What Sets Armour Steel Apart From S355

Over the years, in different workshops, I have seen the same thing happen more than once: a plate passes incoming inspection without a hitch. Dimensions correct, surface clean, the weld looks tidy. And yet the ballistic protection at exactly that weld is already gone – not because anyone was careless, but because heat was mismanaged during welding. You cannot see it on the part. You find out at the ballistic test, or in the worst case, never.

That is the real difference from S355 or any other structural steel. Anyone with twenty years of machining and forming behind them, working with ordinary structural steel, has learned one thing: a defect shows itself. A crack, distortion, a heat-tint colour, a dent – something is visible, and you can rework or scrap the part before it leaves the building. That experience does not carry over to armour steel. Treat the material like a thicker, more expensive version of structural steel, and you lose the ballistic performance without any inspection step in your own shop ever flagging it.

The core idea in one sentence

Armour steel is a quenched and tempered martensitic steel. Its ballistic performance comes entirely from the heat-treatment state the plate received at the mill. Every processing step in your own shop that introduces heat – welding, flame cutting, hot straightening, even overly aggressive grinding – works against exactly that microstructure. The whole of armour-steel manufacturing technique ultimately comes down to one idea: control heat. Not avoid it – control it, in both directions.

With S355, heat is mostly uncritical as long as the final static strength checks out. With armour steel, heat is the lever that can destroy the actual purpose of the part without the geometry, the dimensions, or the surface ever showing it.

Two failure modes you cannot see

Too much heat causes softening of the heat-affected zone. Hardness and strength right next to the weld drop, and that is exactly where the part loses ballistic protection. This only becomes visible during the ballistic test – there is no signal for it during ongoing production.

Too little controlled heat leads to the opposite extreme: cold cracking, also known as hydrogen cracking. Cooling too fast, no preheat, hydrogen sources in the filler metal or the environment – the result is a crack in the heat-affected zone or at the weld toe. It often does not show up immediately, but only hours to days after welding, by which time the part has long since moved on.

What makes both failure modes so treacherous: the part looks flawless either way. No visible anomaly, no dimensional deviation, nothing a normal visual inspection would catch. That is why I give every shop that is new to this material the same sentence in the first hour: the process is the product, not the part. Anyone who only inspects the part is inspecting the wrong half of the work.

Why a hardness class alone tells you nothing

Armour steel is not one uniform material but a family of grades – from tougher, structurally load-bearing classes to very hard, weight-optimised add-on armour. Brands such as Armox, Ramor, or Secure each cover different points within that family, depending on hardness and intended use. What holds across all of them: the harder the grade, the more sensitive it is to heat, and the narrower the window in which welding is permitted. There is also a practical limit beyond which a plate is no longer welded through at all, but only welded on as an overlay, bolted, or bonded – because the risk of destroying the microstructure during welding otherwise becomes too great.

The actual permissible preheat and interpass temperature for a specific plate is not found in a rule of thumb or a note left over from the last project. It is stated on that plate's mill certificate, because the carbon equivalent varies from batch to batch. Anyone calculating from memory or from an old table is working with values that no longer apply to the batch in hand.

What it costs when this gets overlooked

The expensive part is not the single defective weld. The expensive part is that the defect stays invisible until it is not – at the ballistic test, at an audit by an accredited body, or, worst case, never, because the part has already been built in. Weeks or months often pass between the cause and the moment it surfaces. By then, production has continued, delivery has happened, and the part may already be installed further downstream. Traceability, the deviation-approval process, rework, or scrap then come on top of what production already cost in the first place.

For a shop moving into the defense supply chain from classic structural steel or general engineering, this is the real culture shift: the tolerance for "looks fine, should be okay" that exists in structural steel does not exist with armour steel. It was never quite right in structural steel either, but it was forgivable there. With armour steel, it is not.

What to do about it

The consequence is not some hidden science, but it does demand discipline at points that nobody cared about in structural steel.

• Document heat input per weld, do not estimate it – for every process, not only for critical welds.
• Derive preheat and interpass temperature from the mill certificate of the actual batch, not from a general table.
• Treat every heat-introducing step – including seemingly harmless ones such as coating or straightening – as an intervention in the heat-treatment state, not as a side note.
• Train welding supervision and operators until they have internalised the difference between "looks fine" and "is actually fine" for this material.

What this means for your shop

If your business comes from classic structural steel or general engineering and is considering the move into defense manufacturing, this is the first point I look at on site: how is heat controlled and documented in your shop today – and where would an invisible defect slip through right now? The Defense-Readiness-Check examines this systematically, over two days on site, for a fixed fee, and ends with a written roadmap. If you would like to clarify beforehand, with no obligation, whether that is the right starting point for your shop: initial call, 45 minutes, remote, free of charge – book a first conversation.