How is 17-4 PH stainless heat treated?
A single low-temperature age from Condition A: H900 at 482 °C (900 °F) for peak strength, H1025 or H1075 for toughness, H1150 for the most ductility and easiest machining.
17-4 PH (UNS S17400, AMS 5643 bar) is a martensitic precipitation-hardening stainless. It is supplied in Condition A: solution treated at about 1040 °C (1900 °F) and air cooled, which leaves it martensitic at roughly 30-35 HRC and about 1000 MPa (145 ksi) tensile. Hardening is then a single aging step at a temperature between 480 and 620 °C (900 and 1150 °F), held for 1-4 hours and air cooled. The condition name is the aging temperature in Fahrenheit: H900 is aged at 482 °C (900 °F) for 1 hour, H1025 at 552 °C (1025 °F) for 4 hours, H1075 at 579 °C (1075 °F) for 4 hours, H1150 at 621 °C (1150 °F) for 4 hours. No quench, no scale to speak of, and the part comes out looking like it went in.
The conditions trade strength for toughness. H900 gives about 1310 MPa (190 ksi) tensile, 1170 MPa (170 ksi) yield and 40-45 HRC, with modest impact toughness. H1025 gives about 1070 MPa (155 ksi) tensile and 35-38 HRC with much better toughness and is the common choice for shafts, valve parts and aerospace fittings. H1075 and H1150 drop to about 1000 and 930 MPa (145 and 135 ksi) tensile and 31-35 HRC, with H1150 giving the best impact strength and stress-corrosion resistance. H1150-M (a double age) is used where maximum toughness is wanted. Corrosion resistance is best in the overaged conditions and worst in H900.
Dimensional change is small and predictable, which is the reason 17-4 is chosen over quenched alloy steels. Aging contracts the part by about 0.0004-0.0006 mm/mm (0.04-0.06%) for H900 and about 0.0008-0.0010 mm/mm for H1150, and distortion is minimal because there is no quench. The usual practice is to machine in Condition A, age, then finish grind or machine only the critical dimensions. Finish machining after H900 is slow; after H1025 through H1150 it is practical. If a part has been welded or cold worked heavily, re-solution treat (Condition A) before aging, or the properties will not be uniform.
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What to specify
- Material: 17-4 PH per AMS 5643 (bar, Condition A) or ASTM A564 Type 630, with the condition to be delivered stated separately
- Final condition: H900, H1025, H1075 or H1150, per AMS 2759/3, with the hardness range (H900 40-47 HRC, H1025 35-42 HRC, H1150 28-38 HRC are the usual acceptance bands)
- Which dimensions apply after heat treatment and which may be finished afterwards; allow for 0.04-0.10% contraction
- Heat treat after rough machining and welding, and before final grinding of critical features
- Hardness test location and method (HRC on a flat, ground area) and a certification with the furnace record
- Passivation after aging per ASTM A967 or AMS 2700 if the part sees wet service, since aging leaves a light oxide
- Do not specify Condition A for the finished part unless machinability is the only requirement; it is not a stable service condition
Pitfalls
- Ordering "17-4 PH hardened" without the condition; H900 and H1150 differ by 380 MPa (55 ksi) and 10 HRC points
- Expecting to hard-machine H900 economically; rough in Condition A and finish after aging, or choose H1025 or H1150
- Skipping re-solution treatment after welding or heavy cold work, then aging to non-uniform properties
- Using H900 in chloride or hydrogen-charging service where it is prone to stress-corrosion and hydrogen cracking; H1075 or H1150 is safer
- Forgetting the contraction: a bore finished to size in Condition A comes out under size after aging
- Aging in a furnace with poor temperature uniformity; a 15 °C error moves the hardness by a full HRC point or more
Frequently asked questions
- What does H900 mean for 17-4 PH?
- Aged at 900 °F (482 °C) for 1 hour and air cooled. It is the peak-strength condition: about 1310 MPa (190 ksi) tensile and 40-45 HRC, with the lowest toughness and corrosion resistance of the standard conditions.
- Can 17-4 PH be machined after heat treatment?
- Yes. H1150 and H1075 machine reasonably well, H1025 is slower, and H900 at 40-45 HRC is slow and hard on tools. The usual route is to machine in Condition A, age, then finish only the critical features.
- How much does 17-4 PH shrink when aged?
- About 0.04-0.06% for H900 and up to 0.10% for H1150, uniform and predictable. A 100 mm (4") dimension shrinks roughly 0.05-0.10 mm (0.002-0.004"). Distortion is minimal because there is no quench.
- Which 17-4 PH condition should I choose?
- H1025 for most shafts, fittings and valve parts (a good strength-toughness balance). H900 only where maximum strength is worth the toughness and corrosion penalties. H1150 for impact loads, sour service or stress-corrosion exposure.
