Precipitation-hardening stainless for aerospace and defense — machined in Condition A, aged to H900 through H1150, finished at print.
17-4 PH solves a problem the austenitic grades can't touch: it delivers alloy-steel strength — up to roughly 200 ksi ultimate in the peak-aged condition — while keeping genuine stainless corrosion resistance, close to 304's in most environments. The mechanism is what makes it a machinist's alloy. Approximately 17 percent chromium and 4 percent nickel put the base metal in martensitic stainless territory; the magic ingredient is copper, held in solution when the mill solution-anneals the material (Condition A). Age the alloy at modest temperature — 900°F to 1150°F depending on the specified H-condition — and copper-rich precipitates form throughout the matrix, pinning dislocations and multiplying strength. No quench, no violent transformation, and therefore only a small, predictable dimensional change: the entire machine-then-harden workflow that is risky in oil-quenched alloy steels becomes routine. That combination — strength, corrosion resistance, and a heat treatment that doesn't wreck finished geometry — is why 17-4 PH saturates aerospace and defense bills of material: actuator shafts, valve internals, structural fittings, firearm and ordnance components, and instrument hardware that has to be strong, hard, and rust-free at once. Our stainless machining network plans every 17-4 job around the heat-treat schedule from the first quote.
Solution-annealed 17-4 machines more like a well-behaved alloy steel than like gummy austenitic stainless: chips break shorter than 304's, work hardening is far milder, and finishes come out clean. It is harder than annealed 304, so tool loads are real, but the cut is predictable — the network's rigid VMCs (travels to 32.5" × 20.5" × 20.1"), the 5-axis trunnion platform with its 18,000 RPM spindle and 60-tool magazine, and turning centers with 1.625" to 3.05" bar capacity all carry it at steady production feeds.
The standard sequence: rough and semi-finish in Condition A, send parts to qualified heat treaters in the network for aging to the specified H-condition, then bring them back for finish passes on critical features. Aging causes a slight, consistent contraction — small enough to ignore on general tolerances, large enough to matter at ±.0002" — so tight-tolerance bores, threads, and bearing fits are finished after age, in the hardened state, where they stay put. Certified furnace charts travel with the lot.
Finishing H900 or H1025 material is entirely practical with rigid setups and current carbide — speeds drop and edges must be fresh, but the payoff is features machined at final hardness with no post-heat-treat movement left in the part. Grinding stays available for the tightest journals. When a customer supplies pre-aged H-condition bar, we machine it complete in the hard state; it costs more spindle time than the A-then-age route, which is one reason quoting the heat-treat sequence early saves money.
The aging temperature is a dial between strength and toughness. H900 sits at peak strength and hardness; H1025 trades some of that for better fracture toughness and markedly better stress-corrosion resistance, making it the common aerospace pick for loaded structure; H1150 maximizes toughness for shock-loaded and cryogenic-adjacent service. If the drawing leaves condition open, we flag the question at quote time — sustained tensile stress plus a corrosive environment is a combination that argues against H900, and the aging cycle costs the same either way.
17-4 PH is the answer when a stainless part runs out of strength: shafts and pins carrying real bending loads, valve stems and trim in high-pressure fluid systems, aerospace structural fittings and fasteners, pump and gearbox internals, and defense hardware from fire-control components to ordnance parts. Instrumentation programs use it for spindles and load-bearing sensor hardware that 316 would gall or bend. Turned 17-4 details run on the network's Swiss and fixed-headstock lathes; prismatic fittings run the 5-axis platform for single-setup accuracy that survives the trip through heat treat. Where the environment is chloride-heavy — seawater immersion, brines — 316 keeps the corrosion edge and the design conversation is a real trade study.
Routine tolerances on 17-4 ship at ±.0002", with repeatability on precision turned features to ±.0001" — and because critical features are finished after aging, those numbers describe the delivered, full-strength part, not a pre-heat-treat measurement. As-machined finishes run 32 Ra or better on finish passes; passivation per ASTM A967 / AMS 2700 restores the passive surface after machining, and bead blast or tumble finishes are available through the network's qualified processors. Every lot ships with material certifications, heat-treat certifications for the specified H-condition, and full traceability; first article inspection documentation is available as program requirements dictate, and AS9100 partners are added as program requirements warrant.
17-4 is a martensitic stainless containing copper that stays in solution when the alloy is solution-annealed (Condition A). A low-temperature aging cycle — 900°F for H900, higher for H1025 and H1150 — precipitates fine copper-rich particles that block dislocation motion and raise strength dramatically. Because aging happens at low temperature, dimensional change is small and predictable, which is what makes the machine-then-age workflow practical.
The standard strategy is machine in Condition A, then age. Condition A cuts far more easily, and aging produces only a slight, predictable contraction rather than quench distortion. For tight tolerances we rough and semi-finish in A, age to the specified H-condition, then finish critical features in the aged state so they land at print. Simple parts with looser tolerances can be machined complete in A and aged afterward.
The number is the aging temperature in Fahrenheit, and it sets the strength-toughness trade. H900 is peak strength and hardness with the least toughness; H1025 gives up some strength for better toughness and stress-corrosion resistance and is a common aerospace compromise; H1150 is the softest, toughest, most SCC-resistant condition. Sustained stress in a corrosive environment generally argues against H900.
The full stainless family — grades, finishes, and capabilities.
The chloride-resistance champion when environment beats strength.
The general-purpose grade for parts with ordinary loads.
Single-setup fittings that hold accuracy through heat treat.
Critical features finished at ±.0002" in the aged state.
Send the print with its H-condition — the heat-treat plan comes back with the price.
