NM400 vs NM500: Stepping Up Wear Plate Grades

What the numbers mean

In the GB/T 24186 series the number approximates the plate's Brinell hardness: NM400 is delivered around 370–430 HBW, NM500 around 470–530 HBW (exact windows vary with thickness and mill). Both are quenched (or quenched-and-tempered) low-alloy steels whose wear resistance comes from hardness, not from a special alloy trick.

The wear–fabrication trade

	NM400	NM500
Hardness	~400 HBW	~500 HBW
Sliding-abrasion life	Baseline	Typically 1.5–2× NM400 in pure abrasion
Cold forming	Moderate bends possible with generous radii	Very limited; large radii, warm forming preferred
Welding	Standard low-hydrogen practice, moderate preheat	Higher preheat, strict heat input control
Machining/drilling	Difficult but routine with carbide	Slow; plasma/laser cutting and limited drilling
Toughness	Better	Lower; more care needed in impact duty

Laboratory abrasion tests reward hardness, but field life also depends on impact: in high-impact applications (primary crusher feed, excavator lips striking rock) a cracked or spalled NM500 liner can underperform a tougher NM400 one that only wears faster.

Stepping logic

• Use NM400 for dump bodies, liners, chutes, buckets and wear parts that must be bent, welded in place, or repaired in the field — it is the general-purpose workhorse.
• Use NM500 for flat or gently formed liners in sliding abrasion: ore passes, conveyor transfer points, fan blades, and areas where thickness cannot grow but life must.
• Consider the whole fabrication route before stepping up: if the part needs tight bends or extensive site welding, the harder grade may lose more in fabrication and repair cost than it gains in wear life.

A note on thickness

Hardness is verified at the surface; through-hardness and toughness vary with plate thickness in all wear grades. For thick plate in severe duty, confirm the guaranteed hardness range and impact values on the mill certificate rather than relying on the grade number alone.

Field experience in common duties

In truck bodies hauling crushed rock, operators commonly report NM500 liners outlasting NM400 by a wide margin on the floor, where sliding dominates, while the front wall and corners — the impact zones — are often kept in NM400 or protected with bars, because that is where hard plate cracks first. Mixed-grade construction (hard floor, tougher walls) is normal practice, not a compromise. In fixed plant, NM500 earns its premium in ore chutes, transfer liners and mill feed spouts where the material stream is mostly sliding; for primary impact plates and for parts that must be re-bent or built up on site, NM400 remains the safer specification. Cutting follows the same gradient: NM400 can be drilled with carbide tooling at reduced speeds, while holes in NM500 are usually cut by plasma, waterjet or laser rather than drilled. Whichever grade is chosen, wear plate should be costed per operating hour or per million tonnes handled — on that basis the harder grade frequently wins in pure abrasion, and just as frequently loses where impacts and repairs dominate.