Which 17-4 PH stainless steel condition should a machined part call out: H900, H1025 or H1150?

What do Condition A, H900, H1025 and H1150 mean for 17-4 PH?

Condition A is the solution-treated state the bar is sold in, and each H number is the aging temperature in degrees Fahrenheit that follows it. Lower aging temperatures give higher strength and hardness; higher aging temperatures trade strength for toughness, ductility and resistance to stress corrosion cracking.

17-4 PH is furnished annealed, also called solution heat treated or Condition A, which leaves it as untempered martensite. The aging step then forms fine copper-bearing precipitates, and those precipitates are what raise the strength. Per the ATI 17-4 technical data sheet, H900 is a 1 hour hold at 900°F (482°C) and the other H conditions are 4 hour holds at their named temperature, each followed by air cooling. H1150M is a two-step treatment per ASTM A693, as listed in the same data sheet: a 2 hour hold at 1400°F (760°C) followed by the H1150 cycle; the same data sheet notes that forming goes more easily in this overaged state than in Condition A.

Condition A is not a service condition. The ATI data sheet states that annealed material should not generally be put into service, because untempered martensite is less ductile than aged material, can fracture unpredictably, and is more sensitive to hydrogen embrittlement and chloride stress corrosion cracking. A drawing that names 17-4 PH without a condition leaves that choice to whoever reads it.

Two cautions on reading the table. The hardness ranges are the AMS 5604 sheet, strip and plate values that ATI tabulates, so check them against the bar specification on your drawing, which may state a different range or a minimum only. And the toughness column is a ranking, not a number: the ATI data sheet says the alloy is most prone to stress corrosion cracking at peak strength, and the Charpy V-notch minimums in ASTM A564 rise with aging temperature, from none at H900 and 5 ft·lbf at H925 to 30 ft·lbf at H1150.

Should a 17-4 PH part be machined before or after aging?

For conditions at or below H1025, rough and finish machine in Condition A, age, then finish only the features whose tolerance is tighter than the aging shrinkage. For H1100, H1150 and H1150M, buying bar already aged and machining it complete is often the simpler route, because those conditions are generally no harder than Condition A.

Aging shrinks the part. The volume contraction is predictable, and the Precision Machined Products Association puts it at 0.0004 to 0.0006 in/in for Condition A to H900 and roughly 0.0009 to 0.0012 in/in for Condition A to H1150 (PMPA Craftsman’s Cribsheet 67). On a typical 2 in (50.8 mm) bore aged to H900, that is roughly 0.001 in of diameter change, the same order as a close bearing fit. A bore that measures in tolerance before aging can therefore be out of tolerance after it.

That gives three workable routings. Route one: machine everything in Condition A, age, and accept the shrinkage on all features, which works when the tightest tolerance is loose compared with the contraction figure times the feature size. Route two: machine in Condition A leaving stock on the critical bores, seats and threads, age, then finish those features by grinding, honing or a light finishing cut. Route three, for the overaged conditions: start from bar supplied in H1150 or H1150M and machine complete with no size change afterward. The ATI data sheet notes that aging above 1075°F generally leaves the material softer than Condition A, which is why route three usually machines no harder than route one.

Aging also leaves a light oxide tint, which the ATI data sheet notes as surface oxidation, so plan any cleaning and passivation after aging and after the final machining, never before. Machining does not change the condition. Welding does alter it locally, and ATI recommends solution annealing a multi-pass weldment before aging it again; a repeat solution treatment returns the whole part to Condition A. The stress relieving guide covers the related question of where a thermal step belongs in a routing.

How does 17-4 PH compare with 15-5 PH and 316 stainless?

15-5 PH ages to the same H conditions with the same strength minimums but is melted to be essentially free of delta ferrite, which gives better transverse toughness in larger sections. 316 is far weaker but resists chloride pitting better, so it wins where corrosion, not load, governs.

15-5 PH (UNS S15500) is bought to SAE AMS 5659 for bar and forgings, and ATI also lists it under ASTM A564 and A705 (ATI 15-5 product page). It is the usual step up when a 17-4 PH part is large, loaded across the grain, or needs consistent toughness from heat to heat. The aging conditions, the routing logic and the shrinkage behavior described above carry over, so switching alloys rarely changes the drawing beyond the material line.

The comparison with 316 is a different decision. ATI’s test summary puts the general corrosion resistance of 17-4 PH on a par with Type 304 across most environments, while 316 adds molybdenum for better pitting resistance in chlorides. 316 cannot be hardened by heat treatment, so its strength stays far below any aged 17-4 PH condition, and it has no aging shrinkage to plan around. If the part is a lightly loaded wetted fitting, 316 is usually the better choice; the 304 vs 316 guide covers that step.