Springs & spring washers
Stainless springs requiring formability + strength + corrosion. Formed in Condition A then aged.
Semi-austenitic PH stainless · UNS S17700
17-7PH Stainless
Semi-austenitic precipitation-hardening stainless. Forms easily in the annealed condition (Condition A), then ages to UTS 1310 MPa. Standard for stainless springs, washers, formed aerospace fittings, and any part that needs both formability and high strength.
UTS (TH1050)
1310 MPa
Density
7.81 g/cc
Modulus
200 GPa
Mach.
35 /100
When to use
Pick 17-7PH when you need to form first, then harden.
17-7PH’s killer feature: in Condition A (austenitic, annealed), it’s extremely formable — drawn, bent, deep-formed like 304 stainless. Then a two-step heat treatment converts the structure to martensite and precipitates strengthening phases, ramping UTS from 895 MPa to 1310 MPa. You get the formability of austenitic stainless and the strength of an alloy steel in the same part.
The most common use case: stainless springs and spring washers — formed cold while soft, heat-treated to strength after. Also seen on aerospace bellows, diaphragms, and any thin-section formed component requiring stainless corrosion. For parts that don’t need forming, 17-4PH is more common.
Strengths
Where it wins.
- Formable in Condition ARoll, draw, bend, and deep-draw at room temp like 304. Then HT for strength — unique combination.
- High strength after agingTH1050 UTS 1310 MPa — stronger than most martensitic stainless.
- Good fatigueStandard for stainless springs in critical applications. Holds endurance limit at temp.
- High temperature usableUseful service to ~370 °C (700 °F) — better than 17-4PH.
- Corrosion resistanceBetter than 17-4PH due to higher chromium. Pitting resistance comparable to 304/316.
Trade-offs
What you give up.
- Two-step heat treatmentCondition A → conditioning at 760 °C → aging at 565 °C. More complex than 17-4PH's single age.
- Dimensional change through HTLarger than 17-4PH — ~0.001″/inch. Account for it in pre-HT dimensions.
- Less common than 17-4PHMill availability and shop familiarity favor 17-4PH for non-formed parts.
- Sensitive to Condition A handlingCold work or thermal exposure between purchase and HT can shift final properties — track conditions carefully.
- Lower strength than 17-4PH H900TH1050 UTS 1310 MPa equals 17-4PH H900. Both alloys top out around the same UTS.
Specs
Mechanical, physical & chemical data.
Mechanical
Properties (TH1050)
| UTS — Condition A (annealed) | 895 MPa (130 ksi) |
|---|---|
| YS — Condition A | 275 MPa (40 ksi) |
| Elongation — Condition A | 35% |
| UTS — TH1050 (aged) | 1310 MPa (190 ksi) |
| YS — TH1050 (aged) | 1170 MPa (170 ksi) |
| Elongation — TH1050 | 6% |
| Modulus of elasticity | 200 GPa |
| Hardness — TH1050 | 39-44 HRC |
| Hardness — RH950 | 44-48 HRC |
Source: AK Steel datasheet. RH950 (refrigeration-transformed, aged 950 °F; UTS 1450 MPa) is the strongest heat-treated condition. The cold-rolled CH900 condition is the highest-strength overall and is the one used for spring strip.
Physical & thermal
At room temperature
| Density | 7.81 g/cc (0.282 lb/in³) |
|---|---|
| Melting range | 1400 – 1430 °C |
| Specific heat | 0.460 J/g·°C |
| Thermal conductivity | 16.4 W/m·K |
| CTE (20–100 °C) | 10.1 µm/m·°C |
| Electrical resistivity | 0.83 µΩ·m |
| Magnetic — Condition A | Slightly magnetic |
| Magnetic — TH/RH (aged) | Ferromagnetic |
Magnetic transition during HT — Condition A is austenitic and only weakly magnetic; aged condition is fully martensitic.
Composition
Chemistry, weight %
| Chromium (Cr) | 16.0 – 18.0 |
|---|---|
| Nickel (Ni) | 6.50 – 7.75 |
| Aluminum (Al) | 0.75 – 1.50 |
| Manganese (Mn) | 1.00 max |
| Silicon (Si) | 1.00 max |
| Carbon (C) | 0.09 max |
| Phosphorus (P) | 0.040 max |
| Sulfur (S) | 0.040 max |
| Iron (Fe) | balance |
The aluminum is the precipitation-strengthening element (forms Ni-Al phases during aging). Higher Cr + Ni vs 17-4PH improves corrosion.
Specifications
Common purchase specs.
AMS 5528
Sheet, strip — Condition A
AMS 5529
Sheet, strip — Condition C
AMS 5673
Wire — spring temper (cancelled)
AMS 5644
Bar — solution treated (Condition A)
ASTM A693
Sheet, strip, plate
ASTM A564
Bar — heat treated PH stainless
AISI 631
Equivalent designation
UNS S17700
Unified numbering
Machining
Machining considerations.
Always machine in Condition A (annealed). Aged condition is grind-only. Plan dimensions to accommodate the heat-treat dimensional change (~0.001″/inch).
Cutting speed (Cond. A)
100-180 SFM with carbide. Slightly slower than 304 — work-hardens like austenitic stainless.
Feed rate
Aggressive constant feed. Same rules as 316L.
Tooling
Sharp positive-rake carbide. Coated grades for production.
Coolant
Flood with sulfurized cutting oil or high-pressure water-soluble.
No dwell
Like all PH stainless and austenitic, dwell creates a hardened layer.
HT planning
Conditioning at 760 °C → aging at 565 °C × 90 min. Plan ~0.001″/inch dimensional change.
Finish-grind after HT
Final tolerance < ±0.001″ should be ground after aging.
Order Condition A
Always specify Condition A for machining purchase. Aged stock must be ground.
Applications
Where 17-7PH shows up.
Aerospace bellows & diaphragms
Thin-section formed parts in fluid systems requiring corrosion + fatigue resistance.
Formed brackets & fittings
Sheet metal aerospace hardware needing high strength after forming. Where 304 isn't strong enough.
Compare
Related materials.
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