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SUS630 and 17-4 PH Machining: Getting the Heat Treat Sequence Right

Materials / 1 Sept 2026

SUS630 is the JIS designation for 17-4 PH, a precipitation hardening stainless that gives you the strength of a heat treated alloy steel and corrosion resistance close to the 300 series. It shows up on aerospace fittings, pump shafts, valve components and tooling.

It is also the grade where sequence matters more than speeds and feeds. Get the order of operations wrong and no amount of careful machining will save the dimensions.

What it is

Roughly 15 to 17.5 % chromium, 3 to 5 % nickel, 3 to 5 % copper, plus niobium and tantalum. It is martensitic in structure. The strength does not come from quenching, it comes from copper rich precipitates that form during a low temperature aging treatment.

That is what makes it useful: the aging happens at 480 to 620 degC, not at a quenching temperature, so distortion during heat treatment is small compared with a conventional hardening cycle. Small, but not zero, which is the whole point of this article.

JIS calls it SUS630. AISI calls it 630. UNS is S17400. EN is 1.4542. Everyone on the shop floor calls it 17-4.

The conditions, and what each one gives you

Condition	Aging	Typical hardness	Typical tensile	Use it for
A	None, solution annealed	30 to 36 HRC	around 1000 MPa	Buying and roughing
H900	480 degC	around 44 HRC	around 1310 MPa	Maximum strength
H925	495 degC	around 42 HRC	around 1170 MPa	High strength, a little more toughness
H1025	550 degC	around 38 HRC	around 1070 MPa	Balanced
H1075	580 degC	around 36 HRC	around 1000 MPa	Balanced, better ductility
H1150	620 degC	around 31 HRC	around 930 MPa	Toughness, easiest to finish

The H number is the aging temperature in degrees Fahrenheit, which is why they look arbitrary in metric. H900 means aged at 900 degF.

Two practical notes. Higher aging temperature means lower strength and better toughness, and it also means better machinability, so if the service does not demand H900, do not specify it. And H1150M is a double aged variant used where maximum toughness matters, which is softer again.

The sequence that works

Buy in condition A. Stockists supply it this way. Ask for the certificate to state condition A.
Rough everything. Condition A machines at roughly 48 on the standard index, better than SUS316 and close to SUS304. Normal carbide tooling handles it.
Leave finishing stock. Typically 0.2 to 0.5 mm on critical diameters and faces, more on long thin features.
Stress relieve if the part is large or heavily roughed. Optional, and worth it on anything with an unbalanced amount of material removed.
Age to the required H condition. Expect a small dimensional change: the part typically contracts slightly, on the order of 0.0005 to 0.001 mm per mm depending on temper. On a 100 mm dimension that is 0.05 to 0.1 mm, which is far outside a tight tolerance.
Finish the critical features. Turning and milling are workable up to about H1075. At H900, expect to grind or hard turn.
Passivate if specified.

Why machining fully in condition A fails

It is tempting. Condition A machines well, the part comes off the machine complete, and heat treatment looks like a final step.

Then the part shrinks. Not much, and not unpredictably, but enough. A bore that was h7 at 40 mm before aging is out of tolerance after it. A face that was flat within 0.02 mm is not.

Shops that run 17-4 regularly plan for this. Shops that do not will quote the part as if aging were free and discover the problem on the first article. It is one of the clearer questions to ask when you are choosing who cuts the part: do you machine 17-4 before or after aging, and how much do you leave?

Machining notes by condition

Condition A, around 32 HRC. Treat it like a tough stainless. Positive rake, sharp edges, a feed heavy enough to cut under any previous hardened layer. It work hardens less aggressively than SUS304 but it is not immune. Surface speed lower than SUS304, tool life shorter.

H1150 to H1075, 31 to 36 HRC. Still conventionally machinable. Rigidity matters more than it did in condition A. Chatter marks on a face at this hardness are hard to remove afterwards.

H1025 to H925, 38 to 42 HRC. Getting into hard turning territory for finishing. Ceramic or CBN inserts, light depths, high rigidity. Milling is workable with the right tooling but slow.

H900, around 44 HRC. Finish by grinding or hard turning. Do not plan conventional milling on critical features at this hardness. Thread rolling is easier than thread cutting where geometry allows it.

Corrosion, honestly

17-4 PH passivates and resists general corrosion reasonably well, comparable to SUS304 in mild environments and clearly better than SUS410 or SUS420J2.

It is not a chloride grade. The copper and the relatively low nickel put it behind SUS316 in salt, and it is susceptible to stress corrosion cracking, particularly in the higher strength tempers. For a part that is both highly loaded and in chlorides, H900 is the wrong temper. H1150 is more resistant, which is one more reason not to over specify strength.

What to put on the drawing

SUS630 (17-4 PH) per JIS G 4303, UNS S17400
Supply in condition A
Heat treat to H1075 after rough machining
Finish machine critical features after aging
Mill certificate to EN 10204 3.1, state condition
Passivate per ASTM A967

Stating the sequence on the drawing is not overreach. It tells every shop quoting that you know the part needs two machining operations, and it stops the low quote from being the one that ignored the heat treatment.

For where SUS630 sits against the rest of the range, see the JIS to AISI equivalents table. For the hardened martensitic grades that sit above it in hardness, see SUS440C and SUS420J2.

Have a 17-4 part to price? Get a quote. The part is placed with a shop that has run the ageing sequence before, with the contraction planned into the finish machining. Priced quote within 24 business hours.

Questions

Should I machine 17-4 PH before or after aging?

Rough in condition A, age to the required H temper, then finish only the features that must hold tight tolerance. Aging moves the part slightly, so finishing afterwards is what protects your dimensions.

What is condition A?

Solution annealed, the as supplied state from most stockists. Roughly 30 to 36 HRC, machinable with normal carbide tooling. It is not a final condition for service, it is the state you buy and rough in.

Which H temper should I specify?

H900 for maximum hardness and strength, around 44 HRC. H1025 and H1075 trade some strength for toughness. H1150 gives the best toughness and the easiest finishing, around 31 HRC. Pick from the service requirement, not from habit.

Does 17-4 PH resist corrosion as well as SUS304?

Broadly comparable in mild environments, and better than the martensitic grades. It is not a substitute for SUS316 in chlorides. The copper and the lower nickel put it behind the austenitic 300 series in aggressive service.

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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