What Is 1.4542 Stainless Steel? A Guide to 17-4 PH (UNS S17400)

1.4542 stainless steel - the EN name for 17-4 PH, grade 630, UNS S17400 - occupies a unique position in the stainless steel family. It delivers the strength of a hardened martensitic steel together with corrosion resistance approaching that of austenitic 304, and it can be machined soft and aged hard in the same component. These characteristics make it the default precipitation-hardening grade for aerospace, chemical, oil and gas, and medical applications. This article explains what the designations mean, what the standard requires, and how heat treatment controls the final properties.

1. Designation and Equivalents

Standard	Designation
EN (Europe)	1.4542 / X5CrNiCuNb16-4 (EN 10088-1)
ASTM / AISI (USA)	Grade 630, 17-4 PH, UNS S17400 (ASTM A564, A693, A705)
JIS (Japan)	SUS630 (JIS G4303, G4304)
GB (China)	05Cr17Ni4Cu4Nb (GB/T 1220)

The EN steel name X5CrNiCuNb16-4 reads: X = high-alloy steel, 5 = nominally 0.05 % carbon, CrNiCuNb = chromium, nickel, copper and niobium alloying, 16-4 = 16 % chromium, 4 % nickel. The copper and niobium additions are the precipitation-hardening elements.

2. Chemical Composition (ASTM A564 / A693 Type 630)

Element (wt%)	Min	Max
Carbon	-	0.07
Manganese	-	1.00
Silicon	-	1.00
Phosphorus	-	0.040
Sulfur	-	0.030
Chromium	15.0	17.5
Nickel	3.0	5.0
Copper	3.0	5.0
Niobium + Tantalum	0.15	0.45

Copper is the key precipitation-hardening element: fine copper-rich particles form during aging and strengthen the martensitic matrix. Niobium refines the structure and contributes to strength. The carbon is kept low (≤ 0.07 %) so the material can be welded with reasonable precautions and remains tough after hardening.

3. Heat Treatment Conditions and Mechanical Properties

17-4 PH is supplied in Condition A (solution annealed) and hardened by aging. The table lists the minimum properties required by ASTM A564 for the most common conditions.

Condition	Heat Treatment	Tensile Strength (min), MPa	Yield Strength 0.2% (min), MPa	Elongation (min), %	Hardness
A (solution annealed)	1040 ± 15 °C, cool to 32 °C or below	not specified (typically ≈ 900–1000)	not specified	not specified	≈ 30–33 HRC typical
H900	480 ± 6 °C, 1 h, air cool	1310	1170	10	≥ 40 HRC (typical ≈ 44)
H1025	550 ± 6 °C, 4 h, air cool	1070	1000	12	≥ 35 HRC (typical ≈ 38)
H1150	620 ± 6 °C, 4 h, air cool	930	725	16	≥ 28 HRC (typical ≈ 31)

The two-step mechanism is straightforward: solution annealing at about 1040 °C dissolves all alloying elements and produces soft, low-strength martensite on cooling; aging at 480–620 °C then precipitates copper-rich particles that dramatically raise strength and hardness. Higher aging temperatures (H1150) sacrifice some strength for better toughness and stress-corrosion resistance. For maximum toughness, H1150M (a modified 620 °C cycle) is used.

4. Key Characteristics

High strength-to-weight ratio: H900 delivers tensile strength above 1300 MPa, roughly double that of 304, enabling lightweight designs in aerospace and high-performance machinery.

Corrosion resistance comparable to 304: adequate for atmospheric, freshwater and mild chemical service; not intended for marine immersion without protection.

Machinability in the soft condition: Condition A machines similarly to 304, allowing full machining before the final aging step, after which dimensional change is minimal (about 0.0005–0.001 mm/mm depending on section).

Magnetic response: the grade is magnetic in all conditions; it is not suitable where non-magnetic behaviour is required.

5. Applications

Aerospace: landing-gear components, structural fittings, fasteners, turbine blades and shafting.

Chemical and petrochemical: valve stems, pump shafts, impellers and fittings requiring strength plus moderate corrosion resistance.

Oil and gas: downhole tools, wellhead components and instrument housings. For sour (H₂S) service, hardness must be limited per NACE MR0175/ISO 15156, typically using H1150 or higher aging conditions.

Medical: surgical instruments and orthopaedic devices; the alloy is widely used in implantable components such as bone screws.

Nuclear and general engineering: springs, shafts and high-strength fasteners.

6. Machining and Welding Considerations

Machine in Condition A for longest tool life; after aging to H900, machining is abrasive and requires carbide or coated tooling.

Welding is possible with matching 17-4 PH filler; the weld zone must be re-solution-annealed and re-aged if full strength is required, because as-welded martensite is hard and brittle.

For service above about 300 °C, strength falls off and overaging occurs; 17-4 PH is not a high-temperature creep material.

Frequently Asked Questions

What does 17-4 PH mean?

17 % chromium, 4 % nickel, PH = precipitation hardening. The name describes the composition and the strengthening mechanism; the same material is 1.4542, grade 630, UNS S17400 or SUS630 depending on the standard system.

What is the difference between 1.4542 and 1.4568 (17-7 PH)?

1.4568 (631, UNS S17700) is a semi-austenitic precipitation-hardening grade with 17 % Cr, 7 % Ni and 1 % Al. 1.4542 gives slightly lower peak strength but better toughness and machinability; 631 offers higher strength in sheet/spring form and better cryogenic toughness. 1.4542 is the more common and versatile of the two.

Which heat treatment gives the highest strength?

H900 (480 °C, 1 h): minimum tensile 1310 MPa, yield 1170 MPa, hardness ≥ 40 HRC. It is the standard choice for strength-critical parts where toughness requirements are moderate.

Is 17-4 PH suitable for sour service?

Only in overaged conditions. NACE MR0175/ISO 15156 permits 17-4 PH for sour environments when hardness is limited (typically ≤ 33 HRC for most applications), which corresponds to H1150 or higher aging temperatures.

Can 17-4 PH be welded?

Yes, with matching filler, but the joint must be solution annealed and aged after welding to restore strength. Welding without post-treatment leaves brittle untempered martensite in the heat-affected zone.

Is 1.4542 magnetic?

Yes. The martensitic structure is magnetic in the annealed and aged conditions. Where non-magnetic behaviour is mandatory (for example certain instrument housings), austenitic grades must be used instead.