15-5 PH

What is 15-5 PH?

15-5 PH is a martensitic precipitation hardening stainless steel developed as a delta-ferrite-free refinement of 17-4 PH. Its composition contains 14.0 to 15.5 percent chromium, 3.5 to 5.5 percent nickel, 2.5 to 4.5 percent copper and 0.15 to 0.45 percent niobium plus tantalum. Carbon is limited to 0.07 percent. The grade is registered as UNS S15500 and designated Type XM-12 in ASTM A564.

The composition differs from that of 17-4 PH in its chromium and nickel ranges. The chromium range decreases from 15.0 to 17.5 percent in 17-4 PH to 14.0 to 15.5 percent in 15-5 PH. The nickel range increases from 3.0 to 5.0 percent to 3.5 to 5.5 percent. Chromium stabilises ferrite, while nickel stabilises austenite and therefore martensite. These changes reduce the retention of delta ferrite in the finished structure.

15-5 PH was developed to provide greater toughness than 17-4 PH, particularly in the short transverse direction. Delta ferrite stringers and elongated inclusions align with the working direction in rolled or forged bar and reduce transverse toughness. The grade controls delta ferrite content and the size and shape of inclusions. Consumable-electrode remelting is normally used for this purpose. The principal distinction from 17-4 PH is improved transverse toughness rather than increased strength.

Laxcon Steels lists 15-5 PH in its grades reference as a precipitation hardening grade. The same steel is written 15-5PH, 15/5 PH, 1.4545 and S15500.

Ferrite free refinement of 17-4 PH per ASTM A564 (UNS S15500)

What is the chemical composition of 15-5 PH?

Composition limits in weight percent are carbon 0.07 maximum, manganese 1.0 maximum, sulphur 0.03 maximum, phosphorus 0.04 maximum, silicon 1.0 maximum, chromium 14.0 to 15.5, nickel 3.5 to 5.5, copper 2.5 to 4.5 and niobium plus tantalum 0.15 to 0.45.

What is 15-5 PH equivalent to in other standards?

Equivalence means nearest counterpart, not identity: each standards body sets its own composition window, so check the limits of the standard actually named on the order before substituting.

What is the PREN of 15-5 PH?

Computed by Laxcon Steels from the composition above as PREN = Cr + 3.3Mo + 16N, the relationship published in Practical Guidelines for the Fabrication of Duplex Stainless Steels (International Molybdenum Association, third edition, 2014). The published relationship is given for austenitic and duplex stainless steels, so this figure extends it: read it against other precipitation hardening grades rather than across families. The first figure uses the minimum specified chromium, molybdenum and nitrogen, so it is the floor for a conforming heat rather than a typical value; the second uses the maxima, so the two together are the width of the band one compliant grade allows. PREN ranks resistance to chloride pitting and nothing else: it does not predict crevice corrosion, stress corrosion cracking or service life, and it is not a substitute for a corrosion test. The pitting resistance reference carries the full table and the caveats.

Mechanical properties

15-5 PH is supplied after solution treatment in Condition A. A single low-temperature ageing treatment produces the hardened condition. Each condition takes its name from its ageing temperature in degrees Fahrenheit. For H900, aged near 480 degrees Celsius, ASTM A564 and AMS 5659 specify minimum tensile strength of 1,310 MPa and minimum 0.2 percent yield strength of 1,170 MPa. Hardness is about 388 HB or 40 HRC. For H1150, aged near 620 degrees Celsius, the requirements decrease to 930 MPa tensile strength and 725 MPa yield strength. Hardness is about 277 HB or 28 HRC, while elongation and reduction of area increase.

H1025 and H1075 are intermediate conditions. They follow the same decrease in strength and increase in ductility between H900 and H1150. Published minimum values for these conditions vary by source and section size. The applicable specification and section size therefore determine the required values for an order.

Density is close to 7.75 grams per cubic centimetre in Condition A and increases slightly during ageing as copper precipitates form. Electrical resistivity decreases from about 98 microhm centimetres in Condition A to about 77 microhm centimetres in H900. Both changes result from the precipitation reaction removing copper from solution.

Precipitation hardening

Solution treatment dissolves copper in a supersaturated martensitic matrix and leaves the material soft enough for machining. Ageing holds the component at a few hundred degrees Celsius for one to four hours. Copper then forms a fine dispersion of copper-rich particles. These particles obstruct dislocation movement and increase strength without a quench.

The absence of a quench prevents quench distortion. Ageing instead causes a small, predictable and repeatable shrinkage. Long and slender components can be finish machined in Condition A and hardened afterward without straightening or regrinding. This dimensional stability supports the use of precipitation hardening grades in aerospace shafting and actuator hardware.

The ageing condition determines the final mechanical properties and must be stated on the drawing. A solution-treated bar can receive any of the available ageing treatments after machining. Two components from the same length of 15-5 PH can consequently differ by 445 MPa in yield strength and 12 points of Rockwell C hardness. Ageing is therefore part of the process route rather than only a purchasing designation.

Corrosion behaviour

The pitting resistance equivalent number ranges from 14.0 at the specified composition minima to 15.5 at the maxima. The calculation uses chromium alone because the grade contains no molybdenum and has no specified nitrogen. This range is slightly below the 15.0 to 17.5 range of 17-4 PH. The difference results from the lower chromium content used to suppress delta ferrite. General corrosion resistance is comparable to that of martensitic and 400 series stainless steels rather than austenitic grades.

Ageing condition affects corrosion behaviour. Higher ageing temperatures slightly over-age the precipitates, reduce strength and improve resistance to stress corrosion cracking. Components for marine or chemically aggressive service are therefore normally specified in a higher-temperature condition instead of the maximum-strength condition.

Processing and applications

Machining normally takes place in Condition A. In this condition, 15-5 PH behaves broadly like a soft martensitic stainless steel. It does not have the gumminess of austenitic steel and can be machined with ordinary carbide tooling. Standard welding methods can be used without preheat. Post-weld solution treatment and ageing restore parent-metal properties in the joint.

Applications include aerospace structural fittings, actuator and landing gear components, fasteners, gears, valve and pump parts, oil and gas equipment, power generation hardware and food processing machinery. These applications require strength, dimensional stability during heat treatment and reasonable corrosion resistance. The grade is UNS S15500 and ASTM Type XM-12. Aerospace specifications include AMS 5659 for bar and AMS 5862 for plate. The nearest European material number is 1.4545.

In which product forms does Laxcon list 15-5 PH?

Taken from the published specification on each product page. A form that is not listed here is not a form this reference can confirm the grade in; the enquiry route settles what can actually be rolled to an order.

Where 15-5 PH is not the right choice

15-5 PH is unsuitable where chloride exposure governs material selection. Its PREN range of 14.0 to 15.5 is the second lowest of the precipitation hardening grades in this reference. In seawater, coastal atmospheres and chlorinated water, its pitting behaviour resembles that of a 400 series steel rather than an austenitic grade. Where both high strength and chloride resistance are required, super duplex 2507 provides a minimum yield strength of 550 MPa and a PREN range of 37.7 to 47.6.

Condition A is unsuitable for service. Unaged precipitation hardening steel has lower ductility and lower resistance to stress corrosion cracking. A drawing that specifies 15-5 PH without its condition does not fully specify the component. H900 is also unsuitable where environmental cracking is possible because it combines the highest hardness with the lowest resistance to cracking.

15-5 PH is unsuitable for service above its ageing temperature. Continued ageing during service reduces its strength. For components that require only longitudinal strength in a modest section, 17-4 PH is more widely stocked and has the same H900 minimum properties. The additional cost of 15-5 PH applies where short transverse toughness in a heavy section is a design requirement.

Other precipitation hardening grades