P20 vs 718 (1.2738HH): Prehardened Mold Steel Compared

Naming first

"P20" names the AISI family: 1.2311 is the standard grade; adding about 1% nickel gives 1.2738. 718 (ASSAB/ Uddeholm trade name, widely used generically in Asia) is a refined 1.2738-type steel delivered at higher hardness — often marked 1.2738HH, the HH standing for high hardness. Typical delivery conditions:

	P20 / 1.2311	1.2738	718 / 1.2738HH
Delivery hardness	280–325 HB	290–330 HB	330–370 HB
Nickel	—	~1%	~1%
Polish level	Good	Better in thick blocks	Best of the three

Polish and surface

All three are polishable, but they separate at mirror finishes. The nickel grades hold uniform hardness through thick blocks, so a deep cavity polishes evenly without soft spots. 718-type material is usually melted cleaner and supplied specifically for high-gloss and textured (etched) surfaces — automotive lenses, appliance panels and large housings. For SPI A-level mirror work, premium 718 variants are the standard choice in this family; for satin or grained finishes, plain 1.2311 is normally sufficient.

Size and hardness logic

Working hardness sets wear life and edge strength at the parting line. At 280–325 HB, P20 suits small and medium molds and moderate production runs. The jump to 330–370 HB with 718 extends cavity life for long runs and glass-filled resins, and the nickel keeps that hardness uniform in blocks well beyond 400 mm, where standard P20 chemistry would soften toward the core.

Cost logic

718 typically costs more than 1.2311 — and the premium grows with block size because it is bought precisely for large blocks. On a large mold the steel is a small share of total cost next to machining and polishing hours, so the insurance is cheap. On a small mold, P20 delivers the same part quality for less money. Match the grade to block thickness and required finish, in that order.

Maintenance and repair

The two grades also behave alike in the mold shop's daily work, with one exception worth noting. Both weld-repair acceptably with matched P20-type consumables, and welded areas on either will polish slightly differently from the parent steel — a visible witness line is possible on high-gloss cavities regardless of grade. The 718 family's cleaner melting reduces, but does not eliminate, that risk. For texturing, uniform hardness is what keeps etch depth even across a deep cavity, which is another reason large textured tools are specified in the nickel grades. Neither grade is normally nitrided for gloss work, since the case changes polish response. Finally, a caution on names: because "718" is a trade name reused loosely in some markets, purchase orders should state the standard designation (1.2738HH) and the required hardness range in HB, and verify both on the certificate — blocks sold simply as "718" vary more between suppliers than the standard grades do.