Mastering Armor Steel Welding for Combat Vehicles

Modern ground combat vehicles are being redesigned for a battlefield that demands lighter, smarter, and tougher platforms. This shift has accelerated the use of high-strength, low-alloy (HSLA) quenched-and-tempered armor steels, such as MIL-DTL-12560 rolled homogeneous armor (RHA) and MIL-DTL-46100 high hardness armor (HHA).

While these materials offer exceptional ballistic performance, their engineered strength and toughness depend on strict control of heat during welding. This article explains how these steels respond to welding, why heat input and cooling rate matter, and how welders can preserve ballistic integrity by following qualified procedures and disciplined practices with the goal of helping welders understand the metallurgy so armor welds perform exactly as designed.

Today’s threats are unpredictable, and vehicles must adapt. This shift has pushed the industry toward advanced HSLA quenched-and-tempered armor steels that deliver strength and toughness without excessive weight.

However, there’s a catch: These materials demand more from welding. Their properties result from carefully controlled heat treatment, and welding can undo those (heat treatment) processes, if procedures aren’t followed. This article also breaks down how two key armor steels, MIL-DTL-12560 RHA and MIL-DTL-46100 HHA, behave when welded, as well as how welders can preserve their ballistic performance through simple, repeatable weld science.

Armor Steels: Engineered for the Modern Fight

Armor steels aren’t just a hard plate. They’re engineered materials built to deliver ballistic protection without dragging vehicles down with unnecessary weight. Two of the most widely used weldable grades, MIL-DTL-12560 RHA and MIL-DTL-46100 HHA, approach that mission differently.

12560 RHA is the structural backbone. It’s tough and ductile, absorbing blast energy through controlled deformation. Think of it as the shock absorber of the armor world. With hardness typically in the low 400 HV range, it’s well-suited for hulls, frames, and large structural components.

46100 HHA is the specialist. With hardness often exceeding 500 HV, it’s designed to stop high velocity, armor piercing threats. That added hardness comes with a tradeoff — 
it’s less forgiving during welding and far more sensitive to heat input and hydrogen cracking.

Both steels arrive quenched and tempered, meaning their strength and toughness come from a carefully engineered tempered martensite structure. That same structure can be easily damaged if welding heat isn’t controlled.

Metallurgy in Plain Language

Armor steels don’t get their strength by accident. They’re heat treated to form martensite, which is incredibly strong but brittle. Tempering modifies that martensite, restoring toughness while retaining strength.

The process is straightforward.  The steel is heated to form austenite, rapidly quenched to create martensite, and then tempered to balance hardness and toughness. That balance gives RHA its ductility and HHA its high velocity stopping power.

However, welding can undo that heat treatment in seconds.

• Too much heat softens the armor. Over tempering reduces hardness and strength, reducing ballistic performance.
• Too little heat or too rapid cooling makes the steel brittle and prone to cracking. Rapid cooling forms untempered martensite and traps hydrogen, setting the stage for delayed cracking hours after welding.

Armor steels require strict control of heat input and cooling rate. Do that correctly, and the metallurgy works for you instead of against you.

Where Armor Welds Go Wrong and Why

Once you understand how armor steels respond to heat, the welding challenges start to make a lot more sense. These materials aren’t difficult; they’re honest. Treat them properly, and they perform exactly as engineered. If you rush, skip steps, or ignore heat control, they’ll show you the consequences. Take note of the following four factors.

1. Hydrogen Cracking: The Silent Saboteur

The weld may look great when you clock out, but the next morning, there’s a crack running through the heat-affected zone (HAZ) like a lightning bolt. That’s hydrogen cracking. Hydrogen sources include moisture, mill scale, rust, paint, oil, grease, or any shop grime. Combine that with rapid cooling and high residual stress, and cracking becomes inevitable. Skipping preheat isn’t a gamble; it’s a guarantee.

2. HAZ Softening: When the Armor Loses Its Edge

Excessive heat input over tempers the steel. Hardness drops, strength declines, and ballistic performance suffers. The plate may still look fine, but it won’t perform like armor anymore.

3. Excessive HAZ Hardness: Brittle by Accident

Cooling too quickly creates untempered martensite that’s extremely hard, brittle, and prone to cracking. This is where welders see toe cracks, underbead cracks, or delayed cracking hours after welding.

4. Poor Fusion and Porosity: The Fundamentals Still Matter

Armor steels don’t forgive shortcuts. Mill scale, coatings, and oxides are sources of contamination that lead to welding discontinuities. Porosity isn’t just cosmetic; it reduces strength by reducing the effective cross section.

Cleanliness isn’t optional. It’s a part of ballistic performance.

Practical Guidance: Preserve the Metallurgy

Armor steels don’t need complicated welding tricks; they need discipline. The best way to preserve the strength and toughness engineered into 12560 and 46100 is to follow a qualified welding procedure and treat heat like the critical variable it is.

A good procedure already tells you everything the steel expects: cleaning and preparation, controlled preheat, stable interpass temperature, and low hydrogen processes. These steps slow the cooling rate, prevent untempered martensite, and limit hydrogen sources to prevent delayed cracking. In addition, when required, postweld heat treatment helps temper the HAZ and drive out remaining hydrogen.

The bottom line is armor steels perform exactly as designed when welders follow qualified procedures and control the heat.

Conclusion

Armor steels can seem intimidating, but they’re engineered materials built for a modern battlefield. They expect welders to protect the strength and toughness they were designed with.

Welding armor isn’t rocket science. It’s weld science. With disciplined procedures and a solid understanding of how heat affects metallurgy, welders can produce joints that deliver predictable performance and reliable protection.

In the end, armor welding isn’t about doing something complicated. It’s about doing the right things consistently. Follow the procedure, control the heat, and the weld will do its job protecting the protectors.

Works Consulted

• MIL-DTL-12560 and MIL-DTL-46100 (for armor plate, steel, wrought, homogeneous, and high hardness)
• MIL-STD-3040 (for Welding of Steel Structures)
• TACOM 12479550 (legacy welding/fabrication requirements for earlier programs, which is now largely transitioning to MIL-STD-3040)
• AWS B2.1, Specification for Welding Procedure and Performance Qualification
• AWS D1.1, Structural Welding Code — Steel
• AWS Welding Handbook, Ninth Edition, Volume 4, Materials and Applications, Part 1
• Armor plate manufacturer data sheets (the primary source for chemical composition and hardness ranges)
• Filler metal manufacturer technical data sheets (used for hydrogen control strategies and consumable selection)
• U.S. Army Research Laboratory technical reports (ballistic performance, weld zone behavior, and test methodology)

BRENT WILLIAMS (brent.williams@weldfuzion.com) is a CWI and founder and technical director of WeldFuzion LLC, Reed City, Mich. This article is based on a presentation given at the AWS South Florida Section meeting in April 2025.