AR400 vs NM400: Two Standards, One Hardness Class

AR400 and NM400 are routinely treated as equivalents. Their governing standards define hardness differently — this guide explains the overlap and what to verify before substituting.

Different documents, similar plates

AR400 is a commercial designation under the ASTM system (typically ordered to ASTM A514 or simply to a producer's AR specification): the "400" denotes a target Brinell hardness of 400. NM400 is a formal grade in the Chinese standard GB/T 24186, with specified chemistry, hardness and impact requirements.

Where the specs differ

	AR400	NM400
Governing document	ASTM framework / mill spec	GB/T 24186
Typical hardness window	About 360–440 HBW (often 380–430 nominal)	About 370–430 HBW
Impact toughness	Not always guaranteed	Specified at −20 °C
Chemistry	Mill-defined within broad limits	Defined ranges in the standard

The practical trap is the word "AR400" itself: because it is a hardness-class name rather than a tight grade, two suppliers' AR400 can differ in chemistry, toughness and through-hardness, and some AR plate is sold with no impact guarantee at all. NM400's standard at least fixes a minimum impact value and chemistry windows.

Substitution care

For wear liners, dump bodies and chutes, AR400 and NM400 are genuinely interchangeable in service — hardness drives wear life, and the windows overlap almost completely. Before substituting in anything structural, impact-loaded, or low-temperature, verify on the mill certificate (EN 10204 3.1):

Actual surface hardness across the plate, not just the average.
Charpy impact values and test temperature, if the duty needs them.
CEV or carbon equivalent, if the fabrication involves heavy welding.
Through-thickness hardness for thick plate.

If any of these is missing from an AR400 offer, price the two as different products — because in specification terms, they are.

How the two are usually bought

In North American practice, AR400 is frequently purchased against a purchase-order description — hardness range, sometimes chemistry limits — rather than a full standard, and the delivering mill's own datasheet becomes the de facto specification. In Chinese and export trade, NM400 is purchased against GB/T 24186 directly, which pins down chemistry, hardness sampling and impact testing. Neither route is inherently better, but the paperwork burden differs: with NM400 the standard does the work, with AR400 the buyer must write the requirements. Experienced buyers of AR400 therefore add three lines to the order: a surface hardness window (not a single target), a minimum Charpy value with test temperature when the plate will see impact or cold, and a maximum carbon equivalent when welding is heavy. Fabricators also note that both plates behave the same in the shop — low-hydrogen practice, controlled preheat for thick or restrained joints, no cold forming below supplier-recommended radii — so a substitution that passes the certificate check rarely needs any change on the shop floor.