01 — Overview & Material Identity
Quick Answer: AM-355 alloy (UNS S35500, AISI 634) is a semi-austenitic, precipitation-hardening chromium-nickel-molybdenum stainless steel achieving 170–220+ ksi tensile strength through controlled heat treatment, while retaining corrosion resistance superior to all standard martensitic stainless grades. It is the preferred forging material for aerospace gas turbine compressor components, defense structural parts, and high-pressure industrial hardware where strength, 1,000°F temperature resistance, and corrosion performance must coexist in a single alloy.
AM-355 alloy — formally designated UNS S35500 and classified as AISI 634 — occupies a unique structural position in the stainless steel family. As a semi-austenitic precipitation-hardening (PH) grade, it can be supplied in a soft, formable annealed condition similar in workability to austenitic 300-series stainless, then upgraded through controlled heat treatment to tensile strengths rivalling nickel superalloys — all while retaining corrosion resistance superior to the entire quench-hardenable 400-series family.
The "AM" prefix traces to Allegheny Metal (Allegheny Ludlum), the alloy's original developer. Today the material is procured globally under the UNS S35500 designation, governed by SAE AMS aerospace material specifications and multiple ASTM standards. Its semi-austenitic classification is the defining characteristic: unlike fully austenitic grades that resist hardening, or martensitic grades that sacrifice corrosion resistance for strength, AM-355 uses nitrogen and carbon in careful balance to trigger a controlled martensitic transformation during heat treatment — the mechanism that delivers exceptional strength without compromising the passive chromium oxide barrier responsible for corrosion protection.
AM-355 is one of the few engineering alloys that offers the formability of an austenitic stainless in the annealed condition and the high-cycle fatigue strength of a martensitic alloy after heat treatment — in a single material, without changing grade mid-fabrication. This dual-state capability drives its use in gas turbine compressor components where machining must be completed before final hardening.
02 — Chemical Composition of AM-355 (UNS S35500)
The composition of AM-355 stainless steel is tightly controlled to ensure consistent heat treatment response and mechanical performance. Every alloying element serves a specific metallurgical function:
| Element | Range (wt.%) | Metallurgical Function |
|---|---|---|
| Chromium (Cr) | 15.0 – 16.0 | Forms passive Cr₂O₃ layer; oxidation resistance to ~1,000°F (538°C) |
| Nickel (Ni) | 4.0 – 5.0 | Stabilises austenite at room temp; improves toughness and corrosion resistance |
| Molybdenum (Mo) | 2.5 – 3.25 | Pitting and crevice corrosion resistance; solid-solution strengthening |
| Carbon (C) | 0.10 – 0.15 | Primary hardening agent via carbide precipitation; raises martensite-start (Ms) temp |
| Nitrogen (N) | 0.07 – 0.13 | Interstitial strengthening; lowers Ms to promote transformation during sub-zero cooling |
| Manganese (Mn) | 0.50 – 1.25 | Austenite stabiliser; deoxidiser during melting |
| Silicon (Si) | ≤ 0.50 | Deoxidiser; minor oxidation resistance improvement |
| Phosphorus (P) | ≤ 0.040 | Controlled residual; maintained low for toughness |
| Sulfur (S) | ≤ 0.030 | Controlled residual; maintained low for ductility |
| Iron (Fe) | Balance | Matrix element |
The combined carbon (0.10–0.15%) and nitrogen (0.07–0.13%) content in AM-355 is intentionally elevated compared to other PH grades such as 17-4PH (UNS S17400) or 15-5PH (UNS S15500), which use copper as the primary aging agent and carry very low carbon. In AM-355, the C+N content drives precipitation hardening: during aging, fine chromium carbide and nitride particles precipitate within the martensitic lath structure, pinning dislocations and delivering the high-strength properties of AMS 5743 and AMS 5744.
03 — Mechanical Properties of AM-355 Alloy
The mechanical performance of AM-355 (UNS S35500) is strongly condition-dependent. The table covers properties from annealed supply stock through both AMS hardened conditions:
| Property | Annealed (Supply) | AMS 5743 (Eq.+OT) | AMS 5744 (SZ+DA) |
|---|---|---|---|
| Tensile Strength (min) | ~110 ksi (758 MPa) | 170 ksi (1,172 MPa) | 220 ksi (1,517 MPa) |
| 0.2% Yield Strength (min) | ~65 ksi (448 MPa) | 140 ksi (965 MPa) | 195 ksi (1,344 MPa) |
| Elongation (min) | ~25% | 12% | 8% |
| Reduction in Area (min) | ~55% | 25% | 20% |
| Hardness (typical) | 24–28 HRC | 32–40 HRC | 44–48 HRC |
| Elastic Modulus | ~28 × 10⁶ psi (193 GPa) — all conditions | ||
| Density | 0.279 lb/in³ (7.72 g/cm³) | ||
| Max. Service Temperature | 1,000°F (538°C) in hardened conditions | ||
Relative Performance vs. Common PH Stainless Grades (100 = best in class)
04 — AM-355 Heat Treatment Routes (AMS 5743 & AMS 5744)
AM-355 heat treatment is more complex than most PH grades because the alloy requires either a sub-zero cooling step or an equalization step to drive the martensitic transformation before aging. Two principal routes are defined by AMS:
Route A — Equalize and Overtemper (AMS 5743)
Produces a minimum tensile strength of 170 ksi. Standard condition for most structural forging applications.
Route B — Sub-Zero Quench and Double Age (AMS 5744)
Produces a minimum tensile strength of 220 ksi — the highest-strength condition of AM-355.
AM-355 may also be hardened by cold reduction (10–40% area reduction) without heat treatment. Cold work alone produces tensile strengths approaching 200 ksi, sustained at temperatures up to 1,000°F. This route is used for strip, wire, and thin-section bar where cryogenic treatment cycles are impractical. Properties are directional and should be considered in component design.
05 — AM-355 Forging Characteristics & Process Parameters
The AM-355 forging process requires precise temperature management to achieve refined grain structure and avoid delta ferrite formation that compromises toughness and heat treatment response:
- Maximum starting temperature: 2,100°F (1,149°C). Forging above this limit increases delta ferrite content — a brittle BCC phase that reduces transverse ductility and creates non-uniform heat treatment response. Bring the workpiece to temperature slowly for large cross-sections.
- Finishing temperature range: 1,700–1,800°F (927–982°C). Low finishing temperature prevents austenite grain coarsening, which degrades fatigue life, and promotes homogeneous carbide distribution important for consistent heat treatment response.
- Immediate air cool after forging. Cool in still air to room temperature. Neither water quench nor forced-air quench is required; rapid quenching may introduce damaging residual stresses in thick-section forgings at corners and radii.
- Post-forge thermal treatment. Equalization and overtemper (or full AMS 5744 sub-zero cycle) must follow after the forging returns to room temperature and any required rough machining is complete. Heat treating before final machining reduces distortion from hardening stresses.
- Open-die vs. closed-die selection. Our facility uses open-die (press / hammer) forging for large-diameter discs, flanges, rings, and shaft billets. Closed-die (impression die) forging is used for near-net-shape compressor blades, brackets, and housings where dimensional tolerances and material utilisation are critical. Both processes are available for AM-355 forgings (UNS S35500) from Jiangsu Liangyi.
06 — Corrosion Resistance of AM-355 (UNS S35500)
AM-355 corrosion resistance substantially outperforms quench-hardenable martensitic grades (Types 410, 420, 440C) and approaches the performance of austenitic 304/316 — a significant achievement at 170–220 ksi strength levels:
- Atmospheric corrosion: Excellent resistance in industrial, urban, and marine atmospheric environments. The 15–16% Cr content maintains a stable, self-repairing passive oxide film in most above-water exposures.
- Pitting and crevice corrosion: The 2.5–3.25% molybdenum provides meaningful resistance to chloride-induced pitting, making AM-355 viable in splash-zone marine applications — conditions where Types 410 and 440C would be unacceptable.
- Stress corrosion cracking (SCC): SCC resistance is heat-treatment-dependent. The sub-zero cooled condition (AMS 5744) shows better SCC resistance. For optimum SCC resistance, the recommended treatment sequence is: 1,875°F water quench → −100°F sub-zero → 1,700°F air cool → −100°F sub-zero → 1,000°F temper.
- Intergranular corrosion (IGC): Material in the equalized-and-overtempered condition is susceptible to IGC due to chromium carbide precipitation at grain boundaries. Passivation per ASTM A967 mitigates but does not eliminate this risk in severe environments.
- Chemical environments: Suitable for mild organic acids, dilute alkalis, and many organic process streams. Not suitable for reducing mineral acids (concentrated HCl, HF, or concentrated H₂SO₄).
For maximum corrosion performance, all mill scale, heat tint, lubricant, and embedded iron contamination must be removed after any thermal operation. Passivation per ASTM A967 (citric acid or nitric acid) is recommended for marine, food-processing, or chemical service components. Avoid prolonged contact with low-alloy carbon steel tooling in chloride environments to prevent galvanic contamination of the passive film.
07 — Machinability, Forming & Welding AM-355
In the annealed condition, AM-355 machinability closely resembles austenitic Type 304: high work-hardening rate, tendency to be gummy on cutting tools, and requirement for consistent positive cuts to avoid rubbing and glazing:
- Use rigid toolholding and workpiece clamping — deflection accelerates tool wear dramatically in high-work-hardening grades.
- Employ sharp, positive-rake carbide inserts (C5/C6 or coated carbide for interrupted cuts). High-speed steel is viable for low-speed operations but gives much lower productivity.
- Use adequate flood coolant (water-soluble oil emulsion at 8–10%, or sulphurised cutting oil for threading). Never machine dry.
- Maintain consistent chip-forming cuts at all times. Dwelling or spring cuts cause the surface to work-harden rapidly, damaging tool and workpiece.
- When cold forming in the annealed condition, pre-heating to 600–700°F (316–371°C) reduces work-hardening rate and springback on bends.
- In the hardened condition, AM-355 retains sufficient ductility for limited straightening to correct heat treatment distortion, but is not suitable for significant cold forming at 170–220 ksi.
Welding AM-355
- Acceptable processes: Shielded metal arc (SMAW), gas tungsten arc (GTAW/TIG), and resistance welding are all viable.
- Avoid: Oxy-acetylene welding — carbon and oxygen contamination from the flame unpredictably alters heat treatment response and reduces corrosion resistance.
- Filler metal: Use matching-chemistry filler (similar Cr-Ni-Mo-C-N balance) to ensure weld metal responds to subsequent heat treatment similarly to base metal.
- Post-weld heat treatment: Re-aging per the applicable AMS condition is strongly recommended when full mechanical properties are required in the weld zone. Welding in the hardened condition without PWHT leaves the heat-affected zone in a softened state that may not meet strength requirements.
08 — AM-355 Standards & Specifications
The following active specifications govern procurement, testing, and acceptance of AM-355 (UNS S35500):
| Specification | Product Form | Governing Body | Condition / Key Requirements |
|---|---|---|---|
| AMS 5743 | Bars, Forgings | SAE International | Equalized & Overtempered; 170 ksi min UTS; 140 ksi min YS; 12% min elong. |
| AMS 5744 | Bars, Forgings | SAE International | Sub-Zero Cooled & Double-Aged; 220 ksi min UTS; 195 ksi min YS; 8% min elong. |
| AMS 5547 | Sheet, Strip | SAE International | Annealed or equalized; covers thin product forms for forming applications |
| AMS 5549 | Plate | SAE International | Annealed or equalized; structural plate form for machined components |
| AMS 5780 | Wire | SAE International | Cold-drawn wire for high-strength springs and precision fasteners |
| MIL-S-8840 | All forms | U.S. DoD | Military procurement specification; superseded in most current programs by AMS |
| ASTM A579 | Forgings | ASTM International | Grade 65 covers UNS S35500 in the hardened condition |
| ASTM A705 | Bars | ASTM International | Standard for age-hardening stainless and heat-resisting steel bars |
| ASTM A484 | General | ASTM International | General requirements for stainless steel bars, billets, and forgings |
For aerospace and defense contracts, material certifications must confirm the heat number, chemistry report, and mechanical test results against the applicable AMS specification. First Article Inspection (FAI) and material review documentation may additionally be required per your quality plan. Jiangsu Liangyi provides full material traceability, mill test reports, and co-ordinated third-party inspection for all AM-355 forgings on request.
09 — AM-355 Industry Applications
The combination of AM-355 high strength, 1,000°F temperature resistance, and superior corrosion resistance drives its use across multiple demanding sectors:
10 — AM-355 vs. Alternative Stainless Steel Grades
Selecting between AM-355 vs 17-4PH or other PH grades requires understanding both strength requirements and operating environment. The comparison below covers the most commonly evaluated alternatives:
| Alloy | UNS | Max UTS (typ.) | Corrosion Resist. | Max Svc. Temp. | Key Differentiator vs. AM-355 |
|---|---|---|---|---|---|
| AM-355 ★ | S35500 | 225+ ksi | Good–Very Good | 1,000°F (538°C) | Highest svc. temp. of PH stainless; best strength 500–1,000°F |
| 17-4PH | S17400 | 190 ksi | Good | 600°F (316°C) | Simpler one-step aging; wider availability; lower max temp. |
| 15-5PH | S15500 | 190 ksi | Good | 600°F (316°C) | Better transverse toughness than 17-4PH; same temp. ceiling |
| PH 13-8Mo | S13800 | 220 ksi | Very Good | 700°F (371°C) | Best SCC resistance among PH grades; H₂S environments |
| UNS S45500 (e.g., Custom 455®*) | S45500 | 235 ksi | Good | 700°F (371°C) | Lowest carbon; least IGC susceptibility; excellent notch toughness |
| A-286 | S66286 | 150 ksi | Very Good | 1,300°F (704°C) | Fe-Ni-Cr superalloy; required above 1,000°F; lower strength |
| Inconel 718 | N07718 | 180 ksi | Excellent | 1,300°F (704°C) | Ni superalloy; 3–5× cost premium; required above 1,000°F |
* Custom 455® is a registered trademark of Carpenter Technology Corporation. Use here is for informational comparison purposes only.
Choose AM-355 when your design demands tensile strengths above 200 ksi sustained at temperatures approaching 1,000°F combined with corrosion resistance exceeding the 400-series grades. AM-355 is the standard aerospace choice for gas turbine compressor stages where 17-4PH would over-temper and A-286 would be over-engineered and cost-prohibitive.
Choose an alternative when: Service temperature exceeds 1,000°F → A-286 or IN 718; SCC in H₂S or chloride environments is the dominant driver → PH 13-8Mo; or simpler supply chain and one-step aging matter more than maximum service temperature → 17-4PH or 15-5PH.
To discuss your specific AM-355 component requirements with our engineering team, see our AM-355 alloy forgings product page for available forms, tolerances, and lead times.
11 — Frequently Asked Questions: AM-355 (UNS S35500)
12 — Related Topics & Further Reading
Explore more engineering guides on stainless steel forgings from our engineering blog:
Technical data in this guide is drawn from SAE AMS 5743, AMS 5744, ASTM A579/A705 specifications, and peer-reviewed materials engineering literature. Always verify specification requirements against the current revision of the applicable standard before use in design.