17-4 PH Machining: Condition A, Aging Movement and Hole Fit
A drawing that says "17-4 PH" has named an alloy and nothing else that matters. The same grade in Condition A and in H900 behaves like two different materials on the machine and in the assembly: one cuts like soft stainless and moves when it is aged, the other arrives at the machine at 44 HRC and is finished with carbide and patience.
Most of the problems buyers report with 17-4 PH machining are not cutting problems. They are sequencing problems: the part was finished to size before aging, or aged in the wrong condition, or threaded in a way that guarantees galling. This note covers what the condition does to the process, how much a part moves, and where 17-4 is simply the wrong alloy.
The drawing names the alloy, not the condition
17-4 PH (AISI 630, UNS S17400) is a martensitic precipitation-hardening stainless: about 16% chromium, 4% nickel, 4% copper and a niobium addition. It is supplied solution-annealed, which is Condition A, and it gains strength only when it is aged at a temperature between roughly 480 °C and 620 °C.
The aging temperature is the specification that was left off the drawing. H900 is the highest-strength condition, roughly 44 HRC and 1310 MPa yield. H1150 is the toughest, around 28 HRC and 800 MPa. The two are not interchangeable, and the choice changes both the machining sequence and the final dimensions.
| Condition | Aging | Hardness (approx.) | Yield (approx.) | Pick it when |
|---|---|---|---|---|
| A (solution annealed) | none | 30 to 35 HRC | 1000 MPa | The shop needs to machine before hardening |
| H900 | 480 °C, 1 h | 44 HRC | 1310 MPa | Maximum strength, thin sections, wear surfaces |
| H1025 | 550 °C, 4 h | 38 HRC | 1070 MPa | Strength with some ductility |
| H1075 | 580 °C, 4 h | 33 HRC | 1000 MPa | Fatigue-loaded parts that must not be brittle |
| H1150 | 620 °C, 4 h | 28 HRC | 800 MPa | Toughness, sour or chloride service, heavy sections |
If the drawing does not state the condition, the shop has to guess, and the guess will be either expensive or wrong. Write "17-4 PH, H1150" or "17-4 PH, Condition A, age to H900 after machining" and the sequence becomes unambiguous.
The 17-4 PH machining sequence: cut soft, age, finish
The default route is: machine in Condition A, then age. Condition A is the softest state and the only one where 17-4 behaves like a normal stainless in the cut. It is still gummy and work-hardens, but tools survive.
The alternative is to buy the material pre-aged. That removes the distortion question entirely, because the part is machined at final hardness. It costs more per kilo, and it narrows the feature set: at 44 HRC the part can be milled and turned with carbide, but small bores, deep pockets, fine threads and sharp internal corners all get slower and more fragile.
For parts with a handful of tight features and a lot of open geometry, pre-aged stock is usually the cheaper route. For parts with pockets, thin walls and a bore that must be round, machine soft and age at the end, then finish the critical features.
See also: Surgical Instrument Machining: Pivot Alignment, Flatness and Hand Feel and Datum Scheme: How the Datum You Choose Changes the Price of the Part.
What aging does to a bore
Aging is a diffusion process, and the lattice contracts as the copper-rich precipitates form. The movement is small in absolute terms but not small compared with a machining tolerance: expect roughly 0.0005 in/in, or about 0.05%, when aging from Condition A.
On a 100 mm bore that is 0.05 mm of diameter, which is the entire standard tolerance band before the part has been measured. On a 200 mm spigot it is 0.1 mm, and the fit is gone.
| Feature | Movement on aging | What to do |
|---|---|---|
| Bore finished before aging | Closes by roughly 0.05% of diameter | Leave 0.02 to 0.05 mm and finish after aging, or bore after aging |
| Outside diameter | Shrinks by roughly 0.05% | Same rule: finish critical diameters after aging |
| Flat face, thin section | Bows; can exceed the flatness callout | Age before final machining, or rough, age, finish |
| Hole pattern | Position shifts with bulk contraction | Drill before aging only if position tolerance is loose |
| Thread | Pitch diameter shrinks, thread binds | Chase or cut after aging, never before |
The practical rule is simple: any feature whose tolerance is tighter than 0.05 mm should be finished after aging. That usually means a roughing operation, an outside heat-treatment cycle and a finishing operation, which is a scheduling cost as much as a machining cost. It is also why a 17-4 part with tight bores quotes higher than the same geometry in 316.
For parts that need both, rough to 0.3 to 0.5 mm, age, then finish. The roughing leaves enough material to correct the distortion, and the aging relieves the bulk of the residual stress from stock removal.
Chips, work hardening and tool life
Condition A is where 17-4 PH machining is most forgiving, and it still machines at roughly half the surface speed of free-machining stainless. It galls, it work-hardens if the tool rubs instead of cutting, and it produces long stringy chips that wrap around the tool.
Three habits matter. Keep the feed per tooth up so the tool always cuts under the hardened layer. Use a good flow of coolant, because 17-4 conducts heat poorly and the edge temperature climbs fast. And never dwell: a tool that stops in the cut work-hardens the surface it is about to cut again.
In H900 the same part needs carbide with a strong edge, lower speeds again, and a shorter tool life budget. Small-diameter tools in deep pockets become the limiting factor, and that is where the quote either holds or does not.
Threads, galling and the insert question
17-4 PH galls against itself and against stainless fasteners. A 1/4-20 tapped hole in 17-4 that is assembled and disassembled a few times will start to seize, especially in the H900 condition.
Three options, in order of preference for parts that get serviced:
- Threaded inserts. A Keensert or a heavy-duty insert puts a hardened, replaceable thread in the part. The threaded insert note covers when an insert beats a tapped hole.
- Roll-formed threads. Rolling forms the grain instead of cutting it and leaves a smoother flank, which reduces galling. It needs a slightly larger tap drill and a ductile condition.
- Cut threads with an anti-seize specification. Acceptable for one-time assembly, and worth stating on the drawing so the assembly line does not dry-fit the joint.
If the thread is cut before aging, the aging contraction will change the pitch diameter. Cut or chase after aging, and gauge with a thread plug rather than by feel.
Where 17-4 PH is the wrong answer
17-4 is a good default for medium-strength stainless parts that need corrosion resistance better than 410 or 420 and more strength than 316. It is the wrong answer in four common cases:
Hot chloride service. 17-4 is more susceptible to stress corrosion cracking than 316L in warm chloride environments. If the part lives in seawater or a chlorinated washdown, the duplex or 316L route is safer even at lower strength.
Very large sections. 15-5 PH offers similar strength with better toughness and more consistent properties through thick sections. Aerospace buyers often specify it for exactly that reason.
Wear at high hardness. At 44 HRC, 17-4 is hard but not a tool steel. Where the failure mode is abrasion, 440C or a coating does the job with less compromise.
Weight. If the part is flying or moving, titanium or aluminium wins on mass and the strength argument rarely survives the weight calculation.
One more limitation: the useful service temperature tops out around 300 °C. Above that the precipitates overage, and the strength the part was aged for quietly disappears.
Frequently asked questions
Can 17-4 PH be machined after aging?
Yes, with carbide and reduced speeds. At H900 (about 44 HRC) the cut is slower and tool life is shorter, so it is normally reserved for finishing the few features that must be held after heat treatment rather than for the whole part.
How much material should be left for aging?
For tight bores and diameters, leave 0.02 to 0.05 mm per side on the finishing allowance, and 0.3 to 0.5 mm on roughing. The exact figure depends on the section and the aging temperature: H900 moves less than H1150, and a thin wall moves more than a solid block.
Is 17-4 PH magnetic?
In Condition A it is largely non-magnetic; after aging it becomes ferromagnetic. That matters for sensor housings and for any assembly that assumes a non-magnetic component.
Does 17-4 need passivation after machining?
It benefits from passivation to remove free iron from the machined surface, but the effect is smaller than on 304 or 316 because of the alloy's own oxide. If the part is also aged, passivate after aging, not before.
What is the difference between 17-4 PH and 15-5 PH?
Both are precipitation-hardening stainless with similar strength. 15-5 has lower delta ferrite and better toughness in large sections, which makes it the more common aerospace specification; 17-4 is more widely stocked and usually cheaper for small parts.
Next step
Send the drawing with the condition written on it, and the quote will separate the machining that happens before aging from the finishing that happens after. If a bore is specified to a band that aging will consume, that is worth knowing at quote stage rather than at first article.
The materials page lists the stainless grades we hold in stock, and the bearing bore note covers what an H7 seat needs when the housing is aged after machining.
Related reading
Send a drawing for a quote and DFM review.
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