The NM Hardness Series
The Chinese abrasion resistant series defined in GB/T 24186 begins at NM360 and rises through NM400, NM450, NM500 and NM600 as hardness increases. NM550 is also offered by many producers as an intermediate grade. Each step raises surface hardness and therefore wear life, but reduces ductility and tolerance to impact.
| Grade | Typical Surface Hardness HBW | Relative Suitability |
| NM400 | 370 to 430 | General haulage and mixed material |
| NM450 | 420 to 480 | Hard rock and gravel |
| NM500 | 470 to 540 | Sharp abrasive rock |
| NM550 | 530 to 590 | Very severe sliding abrasion |
| NM600 | 570 to 640 | Most severe abrasion with low impact |
The values above are typical published ranges, and the guaranteed minimum hardness for the ordered thickness should always be confirmed on the mill certificate.
Choosing the Grade for a Truck Body
A dump truck body usually combines two or three grades. The floor and the lower part of the side walls take the highest abrasion and are lined with NM450 or NM500. The upper side panels see less sliding wear and can use NM400. The front headboard experiences impact from the load and is often built in a tougher structural steel with a wear liner on the face. Where the material is sharp, hard rock, the higher hardness grades pay for themselves quickly. Where the load is wet sand or soil with little impact, NM400 gives an excellent balance of life and cost.
Thickness Selection
Truck body floors are commonly lined with 6 mm to 12 mm plate, and side liners with 4 mm to 8 mm plate. The thickness should be chosen so that the liner is stiff enough to avoid drumming under load and thick enough to be welded without burn through. A thinner plate on a supporting structure often outperforms a thicker unsupported plate because a sagging floor accelerates wear. Where the body structure is aluminium, a steel wear liner is fastened with countersunk bolts or with a bolted and bonded system rather than welded, because the two metals cannot be welded together directly.
Welding and Fastening to the Body
When the liner is welded to a steel body, a qualified procedure using a low hydrogen consumable is used, with preheat for the harder grades and a stitch welding sequence to control distortion. Slot welds should be avoided in high hardness plate because they concentrate strain. Bolted fixing is preferred where the liner will be replaced. Countersunk bolts give a smooth wear surface, and the bolt spacing is set so that the plate is held flat against the body. A gap between liner and body allows material to enter and causes rapid local wear.
Typical Applications
Dump truck floor and side liners.
Trailer bodies and tipper containers.
Loader and excavator bucket wear plates.
Transfer chute and hopper liners on mobile plant.
Scraper and concrete mixer wear parts.
Frequently Asked Questions
Q: Which grade is used most often for dump truck floors?
NM450 and NM500 are the most common choices, with NM400 used where the material is less abrasive and impact is a concern.
Q: Is NM600 suitable for a whole truck body?
It is normally used only on the highest wear zones because its low ductility makes it unsuitable for areas that receive heavy impact.
Q: Can wear plate be bolted instead of welded?
Yes. Countersunk bolting is common where liners are replaced periodically, and the bolt spacing is chosen to keep the plate flat against the body.
Q: How thick should a truck body liner be?
Floors commonly use 6 mm to 12 mm and side liners 4 mm to 8 mm, with the final choice based on abrasion severity and the supporting structure.
Q: What hardness values are guaranteed?
The mill certificate states the measured hardness, and the buyer should confirm the guaranteed minimum for the ordered thickness before placing the order.