In the H1025 condition, 15-5 PH delivers transverse Charpy impact of 30 to 45 ft·lb, roughly double the 15 to 25 ft·lb range typical of 17-4 PH in the same aging treatment [S3]. That single data row is the practical answer to the 15-5 vs 17-4 question for any forged cross-section loaded in the short-transverse direction.
Both alloys are Cr-Ni-Cu-Nb precipitation-hardening martensitic stainless steels that age-harden to similar tensile and yield strength bands. The mechanical delta is essentially zero in the longitudinal direction. What separates them is microstructure: 17-4 PH retains 5 to 15 percent delta ferrite, while 15-5 PH was developed as a ferrite-free variant through tighter Cr control (14.0 to 15.5 percent Cr vs 15.0 to 17.5 percent in 17-4 PH) [S2][S3]. Ferrite stringers in heavy sections are exactly what cost you through-thickness toughness.
Chemistry Comparison and Why the Delta Ferrite Forms
The two grades overlap heavily on the mill cert. UNS S17400 (17-4 PH) and UNS S15500 (15-5 PH) both contain 0.15 to 0.45 percent niobium, cap carbon at 0.07 percent, and age-harden on the same copper-rich precipitate mechanism [S3]. Copper ranges 3.0 to 5.0 percent in 17-4 PH and 2.5 to 4.5 percent in 15-5 PH; nickel 3.0 to 5.0 percent vs 3.5 to 5.5 percent. The lever that changes the microstructure is the chromium ceiling, set lower in 15-5 PH to keep the ferrite-stabilizer balance out of the two-phase field during solidification.
15-5 PH is also routinely vacuum arc remelted (VAR) for cleaner inclusion populations, which contributes to the uniform fine grain structure credited with the transverse-toughness gain [S1]. For procurement, that means a 15-5 PH heat with a tighter Cr ceiling and a VAR melt practice is what you are paying the premium for, not a different age-hardening curve.
Mechanical Properties in H1025, the Default All-Purpose Condition
Both grades hit 155 ksi minimum tensile and 145 ksi minimum yield in H1025, per published property data summarized in the Anderson Shumaker comparison [S3]. Elongation runs 12 percent minimum for 17-4 PH and 14 percent minimum for 15-5 PH. Longitudinal Charpy impact sits at 35 to 50 ft·lb for 17-4 PH and 40 to 60 ft·lb for 15-5 PH, close enough that designers rarely notice the difference.
The transverse Charpy row is where the alloys separate: 15-5 PH posts 30 to 45 ft·lb vs 17-4 PH's 15 to 25 ft·lb in the same H1025 condition [S3]. That is the spec line an aerospace structural fitting, a valve body forging, or a downhole tool designer reads first when the load path is through-thickness. For higher-strength H900, expect all toughness numbers to drop; for overaged H1150, expect them to rise with a corresponding strength penalty.
When to Spec 15-5 PH and When 17-4 PH Is Good Enough
Spec 15-5 PH for heavy cross-section forgings, aerospace structural fittings with directional loading, and any component carrying short-transverse tensile or fatigue loads [S2][S3]. The tighter Cr balance and ferrite-free martensite make through-thickness properties predictable in sections that would otherwise pool delta ferrite at the billet centerline.
Keep 17-4 PH for general-purpose PH stainless needs, small to medium symmetric parts, and cost-driven work where transverse loading does not govern. 17-4 PH is the most widely stocked PH grade in North America, so lead time and bar/billet availability often decide for you when the design can tolerate it [S3]. Do not substitute one for the other without engineering sign-off, because the 2x transverse Charpy gap is exactly the kind of thing that shows up in a post-incident material review.
Aging Conditions, Corrosion, and Forging Window
Both grades respond to the same H900 / H925 / H1025 / H1075 / H1100 / H1150 aging series, with the number denoting the aging temperature in degrees Fahrenheit [S3]. H900 hits peak strength near 190 ksi UTS at the cost of toughness and chloride SCC resistance; H1025 is the all-purpose default; H1150 is the overaged condition used where chloride stress corrosion cracking is a service risk.
Corrosion resistance is nearly identical between the two, better than type 410 but below 304/316, and both should be coated or cathodically protected before seawater service [S3]. Forging windows also overlap, with both grades worked in the 2100 to 1850 °F range, slow cooled, solution treated at about 1900 °F, water quenched, and then aged [S3]. The processing delta is small, which is why fabricators tend to run both alloys on the same forge floor without retooling.
Decision Matrix: 15-5 PH vs 17-4 PH on Four Spec Criteria
On transverse Charpy (H1025), 15-5 PH scores 30 to 45 ft·lb against 17-4 PH's 15 to 25 ft·lb, roughly a 2x advantage that does not close at any standard aging temperature [S3]. On longitudinal strength in H1025, both hit 155 ksi UTS / 145 ksi yield minimum, so this criterion is a tie [S3]. On corrosion behavior, both sit in the same band, comparable to 304 in mild environments but vulnerable to chloride SCC, again a tie [S1][S3]. On availability and cost, 17-4 PH wins because it is the default stocked PH grade; 15-5 PH carries a premium driven by tighter Cr control and more common VAR melt practice [S1][S3]. The decision thus reduces to: if the part sees short-transverse loading or is a heavy forging, pay the 15-5 PH premium; otherwise, run 17-4 PH and save the budget for machining time. For related material-selection thinking on motor lamination steels, see the 0.27 mm vs 0.35 mm NGO steel motor lamination trade-offs breakdown, which uses a similar through-thickness property lens on a different alloy system.
Failure Modes and Limits Engineers Should Flag
The transverse-toughness advantage of 15-5 PH erodes if the heat is not VAR melted or if the billet cools too slowly from forging, allowing ferrite to re-form at the centerline [S1][S3]. Always require a transverse Charpy coupon from the forging itself, not from a separately heat-treated sample, because the cooling path through the section controls the ferrite population more than the nominal Cr content does.
Neither grade is a seawater material. Both will chloride-SCC in H900 and should be overaged to H1150 or shifted to a duplex stainless if the service is hot, chloride-rich, and loaded [S3]. For procurement, watch for mills quoting 17-4 PH on a 15-5 PH print because the bar is on the shelf, the chemistry overlap lets it pass a basic PMI check, and the transverse Charpy is what you will not catch until the forging is on the CMM. For broader material context, the A413 vs A380 aluminum die casting outdoor housing corrosion comparison covers a similar through-section property story in a non-ferrous system.
Track the next two signals when you have a real 15-5 vs 17-4 decision on the desk: the transverse Charpy value reported on the actual forging test report (not the bar-stock cert), and whether the heat is identified as VAR or ESR remelt on the mill paperwork. Either one of those is the difference between a part that passes a directional-impact qualification and one that does not.
For the relevant spec sheets and selection criteria, see ph meter, checking and testing apparatus, and construction machinery and equipment.