Three Different Steel Families
The grades S355JR, NM400, NM450, NM500 and C70 are all supplied as plate, but they belong to three different families with different metallurgy and uses. Specifying by strength, by hardness or by carbon content changes the plate completely, so the application must be defined before the grade is chosen.
S355JR: The Structural Grade
S355JR is a structural steel defined in EN 10025-2 with a minimum yield strength of 355 MPa for thickness up to 16 mm and a tensile range of 470-630 MPa, with 27 J Charpy impact energy at 20 degrees Celsius. It is weldable, formable and intended for load-carrying members such as beams, columns, frames and general fabrication.
NM400, NM450 and NM500: The Wear Grades
The NM series to GB/T 24186-2009 is quenched and tempered for hardness, not for yield strength. NM400, NM450 and NM500 deliver typical hardness of 370-430, 420-480 and 470-530 HBW respectively, and are used for liners and wear parts where abrasion, not structural load, is the design driver.
C70: The High-Carbon Grade
C70 is a high-carbon steel with about 0.70% carbon, supplied in the annealed or spheroidised condition for further processing. It is used for springs, tools and machine parts that are hardened after fabrication. It is not a structural grade: it has lower weldability and must be handled with preheat and post-weld treatment if welded at all.
Choosing the Right Plate
Load-carrying members: S355JR or the equivalent Q355 grade
Sliding and scouring abrasion: NM400, NM450 or NM500 by severity
Springs, saws and tools: C70, heat-treated after machining
Quality and Documentation
Each family is documented differently: S355JR with tensile and Charpy test certificates, NM grades with Brinell hardness and impact results, and C70 with chemistry and hardness in the supplied condition. All deliveries include mill certificates with heat traceability.
Frequently Asked Questions
Q: Can S355JR be used for wear applications?
S355JR has no special abrasion resistance. For sliding wear, an NM grade is the correct choice even though its yield strength is not the specified property.
Q: Is C70 suitable for structural frames?
No. C70 is a high-carbon steel for springs and tools, with limited weldability. Structural frames should use S355JR, Q355 or equivalent structural grades.
Q: What does S355JR guarantee?
A minimum yield of 355 MPa up to 16 mm, a tensile range of 470-630 MPa and 27 J Charpy impact at room temperature.
Q: Which NM grade should I choose?
Match the hardness to the wear severity: NM400 for general service, NM450 for continuous sliding wear, and NM500 for the most abrasive conditions.
Q: Can these grades be welded together?
Structural-to-wear joints are common, using low-hydrogen consumables and preheat for the NM side. C70 is not normally welded to structural members.
Q: Are mill certificates provided for all grades?
Yes, every plate delivery includes mill test certificates recording chemistry, mechanical properties and the relevant hardness or heat-treatment data.