17-7 PH

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event2025-9-29
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Stainless Steel 17-7 PH is a precipitation-hardening stainless steel (≈17% Cr, ≈7% Ni, with Al additions) developed for high strength, excellent fatigue properties and good corrosion resistance. It is widely used for springs, flat springs and other high-strength components where heat treatment after forming can produce very high tensile strength while preserving good dimensional stability. See related spring product pages and material comparisons below.

Contents

Introduction & quick summary
Core characteristics
Typical applications
Forming into springs & spring types
Alternatives & comparison
Selection guidance
FAQ
References

Introduction & quick summary

17-7 PH (UNS S17700) is a semi-austenitic, precipitation-hardening stainless steel designed to be formed in a ductile (annealed) condition and then heat treated (precipitation hardening) to achieve very high strength and excellent fatigue resistance — a common choice for high-performance springs and flat strips. For spring energizers and canted coil springs it offers an attractive balance of strength, fatigue life and routine corrosion resistance; for extreme corrosion or sour service consider higher-alloy CRAs (e.g., Inconel 625, Hastelloy) or cobalt/nickel alloys (MP35N, Elgiloy).

Need 17-7 PH spring wire/strip? Request a quote — include required temper (e.g., TH 1050 / RH 950), wire/strip diameter, and operating temperature / fluid.

Core characteristics

Typical chemical composition (wt%) — representative

Note: ranges below are typical for commercial 17-7 PH grades (UNS S17700). Fe (iron) is the balance; always confirm exact lot values on the supplier mill certificate.

Element	Typical (wt%)
Iron (Fe)	Balance (≈ 70–75%)
Chromium (Cr)	16.0 – 18.0
Nickel (Ni)	6.5 – 7.8
Aluminum (Al)	0.75 – 1.50
Carbon (C)	≤ 0.09
Manganese (Mn)	≤ 1.00
Silicon (Si)	≤ 1.00
Phosphorus (P)	≤ 0.040
Sulfur (S)	≤ 0.030
Nitrogen (N)	≤ 0.10 (varies)

Key physical data (typical)

Typical physical & mechanical properties for common wrought forms.

Property	17-7 PH (typical)
Density	7.80 g/cm³ — (7800 kg/m³)
Elastic modulus (E)	≈ 200×10⁹ Pa — (≈ 200 GPa)
Shear modulus (G)	≈ 77×10⁹ Pa — (≈ 77 GPa)
Tensile strength (UTS) — annealed / typical	≈ 1034×10⁶ Pa — (≈ 1034 MPa / 150 ksi) typical maximum annealed UTS reported by some suppliers; spring and aged tempers achieve substantially higher values (see next row).
Tensile strength (UTS) — spring / aged tempers (typical)	Up to ≈ ≈ 1.6 – 2.0×10⁹ Pa (≈ 1600–2000 MPa / ~230–290 ksi) depending on diameter & temper (e.g., cold-drawn + TH/RH aging). Always confirm supplier temper charts.
Yield strength (0.2% offset) — typical	Varies with temper — for spring tempers yield can exceed 1000 MPa; confirm supplier tables for diameter/temper.
Min service temperature	≈ −196 °C (≈ −320.8 °F) — good cryogenic toughness in many tempers; verify product form.
Short-term / intermittent max	≈ 482 °C (≈ 900 °F) — certain heat-treat/conditions show useful mechanical behaviour up to ~482 °C (900 °F); specific limits depend on temper and application.
Recommended long-term continuous	≈ 316 °C (≈ 600 °F) — common conservative guidance for long-term mechanical performance of springs; verify with stress-relaxation data for your temper/diameter.

Typical applications

High-performance springs and flat springs (compression, tension, leaf & strip springs) where high strength and fatigue life are required.
Spring-energized seals, canted coil springs and other energizers where dimensional stability after heat treatment is important — see our spring-energized seal and canted coil spring pages.
Aerospace, chemical processing, food-grade springs (after appropriate finish/passivation) and general machine components exposed to moderately elevated temperatures (≤316 °C / 600 °F).

Forming into springs & spring types

Summary: 17-7 PH is normally supplied annealed for forming (ductile) and then subjected to a precipitation-hardening heat treatment (various conditions: RH950, TH1050, CH900 etc.) after forming to develop high strength and excellent fatigue behavior. Common spring forms produced from 17-7 PH wire/strip include canted coil, helical, cantilever (V/U) and flat contact springs. For product examples see Canted Coil Springs, Helical Springs and Full Contact Springs.

Forming & heat-treat notes (practical)

Form in the annealed condition (easy to form). After forming, perform precipitation hardening to the specified condition (e.g., RH950 / TH1050) to reach required tensile/yield; the chosen aging condition controls final strength vs ductility.
Cold work prior to aging increases strength; diameters and cross-section affect achievable UTS in spring tempers — request diameter-by-temper tables from wire/strip suppliers.
Stress-relaxation and creep at elevated temperature must be validated for each temper & diameter — obtain supplier stress-relaxation curves for design.
Surface finishing (electropolish/passivation) improves corrosion fatigue life in aggressive fluids and reduces initiation sites.

Alternatives & comparison

Compact comparison to help select candidates. Temper-dependent values — verify with supplier datasheets and mill certificates for chosen temper/diameter.

Material	Best when	Typical strength (UTS)	Service temp guidance
17-7 PH (UNS S17700)	High-strength springs requiring heat-treatment after forming; excellent fatigue & dimensional stability	~1034 MPa (annealed) → up to 1600–2000 MPa (spring/aged tempers) depending on diameter/condition	Long-term ≤ ≈ 316 °C (≈ 600 °F); short-term up to ≈ 482 °C (≈ 900 °F) depending on temper.
17-4 PH	When similar PH benefits needed but with different aging response; commonly used for many springs	High (precipitation hardened)	Long-term ≤ ≈ 316 °C (≈ 600 °F) — compare specific tempers
302 / 301 / 304 (stainless)	When highest cold-work hardenability (301) or general corrosion resistance is sufficient but PH processing is not desired	Lower than PH alloys in heat-treated state; 301 can be work-hardened	Long-term conservatively ≤ ≈ 260 °C (≈ 500 °F)
Elgiloy / MP35N (cobalt / cobalt-nickel)	When extreme fatigue, SSC resistance or long-term preload retention is required	Very high (work-hardened / wire tempers)	Often lower recommended long-term continuous temperature than some PH or Ni alloys; choose by chemistry & test results.
Inconel 625 / Hastelloy C-276	When corrosion (chloride/oxidizers) or high-temperature creep resistance dominates	Moderate → high depending on cold work	Usually better high-T corrosion resistance; consult each alloy page for details. Inconel 625, Hastelloy C-276

Selection guidance

Direct selection rules — choose the alloy/temper that addresses your dominant constraint:

Dominant: Highest fatigue & spring strength (post-heat treat) — 17-7 PH is a strong candidate (heat-treat after forming for best strength/stability).
Dominant: Corrosion / SSC risk in sour service — consider MP35N or Elgiloy depending on fluid & required hardness control.
Dominant: High continuous temperature (creep/oxidation) — consider high-Ni alloys (Inconel, Hastelloy) if corrosion/creep dominate.

Quick validation checklist (must-do before final selection)

Obtain supplier mill certificate and diameter-by-temper mechanical tables (UTS/yield for the required condition).
Request stress-relaxation curves at intended operating temperature and diameter for the chosen temper/aging cycle.
Run representative corrosion + fatigue tests in the actual fluid & temperature for critical services.

FAQ

Q1: What is the difference between 17-7 PH and 17-4 PH for springs?

A1: Both are precipitation-hardening stainless steels. 17-7 PH is often chosen for strip/spring applications needing good formability plus very high final strength after aging; 17-4 PH has different chemistry and aging response — selection depends on required mechanical profile and processing. Check supplier tempers & tables.

Q2: Can 17-7 PH be formed into canted coil springs?

A2: Yes — 17-7 PH is supplied in annealed (formable) condition then heat-treated after forming to produce high-strength canted coil springs and other energizers. Request supplier guidance for coil diameter, temper and aging cycle. See our canted coil spring page for examples.

Q3: What long-term temperature should I use for 17-7 PH springs?

A3: Conservative guidance for long-term spring mechanical performance is ≤ ≈ 316 °C (≈ 600 °F). For higher temps or continuous elevated-T service, obtain stress-relaxation data for the intended temper/diameter.

Q4: Are there standard specs for 17-7 PH spring wire?

A4: Yes — common specifications and guidance include ASTM/AMS/SAE documents for spring wire (e.g., AMS / SAE J217 and supplier wire specs). Always specify the temper/condition (e.g., RH950, TH1050, CH900) when ordering.