UNS S17400 Precipitation-Hardening Stainless Steel: H900-H1150 Selection

Written by: Emma, Technical Sales Engineer  |  Reviewed by: Ethan, Materials Engineer  |  Updated: August 2026

UNS S17400, better known as 17-4 PH stainless steel, is a precipitation-hardening grade whose properties can be tuned across a wide range by selecting the right aging condition. The designations H900, H1025, H1075, and H1150 are frequently seen in specifications and RFQs, and they are not different grades — they are heat-treatment conditions of the same material.

This article explains how 17-4 PH responds to precipitation hardening, what H900 through H1150 mean for strength, hardness, and toughness, and how to choose between them. It is written for materials engineers, machining and manufacturing engineers, and B2B buyers.

1. Direct Answer

UNS S17400 is a precipitation-hardening stainless steel. After solution treatment, the material is aged to develop different combinations of strength and hardness. The H900 condition is typically associated with higher strength and hardness, while H1150 generally offers better toughness and ductility at a lower strength level.

The choice between H900 and H1150 is not a case of "higher number is better." Each condition serves different service requirements, and the correct selection depends on how the part will be loaded and used.

2. Quick Comparison

This table is a qualitative guide. No absolute mechanical values are quoted, because they depend on the standard, product form, and heat-treatment condition.

3. What Is UNS S17400 / 17-4 PH?

17-4 PH is a precipitation-hardening stainless steel with a martensitic matrix. It is not a conventional martensitic grade, because its strengthening comes from a different mechanism.

• Precipitation-hardening mechanism: After solution treatment and rapid cooling, the material is aged at a moderate temperature, which causes fine precipitates to form and harden the matrix.
• Martensitic matrix: The base structure is martensitic, which provides a useful starting strength.
• Chromium: Provides corrosion resistance and contributes to the passive film.
• Nickel: Stabilizes the structure and participates in the precipitation response.
• Copper: The key precipitation element; copper-rich precipitates form during aging and produce the hardening effect.
• Niobium (columbium): Added to refine the structure and support the heat-treatment response.
• Solution treatment: The first step, which dissolves the alloying elements into a uniform structure.
• Aging precipitation: The second step, which develops the strength and hardness through precipitation.

Classifying 17-4 PH as an ordinary martensitic stainless steel would miss the point: its flexibility comes from the precipitation-hardening response, which is what allows the same grade to serve very different strength and toughness requirements.

4. H900-H1150 Heat Treatment

The heat-treatment route for 17-4 PH has two main stages: solution annealing followed by aging.

• Solution annealing: Heating into the solution range to dissolve alloying elements, followed by rapid cooling;
• Aging: Reheating to a controlled aging temperature to precipitate the hardening phases;
• Aging temperature: The condition designation reflects the aging temperature — for example, H900 refers to aging at approximately 900°F, and H1150 at approximately 1150°F;
• Holding time: Time at the aging temperature affects the final properties;
• Cooling: The cooling practice after aging is also part of the procedure;
• Precipitation: Higher aging temperatures generally produce a softer, tougher result;
• Overaging: Higher aging temperatures move the material toward an overaged condition, trading strength for toughness.

Specific heat-treatment parameters must follow the applicable specification, product form, and the supplier or processor procedure. No universal procedure is provided here, because the correct cycle depends on the actual product and requirements. In practice, the buyer specifies the target condition and the required mechanical properties, and the processor qualifies the cycle that achieves them for the specific product form and section size.

5. Strength, Hardness and Toughness

Different aging conditions shift the balance of properties in a predictable direction:

• Tensile strength: Highest at H900 and decreasing as the aging temperature rises;
• Yield strength: Follows a similar pattern;
• Hardness: Highest at H900;
• Elongation: Generally improves at higher aging temperatures;
• Toughness: Better at H1150 than at H900;
• Dimensional stability: The aging condition can affect dimensional behavior in service.

6. H900 vs H1150: How Should You Choose?

Consider H900 when:

• High strength is the main requirement;
• High hardness is useful;
• Service toughness requirements are satisfied;
• The applicable specification permits the condition.

Consider H1150 when:

• Toughness and ductility are more important;
• Impact loading is relevant;
• Dimensional or service stability favors a more overaged condition;
• Lower strength is acceptable.

H1025 and H1075 sit between these extremes and are used where an intermediate balance of strength and toughness is required. A common practical approach is to start from the service load case and work backward: if the design is limited by strength, H900 is the natural starting point; if it is limited by impact or notch sensitivity, the higher aging conditions deserve the closer look.

7. Corrosion and Service Limits

17-4 PH offers good general corrosion resistance, but it should not be treated as a direct substitute for 316L in aggressive environments.

• Atmospheric corrosion: Good in many environments;
• Chloride exposure: Requires evaluation; chloride levels, temperature, and stress all matter;
• Pitting / crevice corrosion: Can occur where chlorides concentrate;
• Stress corrosion cracking: A consideration in chloride service, especially under tensile stress;
• Surface condition: Surface finish and cleanliness affect corrosion behavior;
• Heat-treatment condition: The aging condition can influence corrosion performance.

In severe chloride environments, the combination of temperature, chloride concentration, stress, and surface condition must be reviewed before selecting 17-4 PH.

8. Fabrication and Machining

Fabrication of 17-4 PH depends on the material condition at each step.

• Solution-treated condition: The normal starting point for machining and forming;
• Aged condition: Harder and stronger, and more difficult to machine;
• Machining: Usually performed before aging where practical;
• Forming: Performed in the solution-treated condition;
• Welding: Possible, but the final properties depend on post-weld heat treatment;
• Post-weld heat treatment: Often required to restore the intended properties;
• Dimensional changes during aging: Aging can cause small dimensional changes that must be allowed for.

The actual sequence depends on the product form, thickness, and fabrication route, and should be agreed between the manufacturer and the buyer.

9. Standards and RFQ Requirements

When buying 17-4 PH, the H-condition must be specified explicitly.

• UNS S17400: The UNS designation;
• ASTM A564 where applicable: Commonly used for bar products;
• ASTM A693 where applicable: Commonly used for plate and sheet;
• Applicable EN standards: Where the European designation system applies;
• Product form: Bar, plate, sheet, or other form;
• Heat-treatment condition: The required H-condition (H900, H1025, H1075, H1150, or as specified);
• Hardness: The target hardness range;
• Mechanical properties: The required strength values;
• MTC: A material test certificate;
• PMI: Positive material identification where required;
• Testing and traceability: Any specified tests and heat-number traceability.

An RFQ should confirm at minimum: grade, UNS designation, product form, standard, dimensions, the H-condition, mechanical requirements, surface condition, and inspection/MTC requirements.

Common Purchasing Mistakes

• Treating H900 and H1150 as if they were different grades;
• Assuming H900 is always "better" than H1150;
• Looking only at tensile strength and ignoring toughness;
• Ignoring the product form and standard;
• Treating 17-4 PH as directly equivalent to other PH grades;
• Using 17-4 PH as a 316L substitute in severe chloride service;
• Ignoring the effect of the aging condition on dimensions and properties;
• Not verifying the heat-treatment condition on the MTC;
• Writing only "17-4PH" without specifying the H-condition.

FAQ

Q1: What is UNS S17400 stainless steel?
UNS S17400 is the UNS designation for 17-4 PH, a precipitation-hardening stainless steel with a martensitic matrix, strengthened by aging after solution treatment.

Q2: What is the difference between H900 and H1150?
H900 and H1150 are aging conditions of the same grade, not different grades. H900 is aged at a lower temperature for higher strength and hardness, while H1150 is aged at a higher temperature for better toughness and ductility.

Q3: Is H900 stronger than H1150?
Generally yes. H900 typically provides higher tensile and yield strength and higher hardness, while H1150 provides better toughness and elongation at lower strength.

Q4: Which 17-4 PH condition has better toughness?
Higher aging conditions such as H1150 generally provide better toughness and ductility than H900. The right choice depends on whether strength or toughness dominates the service requirement.

Q5: Is 17-4 PH precipitation hardening stainless steel?
Yes. 17-4 PH is a precipitation-hardening stainless steel: it is solution treated and then aged to develop its strength through fine precipitates.

Q6: Is UNS S17400 corrosion resistant?
It has good general corrosion resistance, but it is not a direct substitute for 316L in aggressive chloride service. Chloride level, temperature, stress, and surface condition must all be reviewed.

Q7: Can 17-4 PH be used in chloride environments?
It can be used in some chloride environments, but severe chloride service should be evaluated carefully, considering pitting, crevice corrosion, and stress corrosion cracking risks.

Q8: Which H condition is best for machining?
Machining is generally easiest in the solution-treated condition, before aging. Once aged, higher-strength conditions such as H900 are more difficult to machine than lower-strength conditions.

Q9: What ASTM standard covers 17-4 PH stainless steel?
ASTM A564 commonly covers bar products, and ASTM A693 commonly covers plate and sheet. The applicable standard depends on the product form.

Q10: What should be included in a 17-4 PH RFQ?
Include the grade, UNS designation, product form, applicable standard, dimensions, the required H-condition, mechanical requirements, surface condition, MTC, and any inspection or traceability requirements.

Related Reading

• 17-4 PH Stainless Steel Bar: Grades, Condition and Tolerance
• 17-4 PH Stainless Steel Plate: Heat Treatment and Properties
• 17-4 PH Stainless Steel Sheet: Finish and Thickness
• 17-4 PH Stainless Steel Coil: Processing and Selection
• Precipitation-Hardening Stainless Steel: Grades and Selection
• 420 Stainless Steel Heat Treatment: Hardness, Tempering and Uses
• 440C Stainless Steel Hardness: Heat Treatment and Bearings

Sources and Further Reading

For authoritative standards, grade data, and heat-treatment information, the following organizations provide technical material on stainless steel grades:

• ASTM International — https://www.astm.org/
• worldstainless — https://worldstainless.org/
• British Stainless Steel Association (BSSA) — https://bssa.org.uk/
• Nickel Institute — https://nickelinstitute.org/
• Outokumpu — https://www.outokumpu.com/
• Alleima — https://www.alleima.com/
• ISO — https://www.iso.org/

Where specific H900 or H1150 mechanical-property or heat-treatment data is required, it should be checked against the applicable ASTM specification or reliable steel-mill technical data, and the applicable conditions should be stated.