Hardox 400 is a trademarked abrasion-resistant structural steel plate from SSAB, and it's the most widely used grade in the entire Hardox line — the reference point most fabricators think of first when someone says "wear plate." It's produced through quenching and tempering to a nominal Brinell hardness of roughly 400 HBW, which is high enough to resist gouging and abrasive wear far better than mild or structural steel, while still retaining enough ductility and weldability to be cut, formed, and welded into fabricated wear parts using conventional shop equipment.

Because it sits in the middle of SSAB's hardness range — harder than general structural plate but noticeably tougher than the highest-hardness Hardox grades — Hardox 400 is the default choice for a huge range of wear applications: truck and trailer beds, excavator and loader buckets, chutes, hoppers, screens, and general wear liners across mining, construction, agriculture, and recycling equipment. Its balance of hardness, toughness, and formability is what makes it the highest-volume grade in the family rather than the most extreme one.

Hardox is a trademarked SSAB product brand, not a designation covered by AISI, DIN, JIS, or other national steel standards. There is no direct international standard equivalent — comparable through-hardened wear plates are marketed by other producers under their own trade names (e.g. AR400-type plate), but chemistry and guaranteed properties are not identical across brands.

At roughly 400 HBW, Hardox 400 is dramatically more abrasive to a cutting edge than mild or medium-carbon structural steel. The through-hardened, tempered martensitic microstructure that gives this plate its wear resistance in service is exactly what makes it hard on tooling in the machine shop — there's no soft, easily sheared matrix to work with, so every pass is cutting against a uniformly hard structure rather than isolated hard spots.

Machining Hardox 400 successfully means treating it like a hardened alloy steel rather than a mild plate: rigid workholding and tooling to avoid deflection and chatter, wear-resistant coated carbide grades with good hot hardness, and cutting speeds well below what the same equipment would run on an uncoated mild steel plate. Most Hardox fabrication work is limited to flame or plasma cutting, drilling, and edge preparation rather than heavy stock removal, since the plate is typically used at or near its as-supplied thickness. Where machining is required — drilling mounting holes, boring, or edge finishing — conservative feeds and generous tool-life allowances are the norm.