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17-7Ph / 1.4568 / UNS S17700 / AMS 5528 / AMS 5529

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Corrosion resistance: 17-7 PH is an age-hardened precipitation hardening stainless steel with the highest chromium (17%) and nickel (7%) content of the PH grades compared with 15-5 PH and 17-4 PH. 1.4568 offers excellent strength properties, high hardness and elasticity, good corrosion resistance and resistance to high temperatures and to deformation.

Weldability: 17-7 PH steel presents certain difficulties and points to observe in terms of weldability. Although this steel alloy is high strength, stainless and hardenable by heat treatment, it requires some particular precautions during welding.​‌​​‌​

Machinability: In terms of chemical composition its corrosion resistance is equal to that of AISI 301 / 1.4310 steel, while in terms of mechanical properties it is a much better choice for critical parts and components subject to fatigue. 1.4568 has an austenitic structure in the annealed condition and transforms to martensite after the ageing heat treatment, increasing its strength properties while also acquiring magnetic properties.

Heat treatment: 17-7 PH steel is processed by solution treatment and ageing. These treatments raise the hardness of the steel and give it the required mechanical properties.​‌​​‌​

Solution treatment: The steel is heated to approximately 1040-1065 °C (approximately 1900-1950 °F). It is held at this temperature for a sufficient period and is then cooled rapidly, generally in water or air. This treatment softens the steel and stabilises the martensitic phase in its internal structure.

Ageing: After solution treatment, the steel is aged for a set period at elevated temperature, between 480 and 620 °C. This temperature range is the stage that raises the hardness of the steel and develops the required mechanical properties. The ageing time and temperature directly affect the final hardness and durability of the steel, and the ageing time can generally be between 1 and 4 hours. This treatment gives the steel high strength and hardness, but its brittleness must not be increased at the same time, which is why careful control of the ageing parameters is important.​‌​​‌​

Applications: 1.4568 is used in valve components, tanks, springs, spring washers, food-industry blades, diaphragms, low and high temperature parts, heat exchangers, special fittings and installations, and machine structures and components. The steel is produced as strip, wire, plate and bar for the aerospace, oil, chemical and petrochemical industries and for the energy, food and cryogenic industries.

Standards by Product Form​‌​​‌​

17-7 PH (UNS S17700 / AISI 631 / 1.4568) is a semi-austenitic precipitation-hardening stainless steel. Its reason for existing, in one sentence: as delivered (Condition A) it is austenitic, soft and formable like Type 301; once the part is formed it is heat-treated to transform it to martensite and then aged to spring or structural strength. That is exactly what a fully martensitic grade such as 17-4 PH cannot do.​‌​​‌​

Composition (ASTM A693): C ≤0.09% · Mn ≤1.00% · Si ≤1.00% · P ≤0.040% · S ≤0.030% · Cr 16.00–18.00% · Ni 6.50–7.75% · Al 0.75–1.50%. AISI Type 631, UNS S17700, EN 1.4568 and the trade name “17-7 PH” all denote the same grade; some catalogues list them as if they were different products.​‌​​‌​

What the Condition Letters Mean

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The condition designations are the most confusing feature of this grade. 17-7 PH cannot be aged directly: in Condition A the matrix is austenite and there is no martensite for the precipitates to form in. First comes an intermediate anneal called austenite conditioning, which precipitates chromium carbides at the grain boundaries, locally depleting the matrix of chromium and carbon and thereby raising the alloy’s martensite start and finish temperatures. Only then does the material transform to martensite and become ageable.

WARNING — two catalogue traps. (1) For this grade the “typical” mill values and the specification limits are different figures, and datasheets of different vintages diverge from each other as well. State which is which on anything customer-facing or contractual; the table above gives specification limits. (2) Copied tables have been observed with the RH 950, C and CH 900 rows shuffled. Quick check: hardness must always follow the order A < TH 1050 < RH 950 ≈ C < CH 900; if it does not, the table has been mis-transcribed.​‌​​‌​

Forming, Welding and Machining

Forming — the whole point of the grade​‌​​‌​

In Condition A, formability is comparable to Type 301: it deep-draws and bends with standard tooling. Because it work-hardens rapidly, intermediate anneals may be needed for complex or multi-stage drawing. Dimensional change on hardening must be designed in: the A → TH 1050 / RH 950 route gives a net expansion of about 0.004 in./in. (a transformation expansion of ~0.0045 in./in., partly offset by ~0.0005 in./in. of contraction during ageing); the C → CH 900 route gives a contraction of about 0.0005 in./in.

Welding​‌​​‌​

GTAW/GMAW and resistance welding are usable, and parts are normally welded in Condition A. Weldability is poorer than that of 17-4 PH: the 0.75–1.5% aluminium oxidises readily in the weld pool, degrading penetration and producing oxide or slag, so inert-gas shielding is essential. The matching filler is AMS 5824 (UNS S17780). The full austenite-conditioning and ageing sequence must be re-applied after welding — otherwise the HAZ and weld metal remain unhardened. No verified preheat or interpass temperature was found for this grade.

Machining​‌​​‌​

Machinability is about 75% of B1112. Chips are long and gummy, so chip breakers, slow speeds and steady feeds are needed. Bulk machining is normally done in Condition A (the softest state); where critical dimensions must hold in the hardened state, finish machining is left until after heat treatment — standard practice for precipitation-hardening grades.

Applications and Limits​‌​​‌​

Frequently Asked Questions​‌​​‌​

Should I order TH 1050 or RH 950?

Both start from the same Condition A stock and diverge at the austenite-conditioning temperature (760 °C versus 954 °C), giving different strength/toughness trade-offs. By the specification limits, RH 950 gives higher guaranteed strength (tensile ≥1448 MPa, yield ≥1310 MPa, ≥44 HRC), while TH 1050 stops at ≥1241 MPa tensile and ≥1034 MPa yield (≥38 HRC); minimum elongation is 6% for both. Where maximum strength and hardness matter (springs, fasteners), order RH 950. TH 1050’s lower conditioning temperature and simpler cooling regime — there is no −73 °C sub-zero step — reduce distortion risk and heat-treatment logistics, so where toughness and dimensional consistency matter more than the last 15% of strength, TH 1050 is the easier choice.​‌​​‌​

Why must I order Condition A if the part will be formed?

Because Condition A is the only condition in this grade that is still austenitic and ductile — it deep-draws, bends and forms comparably to Type 301. Every other condition (C, CH 900, TH 1050, RH 950) has either been cold worked or transformed to martensite and aged, and is correspondingly hard, limited in ductility and unsuitable for significant forming. That is precisely why this semi-austenitic grade is chosen over a fully martensitic PH steel: form the part cold while it is still austenitic, then heat-treat it to final spring or structural strength. So whenever forming is downstream, the mill condition ordered must be Condition A.​‌​​‌​

How much dimensional change should I plan for during heat treatment?

Per the mill’s own data, the A → TH 1050 / RH 950 route produces a net expansion of about 0.004 in./in.: the transformation to martensite gives about 0.0045 in./in. of expansion, partly offset by roughly 0.0005 in./in. of contraction from precipitation during ageing. The C → CH 900 route produces a small contraction of about 0.0005 in./in. Build these figures into tolerances and fixture allowances before heat treatment: parts that must hold tight dimensions afterwards are either formed deliberately over- or undersize to compensate, or finish-machined after the heat-treat cycle rather than before it.​‌​​‌​

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