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SUS440C and SUS420J2: Machining Stainless That Has to Be Hard

Materials / 3 Sept 2026

Most stainless machining is austenitic: SUS304, SUS316, the grades that work harden and never get hard. The martensitic grades are a different job. SUS420J2 and SUS440C are specified because the finished part has to resist wear, hold an edge or carry a bearing load, and that means heat treatment sits in the middle of the process rather than at the end.

Get the sequence right and these parts are straightforward. Get it wrong and you are grinding features that should have been turned.

What these grades are

SUS420J2 carries roughly 0.26 to 0.40 % carbon and 12 to 14 % chromium. It hardens to around 50 to 55 HRC. Blades, mould inserts, surgical instruments, wear plates. EN 1.4028. Its sibling SUS420J1, at 0.16 to 0.25 % carbon, stops around 45 to 50 HRC and is EN 1.4021.

SUS440C carries roughly 0.95 to 1.20 % carbon, 16 to 18 % chromium and a little molybdenum. It hardens to around 58 to 60 HRC, the highest of the common stainless grades. Bearing races, valve seats, nozzles, cutting edges. EN 1.4125, UNS S44004.

SUS410, for context, sits below both at 11.5 to 13.5 % chromium and low carbon, reaching around 40 to 45 HRC. It is the cheap martensitic option for shafts and fasteners where moderate hardness is enough.

The AISI system collapses SUS420J1 and SUS420J2 into “420”. If your drawing says SUS420J2 and the quote comes back saying 420, confirm which carbon range the shop will buy. That is not a formality, it is the difference between 48 HRC and 55 HRC.

The sequence

Buy annealed. Both grades are supplied annealed, around 200 to 250 HB. This is the only state in which they machine conventionally.
Rough and semi finish everything. Every feature that can be cut now, cut now. Drilling, tapping, pockets, slots, anything with an internal corner.
Leave grinding stock on critical features. Typically 0.1 to 0.3 mm on diameters, more on long parts and on faces that will distort.
Heat treat. Harden and temper to the specified hardness. Expect distortion. A long thin part will move more than a compact one, and an unbalanced part will move more than a symmetric one.
Stress relieve or double temper if specified. SUS440C in particular often gets a sub zero treatment to convert retained austenite, followed by tempering.
Grind or hard turn the critical features.
Passivate if specified.

The rule of thumb is simple: anything with a corner, a thread or a hole gets done before heat treatment. Grinding an internal corner is slow. Tapping at 58 HRC is not a thing.

Machining annealed

Annealed martensitic stainless is not difficult, but it is not free machining steel either.

It work hardens less than the austenitics, which is a relief
The carbides, particularly in SUS440C, are abrasive. Tool wear is by abrasion rather than by built up edge
SUS440C’s high carbide content makes it the more abrasive of the two by a wide margin
Machinability index sits around 45 for SUS420 annealed and around 40 for SUS440C annealed

Practical points: coated carbide, moderate surface speed, a feed heavy enough to get under the skin, and plan on replacing inserts on wear rather than on catastrophic failure. Flood coolant. The chips are shorter and better behaved than austenitic chips, which is one of the few things easier about these grades.

After hardening

Up to about 45 HRC hard turning with CBN or ceramic is comfortable. Milling is possible with the right tooling.

45 to 55 HRC, SUS420J2 territory. Hard turning works on external features with a rigid setup. Grinding is normal for anything with a real tolerance. Drilling is out.

Above 55 HRC, SUS440C territory. Grinding for almost everything critical. Hard turning is possible on simple external diameters with CBN and a very rigid machine, and many shops would rather grind it. EDM for features that cannot be reached by a wheel.

This is why the sequence matters so much. Every feature you leave for after hardening is a feature that gets ground.

Distortion, and how to plan for it

Heat treatment moves the part. How much depends on geometry, section change and how the part is supported in the furnace.

What helps:

Symmetry. A part with balanced material distorts less than one with a heavy boss on one side
Stress relieving after roughing, particularly when a lot of material came off one side
Grinding stock that matches the risk. 0.1 mm on a compact part is plenty. A 300 mm shaft may need 0.4 mm and a straightening operation
Telling the heat treater the geometry. Fixturing and orientation in the furnace are their decision, and they make a better one with information

What does not help: hoping. If the drawing has a flatness or a cylindricity call-out and the part is going through a hardening cycle, plan a finishing operation for it.

Corrosion resistance, stated plainly

These grades are corrosion resistant, not corrosion proof, and clearly behind the austenitics.

The chromium that protects the surface has to be dissolved in the matrix. In a high carbon martensitic grade a large fraction of it is locked into chromium carbides instead, which is exactly what delivers the hardness. The surface therefore has less free chromium available to form a passive film.

Practically:

SUS440C hardened and tempered resists mild atmospheric corrosion and most food environments
Neither grade belongs in chlorides or in continuously wet service without a coating
Passivation after machining matters more here than on austenitic parts, because embedded iron from tooling has a worse surface to sit on
Over tempering above about 400 degC degrades corrosion resistance further, so tempering temperature is a specification, not a suggestion

What to put on the drawing

SUS440C per JIS G 4303, UNS S44004
Supply annealed
Harden and temper to 58 to 60 HRC after rough machining
Grind critical diameters after heat treatment
Passivate per ASTM A967
Mill certificate to EN 10204 3.1

Specify the hardness as a range, not a single number. A drawing that says “58 HRC” with no tolerance is a drawing that will be argued about.

For where these grades sit in the wider range, see the JIS to AISI equivalents table. For the precipitation hardening alternative that avoids a full quench, see SUS630 and 17-4 PH.

Have a hardened stainless part to place? Get a quote. The rough, harden, grind sequence is quoted as one order and placed with a shop that owns the heat treat relationship, not just the mill. Priced quote within 24 business hours.

Questions

What hardness do these grades reach?

SUS420J2 reaches roughly 50 to 55 HRC after hardening and tempering. SUS440C reaches roughly 58 to 60 HRC. SUS420J1, with lower carbon, stops around 45 to 50 HRC.

Can I machine them after hardening?

Only with hard turning or grinding. At 55 HRC and above, conventional milling and drilling are not practical on critical features. Plan the sequence so everything that can be cut soft is cut soft.

Are they as corrosion resistant as SUS304?

No. Martensitic grades trade corrosion resistance for hardness. They need the chromium tied up in the matrix rather than in carbides, and the high carbon works against that. Treat them as corrosion resistant, not stainless in the 304 sense.

Why does my drawing say SUS420J2 rather than 420?

AISI 420 covers a wide carbon range in one designation. JIS splits it: J1 is roughly 0.16 to 0.25 per cent carbon, J2 is roughly 0.26 to 0.40 per cent. The split matters, because it sets the achievable hardness.

Have a drawing for a stainless part?

Send it and an engineer reads the title block, quotes it within 24 business hours, and if you order we place, manage, inspect and ship the parts on one PO and one invoice.

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