What is AR500 steel?
AR500 steel is the industry designation given to Abrasion Resistant steel that has a Brinell hardness rating of 500. The “AR” in AR500 steel stands for “Abrasion Resistant”. While this may seem rather odd, using abrasion resistant steel for ballistic purposes like armor and steel targets actually makes perfect sense. The same abrasive forces that would deteriorate softer steel in manufacturing and mining applications is roughly equivalent to the ballistic forces introduced by a bullet. While these two forces may not seem to be remotely comparable, the solution to both is hardening the steel. The harder the steel, the less it will deform under a specified force, in this case a bullet. The harder the steel, the faster and heavier rounds it is able to stop effectively. This trend continues in steel until around the 500 level on the Brinell scale, while it would seem reasonable to assume that AR600 or 700 would be capable of stopping even tougher rounds, field-testing has shown that past 500, the steel becomes very brittle. While these extremely hard steels may not deform when faced with large magnum class rifle rounds, they wind up shattering catastrophically, usually still letting the projectile through in the process. AR500 is the preferred middle ground between hardness and flexibility for ballistic applications because it stops the greatest number of calibers while maintaining its structural integrity.
Hardness is measured typically using the Brinell hardness scale. The Brinell scale describes the indentation hardness of various materials through the degree of penetration caused by an indenter. Basically, pressure is applied to a steel or carbide ball on the material and the indentation caused by this force is measured and then translated via an equation to a numerical score. One of several definitions used to describe the property of “hardness” in the material science community; it is the scale most commonly used in the steel industry and, by extension, in the steel ballistic plate-manufacturing sector. Typical alloys include roughly around C-.30%, Si-.70%, Mn-1.70%, Cr-1.00%, Ni-.80%, Mo-.50% and B-.004%, though each manufacturer uses their own proprietary blend, this steel makeup is a rough average. AR500 steel on the molecular level then does not seem particularly special, and it really is not as far as steel goes, but it does have a few special properties. Starting from this stock blend steel, it is then heat and quench hardened to its specified 500 Brinell rating. To put that number into perspective, ordinary mild steel only has a Brinell hardness of 120. A square inch of AR500 steel can withstand up to 110 tons of pressure. AR-500 steel is also about the hardest workable material available, everything beyond it on the Brinell scale, such as glass and ceramics, have a tendency to shatter when worked and are therefore susceptible to the catastrophic failure discussed earlier.
The advantage of using such a hard variety of steel is that very little of it is necessary to defeat even very fast rifle rounds. As a result, the overall weight of an AR500 steel bulletproof vest can be kept well below 20lbs while offering complete torso and back protection. In comparison, the ceramic bulletproof vests currently in use by the US Army weigh up to 35lbs, or roughly double the weight of an average NIJ Level III+ AR500 steel vest. Even very thin sheets of AR500 are capable of defeating pistol calibers. Even fully rifle rated plates are relatively thin compared to their Kevlar or Ceramic counterparts. This makes them easier to move around in and less cumbersome. This makes AR500 steel bulletproof vests perfect for those working inside or in tight quarters like vehicles.
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