Steel grade guide

SUS420J2 (AISI 420 / X30Cr13) — the stainless you can actually harden

How martensitic stainless SUS420J2 is hardened and tempered, why the tempering temperature decides corrosion resistance as well as hardness, and why zinc plating is usually the wrong finish for it.

Grade data

SUS420J2 — Grade data
JIS designation	SUS420J2
Closest equivalents	AISI 420 / DIN X30Cr13
Carbon (%)	0.26–0.40
Steel family	Stainless steel
Heat-treatment route	Quench and temper
Typical surface hardness (HRC)	48–55 HRC
Typical hardness specified in service (HRC)	45–52 HRC
Hardenable by heat treatment	Yes

Hardness values are typical industry ranges for the small sections we process, and depend on the part, the section size, the quench and the governing specification. They are not a guarantee and do not replace your drawing or standard.

Reviewed: 2026-08-04

Hardness by grade — reference table Related service: Heat Treatment

What it is

SUS420J2 is a martensitic stainless steel — the family that hardens by heat treatment, unlike the austenitic grades. Roughly 13% chromium gives it corrosion resistance well below SUS304, and that resistance depends on the chromium staying in solution, which means it depends on how the part was heat treated and whether it was passivated afterwards.

How it is heat treated

Harden, then temper — and the tempering temperature is a materials decision, not just a hardness dial. Tempering low keeps hardness and corrosion resistance; tempering in the intermediate range costs both toughness and corrosion resistance through carbide precipitation; tempering high recovers toughness but gives away the hardness the grade was chosen for. Slow cooling after hardening does the same damage as a bad temper.

Plating and hydrogen embrittlement

Zinc plating a stainless part defeats the reason it was specified, and it is rarely the right answer here — passivation after machining and heat treatment is. Where an acid or electrolytic step is unavoidable, remember that martensitic stainless is genuinely susceptible to hydrogen embrittlement, unlike the austenitic grades, and the part is above the ISO 4042 hardness trigger.

ISO 4042 de-embrittlement bake

Indicated after electroplating

ISO 4042 calls for embrittlement-relief baking on electroplated fasteners at property class 10.9 and above, or at roughly 390 HV (≈ 40 HRC) and above. Many customer specifications set a lower trigger — the drawing decides, not this page.

Hydrogen de-embrittlement bake planner

Typical ISO 898-1 property classes

Not applicable

When to choose it

Choose it when a part has to be both hardened and corrosion resistant — stainless screws and pins that need wear resistance, cutting or forming edges, and small parts in wet service.

When not to choose it

Do not choose it when you need SUS304-level corrosion resistance; 13% chromium in the hardened condition is a compromise and it will corrode in service that 304 shrugs off. Do not choose it for welded assemblies, and do not assume it can be tempered anywhere convenient on the temperature scale.

Parts we run in this grade

Screws Pins By Part →

What tends to go wrong

Quench crackingStraight or slightly curved cracks that appear after quenching — under the head fillet, along the shank, across a head recess, at a thread root, or radially at the corner of a nut. They are open and sharp-tipped, and after plating they often show as a dark line that bleeds. Cracked parts may ring dull instead of clean, and a cracked lot normally shows several pieces cracked in the same geometric position.

Low hardness after temperingThe lot reads below the drawing minimum — for example a class 10.9 bolt at 28 HRC where 33–39 HRC is specified — and the readings are consistent rather than scattered. Proof load and tensile results follow the hardness down, threads deform at nominal torque, and the parts feel soft to a file. Because the readings are uniform, the cause is normally a whole-lot condition rather than a local quench fault.

Hydrogen embrittlement — delayed fracture after platingHeads pop off or bolts split longitudinally hours to days after plating — often 2 to 48 hours after tightening, sometimes weeks later on the shelf. The fracture is brittle with no necking, usually starting under the head fillet, at the first engaged thread or at a thread root. The same lot passed hardness and tensile before plating, there is no overload evidence on the joint, and the failures cluster in the hardest parts of the lot.

Distortion after heat treatment — bend, ovality, pitch driftParts that were straight and in tolerance before the furnace no longer are. Long screws bow, thin shanks bend, nuts and washers go out-of-round or dish, flatness and perpendicularity fail, and threads that gauged correctly now fail the GO gauge because pitch diameter and helix have moved. Distortion is systematic — the whole lot leans the same way — where handling damage is random.

Frequently asked questions

Is SUS420J2 as corrosion resistant as SUS304?+

No, and it is not close. It is a hardenable stainless bought for hardness with useful corrosion resistance, not for maximum corrosion resistance. Choosing between them is choosing which property you actually need.

Does martensitic stainless need de-embrittlement baking?+

If it has been through an acid or electrolytic process at hardened strength, treat it as susceptible — martensitic stainless is not immune the way austenitic 304 is, and at this hardness it is above the ISO 4042 trigger. The specification sets the requirement.

Other grades we run

S20C S45C SCM435 SCM440 SCr440 SWCH10B21 SWCH10B33 SUP9 SK85 (SK5)SUJ2 SKS3 SUS304 All steel grades →

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