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13-8Ph / 1.4534 / UNS S13800 / AMS 5629 / AMS 5864

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Corrosion resistance: Very good in terms of corrosion resistance, grade 1.4534 / 13-8 PH is used where high mechanical strength and high corrosion resistance are required together. One of the age-hardened (PH) materials, this grade has considerably better corrosion resistance than grade 630 (17-4 PH, 1.4542) and 15-5 PH (1.4545). PH 13-8 Mo is chosen where very high mechanical strength and very good corrosion resistance are required.

Weldability: As with all PH (age-hardened) materials, this grade has limited weldability.​‌​​‌​

Machinability: The purpose of this material is to deliver the highest level of mechanical strength together with very good corrosion resistance in extremely critical parts. Machinability is one of the last properties sought in this material, and the machinability of this grade is low.

Heat treatment: 13-8 PH / 1.4534 steel is put through special heat treatment techniques in order to gain hardness and durability. The treatment is generally carried out by solution treatment followed by air cooling.​‌​​‌​

Applications: The most common application area for PH 13-8 Mo stainless steel is aerospace. It is chosen for aircraft landing gear parts, aircraft landing gear gears, parts requiring high mechanical strength, valve and shaft components, shafts in contact with petrochemical fluids and working under load, and nuclear power plant shafts.

Because it is expensive, this grade is chosen in environments where the alternatives are not sufficient. As a highly specialised material, it is generally used in very specific components.​‌​​‌​

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Standards by Product Form

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PH 13-8 Mo (UNS S13800 / XM-13 / 1.4534) sits at the toughness end of the precipitation-hardening stainless family. Two features set it apart: it hardens not through copper but through a NiAl (nickel-aluminide) intermetallic, and it must be double melted (VIM + VAR or ESR) — which is where its transverse and through-thickness toughness comes from.

WARNING — catalogue traps. (1) AMS 5659 is not a 13-8 Mo specification — it is the 15-5 PH bar/wire/forging spec, and it is sometimes miscopied onto 13-8 Mo listings. (2) Some secondary datasheets publish “typical” mill test results as though they were specification values; the table below gives the AMS 5629 minimums (guaranteed) — typical values sit above these but cannot be held to contractually. (3) Published Charpy figures for 13-8 Mo are largely single-sourced; where a guaranteed impact or fracture toughness is required, the specification to cite is AMS 5934, not AMS 5629.

Composition (ASTM A564 / AMS 5629): C ≤0.05% · Cr 12.25–13.25% · Ni 7.50–8.50% · Mo 2.00–2.50% · Al 0.90–1.35% · Mn ≤0.10% · Si ≤0.10% · P ≤0.010% · S ≤0.008% · N ≤0.010%. Why the cleanliness matters: the very low carbon keeps chromium out of carbides, so it stays in solid solution for corrosion resistance and the alloy strengthens almost entirely through NiAl precipitation. The P and S limits — far tighter than typical 300-series levels — minimise grain-boundary segregation and sulphide inclusions, which is the real source of the grade’s high, uniform toughness in heavy sections, and is only achievable with VIM+VAR/ESR melting.​‌​​‌​

Conditions — AMS 5629 Minimums

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The reduction of area is specified separately in three directions: longitudinal (L), transverse (T) and short-transverse / through-thickness (ST). The signature of this grade is that the three values stay close together — in many wrought steels the short-transverse figure falls away sharply, and in 13-8 Mo it does not. That isotropy is the direct payoff of the clean melt and the very low P and S limits.

Heat Treatment, Welding and Machining​‌​​‌​

Heat treatment

The mechanism is precipitation of a finely dispersed, coherent NiAl intermetallic in a low-carbon martensitic matrix. This is a different mechanism from the copper-rich precipitates of 17-4 PH and 15-5 PH, and it is why 13-8 Mo has higher, more isotropic toughness. Solution treatment (Condition A): about 927 °C (1700 °F ±15–25 °F), held according to section thickness, then air or oil cooled to below 16 °C — no separate deep-freeze step is required. Ageing: 4 hours ±15 min at the nominal temperature, air cool: H950 = 510 °C, H1000 = 540 °C, H1025 = 550 °C, H1050 = 565 °C, H1100 = 595 °C, H1150 = 620 °C. H1150M: 760 °C for 2 hours, then 620 °C for 4 hours, air cool. Standard delivery is Condition A.​‌​​‌​

Welding

Weldability is good: no preheat is required and no special interpass cooling is needed. GTAW is standard; resistance welding is also suitable. The matching filler is AMS 5840 bare wire — which must itself be vacuum melted with controlled interstitials, or the toughness match is lost. Post-weld ageing is required (roughly 480–620 °C for 1–4 hours, i.e. one of the standard H cycles). Where possible, weld in Condition A and then solution-treat and age the whole assembly; welding already-aged material locally over-ages the HAZ.​‌​​‌​

Machining

Machinability tracks hardness directly. In Condition A, cutting speeds run roughly 20–30% lower than for 17-4 PH or 302/304 austenitic stainless. The best machinability of the standard conditions is the overaged H1150M. Where complex features must be machined before final strengthening, the practical route is to machine in H1150M or Condition A and then re-solution and re-age; H950 and H1000 are the hardest conditions to machine.​‌​​‌​

When to Choose 13-8 Mo

Frequently Asked Questions

When does 13-8 Mo justify its premium over 17-4 PH or 15-5 PH?​‌​​‌​

Choose 13-8 Mo when the part is thick-section and load-bearing and needs guaranteed properties in the short-transverse (through-thickness) direction: large valve bodies, shafts, structural forgings — anywhere a crack could propagate along the weak axis of the material. Two things buy that isotropy: VIM + VAR (or ESR) double melting and the P ≤0.010% / S ≤0.008% limits, far tighter than standard-melt 17-4 or 15-5 PH. 17-4 PH and 15-5 PH are cheaper and machine faster, and are the right call for smaller parts or where through-thickness toughness is not the governing design case. The 2.0–2.5% molybdenum in 13-8 Mo also buys measurably better pitting and crevice resistance than either alternative, which can be decisive in oil-and-gas and marine-adjacent service.

Which H condition, and what is the strength–toughness trade-off?​‌​​‌​

H950 gives the highest strength (yield ≥1413 MPa, tensile ≥1517 MPa) and the best general corrosion resistance of the standard conditions, but the lowest ductility and toughness. Moving up the ageing ladder trades strength for ductility and impact toughness: H1000 and H1025 are the common “balanced” choices; H1100 and H1150 give up substantial strength for maximum ductility and machinability; and H1150M (double age) is the most ductile and machinable but lowest-strength condition, typically chosen for parts needing extensive machining or where fracture toughness governs. One important detail: if the application requires a guaranteed impact or fracture-toughness minimum, specify AMS 5934 rather than AMS 5629.

Does VIM-VAR need to be called out separately on the order?​‌​​‌​

Yes — and by specification number. AMS 5629 permits VAR or ESR as the second melt, so writing “13-8 Mo” alone does not guarantee VIM-VAR. For the tightest cleanliness and toughness guarantee, specify AMS 5934 (“extra high toughness”), which calls out VIM plus consumable-electrode vacuum melting and tighter residual limits. Relying on a distributor’s generic “13-8 Mo” listing without citing AMS 5629 or 5934 risks receiving single-melt or non-aerospace-grade material. Always verify the melting route and the specification number on the certificate.

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