several requirements:

• Specified mechanical properties

• Material hardenability

• Cold-forming or machining suitability

• Fastener geometry and section size

• Operating temperature

• Corrosion exposure

• Joint loading and fatigue requirements

• Surface treatment compatibility

• Material availability and purchasing cost

JUXIN FASTENERS supplies high-strength bolts, alloy steel screws, stainless steel fasteners, locking nuts, weld nuts, 

threaded inserts and drawing-based custom fastening components for automotive and industrial OEM applications.

This guide explains how to compare common automotive fastener materials and identify the information required for engineering approval and commercial sourcing.

Why Automotive Fastener Material Selection Matters

Fasteners operate in different mechanical and environmental conditions throughout a vehicle.

Applications may include chassis and suspension structures, engine mounting, transmission assemblies, battery packs, electrical equipment, interior components and body structures.

Each application creates a different combination of material requirements.

For example, a high-strength steel bolt in a loaded structural joint may require defined tensile and proof-load properties.

A stainless steel screw used in a corrosion-exposed enclosure may be selected primarily for environmental resistance.

A weld nut must satisfy both the requirements of the threaded connection and the intended welding process.

A cold-headed custom bolt must also use material suitable for the required forming deformation.

Material selection therefore influences both the final product and the manufacturing route.

Steel Grade, Property Class and Fastener Performance Are Different Concepts

A common sourcing mistake is to treat a steel designation and a finished-fastener property class as interchangeable.

They are not.

A steel grade identifies a material specification or composition range.

A fastener property class identifies defined mechanical requirements for the finished fastener.

For metric carbon steel and alloy steel bolts, screws and studs within its scope, ISO 898-1 provides relevant mechanical property requirements.

Property classes such as 8.8, 10.9 and 12.9 do not identify one mandatory steel grade.

Different suitable steels may be used when the finished fastener meets all applicable requirements.

Engineering Example

A drawing specifies an M12 property class 10.9 bolt.

The supplier proposes a particular boron-containing steel.

The material cannot be approved solely because that steel is commonly associated with high-strength bolts.

The review should consider the approved material requirements, bolt geometry, hardenability, manufacturing route and verification of the finished mechanical properties.

This distinction is especially important when qualifying an alternative supplier.

Carbon Steel Automotive Fasteners

Carbon steel is widely used for suitable automotive and industrial fasteners because it offers a range of mechanical properties and manufacturing characteristics.

The appropriate composition depends on the required product.

Broad carbon-content categories are useful for introductory comparison, but the actual steel specification must control the material selection.

Low-Carbon Steel

Low-carbon steels are often suitable for products requiring good formability.

Potential applications include selected screws, sheet-metal components, brackets and formed fastening parts.

Their advantages may include good forming characteristics and cost efficiency.

However, low-carbon steel is not automatically suitable for a high-strength bolt.

The required mechanical properties and manufacturing route must be evaluated.

Medium-Carbon Steel

Medium-carbon steels may be considered for bolts and other components requiring higher strength after suitable processing.

Their suitability depends on composition, hardenability, section size and heat treatment.

A medium-carbon designation alone does not establish a particular property class.

High-Carbon Steel

Higher-carbon steels may be used for selected spring and wear-related components.

They are not automatically the preferred material for conventional high-strength automotive bolts.

Greater carbon content can affect forming behavior, toughness and processing requirements.

The selection should be based on the intended component rather than the assumption that more carbon always means better performance.

Cold-Heading Steel for Automotive Bolts and Screws

Cold heading is widely used to manufacture suitable bolts, screws and other fasteners efficiently.

Cold-heading steels are selected for their ability to undergo the required plastic deformation while supporting the intended finished properties.

Relevant considerations include:

• Formability

• Surface quality

• Chemical composition

• Inclusion characteristics

• Hardenability

• Consistency of the supplied material

ISO 4954 provides requirements for steels used in cold heading and cold extrusion within its scope.

SAE J403, SAE J404 and applicable JIS material standards may also be relevant when the drawing or purchasing specification calls for those material systems.

Why Cold-Heading Suitability Matters

A steel may be capable of achieving a specified mechanical property class but still be unsuitable for an economical cold-heading process.

For example, a complex head geometry may require substantial deformation.

If the material has insufficient formability for the intended operation, forming cracks or excessive tool wear may become concerns.

Manufacturing engineers should therefore evaluate material selection together with fastener geometry.

Cold Upsetting and Formability Evaluation

Cold upsetting tests can provide useful information about a material's deformation behavior under specified conditions.

ISO 898-5 is not a general substitute for steel formability testing, and finished-fastener mechanical testing does not automatically establish cold-heading suitability.

Where an upsetting test is required, the customer and supplier should agree on an applicable test method, specimen condition and acceptance criteria.

A successful laboratory upsetting test also does not independently guarantee defect-free mass production.

Boron Steel Automotive Fasteners

Boron-containing steels are used in certain automotive fastener applications because controlled boron additions can improve hardenability under suitable metallurgical conditions.

This may allow selected compositions to achieve the required heat-treatment response.

However, the effectiveness of boron depends on the complete composition and processing history.

Boron steel is not a universal substitute for alloy steel.

10B21 Steel for Automotive Fasteners

SAE 10B21 is a boron-containing steel designation that may be considered for selected cold-formed fasteners.

Its potential advantages can include forming suitability and hardenability when the material and processing conditions are appropriate.

Whether a finished bolt can meet property class 8.8 or 10.9 depends on the complete product and manufacturing specification.

It would be incorrect to assume that every 10B21 bolt automatically qualifies as class 10.9.

When Is Boron Steel a Suitable Alternative?

Boron-containing steel may be considered when:

• The approved drawing permits the proposed material

• The required section size is compatible with the heat-treatment response

• The forming process is feasible

• The finished mechanical requirements can be verified

• The customer accepts the material and manufacturing route

A material substitution should be evaluated technically before any cost comparison is finalized.

Alloy Steel Automotive Fasteners

Alloy steels are commonly considered for high-strength fasteners where hardenability, section size and mechanical performance require careful control.

Alloying elements can influence material behavior, but their effects depend on concentration and interactions with other elements.

Chromium

Chromium can contribute to hardenability and other material characteristics in suitable steel compositions.

Its presence in an alloy steel does not automatically make an uncoated fastener corrosion-resistant.

Molybdenum

Molybdenum can contribute to hardenability and tempering response in appropriate alloy steels.

Its value must be considered within the full material specification.

Nickel

Nickel may influence toughness and hardenability in certain alloy systems.

The effect depends on the composition and processing conditions.

Vanadium

Vanadium can contribute to grain refinement or precipitation strengthening in appropriately designed steels.

These effects should not be generalized to every vanadium-containing fastener material.

Common Automotive Fastener Steel Grades

The following designations may appear in automotive or industrial fastener material discussions.

These material names are not automatically interchangeable.

Different standards may specify different composition limits, product forms, processing conditions or other requirements.

SCM435 vs. SAE 4140

SCM435 and SAE 4140 are both chromium-molybdenum alloy steel designations, but they should not be treated as direct equivalents without comparison.

A substitution review should examine:

• Chemical composition limits

• Product form and supply condition

• Hardenability requirements

• Heat-treatment response

• Mechanical requirements

• Customer approval conditions

A similar material family is not proof of complete specification equivalence.

SCM435 vs. SCM440

These are distinct JIS steel grades.

The selection should reflect the required composition and manufacturing response.

Neither designation independently guarantees that a finished bolt meets class 10.9 or 12.9.

Avoiding Unverified Material Equivalency Claims

When a drawing specifies a steel grade under one national or international material system, an alternative grade from another system must be evaluated against the actual specification.

Commercial descriptions such as "equivalent to" are not sufficient for engineering approval.

Property Class 8.8, 10.9 and 12.9: Choosing the Appropriate Strength Level

For applicable metric steel fasteners, property classes identify defined mechanical requirements.

The appropriate class depends on the joint design and application.

Property Class 8.8

Class 8.8 may be suitable for many general mechanical and automotive connections when the approved design calls for it.

Selection must still account for preload, joint geometry and environmental conditions.

Property Class 10.9

Class 10.9 is commonly specified for suitable high-strength applications.

Potential uses include selected chassis connections, mounting structures and other joints requiring higher specified mechanical performance.

However, its suitability must be determined from the approved design.

Property Class 12.9

Class 12.9 provides higher specified strength characteristics for applicable products.

It should not automatically replace class 10.9.

Higher-strength fasteners may require additional attention to material susceptibility, processing conditions and environmental risks.

Why Higher Strength Is Not Always Better

Increasing the property class does not automatically improve joint reliability.

A bolted joint may be limited by the strength of the internally threaded component, bearing surface pressure, fatigue loading or the clamped material.

For example, installing a higher-strength bolt into a weak threaded aluminum component does not necessarily increase the allowable joint load.

The complete connection must be evaluated.

Hardenability and Bolt Diameter

Hardenability is particularly important when selecting steel for larger or more demanding fasteners.

A material that responds adequately to heat treatment in a small section may not produce the same internal condition in a larger section.

This means material substitution cannot be evaluated solely from nominal tensile strength.

Fastener diameter, geometry and the required finished properties must also be considered.

Stainless Steel Automotive Fasteners

Stainless steel fasteners may be selected for corrosion-exposed automotive applications, electrical enclosures, equipment mounting and other suitable assemblies.

Common austenitic stainless steel families include A2 and A4 under ISO 3506 for applicable fasteners.

Material designations such as JIS SUS304 and SUS316 are also widely recognized.

304 / A2 Stainless Steel

304-family stainless steels can offer useful corrosion resistance in many environments.

However, their suitability depends on chloride exposure, surface condition, temperature and other operating factors.

316 / A4 Stainless Steel

316-family stainless steels contain molybdenum and can offer improved resistance to certain localized corrosion environments compared with 304-family materials.

This does not mean that 316 stainless steel is immune to chloride corrosion or universally superior in every application.

Stainless Steel vs. High-Strength Alloy Steel

Stainless steel and high-strength alloy steel fasteners should be compared against the required mechanical properties.

A stainless steel screw with the same dimensions as a property class 10.9 steel bolt is not automatically a direct replacement.

The relevant strength specifications, joint loading and corrosion requirements must be evaluated.

Galling and Assembly Considerations

Austenitic stainless steel threaded components can be susceptible to galling under certain installation conditions.

Thread condition, mating material, lubrication and assembly procedures may influence the risk.

For production managers, these factors can affect tightening consistency and assembly efficiency.

Material and Coating Must Be Selected Together

Material selection alone does not determine corrosion performance.

Carbon and alloy steel fasteners may require a suitable surface treatment for their intended environment.

Possible coating systems include zinc electroplating, zinc-nickel electroplating and non-electrolytically applied zinc-flake coatings.

The choice should consider corrosion exposure, mechanical property class, coating thickness, thread fit and tightening friction.

Zinc and Zinc-Nickel Coatings

These coatings may be used for suitable automotive steel fasteners.

Performance depends on the complete coating system and applicable specification.

A zinc-nickel coating should not automatically be described as RoHS-compliant without confirming the actual coating chemistry and relevant regulatory requirements.

Zinc-Flake Coatings

Non-electrolytically applied zinc-flake coatings may be appropriate for selected high-strength steel fasteners.

ISO 10683 provides relevant requirements for applicable coating systems.

These processes may reduce hydrogen introduction associated with electroplating, but they do not eliminate every possible hydrogen-related risk.

PTFE-Based Coatings

PTFE-based coatings may be used for selected friction-control or surface applications.

They are not a universal substitute for corrosion protection, nor are they automatically suitable for every high-temperature automotive assembly.

Black Oxide

Black oxide may be suitable for certain components where appearance or other specified characteristics are important.

Its corrosion protection is generally limited compared with many engineered zinc-based coating systems unless additional protection is used.

Hydrogen Embrittlement and High-Strength Steel

Hydrogen-assisted cracking is an important consideration for susceptible high-strength steel fasteners.

Risk depends on material susceptibility, stress and hydrogen exposure.

Certain cleaning and electroplating operations may introduce hydrogen.

Material and coating selection should therefore be coordinated with the approved manufacturing and surface-treatment requirements.

A specified hydrogen-relief treatment may reduce risk under suitable conditions, but it cannot guarantee complete elimination.

EV Battery Fastener Material Selection

Electric vehicle battery assemblies may combine steel fasteners, aluminum structures, stainless steel components and threaded inserts.

Material selection should address:

• Mechanical loading

• Thread engagement

• Mating-material strength

• Corrosion compatibility

• Thermal cycling

• Assembly access

• Electrical requirements where applicable

Steel Bolts in Aluminum Structures

A steel bolt may provide suitable strength, but the joint can still be limited by the aluminum thread or bearing surface.

Thread engagement and the intended tightening process must be evaluated.

Galvanic Corrosion

When dissimilar metals are electrically connected in the presence of an electrolyte, galvanic corrosion may occur.

The severity depends on the material combination, exposure conditions and joint design.

Material selection should therefore consider coatings, isolation methods and environmental sealing where applicable.

Stainless Steel Is Not Always the Best EV Choice

Stainless steel may improve corrosion resistance in selected applications, but its mechanical characteristics and galling behavior may create other design considerations.

The best material is the one that satisfies the complete assembly requirements.

Material Selection for Cold Heading vs. CNC Machining

The intended manufacturing route can influence material choice.

A steel selected for excellent machinability may not be the most suitable material for demanding cold forming.

Likewise, a cold-heading grade may require additional processing to meet a precision-machined feature requirement.

Engineering Decision Table

This framework should be applied to the actual drawing and customer specification.

Material Substitution: What Procurement Teams Must Verify

Global automotive procurement teams may seek alternative steel grades to reduce cost, improve material availability or develop a second supplier.

However, a proposed material substitution should not be approved solely on price or nominal strength.

Material Substitution Review

A useful review should address:

• Original drawing and revision

• Original material specification

• Proposed alternative material

• Chemical composition differences

• Hardenability and section-size suitability

• Cold-forming or machining feasibility

• Heat-treatment requirements

• Finished mechanical properties

• Coating compatibility

• Customer validation requirements

• Supply availability and commercial impact

A material change may also require review of tooling, heat treatment or inspection requirements.

Example: Alternative Material for a Class 10.9 Flange Bolt

An OEM drawing specifies a class 10.9 flange bolt with a defined material requirement.

A supplier proposes another steel grade that may reduce purchasing cost.

The engineering team should first determine whether the drawing permits the substitution.

If the material is not already approved, the proposed alternative requires technical evaluation.

Even if the finished bolt meets specified tensile and hardness requirements, other applicable material, manufacturing or customer approval requirements may remain.

Only after the technical review should the procurement team compare price, MOQ and lead time.

Supplier Development and Long-Term Material Availability

A suitable material must also be commercially obtainable in the required form and quantity.

Supplier development teams should consider:

• Material sourcing consistency

• Approved material specifications

• Minimum order requirements

• Manufacturing feasibility

• Tooling requirements

• Sample approval

• Lead time

• Lot identification

• Change-control expectations

These factors can be important when moving from prototype orders to repeat production.

Frequently Asked Questions

What is the best steel for automotive bolts?

There is no universal best steel. The selection depends on mechanical requirements, geometry, manufacturing process, operating environment and customer specifications.

Can SAE 10B21 be used for property class 10.9 bolts?

It may be suitable for certain products and manufacturing conditions, but the finished bolt must meet the applicable mechanical and customer requirements.

Are SCM435 and SAE 4140 equivalent?

They are related alloy steel designations, but they should not be treated as automatically interchangeable. 

The applicable specifications must be compared.

Does property class 12.9 always provide better automotive performance than 10.9?

No. The correct class depends on the complete joint design and service conditions.

What is the difference between hardenability and hardness?

Hardness describes resistance to indentation under a defined test.

 Hardenability describes a steel's ability to develop a hardened structure through its section under specified conditions.

Is stainless steel stronger than alloy steel?

Not necessarily. Strength depends on the actual material and finished-fastener specification.

Is 316 stainless steel completely corrosion-proof?

No. It offers useful corrosion resistance in many environments but remains susceptible to certain corrosion mechanisms.

Does zinc-nickel coating make a steel bolt suitable for every EV battery application?

No. The coating and fastener must be evaluated against the complete assembly requirements.

Can a steel grade be changed without modifying the drawing?

Only when permitted by the approved specification and customer change-control requirements.

What should be included in an automotive fastener material RFQ?

The drawing, material requirements, mechanical property class, thread designation, coating, quantity and any applicable approval or inspection requirements.

JUXIN FASTENERS: Automotive Fastener Material and OEM Sourcing Support

JUXIN FASTENERS supplies industrial and automotive fastening components for drawing-based OEM projects.

Relevant product families include:

• High-strength hexagon bolts

• Property class 8.8, 10.9 and 12.9 steel fasteners, where specified and feasible

• Alloy steel flange bolts

• Socket head screws

• Stainless steel screws and nuts

• Nylon-insert locking nuts

• All-metal prevailing torque nuts

• Weld nuts

• Blind rivet nuts

• Threaded inserts

• Drawing-based custom fasteners and CNC components

Material availability, proposed substitutions, manufacturing feasibility, coating options and required documentation are evaluated according to the specific project.

Any requested test reports, certifications or customer-specific approvals must be confirmed before order acceptance.

Request an Automotive Fastener Material Review and OEM RFQ

Are you selecting steel for a new automotive bolt, comparing boron steel with alloy steel, evaluating stainless steel alternatives or developing a second supplier for high-strength fasteners?

JUXIN FASTENERS welcomes inquiries from automotive OEMs, Tier-1/Tier-2 suppliers, design and structural engineers, manufacturing engineers, 

production managers, procurement specialists and supplier development teams.

For an efficient engineering and quotation review, please provide:

Approved 2D drawing + material specification + mechanical property class + thread designation + critical dimensions + surface treatment 

+ intended application + proposed alternative material, if any + sample quantity + annual purchasing volume + inspection requirements + target delivery schedule

Our team can review material and manufacturing feasibility, sample arrangements, MOQ, lead time and quotation details.

Company: JUXIN FASTENERS

Website: https://www.juxinfasteners.com/

Email: info@juxinfasteners.com

Send your automotive fastener drawings and material requirements to JUXIN FASTENERS for technical evaluation and quotation.