When you specify a high-strength steel plate, such as S690QL or armour plate, on a drawing, you set yourself a fabrication challenge. Both are quenched and tempered (Q&T) steels, and the same metallurgy that gives them strength also makes them more difficult to work than standard carbon steel.

When you specify a high-yield steel plate such as S690QL or armour plate, you’re not simply selecting a material; you’re defining the fabrication challenge that follows. Both grades are quenched-and-tempered (Q&T) steels, engineered to deliver exceptional strength. However, the same metallurgical properties that make them suitable for demanding structural and defence applications also make them significantly more difficult to process than standard carbon steel.

Incorrect machining parameters can rapidly wear out tooling, increase costs, and, more importantly, compromise the mechanical properties of the selected material. Excessive heat, poor cutting practices, or unsuitable processing methods can alter the microstructure at the cut edge, creating unnecessary risks for critical applications. For defence fabrication in particular, compromising the mechanical specification is more than a quality factor; it’s a certification risk.

This guide explains the key considerations when machining high-strength steel plate and armour-grade materials, and how the right combination of waterjet cutting and CNC machining helps preserve both performance and certification.

Understanding Material Properties for Ballistic Protection

Armour plate steel properties for ballistic protection are based on a carefully engineered balance between hardness and toughness.

Hardness is essential because it helps disrupt, deform, or deflect incoming projectiles. However, hardness alone does not guarantee effective protection. If a plate becomes too hard and brittle, it may crack or generate dangerous rear-face spalling during impact.

Toughness provides the ability to absorb and dissipate energy without catastrophic failure. Defence-grade armour plate is therefore designed to achieve both properties simultaneously through tightly controlled manufacturing processes.

Materials such as Miilux Protection armour plate achieve this balance through a quenched-and-tempered martensitic microstructure. The manufacturing process is carefully controlled to maintain high hardness levels while preserving impact resistance.

Consistency throughout the plate is equally important. Internal defects, inclusions, or irregular grain structures can become crack initiation points during ballistic impact. This is why armour plate is subjected to rigorous inspection procedures, including ultrasonic testing and mechanical testing, before entering service.

High-yield structural steels such as S690QL are also quenched-and-tempered grades, although they are designed for a different purpose. Rather than ballistic protection, S690QL provides exceptional strength-to-weight performance, allowing engineers to reduce structural weight without sacrificing load-bearing capacity.

In many defence and specialist engineering applications, armour plate and S690QL are used together. Armour plate provides protection against threats, while S690QL forms the supporting structure behind it.

Although their applications differ, both materials present similar fabrication challenges. Their elevated hardness, strength, and heat sensitivity demand specialist processing techniques to maintain the certified properties of the original plate.

Key Armour Plate Steel Properties for Ballistic Protection

When evaluating the properties of armour plate steel for ballistic protection, several characteristics must be considered.

Hardness

The strike face must be hard enough to resist penetration and disrupt incoming projectiles. Consistent hardness across the plate is essential for predictable performance.

Toughness

Impact energy must be absorbed without excessive cracking or fragmentation. Toughness is particularly important where personnel protection is required.

Through-Thickness Integrity

Mechanical properties should remain consistent throughout the section. This is why defence-grade steels undergo extensive quality control and ultrasonic testing.

Ballistic Performance

Ultimately, ballistic performance is verified through controlled testing against recognised military or defence standards rather than theoretical calculations alone.

Fabrication Compatibility

The processing route must preserve the properties achieved during manufacture. Poor machining practices or excessive heat input can compromise the armour plate steel’s ballistic-protection properties.

Technical Machining Guidelines

High-yield steel and armour plate require different machining parameters than standard carbon steel. Proper management can maintain the structural integrity of the original plate specification while complying with certification requirements.

Tooling Selection

Machining high-strength steel plate requires tooling capable of withstanding significantly higher cutting forces than those for standard carbon steel.

Carbide tooling is generally essential for S690QL and armour-grade materials. High-speed steel tools typically wear too quickly to maintain productivity or dimensional accuracy. Modern coated carbide inserts, including TiAlN-coated grades, can improve wear resistance and tool life when machining difficult materials.

Tool rigidity is equally important. Excessive overhang, poor workholding, or machine vibration can lead to chatter, inconsistent surface finishes, and premature tool failure. Short, rigid setups help maintain dimensional accuracy while reducing cutting forces.

At Brown McFarlane Precision Engineering, specialist CNC machining equipment and CAD/CAM programming systems enable the manufacture of complex components from high-strength structural steels, wear-resistant grades, and defence materials, while maintaining full material traceability throughout production.

Speeds and Feeds

One of the most common mistakes when machining S690QL is using overly conservative feed rates.

If the cutting edge rubs rather than shears, the material’s surface can become work-hardened. Subsequent passes must then cut through a harder layer, increasing tool wear and reducing process efficiency.

Maintaining appropriate chip load, avoiding excessive finishing passes, and replacing worn tooling before performance deteriorates are all important considerations when machining quenched and tempered steels.

Thermal Management

Heat control is critical when machining high-strength steel plate.

The mechanical properties of Q&T steels are achieved through carefully controlled heat treatment during manufacture. Excessive heat generated during machining can locally alter the material’s microstructure, potentially affecting hardness, toughness, or yield strength.

Effective thermal management relies on appropriate cutting speeds, adequate coolant delivery, sharp tooling, and efficient chip evacuation. Preventing chips from being recut helps minimise unnecessary heat generation and protects both tooling and material performance.

Defining Military Grade Steel

What Qualifies as Military Grade?

There is no single global definition of military-grade steel. Instead, the term refers to compliance with a specific specification or standard. Depending on the application, this may include NATO STANAG requirements, UK DEF STAN standards, or US MIL-DTL specifications.

The governing standard defines the required chemical composition, mechanical properties, testing procedures, and ballistic performance criteria.

How Is Military Grade Steel Tested?

Military-grade materials are validated through a range of testing procedures. Ballistic testing, including V50 testing, measures the velocity at which a projectile has a 50% probability of penetration. This provides a quantifiable assessment of protective performance.

Ultrasonic testing is used to identify internal defects such as laminations and inclusions, while Charpy impact testing verifies toughness under low-temperature conditions.

Together, these tests ensure the material performs as intended in demanding operational environments.

For a fuller breakdown of these standards and how they map to specific grades, our Steel for Defence Applications FAQ Hub covers the material specification angle in more depth.

Machine Shop vs Waterjet Cutting

Selecting the correct processing route is just as important as selecting the right material.

Waterjet cutting and CNC machining should not be viewed as competing processes. Instead, they often work most effectively together.

Waterjet cutting is a cold process that introduces no significant heat into the material and therefore avoids the creation of a heat-affected zone. For S690QL and armour plate, where maintaining the original material properties is critical, this offers a significant advantage.

Waterjet cutting is particularly well-suited to profile cutting and blank production. It can create complex shapes directly from a certified plate while preserving the metallurgical characteristics of the parent material.

CNC machining then provides the precision required for finished components. Features such as bores, threaded holes, mating faces, and tight-tolerance surfaces can be produced accurately while maintaining dimensional consistency.

Although machining typically incurs higher tooling and production costs than profile cutting alone, it delivers precision that waterjet cutting cannot achieve on its own.

For many applications, the most cost-effective solution is a combined approach. Waterjet cutting produces a near-net-shape blank, while CNC machining is reserved for critical features. This reduces machining time, minimises tool wear, and helps maintain overall project efficiency.

By offering both waterjet cutting and precision machining capabilities, Brown McFarlane provides a controlled processing route that helps preserve both material certification and component performance from stock plate to finished part.

Reducing Risk in Defence Supply Chains

Every transfer between suppliers introduces potential risk. Traceability can become fragmented, certification records may be separated from the material, and processing decisions may be made without a full understanding of the grade being supplied.

For high-strength steel plate and defence-grade materials, these risks can affect quality, compliance, and project schedules.

Brown McFarlane reduces those risks by providing a turnkey solution. We combine specialist steel stockholding with advanced processing capabilities, including waterjet cutting, CNC machining, profiling, drilling, rolling, and precision engineering services. Operating within a single quality-controlled environment helps maintain traceability from the original mill certificate through to the finished component.

This integrated approach also reduces lead times by eliminating unnecessary subcontracting and material handling between multiple suppliers.

When specifying S690QL, armour plate, or other high-performance materials, material selection and fabrication strategy should be considered together. The same properties that make these steels valuable in service require specialist expertise during processing.

By combining technical knowledge, specialist stockholding, and precision engineering capabilities, we help ensure that high-strength steel plate retains its certified performance throughout the manufacturing process.

Need advice on machining S690QL or armour plate? Contact Brown McFarlane’s technical team to discuss material selection, waterjet cutting, CNC machining, and processing strategies for your next project. Visit our downloads page for certification and specification documentation.