420 vs 1.2083 vs 1.2316: Choosing a Mold Stainless Steel

One family, three jobs

Martensitic stainless mold steels are hardenable 13–17% chromium steels. The three grades most often compared differ more than their "420-type" label suggests:

	AISI 420 (1.4021/1.4028)	1.2083	1.2316
Carbon	~0.2–0.3%	~0.40%	~0.36%
Chromium	~13%	~13%	~16%
Molybdenum	—	—	~1.1%
Typical working hardness	40–52 HRC, grade-dependent	48–52 HRC	48–52 HRC hardened, or prehardened

420: the economy grade

Plain 420 covers the widest range, from soft 1.4021 to harder 420J2-type variants. It machines easily, hardens adequately, and gives basic rust resistance for mold bases, ejector plates, support pillars and cavities where the resin is not corrosive and the polish requirement is modest. Its ceiling is real: polish quality and pitting resistance are limited by its cleanliness and chromium level.

1.2083: the polish grade

1.2083 (X40Cr14, close to AISI 420 modified) is the classic cavity stainless: enough carbon for 48–52 HRC, fine carbides, and good etch and polish response. For mirror finishes, optical and medical parts, specify the ESR (electro-slag remelted) version. Remelting strips out sulfide and oxide inclusions that otherwise show up as pinholes and drag lines in a polished cavity; ESR material also pits less in service because pits usually start at inclusions.

1.2316: the corrosion grade

With roughly 16% chromium plus molybdenum, 1.2316 has the strongest corrosion resistance of the three — the standard answer for PVC and other halogen-releasing resins, whose hydrogen chloride fumes attack lesser stainless grades. It is often supplied prehardened for large molds, trading a little peak hardness for zero heat-treatment risk on big blocks. Its mirror polish is good but, in standard (non-ESR) condition, usually a step below ESR 1.2083.

Choosing

• Low cost, mild resins, moderate finish → 420.
• Highest polish, medical/optical/food contact, humid storage → 1.2083, ESR for mirror work.
• PVC, flame-retardant or otherwise corrosive resins, large prehardened blocks → 1.2316.

All three still need sensible mold practice: corrosion starts in cooling channels and parting lines first, so water treatment and shutdown protection matter as much as the grade.

Working notes

Hardened stainless mold steels are usually bought annealed and hardened after rough machining, because distortion at 48–52 HRC is small but not zero; finish grinding and polishing stock must be planned. Two practical cautions:

• Repairs: weld repairs on stainless cavities are possible but demanding — matching fillers and full re-heat-treatment are needed to avoid soft or rust-prone patches.
• Nitriding: gas nitriding a stainless cavity raises surface wear resistance but ties up free chromium and can reduce the corrosion resistance the steel was bought for; it is a deliberate trade, not a free upgrade.
• Cooling channels: drill and finish quality inside water channels matters — rough, scaled channels in any of the three grades are where corrosion staining of parts usually begins.