What is armour steel?

23 August 2019

Armour steel, protection steel or “ballistic protection” steel is as it sounds – a steel that shall protect against an external threat in form of incoming projectiles.

There are different steels used for armour applications such as stainless steels, manganese steels, Hadfield steels and others however the most used armour steels are martensitic – hardened steels. The main active chemical compounds in hardened steels are iron (Fe) and Carbon (C). The properties are in general achieved by different amount of carbon, other alloying elements such as Chromium, Nickle and Molybdenum, and specialised heat treatment processes.

The heat treatment process normally consists of hardening and, depending on the grade, sometimes tempering. Hardening consist of heating the steel up to a temperature where the cubical iron crystals change from room temperature form as ferritic (body centred) structure to its high temperature form as austenitic (face centred) structure. This temperature is normally between 700 to 900 degrees depending on alloying content and equipment used.

When the steel has entered the austenitic state, the steel is quenched – this means that the steel is rapidly cooled down to almost room temperature. This rapid cooling “freezes in” solid state solution of carbon atoms in the iron matrix and forms twisted ferritic cubical crystals. The twisted crystals give a very high strength. That twisted cubical version of iron crystals are called martensite, thus the name “martensitic armour steels”.

A second heat treatment process, called tempering, can be applied in order to make the steel more ductile, formable in cold state, and less sensitive to fatigue cracking. Over the past two decades, and steel manufacturing technology has improved, it has become possible to make steels that are lower in impurities – this has removed the need to always temper steel to meet key mechanical properties.

Generally, armour steels are defined by hardness which in most cases are measured and named in Brinell Hardness (HB).

Different armour steels – classes and use

Blast protection steel
Blast protection steels are steels in the range of 370 – 460 Brinell. They are specifically designed to protect against high energy impacts and waves from mines, improvised explosive devices (IEDs) and even grenades. Today most blast protection steels used are 440 Brinell Which offer a good combination of hardness and toughness.

Blast protection steels are normally used for the floor/hull of armoured vehicles. In order to maintain the highest level of protection and integrity in a vehicle, the hull or floor section is manufactured from one section of very wide plate. These plates are bent in strong and long presses (preferably 800 tonnes or larger), and in many cases up to six meters longer. The most common shape of the hull is a “V-shape” – this specific shape under a vehicle is able to redirect the blast forces.

Blast protection steels needs to be very tough in order to absorb large amount of blast energy. It is difficult to undertake realistic blast tests so in order to verify the material impact toughness testing at minus 40 degrees Celsius is the most common way to test and compare blast protection steels. These steels are mostly referred to the blast classes in the NATO standard STANAG 4569. Carbon content varies from 0.12 to 24%.


Add on perforated armour

 

Ballistic protection steels

Construction steels
For the load carry section of a vehicle, a construction steel is typically used – this is because the steel must not only be bendable and weldable; but must have the highest resistance to cyclical loading and fatigue. All “blast steels”, as well as 500 Brinell and 550 Brinell grades of steel should be suitable for “construction steel” applications. Vehicle manufacturers will immediately identify inferior brands and types of steel based on workshop characteristics and usability – cracking, inability to bend/form and limited weldability are all good examples of poor steels that should be avoided.

High Hardness Armour (HHA) – 500 Brinell steel (HB477-540)

High Hardness armour steels (HHA or HH) are the most common armour steels globally, and their properties were originally defined by the United States military norm MIL-DTL-46100.

The HHA steels shall be bendable, weldable and not sensitive to fatigue. They are normally used as load carrying construction steels. Carbon content in HHA steel are normally around 0.27%. To stop NATO standard 5.56 mm calibre bullet SS109 normally 6.5 mm thick steel is needed.

Very High Hardness Armour (VHH) – 550 Brinell steel (HB530-590)

These steels are in principal a 500-type steel with slightly higher carbon content- around 0.31%.The HHA steels shall be bendable weldable and not sensitive to fatigue.

To stop NATO standard 5.56 mm calibre bullet SS109 normally 5.5 mm thick steel is needed.

“Add on steel”

As it sounds, an add on steel is a steel that you add on an existing body/construction. Add on steels are in many cases bolt on as a “spaced armour” solution.

Ultra High Hardness Armour steel (UHH) – 600 Brinell (HB580-640)

600 Brinell type armour steel is still considered quite exotic by some users, and no steel supplier recommends 600 type steels as construction material even if many customers today actually make load bearing bodies/constructions out of these types of steel.

Combined with the recent advancements in steel manufacturing capabilities, and refinement of chemical compositions through research and development, 600 Brinell steels should be bendable and generally usable without immediately cracking during use.

Unlike the HHA and VHH grades, UHH chemical compositions vary more between different manufacturers as each takes a different approach and attempt to achieve different mechanical properties.

To stop NATO standard 5.56 mm calibre bullet SS109 normally 5.0 mm thick steel is needed.

Extreme High Hardness Armour steel (XHH) – 650 Brinell (HB630-700)

XHH armour steels are considered today rare and exotic. XHH armour steels are not being manufactured by all steel companies and are currently mainly made for add on purposes. Welding and bending are possible to some extent however are normally not recommended. XHH steels starts to show fracture behaviour like ceramic materials.

Spaced armour

Spaced armour is mainly to protect against armour piercing ammunition. The most important feature in spaced armour is the air gap or ‘space’. Designs may be considered relatively simple as two steel plates are placed in parallel with a space in between. In many cases a 10 mm air gap is enough to stop very high threat levels.

Typical mounting methods of spaced armour include bolting on a plate of armour steel to a base construction steel. A very common example is when you start with an EN1522 FB6 protection (SS109) – 6.5 mm 500 Brinell type steel and add on a 4.0 mm 500 Brinell type plate with a 10 mm air gap. This solution enables protection against EN1522 FB7 – 7.62 x 51 AP. With this solution you go from (including tolerances) 15 mm stand-alone steel to an 11 mm combination offering a weight saving of 25 percent and a cost reduction for steel.

How spaced armour really works is not stated. There are theories that the bullet is disturbed and twists in the air gap after impacting the first plate of the two plate solution, however a more likely assumption is that the first layer slows down the bullet enough so it will not penetrate the inner plate. It is very common that the inner plate is a 500 Brinell type steel (construction steel) and the outer is a 600 Brinell or 650 Brinell type steel as it is “add on”.

 

 

Perforated armour

Perforated armour is in some aspect similar to spaced armour, but the front layer consists of one or more plates with holes (perforations). The holes are in a regular pattern and are typically smaller than the calibre of the ammunition it has been designed to protect against.

Perforated armour works through the mechanism that the bullet is disturbed and twists so the armour piercing capability is reduced. Making the perforations can be costly as it must be done either before hardening of the armour, orif done after hardening of the armour, completed in such a way so the material is not heated up and the hardness is destroyed.

Before hardening, laser cutting or punching are methods for perforations, however few manufacturers can do this and successfully harden the steel afterwards. Drilling and laser cutting may be done but as there are thousands of holes it can be a slow and expensive process.

After hardening, drilling and water jet cutting are options. Both methods are time consuming and expensive due to the amount of holes required.
A typical example of the use of perforated armour is protection against 7.62 x 54 mm AP (Dragunov). Instead of using a16 mm solid 500 Brinell armour type steel, a 6.5mm 500 Brinell type armour steel plus 4.0 mm perforated 600 Brinell type armour steel can be used. More than 40 percent weight reduction can be made by this solution.

 

 

How to buy armour steel – a simple guide

Globally there are multiple manufacturers offering blast or ballistic grade steel plate with the majority of them located in Europe. . In order to make an informed decisions around the type of armour steel you shall use, the following questions shall be answered:

  • What protection do I need? What type of ammunition (calibre, geometry, armour piercing etc)? What strike speed and angle shall the armour protect against?
  • What weight can the steel be allowed to have? Is there a weight limit/restriction on the vehicle/application being protected?
  • What cost can I bare? Can I go for a more expensive 550 Brinell or 600 Brinell type armour steel over a 500 Brinell type?
  • How shall the steel be fabricated? Here you must consider Cutting, welding and bending of the material
  • Shall I buy directly from the steel producer or a distributor/stock-holder. Most serious armour steel producers you can buy directly from in order to save money, obtain full certification and guarantees, and product support
  • From stock or direct order? Is my quantity big enough and do I have time to buy a production order in order to save money or do I need to buy from stock?
  • How can I get help? A serious armour steel company have people available for you that can help with both what to buy and how to use the steel
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