17-4 PH: why the condition code matters more than the alloy name
17-4 PH is the most widely used precipitation-hardening stainless steel, and the most commonly under-specified. A drawing that says “17-4 PH” and nothing else leaves out the decision that controls strength, toughness, corrosion behaviour, machining cost and dimensional stability.
The condition code is not a detail. It is the specification.
What the codes mean
17-4 PH is supplied solution annealed — Condition A — and then aged at a temperature for a set time. The condition name is simply the aging temperature in degrees Fahrenheit.
| Condition | Aging temp | Typical hardness | Yield strength | Character |
|---|---|---|---|---|
| A (solution annealed) | none | ~363 HB / 38 HRC | ~760 MPa | Supply condition, not a service condition |
| H900 | 482 °C | 40–47 HRC | ~1170 MPa | Strongest, least tough |
| H925 | 496 °C | 38–44 HRC | ~1070 MPa | Slight toughness gain |
| H1025 | 552 °C | 35–40 HRC | ~1000 MPa | Balanced, common choice |
| H1075 | 579 °C | 32–37 HRC | ~930 MPa | Better toughness and corrosion |
| H1150 | 621 °C | 28–33 HRC | ~795 MPa | Toughest, best stress-corrosion resistance |
| H1150-M / double | 760 then 621 °C | ~24–28 HRC | ~620 MPa | Maximum toughness, for sour service |
The trade is monotonic: higher aging temperature gives lower strength and higher toughness, with better resistance to stress corrosion cracking at the top end.
The mistakes buyers make
Specifying H900 by default. H900 is the strongest, so it feels like the safe choice. It is also the most notch-sensitive and the most susceptible to stress corrosion cracking in chloride environments. For anything seeing seawater, sour gas, or cyclic loading with stress raisers, H1025 or H1150 is usually the correct answer even though the datasheet number looks worse.
Leaving the part in Condition A. Condition A is a supply condition. It is metastable, it has poor stress-corrosion resistance, and its properties are not what any design calculation assumed. Parts do get shipped in Condition A by accident when the drawing does not say otherwise.
Forgetting that aging moves the part. 17-4 PH contracts during aging — roughly 0.05 % to 0.10 % depending on condition. On a 200 mm part that is 0.1 to 0.2 mm. If critical features were finished before aging, they are out of tolerance afterwards.
Machining implications
Condition A machines reasonably — similar to 410 stainless, a little gummy, work-hardens moderately. Around 60–90 m/min with coated carbide.
H900 and H925 at 40–47 HRC are hard-material jobs. Roughly 40–70 m/min, rigid setups, and CBN for finishing turning operations. See hard turning or grinding for where the process line sits.
H1075 and H1150 at 28–37 HRC machine noticeably more easily than H900 and are often the cheaper part for that reason alone, on top of being tougher.
This creates the same route decision as nickel alloys: machine in Condition A and age afterwards, accepting the dimensional change and adding a finishing operation; or buy pre-aged material and machine it hard in one operation.
For a part with a handful of tight features, buying pre-aged bar and machining hard is usually cheaper overall even though cutting is slower. For a large part with many features, Condition A machining plus a finishing pass after aging usually wins. Material availability often decides it — pre-aged bar is stocked in common sizes and conditions only.
Corrosion, briefly
17-4 PH is a martensitic stainless with roughly 15–17 % chromium and 3–5 % nickel. Its corrosion resistance is better than 410 and clearly below 316. It handles atmospheric exposure, fresh water and mild chemical service. It is not a seawater alloy in the high-strength conditions.
If the part sees chlorides under stress, either move up the aging temperature or move to a different alloy family — duplex stainless is often the right answer for strength plus chloride resistance.
What to write
Material: 17-4 PH stainless per AMS 5643, supplied Condition A, heat treated to Condition H1025 after rough machining. Hardness 35–40 HRC verified on the finished part.
That is quotable. It states the alloy, the standard, the supply condition, the final condition, the sequence and the acceptance criterion.
If you are unsure which condition the design needs, say what the part does — load, environment, consequence of failure — in the RFQ. That is a question a supplier can answer, and answering it upfront is cheaper than discovering it in service.
Background: precipitation hardening.
Send the drawing and we will flag the condition callout if it is missing.