17-4PH (UNS S17400) and 15-5PH (UNS S15500) are the two workhorse precipitation-hardening stainless steels. Their published minimum strengths are almost identical. The difference that matters sits in the direction you are not testing.

Chemical Composition Compared

Both grades harden by the same mechanism. Copper stays in solution after solution annealing, then precipitates as a copper-rich phase during a single low-temperature ageing treatment between 482 and 621 °C. Niobium is present in both to tie up carbon and refine the grain. What separates them is how much room the composition leaves for delta ferrite to form during solidification.

Read the chromium and nickel rows together. 17-4PH permits up to 17.5% chromium while allowing nickel to fall to 3.0%. High chromium with low nickel is exactly the combination that leaves delta ferrite in the as-cast structure, and delta ferrite survives hot working as stringers aligned with the rolling or forging direction. 15-5PH caps chromium lower and lifts the nickel floor, which closes that window. Carpenter describes the alloy as "chemically balanced to eliminate all but trace amounts of delta ferrite, thus providing superior transverse toughness and ductility plus a high degree of forgeability." The mills that produce 15-5PH by vacuum arc remelting describe the result as an "essentially ferrite-free microstructure."

Mechanical Properties by Condition — Why the Minimums Match

This is the part that surprises most buyers. Take the minimum values published for ASTM A564 Type 630 bar and the minimum values published for A564 Type XM-12 bar and put them side by side. They are the same numbers. The 17-4PH figures below come from the Cleveland-Cliffs mill data bulletin for ARMCO 17-4 PH; the 15-5PH figures are the A564 XM-12 minimums published by the mills that stock the grade.

So if your design calculation only reads the longitudinal minimum, the two grades are interchangeable and the cheaper one wins. That conclusion is correct for a pump shaft loaded in tension along its axis. It is wrong for a landing-gear fitting, a thick forged block, or anything with a notch that opens across the working direction.

Transverse Ductility and Delta Ferrite — the Number That Decides

15-5PH is the only one of the two for which transverse minimums are routinely published next to the longitudinal ones. The gap between the two directions is large, and it is the clearest available measure of what delta ferrite costs you.

At H900, longitudinal elongation is 10% and transverse is 6%. Reduction of area drops from 35% to 20%. That is a 43% loss in ductility purely from the direction the coupon was cut. It is also the condition where 17-4PH is weakest: the mill data bulletin for ARMCO 17-4 PH lists no Charpy minimum at all for H900 or H925, and it states plainly that where superior transverse mechanical properties are important, 15-5 PH should be considered.

For 17-4PH, the same bulletin gives minimum Charpy V-notch values that climb steeply with ageing temperature: no minimum at H900, 7 J at H925, 20 J at H1025, 27 J at H1075, 34 J at H1100, 41 J at H1150 and H1150-D, and 75 J at H1150-M. If the part has to survive an impact event, 17-4PH can do it, but only in the overaged conditions, and that costs you roughly 450 MPa of yield strength against H900.

Melting Practice — Air Melt, ESR and VAR

Standard commercial 17-4PH is air melted. The Cleveland-Cliffs bulletin puts it this way: the grade is generally air melted, and consumable-electrode electroslag remelting is available when the application demands it. Air-melted 17-4PH meets the magnetic particle inspection requirements of AMS 2303. Remelted 17-4PH meets the stricter AMS 2300.

15-5PH runs the other way round. AMS 5659 is titled for consumable-electrode remelted product, and it is split into Type 1 (vacuum arc remelt or consumable electrode vacuum melt) and Type 2 (electroslag remelt). In practice that means a 15-5PH bar ordered to AMS 5659 arrives remelted as a matter of course, not as an extra. Remelting reduces alloy segregation during solidification and gives a finer, more uniform inclusion population, which is the mechanism behind the transverse property improvement.

If your drawing says 17-4PH and you want remelted material, you have to ask for it explicitly. The specification route is AMS 5622 rather than AMS 5643, or A564 Type 630 with a supplementary requirement for consumable-electrode remelting.

Specifications and Designations

Heat Treatment Windows

Both grades use the same eight-condition ladder, and the ageing temperatures are identical within a few degrees. The H-code refers to the ageing temperature in degrees Fahrenheit, which is why H900 sits at 482 °C and H1150 at 621 °C.

The 32 °C figure on the solution treatment is not a typo and it is not a suggestion. Martensite finish for this alloy sits near 32 °C, and if the part is aged before the transformation completes, the properties will not be what the certificate claims. Aerospace specifications tighten the window further, so if the purchase order carries an AMS number, expect the heat-treat route to be audited.

Corrosion Resistance and Chloride Stress Corrosion Cracking

In atmospheric and mild chemical service the two grades behave similarly, and published guidance for 15-5PH places its corrosion resistance close to 304 in many media. Neither grade contains molybdenum, so neither should be specified where chlorides are concentrated and the temperature is elevated.

Where they differ is in the stress corrosion cracking record, and the test data behind it is unusually clear. Bent-beam specimens of 17-4PH stressed to the 0.2% yield strength and exposed on a marine lot 25 m from the waterline failed in every case in Condition H900, in 68 days or less, whether welded or not. Specimens aged at 552 °C and above did not fail after more than 25 years of exposure. The mill recommendation follows directly: for maximum resistance to chloride stress corrosion cracking, age at the highest temperature that still delivers the required properties, and never below 552 °C. 15-5PH carries the same dependence on ageing temperature, but the ferrite-free structure removes the stringer paths that accelerate crack growth once a crack has started.

For sour service, NACE MR0175 / ISO 15156 lists only the double-aged conditions, H1150-D and H1150-M, with a 33 HRC maximum hardness on the finished component. That rule applies to 17-4PH. If the enquiry carries an H₂S requirement, the condition field is decided for you. See our note on NACE MR0175 and the standard it is compared against.

Machining, Welding and Dimensional Stability

Both grades can be machined in Condition A or in the aged condition, and both are weldable by the commercial processes without preheat in sections up to 100 mm, because the low carbon content keeps the rapidly cooled weld zone soft enough to avoid cracking. Published cutting-speed guidance puts annealed 15-5PH at 80 ft/min with a machinability rating around 50% of B-1112, and H1150 at 125 ft/min, figures that track 17-4PH closely.

Dimensional change on ageing is small and predictable, and it is worth building into the machining allowance. From Condition A, 17-4PH contracts 0.00045 mm/mm at H900, 0.00053 at H1025, 0.0022 at H1150, and 0.00243 through the full H1150-M cycle. On a 500 mm shaft, that is 0.22 mm at H900 and 1.1 mm at H1150. Air cooling rather than quenching is what keeps distortion down, and it is the main reason these alloys are chosen over quench-and-temper martensitics when the part is already near final geometry.

Cost, Availability and Lead Time

17-4PH is the higher-volume grade and it is stocked in a wider range of sizes by more mills. That translates into shorter lead times, more competition on price, and a better chance of finding an odd diameter from stock. 15-5PH carries a premium driven by the remelting step and by lower production volume, and thin plate in particular tends to be a mill-order item rather than a stock item.

The premium is real but it is not usually the deciding factor. A remelted 17-4PH ordered to AMS 5622 costs more than standard A564 Type 630 and closes much of the gap, at which point the transverse properties and the published transverse minimums make 15-5PH the cleaner specification. Ask for both quotations on the same condition and the same certification level before concluding that one is expensive.

When to Upgrade from 17-4PH to 15-5PH

What HT PIPE Supplies in Both Grades

HT PIPE supplies 17-4PH and 15-5PH as round bar and rod, plate and sheet, forged flanges, forgings and machined parts and fasteners, in Condition A or mill-aged to any of the eight standard conditions, with EN 10204 3.1 certification as standard and 3.2 available.

FAQ — 17-4PH vs 15-5PH

Is 15-5PH stronger than 17-4PH?

No. The published minimum tensile and yield strengths are the same at every condition, within 2 MPa. 15-5PH is tougher and more ductile across the grain, not stronger along it.

Can 15-5PH replace 17-4PH on an existing drawing?

Usually yes, and it is a common substitution in one direction only. Replacing 15-5PH with 17-4PH requires the designer to accept lower transverse ductility, which should not be done without reviewing the load path.

Why does 17-4PH have no Charpy minimum at H900?

The mill data bulletin states that minimum impact properties cannot be accepted in Conditions H900 and H925. Toughness only becomes specified from H1025 upward, reaching 75 J at H1150-M.

What does the H number mean?

It is the ageing temperature in degrees Fahrenheit. H900 is aged at 900 °F (482 °C), H1150 at 1150 °F (621 °C).

Are both grades magnetic?

Yes. Both are martensitic in every condition, including Condition A, and both stay magnetic after ageing.

Which one should I use for a pump shaft in seawater?

Neither at H900. Marine exposure tests on 17-4PH at H900 produced failures in 68 days or less. Age to H1150 or H1150-M, or move to a molybdenum-bearing grade such as duplex 2205 if chlorides and strength are both critical.

Is remelted 17-4PH equivalent to 15-5PH?

Closer, but not equivalent. Remelting improves cleanliness and reduces segregation, but 17-4PH retains the composition window that permits delta ferrite. Order remelted 17-4PH to AMS 5622 if the drawing cannot change.

Do both grades need preheat before welding?

No. The low carbon content keeps the heat-affected zone soft enough that preheat is not required in sections up to 100 mm. Post-weld ageing restores properties in the weld zone.

Which is easier to get quickly?

17-4PH. It is produced in higher volume and stocked in more sizes. 15-5PH thin plate is the item most likely to be a mill order.