What NM500 Is and Where the Grade Comes From
NM500 is a quenched and tempered abrasion resistant plate defined by GB/T 24186, the Chinese national standard for wear resistant steel plates. The letters NM stand for nai mo, the pinyin abbreviation for wear resistance, and the number indicates a nominal surface hardness of 500 on the Brinell scale. Plates delivered to this grade are specified with a hardness range of 470 to 540 HBW measured on the plate surface. That places NM500 at the top of the general hardness band covered by the standard series, above NM360, NM400 and NM450, and below the specialised NM550 and NM600 grades.
For a fabrication buyer the practical meaning of the grade is simple. Wear rate under sliding abrasion is governed mainly by surface hardness, so a plate at 470 to 540 HBW outlasts mild steel and structural plate by a wide margin in the same duty. The trade-off is formability, so NM500 is normally used as a wear liner or as a wearing component that is supported by a structural frame rather than as the primary load path.
Why Heat Treatment Decides NM500 Performance
NM500 is produced on a quench and temper route. After rolling, the plate is reheated to an austenitising temperature and then quenched at a controlled rate, which transforms the structure into martensite and produces the high hardness. Quenching alone leaves the plate hard but highly stressed and brittle, so a tempering treatment follows. Tempering is performed in the low temperature range so that hardness is largely retained while residual stress and brittleness are reduced.
| Stage | Purpose | Effect on the delivered plate |
|---|---|---|
| Austenitising | Dissolve carbides and homogenise the structure | Prepares the steel for a full martensitic transformation |
| Quenching | Rapid cooling above the critical rate | Raises hardness towards the 470 to 540 HBW band |
| Tempering | Stress relief after quenching | Restores toughness and dimensional stability |
Because tempering temperature drives both hardness and toughness, the tempering practice is recorded in the mill test certificate together with the measured hardness of each mother plate. A plate that is re-heated above its original tempering temperature during downstream processing loses hardness permanently, which is why flame cutting and welding parameters matter for wear plate.
Delivery Condition and Machinability
Wear plate is normally supplied in the quenched and tempered condition with a nominal hardness guarantee. In practice, mills also offer a high temperature tempered delivery state, in which the plate is softened deliberately so that it can be machined, drilled, milled or formed with conventional tooling. That state produces a tempered sorbite structure with a more uniform distribution of carbides, which gives markedly better cutting and machining behaviour than a hard quenched plate, at the cost of wear resistance.
Hardness guaranteed state: used where the plate is the wear surface and is cut by thermal processes.
Softened or high temperature tempered state: used where drilling, counterboring or heavy machining is required.
Plates are routinely supplied with hardness tested at several positions per plate, since hardness can vary across the width of a wide plate.
Cutting, Forming and Welding Practice
Thermal cutting of abrasion resistant plate should follow a preheat rule based on thickness: thinner plates below roughly 20 mm can be cut without preheat, while heavier sections are normally preheated to reduce the risk of a hard, crack sensitive cut edge. Plasma, laser and waterjet cutting are all used for wear plate; waterjet is preferred where the heat affected zone must be avoided entirely. Cold forming is limited because of the high yield strength, and bend radii used for wear plate are considerably larger than those used for structural steel of the same thickness.
Welding of NM500 is done with low hydrogen consumables and controlled heat input. Joints in wear plate are usually fillet or partial penetration joints rather than full strength butt joints, since the plate is attached to a structural backing. Interpass temperature is limited, and preheat is applied for thicker sections to reduce hydrogen cracking risk. Where a wear plate is used on impact loaded equipment, the welding procedure is normally qualified by impact testing of the joint.
Inspection, Certification and Supply Range
NM500 plate is supplied with a mill test certificate that reports chemical composition, hardness results per plate and, where requested, tensile properties. Third party inspection can be arranged for overseas projects, and ultrasonic testing is available for plates used in critical structures or where through thickness properties are relevant. Typical stocked thickness for abrasion resistant plate runs from 3 mm up to 150 mm, with hardness held across the working range.
Thickness tolerance, flatness and edge condition are specified on the purchase order, because wear plate is often installed with bolted or welded fastening and the flatness of heavy sections directly affects installation time. Plates can be supplied in cut to length sizes, in full width plates or as laser cut finished liners according to drawings.
Frequently Asked Questions
Q: What hardness should an NM500 plate certificate show?
A: The GB/T 24186 requirement is a surface hardness of 470 to 540 HBW, and the actual value is measured per plate and reported on the mill test certificate.
Q: Can NM500 plate be cut with a torch without preheat?
A: Thin plates can generally be plasma or flame cut cold, but thicker sections should be preheated to control the hardness of the cut edge and reduce cracking risk.
Q: Does NM500 lose hardness during welding?
A: Yes. The heat affected zone and any region heated above the original tempering temperature will soften, so weld size and heat input are kept under control in wear applications.
Q: Is NM500 suitable for forming into a curved liner?
A: Limited curvature is possible with a generous bend radius and a defined forming procedure, but tight radii require a softer delivery state or a different grade.
Q: What thickness range is normally available?
A: Abrasion resistant plate is commonly stocked from 3 mm up to around 150 mm, with the most frequently requested sizes between 6 mm and 60 mm.
Q: What certificate accompanies an export order?
A: A mill test certificate with heat number traceability is standard, and third party inspection or ultrasonic testing can be added on request.
