What Is AM 355 Stainless Steel?
A Complete Metallurgical Guide to AMS 5743 / AMS 5744

1 What Is AM 355 Stainless Steel?

AM 355 (UNS S35500) is a precipitation-hardenable, semi-austenitic stainless steel alloy developed in the mid-20th century as a high-performance structural material for aerospace and gas turbine applications. The designation "AM" refers to its origin at Allegheny Metals (now ATI Specialty Alloys & Components), and "355" is the internal alloy number -- not a chemical shorthand.

What distinguishes AM 355 from all other stainless steel families is its deliberately engineered dual-phase microstructure: a controlled balance of martensite and retained austenite, stabilized through precisely managed nitrogen, chromium, nickel, and molybdenum content. This is fundamentally different from 300-series austenitic grades such as 304 or 316 (which are single-phase fully austenitic) and different from conventional 400-series martensitic grades such as 410, 416, or 420 (which are single-phase martensitic).

The result is a material that can be formed in its soft, austenitic condition after solution annealing, and then hardened -- via sub-zero treatment followed by aging, or via conditioning and direct aging -- to reach tensile strengths that would be unachievable in a conventional martensitic stainless steel without sacrificing corrosion resistance.

2 AMS 5743 vs. AMS 5744: Understanding the Two Specifications

AM 355 stainless steel is governed by two separate SAE Aerospace Material Specifications (AMS). While both cover the same alloy chemistry, they address different product forms with distinct test requirements, dimensional tolerances, and NDT coverage rules. Selecting the wrong specification on an engineering drawing is a common error with real qualification consequences.

AMS 5743 -- Bars, Billets, and General Forgings

AMS 5743 covers AM 355 in the form of bars, billets, and open-die forgings. This specification is cited when ordering round bars, flat bars, rectangular bars, step shafts, gear shaft blanks, disc blanks, and general open-die forging shapes. Key requirements include:

• Full chemical composition limits for all 9 specified elements (see Section 3)
• Heat treatment condition: annealed (Condition A), sub-zero treated, or fully aged (SCT)
• Tensile test requirements by product diameter/thickness and test direction (longitudinal and transverse)
• Brinell or Rockwell hardness acceptance limits
• Ultrasonic testing (UT) requirements for bars above specified diameters
• EN 10204 3.1 or 3.2 material test certificate (MTC) available upon request, with full heat traceability

AMS 5744 -- Seamless Rolled Rings and Ring Forgings

AMS 5744 covers AM 355 in seamless rolled ring and ring forging product forms. The chemical composition requirements are identical to AMS 5743, but mechanical test coupon orientation, dimensional tolerance tables, and UT coverage differ to reflect the circumferential grain flow inherent in ring-rolled products:

• Test coupon orientation: circumferential (tangential) direction, not longitudinal
• Ring dimensional tolerances: OD, ID, face width, wall thickness, and out-of-roundness limits
• UT scanning coverage adapted for annular geometry, with specific flange root requirements
• Seamless (no-weld) construction is explicitly required -- rolled ring construction preferred

3 Chemical Composition of AM 355 Stainless Steel

The following composition limits apply to both AMS 5743 and AMS 5744. All values are weight percent. The table reflects the requirements of the current SAE specification revision.

Source: SAE AMS 5743 / AMS 5744 specification tables. All values in weight percent. Iron (Fe) is the balance.

Why the Carbon-Nitrogen Balance Is the Key to AM 355

The simultaneous specification of carbon (0.10-0.15%) and nitrogen (0.07-0.13%) is the chemical engineering decision that gives AM 355 its unique capabilities. Both elements are powerful austenite stabilizers and interstitial strengtheners, but they interact with the chromium, nickel, and molybdenum matrix across different temperature ranges.

Their precise combined level -- together with the Cr and Ni contents -- sets the martensite start temperature (Ms) and martensite finish temperature (Mf) of the alloy. The AMS composition limits are specifically designed to keep AM 355 in a metastable austenitic condition at room temperature after solution annealing. The alloy is then capable of transforming to martensite either by cooling below Mf (sub-zero treatment) or by direct aging under appropriate conditions. If C+N is too high, the alloy stays fully austenitic and resists hardening. If too low, transformation is incomplete and excess retained austenite degrades fatigue strength.

4 Dual-Phase Microstructure of AM 355

Unlike single-phase stainless steels, AM 355 is specifically engineered to exist in a mixed microstructural state. Understanding the three phases present and their functions is essential to specifying, processing, and using the alloy correctly.

Martensite -- The Strength-Bearing Phase (~70-80%)

The martensitic phase in AM 355 is a body-centered tetragonal (BCT) crystal structure formed when austenite is cooled below the martensite finish temperature (Mf). In AM 355, it takes the form of lath martensite -- a fine, interlocking needle-like structure that is substantially tougher than the plate martensite found in high-carbon steels. The lath martensite contains dissolved nitrogen and carbon in its crystal lattice, which generates the high-strength response on aging through precipitation of fine alloy carbides and nitrides.

Retained Austenite -- The Toughness Buffer (~15-25%)

Some austenite remains thermodynamically stable even after the full heat treatment cycle, persisting as thin films between martensite laths. This retained austenite acts as a crack arrest mechanism under dynamic loading: when a fatigue crack tip approaches the austenite film, the stress field can trigger a localized martensite transformation that absorbs crack energy -- a phenomenon known as transformation-induced plasticity (TRIP). This is why AM 355 exhibits superior impact toughness compared to conventional martensitic grades like 410 or 422 at similar strength levels. However, retained austenite above approximately 25% degrades fatigue strength and causes dimensional instability, which is why the sub-zero treatment step is used to reduce it.

Delta Ferrite -- The Defect to Control (<5%)

Delta ferrite (delta--ferrite) forms at the highest temperatures during solidification and, if not adequately broken down during forging, persists as elongated stringers aligned with the deformation direction. These stringers are mechanically weak in the transverse direction and act as fatigue crack initiation sites under cyclic loading. Premium AM 355 forgings control delta ferrite to below 5% through: (1) careful chemistry control of the Cr/Ni equivalency ratio; (2) minimum forging reduction ratios of 4:1 or higher; and (3) ESR or VAR remelting of the ingot to homogenize the as-cast dendritic segregation.

5 Heat Treatment Paths for AM 355

AM 355 can be heat treated via two primary paths, designated in the AMS specifications. The chosen path determines the final microstructure, hardness, strength level, ductility, and dimensional stability of the finished component.

Path A -- Condition SCT (Sub-Zero Cold Treat + Age): The Standard High-Strength Path

Path B -- Condition A (Solution Anneal + Direct Age)

In this alternative path, the sub-zero treatment step is omitted. The solution-annealed (metastable austenitic) material is directly aged. During aging, the austenite begins to transform to martensite at the aging temperature, producing a partially martensitic structure. This path yields lower strength than SCT + aging (~900-1,100 MPa tensile depending on aging temperature) but better toughness and lower residual stress. Common in secondary structures and non-critical industrial applications.

6 Mechanical Properties of AM 355 Forgings

Tensile Properties by Condition and Orientation

Minimum guaranteed values per AMS 5743 for bar product forms. Production forgings from ESR ingots routinely exceed these values by 3-8%.

Hardness by Condition

Typical production ranges for well-processed AM 355 forgings. Hardness uniformity across the cross-section (max ±2 HRC) is a key indicator of heat treatment effectiveness.

Other Key Physical Properties

Typical values from literature. Verify with MTC data for critical design calculations.

7 Corrosion Resistance of AM 355

With 15-16% chromium and 2.5-3.25% molybdenum, AM 355 offers corrosion resistance substantially better than conventional 400-series martensitic grades (410, 420, 422) and broadly comparable to 316L austenitic stainless in many environments. The molybdenum content is decisive: it raises the pitting resistance equivalence number (PREN) of AM 355 to approximately 20-23, compared to 15-17 for 17-4 PH and 12-14 for Type 410. In practical terms, this represents meaningful protection against chloride-induced pitting in coastal, offshore, and chemical process environments.

*AM 355 in SCT + 850 °F condition exceeds NACE MR0175 / ISO 15156 hardness limits for sour service. Use 1,000 °F aging condition and confirm with project-specific testing. Performance ratings are qualitative; actual corrosion rates depend on temperature, concentration, and flow velocity.

8 Industry Applications of AM 355 Forgings

AM 355 stainless steel forgings serve critical applications wherever the combination of high strength, corrosion resistance, and moderate temperature capability (to ~400 °C) is required simultaneously. The following industries represent the primary markets for AM 355 forged components. Jiangsu Liangyi supplies AM 355 open-die forgings and seamless rolled rings in weights from 30 kg to 30,000 kg for all of these sectors.

9 Machinability and Welding of AM 355

Machinability

AM 355 is most efficiently machined in the annealed condition (Condition A), where hardness is approximately 28-32 HRC. In this state, it machines similarly to 300-series austenitic stainless steels -- moderate work-hardening tendency, requiring rigid setups, carbide tooling, aggressive chip-breaker geometries, and high-pressure through-coolant where available.

• Rough turning (annealed): Cutting speed 25-40 m/min, feed 0.2-0.4 mm/rev, depth of cut 2-5 mm, carbide grade P25-P35
• Finish turning (annealed): 30-55 m/min, feed 0.05-0.15 mm/rev, depth 0.2-0.8 mm, carbide P10-P20
• Drilling: Cobalt-HSS or solid carbide; reduce speed 30-40% vs. carbon steel; high torque through-coolant preferred
• Grinding (aged condition): Aluminum oxide or CBN wheels; generous coolant flow; light passes (max 0.05 mm radial per pass) to prevent surface re-tempering zones that reduce fatigue life

Machining in the hardened SCT + aged condition (44-48 HRC) is significantly more challenging and should be avoided except for light finishing passes and grinding operations. Sequence components to maximize machining in the annealed state before aging.

Weldability

AM 355 is weldable, but the process complexity and the mandatory PWHT cycle make integral forging the preferred design choice for most aerospace and turbine applications. When welding is unavoidable:

• Pre-heat temperature: 150-200 °C minimum; maintain interpass temperature below 250 °C
• Filler metal: AM 355 matching filler wire, or ER309L for dissimilar metal joints
• Process: GTAW (TIG) preferred for root passes; GMAW acceptable for fill and cap
• PWHT required: Full solution anneal + complete re-aging cycle required to restore base metal properties and corrosion resistance in the HAZ
• Never weld aged AM 355 without subsequent full PWHT -- the HAZ will be sensitized (chromium carbide precipitation at grain boundaries) and susceptible to intergranular corrosion attack

10 Grade Comparisons: AM 355 vs. Other High-Strength Stainless Steels

Choose AM 355 (AMS 5743 / AMS 5744) when: you need tensile strength above 1,300 MPa; the environment contains chlorides and pitting resistance is critical; service temperature stays below ~400 °C; and the budget allows for the more complex SCT heat treatment cycle.

Choose 17-4 PH or 15-5 PH when: the component geometry is complex and single-step aging (no sub-zero treatment) is preferred; maximum strength is 1,100-1,310 MPa; and chloride exposure is moderate.

Choose Type 422 (AMS 5655) when: the application requires service temperatures above 400 °C. Type 422 retains useful creep resistance and fatigue strength up to 649 °C due to its tungsten, vanadium, and molybdenum additions -- a regime where AM 355 would over-temper and lose strength.

11 Forging Process Notes for AM 355

Forging Temperature Window and Process Controls

AM 355 has a narrower forging temperature window than carbon steels or austenitic stainless grades, demanding tighter furnace control and more frequent temperature monitoring during the forging sequence. The recommended forging range is 1,050-1,180 °C (1,922-2,156 °F).

• Soak uniformity: ±15 °C maximum across the billet cross-section. Uneven soak produces variable deformation resistance and inconsistent final microstructure across large forgings.
• Minimum finish temperature: 980 °C. Working AM 355 below this temperature, in the two-phase (austenite + ferrite) region, causes adiabatic shear banding and creates persistent delta ferrite stringers that cannot be eliminated by subsequent heat treatment.
• Reduction ratio: Minimum 4:1 cross-sectional area reduction from ingot to final billet. For premium aerospace forgings where low anisotropy and fine grain size are specified, 6:1 minimum is recommended.
• Post-forging cooling: Air cool or forced-air cool only. Water quenching directly from the forging temperature causes severe thermal gradient cracking in sections above ~80 mm. The air-cooled structure is predominantly austenitic and relatively soft -- ideal entry condition for the solution annealing step.

Why Jiangsu Liangyi Specifies ESR Ingots for Critical AM 355 Forgings

Electroslag remelting (ESR) of the primary ingot delivers four key benefits for AM 355 production: (1) reduces non-metallic inclusion content (oxides, sulfides) by up to 80% vs. conventional casting; (2) eliminates macro-segregation of chromium and molybdenum across the ingot radius; (3) removes porosity and shrinkage defects present in bottom-poured ingots; and (4) produces a columnar-to-equiaxed grain structure more amenable to uniform forging deformation. The result is higher and more consistent transverse mechanical properties -- critical for ring forgings tested in the circumferential direction per AMS 5744. For all critical aerospace and turbine orders, our AM 355 forgings are produced from ESR-certified ingots with complete heat-to-part traceability.

12 Quality Assurance and Inspection Requirements

Every AM 355 production forging from Jiangsu Liangyi undergoes the following mandatory inspection operations, all documented on the EN 10204 3.1 or 3.2 MTC shipped with the order.

• Chemical Analysis (per melt heat): Optical emission spectrometry (OES) on each melting heat. All 9 AMS-specified elements verified against composition limits. Nitrogen content measured by inert gas fusion (IGF) method -- essential because OES has limited nitrogen sensitivity at low concentrations.
• Tensile Testing (per AMS 5743 / 5744): Test coupons machined from production forgings or representative bar sections from the same heat and heat treatment lot. L and T direction tests performed where required by the applicable specification and product size table.
• Hardness Testing: Brinell (HBW) or Rockwell C (HRC) measured at 4-6 locations across the forging cross-section to verify heat treatment uniformity and confirm compliance with specification acceptance limits.
• Ultrasonic Testing (UT): Immersion or contact UT performed to customer-specified standard (e.g. AMS 2630, EN 10228, or equivalent per purchase order). Rejection level is agreed with the customer at order stage -- typically 2.0 mm flat-bottom hole (FBH) equivalent for aerospace-grade forgings. 100% volume coverage for rings and discs above 150 mm bore diameter.
• Magnetic Particle Inspection (MPI): Applied to finish-machined surfaces to detect surface-breaking and near-surface linear indications. Performed to customer-specified standard (e.g. ASTM E1444, EN 10228-1, or equivalent). Acceptance criteria confirmed per customer drawing or purchase order.
• Dimensional Inspection: All critical dimensions verified against customer drawing at incoming, in-process, and final stages. CMM measurement available for near-net-shape forgings and precision-machined components.
• Metallographic Examination (available on request or per drawing): Grain size (ASTM E112 method), delta ferrite content (point-count method per ASTM E562), non-metallic inclusion rating (ASTM E45), and microstructure assessment for phase identification.

13 Frequently Asked Questions About AM 355 Stainless Steel