420SS Stainless Steel Plate: Properties, Equivalent Grades & Uses

The short version: 420SS is a martensitic stainless steel — 12–14% chromium, hardenable by heat treatment. It is the same family as AISI 420, DIN 1.4021 and JIS SUS420J1/J2. You buy it when you need corrosion resistance and hardness in the same plate.

What is 420SS?

420SS is a straight-chromium martensitic stainless steel. Unlike 304 or 316, which are austenitic and cannot be hardened, 420 can be quenched and tempered to a usable hardness — typically 45–55 HRC depending on carbon content. That combination of moderate corrosion resistance and hardness is why it ends up in cutlery, plastic moulds, pump shafts and valve parts.

One thing buyers often miss: “420” is a family, not a single grade. Carbon ranges from about 0.15% to 0.40%, and the two common sub-grades behave very differently.

Typical chemical composition

Values below are typical for the 420 family. Always confirm the exact heat against the mill test certificate (MTC) — carbon in particular decides how hard the final part can go.

Equivalent grades

Cross-standard naming is the main source of confusion. The table maps the common labels. Note that a “420” label alone does not tell you the carbon level — always ask for the full designation.

Hardness and heat treatment

• Annealed (supply condition): typically ≤ 235 HBW — soft enough to machine and cut.
• Hardening: austenitise around 980–1050 °C, then oil or air quench.
• Tempering: 150–370 °C depending on the hardness/toughness balance you want. Higher tempering = tougher but softer.
• Usable hardness: roughly 45–50 HRC for 420J1, up to ~52–55 HRC for higher-carbon 420.

For corrosion-critical parts, avoid tempering in the 400–600 °C range — it can reduce corrosion resistance. If both hardness and corrosion matter, discuss the temper with your heat treater before committing.

Common uses

• Plastic moulds where mild corrosion resistance is needed (e.g. PVC, some filled resins)
• Cutlery, kitchen tools and blades
• Pump shafts, valve stems, screws and bushings
• Measuring tools and jigs that need wear resistance plus some corrosion resistance
• Medical and food-contact parts (check polishability and corrosion spec first)

How to specify 420SS plate correctly

1. Give the full grade — e.g. “1.4021 / SUS420J1” or “1.4028 / SUS420J2”, not just “420”.
2. State the condition — annealed for machining, or pre-hardened at a target HRC.
3. Give thickness and tolerance — plate is usually supplied 3–100 mm; flatness and thickness tolerance affect machining allowance.
4. Ask for the MTC — chemistry plus hardness, so you can verify carbon before heat treatment.