Custom 455 and 17-4PH are both precipitation-hardening (PH) martensitic stainless steels: they are solution treated and then hardened by ageing. The difference between them is the trade-off made between strength and corrosion resistance.
The fundamental distinction: chromium
17-4PH’s chromium is 15.00–17.50 %. Custom 455’s is 11.00–12.50 % — markedly lower. In exchange, Custom 455 raises nickel to 7.50–9.50 % and hardens with titanium; 17-4PH hardens with copper and niobium.
Carpenter Technology’s own two datasheets show the consequence clearly: it benchmarks Custom 455’s general corrosion resistance against Type 430, and Custom 630’s (17-4PH) against Types 302/304. Same mill, two documents, a clear ranking.
In short: Custom 455 buys its strength by giving up chromium.
Compared in figures
| PROPERTY | Custom 455 (S45500) | 17-4PH (S17400) |
|---|---|---|
| Chromium (Cr) | 11.00 – 12.50 % | 15.00 – 17.50 % |
| Nickel (Ni) | 7.50 – 9.50 % | 3.00 – 5.00 % |
| Copper (Cu) | 1.50 – 2.50 % | 3.00 – 5.00 % |
| Niobium + Tantalum | 0.10 – 0.50 % | 0.15 – 0.45 % |
| Molybdenum (Mo) | 0.50 % max | not specified |
| Manganese (Mn) | 0.50 % max | 1.00 % max |
| Silicon (Si) | 0.50 % max | 1.00 % max |
| ASTM A564 type | XM-16 | 630 |
| Hardening mechanism | titanium precipitation | copper precipitation |
| General corrosion resistance | Type 430 level | Type 302/304 level |
The mechanical properties of Custom 455
The values below are the specified minima of ASTM A564 Type XM-16 — not typical values but guaranteed floors. Reproductions of three separate editions of the standard (2004, 2007/ASME SA-564 and 2019) were compared; the figures are identical across all three. Validity: bar and shapes up to 150 mm, longitudinal direction.
| CONDITION | Rm min | Rp0.2 min | A min | Reduction of area min | Hardness min |
|---|---|---|---|---|---|
| H900 | 1620 MPa (235 ksi) | 1515 MPa (220 ksi) | 8 % | 30 % | 47 HRC / 444 HBW |
| H950 | 1515 MPa (220 ksi) | 1415 MPa (205 ksi) | 10 % | 40 % | 44 HRC / 415 HBW |
| H1000 | 1415 MPa (205 ksi) | 1275 MPa (185 ksi) | 10 % | 40 % | 40 HRC / 363 HBW |
| Condition A (solution treated) | not specified | not specified | — | — | 331 HBW / 36 HRC max |
ASTM A564 has no H1050 condition for XM-16.
All three editions of the standard define only H900, H950 and H1000 for XM-16. The H1050 figures circulating in the market come from producers’ typical tables and are not specification minima. Writing “H1050 to ASTM A564” on an order has no basis.
Heat treatment
Solution treatment is 830 ± 15 °C (1525 ± 25 °F) with rapid cooling. Ageing is a single step lasting four hours:
| CONDITION | Ageing temperature | Time | Cooling |
|---|---|---|---|
| H900 | 480 °C (900 °F) | 4 hours | air |
| H950 | 510 °C (950 °F) | 4 hours | air |
| H1000 | 540 °C (1000 °F) | 4 hours | air |
Unlike 17-4PH’s two-stage cycles such as H1150M, no refrigeration and no second stage are needed here. The hardening mechanism of the 540 °C / 4 hour cycle — copper-rich precipitates, Ni₃Ti (η) and G-phase — has been studied independently in a 2026 paper from the Beijing University of Technology.
17-4PH heat treatment
Solution treatment is 1038 ± 14 °C, followed by cooling below 32 °C (this step is mandatory). The ageing temperature defines the condition: H900 = 482 °C, H1025 = 551–552 °C, H1075 = 580 °C, H1100 = 593–595 °C, H1150 = 620–621 °C.
The H900 trap — the most important decision when buying 17-4PH
Maximum hardness is not always the right choice.
Carpenter’s own typical values show how toughness rises as the ageing temperature goes up: 21 J at H900, 75 J at H1150 and 136 J at H1150M — while proof strength falls from 1262 to 869 to 600 MPa. Rolled Alloys reports the same trend.
More critical is stress corrosion cracking. In Cleveland-Cliffs’ marine exposure tests, welded specimens loaded to 90 % of yield showed H900 failing within 68 days, while H1025, H1075 and H1150 survived more than 25 years without failure. Carpenter, Cleveland-Cliffs and BÖHLER set the same rule: age at the highest temperature that meets the strength requirement, and in no case below 552 °C.
Two chemistry sets circulate for Custom 455
Two separate composition limits are published for Custom 455, and the difference affects the certificate:
| ELEMENT | ASTM A564 XM-16 / ASTM F899 | Carpenter datasheet |
|---|---|---|
| Carbon (C) | 0.03 % max | 0.05 % max |
| Phosphorus (P) | 0.015 % max | 0.040 % max |
| Sulphur (S) | 0.015 % max | 0.030 % max |
| Titanium (Ti) | 0.90 – 1.40 % | 0.80 – 1.40 % |
So material certified to Carpenter’s own type analysis can fail ASTM A564 XM-16. State on your order which one you require.
A correction is also due: this alloy is not single-source. Universal Stainless and Fort Wayne Metals also melt it by VIM+VAR, it has its own European material number (1.4543 / X3CrNiCuTiNb12-9), and it is listed under ASTM F899 Class 5 as a surgical instrument material. The accurate description is not “single-source” but “melted by a limited number of producers”.
How to read this table. Rows without a source name have been confirmed by at least four independent organisations. Rows with an organisation named in brackets were found in fewer sources and are therefore given with attribution. No figure we could not verify has been published.
Two further points
1. H1150M and H1150D are not the same thing. H1150M is 760 °C for 2 hours followed by 621 °C for 4 hours; Rolled Alloys defines H1150D as 621 °C for 4 hours, twice. Different cycles, different properties.
2. Condition A is not a service condition. 17-4PH must be aged before use.
Which one is right for your job?
Custom 455: where very high strength is the priority and the corrosive environment is mild — aerospace fittings and highly stressed shafts, surgical and orthopaedic hand instruments, drill bits, drivers.
17-4PH: where strength and corrosion resistance are both required — aerospace and missile fittings, oilfield valve components, pump and valve shafts, gears, fasteners, chemical process equipment, marine shafting, food processing.
Designations, standards and AMS equivalents
The same steel is called different things depending on which system you are speaking in. Most ordering and certification confusion starts here, so a short summary:
- AISI — the American Iron and Steel Institute’s grade naming (410, 316L and so on). It is the most widely used name in everyday use, but on its own it is not a purchasing specification: composition and mechanical requirements are defined in the ASTM standards.
- UNS — the Unified Numbering System operated jointly by ASTM and SAE (S41000 and so on). It is the number used for ordering and certification in North America, and it is more precise than the AISI name.
- W.Nr / EN — the European material number (1.4006 and so on) and EN name (X12Cr13 and so on). This is the governing designation in Europe. It is usually only an approximate equivalent of the AISI grade; carbon, sulphur or molybdenum limits frequently differ.
- AMS — SAE’s Aerospace Material Specifications. An AMS specification binds not only the composition but also the melting practice, the product form, the heat treatment condition and the inspection requirements. In aerospace and defence orders this is the designation that governs.
| DESIGNATION | Custom 455 | 17-4Ph / AISI 630 |
|---|---|---|
| AISI / trade name | Custom 455 | 17-4Ph / AISI 630 |
| EN material no. (W.Nr) | — | 1.4542 |
| UNS number | S45500 | S17400 |
| AMS specifications | AMS 5617 · AMS 5860 | AMS 5604 · AMS 5622 |
| Product page | Custom 455 technical page | 17-4Ph / AISI 630 technical page |
Two reminders. First: an AMS number is specific to a product form and condition. The same grade may have separate AMS specifications for bar, sheet and forgings, so establish which product form you need before calling up an AMS number.
Second: the AISI name, material number, UNS and AMS data in the table above are the designations shown on Defence Metal’s product pages. If you are working to a different edition of a standard or to a different product form, state the material number and the specification edition explicitly on your order.