When to Machine 17-4 PH and When to Age It
September 21, 2026
17-4 PH is the grade buyers reach for when a part has to be hard and still resist some corrosion. It is also the grade where the drawing most often arrives incomplete, because the alloy name on its own does not tell a shop what to make.
What precipitation hardening actually does
17-4 PH is a martensitic stainless with copper and niobium added. Solution treated, it is relatively soft and machinable. Held at a moderate temperature for a few hours, copper rich particles precipitate through the structure and the material hardens. There is no quench and no carburising, which is why distortion is small compared with through hardening a tool steel. Wikipedia has a summary of the alloy chemistry.
The practical consequence is a choice. You can machine it soft and then age it, or you can buy it already aged and machine it hard. Which one is right depends on the tolerance and the feature.
The conditions, in the order you will meet them
| Condition | Ageing | Typical hardness | Typical tensile | What it is for |
|---|---|---|---|---|
| A | none, solution treated | 30 to 36 HRC | around 1100 MPa | The delivery state. Machine here. |
| H900 | 480 C | 40 to 44 HRC | around 1310 MPa | Maximum strength and hardness |
| H1025 | 550 C | 35 to 38 HRC | around 1070 MPa | Strength with better toughness |
| H1075 | 580 C | 32 to 36 HRC | around 1000 MPa | The common middle choice |
| H1150 | 620 C | 28 to 33 HRC | around 930 MPa | Toughness, ductility and the best corrosion resistance of the set |
These are published typical values for the alloy. They are not a promise about your part, and a shop should report the hardness it actually achieved.
Two things move in opposite directions down that table. Hardness and tensile strength fall. Toughness, ductility and corrosion resistance rise. H900 is the strongest and the least corrosion resistant. H1150 is the softest of the aged conditions and holds up best in service.
That is the whole decision. If nobody has written a condition on your drawing, nobody has made it.
Machine soft, then age, or machine hard?
Machine in Condition A, then age. This is the normal route and the cheaper one. Condition A cuts at a machinability rating near 50 against free-machining carbon steel B1112 at 100, which is better than 316L. The part then goes to a furnace and comes back slightly smaller.
The shrinkage is real and it is predictable. H900 pulls a part in by roughly 0.04 to 0.05 percent on length, so a 200 mm dimension loses about 0.08 to 0.10 mm. Higher ageing temperatures shrink less. A shop that runs this grade regularly compensates in the program. A shop that does not will hand you parts that were in tolerance before the furnace.
Machine after ageing. Slower, harder on tooling and more expensive, but the size is final. Choose this when a feature is tighter than about ±0.02 mm, when a bore has to stay round or when the part is thin and would distort.
The mixed route. Rough in Condition A, age, then finish the critical features only. It costs two setups and it is what most tight tolerance 17-4 PH parts actually get. Say on the drawing which features are critical and the shop can plan it without asking.
Where 17-4 PH stops being the right answer
Its PREN sits near 16. That is below 304 at 19 and well below 316L at 24. The number falls further at H900, because the same precipitation that adds strength takes chromium out of solution locally.
So 17-4 PH is not a chloride grade. For a pump shaft in fresh water it is excellent. For the same shaft in sea water it is the wrong choice and 2205 duplex or a higher alloy is the conversation. If the part needs corrosion resistance more than hardness, you are looking at 316L instead.
Two more limits worth knowing. H900 parts are sensitive to hydrogen embrittlement, so plating after ageing needs a bake. And 17-4 PH is magnetic in every condition, which rules it out where a non magnetic part is specified. 15-5 PH is the close cousin with less delta ferrite and better transverse toughness, specified the same way.
What belongs on a 17-4 PH drawing
- The condition. H900, H1025, H1075 or H1150. Not just the alloy.
- The hardness you expect back, as a range, with the test method.
- Which features are critical after ageing, so the shop knows what to finish last.
- Any plating or coating, with a note about hydrogen bake if the condition is H900.
- Passivation, with the standard. 17-4 PH passivates well in citric. See citric against nitric for why the choice matters on this grade.
- Material certificate with the heat number if the part is going into a regulated product.
The short version
Pick the condition from what the part has to survive, not from the highest number in the table. H1075 and H1150 cover most industrial work. H900 is for parts that need every megapascal and will stay dry. Write the condition on the print, say which features matter after the furnace and expect the shop to allow for the shrink.
If you are still deciding between this and a softer austenitic grade, the buyer guide sets out the families side by side, and the grade table has the numbers.
Frequently asked questions
Should the drawing say 17-4 PH or 17-4 PH H1075?
Always the condition. 17-4 PH on its own is not a specification, it is a family. A shop that receives a print with no condition has to ask, and if it does not ask it will guess, usually at Condition A because that is what the bar stock arrives as.
Does the part change size when it is aged?
Yes, and the direction depends on the condition. H900 shrinks the part slightly, around 0.04 to 0.05 percent on length. The higher ageing temperatures shrink it less. A shop that runs 17-4 PH knows the number for its own process and allows for it.
Is 17-4 PH as corrosion resistant as 316L?
No. Its PREN sits near 16 against 24 for 316L, and ageing at H900 lowers it further. It is chosen for strength with useful corrosion resistance, not for chloride service.
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Get matched with a stainless shopFigures in this guide are published specification limits or published typical values for the alloy, not measurements of any part. Machinability is rated against free-machining carbon steel B1112 at 100 and PREN is chromium plus 3.3 times molybdenum plus 16 times nitrogen. The conventions are set out in thereference section and the editorial standards.