What Do the NM Designations Mean?
NM is the Chinese abbreviation for wear-resistant steel, and the number in the grade name refers to the approximate Brinell hardness in HBW: NM400 is about 400 HBW, NM450 about 450 HBW, NM500 about 500 HBW, NM550 about 550 HBW, and NM600 about 600 HBW. These plates are quenched and tempered to develop a hard, abrasion-resistant surface with a tempered martensitic structure, and they are commonly supplied to GB/T 24186. As hardness increases, wear resistance improves, but toughness and weldability gradually decrease, so the optimum grade always depends on the operating conditions.
Hardness and Wear Resistance Comparison
Typical hardness ranges for the five grades are approximately: NM400 at 360-440 HBW, NM450 at 420-480 HBW, NM500 at 450-540 HBW depending on thickness, NM550 at 500-550 HBW, and NM600 at 570-640 HBW. Exact ranges vary with plate thickness and the applicable standard edition, and the mill certificate should be used for acceptance. In sliding abrasion against sand, gravel, and ore, the harder grades wear more slowly: NM600 typically offers the longest service life of the series, while NM400 provides the best balance of hardness and toughness for impact-dominated applications.
Chemical Composition Differences
All five grades are low-alloy steels with carbon, manganese, silicon, chromium, nickel, molybdenum, boron, and often titanium and aluminum additions, with tight phosphorus and sulfur limits. NM400 uses a carbon level around 0.30% maximum and a manganese limit near 1.60%. NM500 raises carbon to about 0.38% maximum and manganese to about 1.70%, with chromium up to 1.50% and nickel up to 1.00%. NM550 and NM600 use higher alloying to reach their hardness, and the boron addition in the range of about 0.0005-0.006% promotes hardenability during quenching. The exact limits should be taken from GB/T 24186 and the mill certificate for the ordered thickness.
Mechanical Properties and Toughness
Typical mechanical values for the series are: NM400 with yield strength around 1000 MPa, tensile strength around 1200 MPa, and elongation about 10%; NM450 with yield strength around 1100 MPa and elongation about 10%; NM500 with tensile strength around 1350 MPa and elongation about 7%, with Charpy testing at -20 deg C or -40 deg C as agreed; NM550 with tensile strength around 1400-1700 MPa and elongation about 8%; and NM600 with tensile strength around 1400 MPa and elongation about 5%. Higher hardness grades show lower elongation, so impact-critical parts should not be designed with the hardest grade unless the impact level is moderate.
How to Choose the Right Grade
There is no single best grade, only the most suitable one for the operating conditions. Choose NM400 or NM450 for impact resistance, easier welding, and better formability in excavator buckets, crusher components, and structural wear parts. Choose NM500 for balanced performance where both wear resistance and moderate toughness are needed, such as dump truck bodies and chutes. Choose NM550 or NM600 for extreme sliding abrasion and maximum service life in high-wear zones such as transfer points, hoppers, and heavy-duty liners. Always consider fabrication: ultra-hard grades need plasma or laser cutting, controlled heat input welding with low-hydrogen consumables, and mechanical fastening where maximum wear life is required at joints.
Frequently Asked Questions
What does NM400 stand for?? NM means wear-resistant steel in Chinese, and 400 refers to the nominal Brinell hardness of about 400 HBW. The same logic applies to NM450, NM500, NM550, and NM600.
Which NM grade is the hardest?? NM600, with a typical hardness of about 570-640 HBW, is the hardest and most wear-resistant grade in the common NM series, followed by NM550 and NM500.
Can NM500 be welded?? Yes, NM500 can be welded with low-hydrogen consumables and controlled heat input, but the weld zone will not match the base hardness and may wear faster in abrasion service.
Which grade should I choose for heavy impact?? For severe impact, NM400 or NM450 is preferred because their higher toughness reduces the risk of cracking. The hardest grades are better for pure sliding abrasion with moderate impact.
What is the difference between NM500 and AR500?? NM500 (GB/T 24186 practice) and AR500 have similar hardness and composition concepts, but they are defined by different standards. The governing standard of the project should be followed.
What thickness ranges are available for NM plates?? NM plates are commonly supplied from about 8 mm up to 100 mm or more depending on grade and mill. Hardness requirements vary with thickness, so the available range should be confirmed with the supplier.
