13-8 Mo PH stainless steel — formally designated 1.4534 (X3CrNiMoAl13-8-2 / UNS S13800) — is a precipitation hardening martensitic stainless steel that achieves yield strengths above 1,400 MPa while maintaining superior pitting resistance and fracture toughness compared to other PH grades. It is manufactured by solution annealing at 927°C followed by age hardening at 510–621°C. This material is widely specified in energy, aerospace, and subsea industries, and is supplied by Jiangsu Liangyi Co., Limited — an ISO 9001:2015 certified forging manufacturer located in Jiangyin, Jiangsu, China.
What Exactly Is 13-8 Mo PH Steel?
13-8 Mo PH stainless steel (EN: 1.4534 / UNS: S13800 / EN short name: X3CrNiMoAl13-8-2) is a martensitic precipitation hardening stainless steel whose properties are achieved through a two-stage thermal treatment: solution annealing at 927°C to form a soft, uniform martensitic matrix suitable for machining, followed by age hardening at 510–621°C to precipitate fine NiAl and Ni₃Al intermetallic particles that dramatically increase strength and hardness with minimal dimensional distortion. The result is a steel that can be machined at 28–34 HRC and aged to 44–48 HRC (H950 condition).
The "13-8 Mo" designation describes the alloy's strategy: approximately 13% chromium for passivation, 8% nickel for martensite stability and toughness, 2–2.5% molybdenum for pitting resistance in chloride environments, and 0.8–1.35% aluminum as the principal precipitation hardening agent. Carbon is kept at ≤ 0.05% to preserve weldability.
The alloy's strictest discipline is in what it limits: EN 10088-3 specifies phosphorus ≤ 0.01% and sulfur ≤ 0.008% — roughly half the levels permitted in standard corrosion-resistant grades. These tight ceilings typically require a premium melting practice to achieve the required purity, which reflects in the alloy's higher cost compared to 17-4PH but delivers significantly higher cleanliness and fatigue resistance.
European (EN 10088-3): 1.4534 / X3CrNiMoAl13-8-2 | UNS (ASTM A564/A693): S13800 | AMS: 5629 and 5864 | MIL-SPEC: MIL-S-81591 | JIS G4303: SUS XM-13 (similar) | Trade names: 13-8 Mo PH, Custom 13-8 Mo.
Note: minor compositional differences may exist between standards. Always verify the specific standard applicable to your project.
Chemical Composition of 1.4534 (EN 10088-3)
Every element in the 1.4534 composition has a specific structural role. Understanding those roles helps engineers predict performance under non-standard conditions and explains why minor P and S deviations have disproportionate consequences on fatigue life and toughness at high strength levels.
At the yield strengths achieved in H950 condition (1,413 MPa), trace phosphorus and sulfur segregate to grain boundaries and can significantly reduce Charpy impact energy. Meeting these impurity limits requires careful melt practice control — typically vacuum induction melting (VIM) followed by a secondary remelting step such as vacuum arc remelting (VAR) or electroslag remelting (ESR). This additional processing is the reason 1.4534 costs more than 17-4PH and why buyers should always verify that actual P and S values are reported on the mill test certificate, not merely stated as "per standard."
Heat Treatment Guide: H950 Through H1150
Unlike conventional steels hardened by water quenching, 1.4534's final properties arise entirely from controlled precipitation of intermetallic phases. The two mandatory stages are solution annealing and age hardening.
± 14°C
(1,700°F)
Heat to 927°C ± 14°C. Hold for minimum 30 minutes plus 1 h per 25 mm of maximum cross-section thickness until the entire part is at temperature. Cool rapidly to below 32°C (90°F) — air, fan-assisted, or water quench for heavy sections. Austenite transforms to martensite during cooling. Result: 28–34 HRC, ideal for all major machining operations.
(H950 to
H1150)
Fine β-NiAl and Ni₃Al intermetallic particles precipitate within the martensitic matrix, blocking dislocation movement and dramatically increasing strength. Aging temperature is the single variable that sets the final strength / toughness balance. Air cool after the full soak.
Aging Condition Selection Guide
| Condition | Aging Temp. | Hold | Min. Yield Strength | Min. Tensile | Hardness | Typical Application |
|---|---|---|---|---|---|---|
| H950 MAX STRENGTH | 510°C / 950°F | 4 h + AC | 1,413 MPa (205 ksi) | 1,517 MPa | 44–48 HRC | Maximum static load; tooling; dies |
| H1000 | 538°C / 1,000°F | 4 h + AC | 1,310 MPa (190 ksi) | 1,413 MPa | 42–46 HRC | High-strength structural parts; turbine components |
| H1025 | 552°C / 1,025°F | 4 h + AC | ~1,241 MPa | ~1,310 MPa | 40–45 HRC | Valve bodies; pump shafts; balanced condition |
| H1050 | 566°C / 1,050°F | 4 h + AC | 1,138 MPa (165 ksi) | 1,207 MPa | 39–43 HRC | Optimum toughness/strength ratio; dynamic loading |
| H1100 | 593°C / 1,100°F | 4 h + AC | ~965 MPa | ~1,034 MPa | 35–40 HRC | Maximum impact resistance |
| H1150 | 621°C / 1,150°F | 4 h + AC | ~793 MPa | ~862 MPa | 31–36 HRC | NACE MR0175 sour service; post-weld PWHT |
Each step upward in aging temperature (H950 → H1000 → H1050) trades approximately 100–170 MPa of yield strength for a measurable gain in Charpy impact energy and fracture toughness (KIc). For parts subject to shock loading, dynamic fatigue, or low-temperature service, H1050 or H1000 typically provides greater reliability than H950.
Mechanical & Physical Properties
| Property | Value | Unit | Notes |
|---|---|---|---|
| Density | 7.78 | g/cm³ (0.281 lb/in³) | Room temperature |
| Elastic Modulus | 197 | GPa (28.6 × 10⁶ psi) | Room temperature |
| Shear Modulus | 76 | GPa (11.0 × 10⁶ psi) | Room temperature |
| Poisson's Ratio | 0.30 | — | |
| Thermal Conductivity | 14.0 | W/(m·K) | At 21°C (70°F) |
| Thermal Conductivity | 17.8 | W/(m·K) | At 538°C (1,000°F) |
| Mean CTE (0–260°C) | 10.6 × 10⁻⁶ | /°C (5.9 × 10⁻⁶ /°F) | Notably lower than 316L (~16 × 10⁻⁶/°C); closer to carbon steel and titanium |
| Mean CTE (0–538°C) | 11.2 × 10⁻⁶ | /°C (6.2 × 10⁻⁶ /°F) | |
| Specific Heat Capacity | 460 | J/(kg·K) | Room temperature |
| Electrical Resistivity | 0.80 | µΩ·m | At 21°C |
| Magnetic Permeability | > 1 (ferromagnetic) | — | Increases further after aging |
| Melting Range | 1,400–1,440 | °C | Approximate solidus–liquidus range |
Thermal Expansion — A Key Design Advantage
The coefficient of thermal expansion (CTE) of 1.4534 at moderate temperatures — 10.6 × 10⁻⁶/°C — is significantly lower than austenitic grades such as 316L (~16 × 10⁻⁶/°C) and far closer to carbon steel and titanium alloys. In multi-material assemblies such as turbine discs or high-pressure pump casings, this closer CTE match substantially reduces differential thermal stress under cyclic heating and cooling — a primary driver of fatigue failure at component interfaces.
Corrosion Resistance
In properly heat-treated conditions, 1.4534 (UNS S13800) exhibits good resistance to:
- Sodium chloride (NaCl) — solutions up to 10% concentration
- Nitric acid — up to 65% concentration at room temperature
- Sodium hydroxide — up to 50% concentration at room temperature
- Sour service (H₂S + CO₂) — in the H1150 condition per NACE MR0175 / ISO 15156-3 at maximum 36 HRC
- Chloride-rich environments — superior to 17-4PH due to the 2–2.5% Mo addition (PRE ~19.5 vs ~16.3)
Pitting Resistance: Why Molybdenum Is the Decisive Factor
The Pitting Resistance Equivalent (PRE) is calculated as PRE = %Cr + 3.3 × %Mo. The 2.0–2.5% molybdenum addition in 1.4534 provides a clear advantage over copper-hardened PH grades, which contain no molybdenum:
1.4534's PRE of ~19.5 represents approximately a 20% improvement over 17-4PH and nearly a 32% improvement over 15-5PH. In applications where chloride, H₂S, and CO₂ coexist — such as subsea valves or downhole tools — this margin significantly reduces the risk of pitting and stress corrosion cracking compared to copper-hardened alternatives.
Grade Comparison: 1.4534 vs 17-4PH, 15-5PH, and 17-7PH
Engineers specifying precipitation hardening stainless steel frequently evaluate these four grades. The comparison below focuses on decision-relevant engineering differences.
1.4534 (S13800) ★
13-8 Mo PH / X3CrNiMoAl13-8-2- Highest yield strength (>1,400 MPa H950)
- Best pitting resistance — PRE ≈ 19.5 (Mo addition)
- Best fracture toughness at equivalent strength
- Best weldability (lowest delta-ferrite)
- Strictest P/S limits — highest cleanliness
- Higher cost due to premium melting requirements
- Specify for: subsea valves, downhole shafts, sour service, demanding structural applications
17-4PH (S17400)
1.4542 / X5CrNiCuNb16-4- Most widely available PH grade globally
- Lowest cost in this comparison group
- Cu-hardened (3–5% Cu) — no Mo addition
- PRE ≈ 16.3 — limited chloride resistance
- Higher delta-ferrite → reduced weld toughness
- Specify for: general structural parts, non-sour oil & gas
15-5PH (S15500)
1.4545- Lower delta-ferrite than 17-4PH → better weld toughness
- Cu-hardened — no Mo addition
- PRE ≈ 14.8 — lowest of the four
- Similar strength to 17-4PH
- Specify for: applications where 17-4PH delta-ferrite is a concern but high chloride resistance is not required
17-7PH (S17700)
1.4568- Semi-austenitic PH — more complex processing
- Primarily for flat-rolled strip and springs
- PRE ≈ 17.0
- Rarely specified for heavy forgings
- Specify for: springs, diaphragms, precision strip applications
1. Chloride / sour-service environments: PRE ≈ 19.5 vs 16.3 is a meaningful advantage for subsea valves, downhole tools, and marine shafts where chloride-induced pitting and stress corrosion cracking are primary failure modes.
2. High fracture toughness requirements: Higher Ni content (7.5–8.5% vs 3–5%) and lower delta-ferrite provide better Charpy impact values at equivalent strength levels.
3. High-purity applications: The strict P ≤ 0.01% / S ≤ 0.008% limits in EN 10088-3 are required by certain customer specifications that 17-4PH cannot satisfy without special melting arrangements.
4. Weldable assemblies at high strength: Lower delta-ferrite content reduces hot-cracking susceptibility in thick-section weld joints.
Industrial Applications of 13-8 Mo PH Stainless Steel
This material is widely specified for demanding applications where conventional stainless steels cannot simultaneously satisfy strength, toughness, and corrosion resistance requirements. Common industries and application areas include:
Aerospace & Defense
Gas compressor and turbine blades, turbine disks, impellers, aircraft structural components requiring high strength-to-weight ratio, landing gear parts, and hydraulic actuator components. The low CTE of 1.4534 (10.6 × 10⁻⁶/°C) is compatible with titanium alloys and carbon steel in multi-material assemblies, reducing thermally driven fatigue at component interfaces during cyclic operation.
Oil & Gas and Subsea
High-pressure valve bodies, balls, bonnets, stems, and seat rings; subsea hydraulic cylinders and accumulators; high-pressure pump components for offshore platforms; heat exchanger tube sheets and flanges; wellhead and Christmas tree components. In the H1150 condition, 1.4534 (UNS S13800) satisfies the hardness limits in NACE MR0175 / ISO 15156-3 for sour (H₂S-containing) service, where the Mo-enhanced PRE of ~19.5 outperforms 17-4PH in combined chloride + H₂S environments.
Power Generation
Gas and steam turbine components, turbine seal rings, packing seal diaphragms, rotor end rings, impellers for industrial pumps and compressors, and casing rings for turbine assemblies. The alloy's oxidation resistance to 593°C and its low thermal expansion coefficient make it suitable for high-temperature rotating components where dimensional stability is critical.
Downhole Drilling
Mud motor splined drive shafts, electric submersible pump (ESP) motor shafts, integral mud flanges, flanged outlets, wellhead assembly components, and downhole tooling. Downhole environments combine mechanical shock loading, chloride-laden formation brines, H₂S, CO₂, and elevated temperatures — a combination that highlights the advantages of 1.4534's higher PRE and toughness compared to copper-hardened PH grades.
Marine
Propeller shafts for commercial and naval vessels, marine engine components, offshore platform equipment, subsea valve parts, and marine hydraulic system components requiring corrosion resistance in seawater environments.
Weldability & Machinability
Welding Guidelines
1.4534 is weldable by conventional fusion processes. The preferred method is GTAW (TIG welding) using ER13-8Mo matching filler wire (AWS A5.9). Key requirements:
- Pre-heat: not required for sections ≤ 25 mm; 65–93°C for 25–75 mm; 93–150°C for sections > 75 mm
- Maximum inter-pass temperature: 150°C (300°F)
- Post-weld heat treatment (PWHT): recommended for structural and pressure-retaining applications — full re-solution anneal at 927°C + re-age, or H1150 aging at 621°C for 4 h (required for NACE MR0175 sour service compliance)
- Lower delta-ferrite content compared to 17-4PH reduces hot-cracking susceptibility in thick-section weld joints
- For NACE-compliant welds: weld metal hardness must not exceed 36 HRC; H1150 PWHT is required to achieve this
Machinability Guidance
Perform all major material removal in the solution-annealed condition (28–34 HRC), where machinability is approximately 45% of AISI 303 free-machining reference. Key guidelines:
- In the aged H950–H1050 condition: machinability drops to 25–35% of AISI 303 — achievable with TiAlN-coated submicron carbide or CBN inserts above 45 HRC
- Cutting speed (turning, annealed): 60–100 m/min; aged condition: 30–60 m/min
- Feed rate: 0.10–0.25 mm/rev roughing; 0.05–0.12 mm/rev finishing
- Flood coolant (8–10% soluble oil) is mandatory — dry cutting causes rapid tool wear and surface work hardening
- Avoid rubbing passes or insufficient feed rates — these cause surface work hardening that reduces tool life on subsequent cuts
Sourcing 1.4534 Forgings: Buyer's Verification Checklist
Because 1.4534 performance is entirely dependent on chemistry control and heat treatment precision, procurement carries a higher verification burden than standard stainless grades. Confirm the following before placing an order:
- Melt practice: Ask how the material achieves the P ≤ 0.01% and S ≤ 0.008% limits. Premium melting practice (typically VIM+VAR or VIM+ESR) is normally required. Confirm actual P and S values are reported on the MTC.
- Mill test certificates (MTC): EN 10204 Type 3.1 is the standard minimum. Type 3.2 (independently witnessed by a third-party inspection body) is available from Jiangsu Liangyi upon request.
- Heat treatment documentation: Request full thermal cycle records including actual metal temperature readings for heavy cross-sections, not just furnace set-point temperatures.
- Non-destructive testing (NDT): Confirm ultrasonic testing (UT) per ASTM A388 or equivalent at both rough-machined and final-machined stages.
- NACE sour service: Verify H1150 condition and hardness ≤ 36 HRC per NACE MR0175 / ISO 15156-3 Table B.3. Ensure weld metal also meets the limit.
- Third-party inspection: Third-party inspection by SGS, Bureau Veritas (BV), TÜV, or other international inspection bodies is available upon request. Confirm scope and timing before order placement.
- Chemical composition report: Confirm P and S actual values are listed — not just a general "per standard" statement.
Located in Chengchang Industry Park, Jiangyin City, Jiangsu Province, China. ISO 9001:2015 certified. Capacity: single-piece weights 30 kg to 30,000 kg; seamless rolled rings up to 6,000 mm OD; bars and shafts up to 15 m length. Equipment: 2,000T, 4,000T, and 6,300T hydraulic forging presses; 1-meter and 5-meter seamless ring rolling machines. EN 10204 3.1 MTC standard; 3.2 available upon request. Third-party inspection by SGS, BV, TÜV, and others available on request.
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For complete technical specifications, available product forms, and to request a quotation, visit the 1.4534 X3CrNiMoAl13-8-2 forged parts page.
Frequently Asked Questions
What does "PH" mean in 13-8 Mo PH stainless steel?
"PH" stands for Precipitation Hardening — the strengthening mechanism that gives this steel family its combination of high strength and corrosion resistance. In 1.4534, precipitation hardening is achieved by aging the solution-annealed martensitic matrix at temperatures between 510°C (H950) and 621°C (H1150). Fine β-NiAl and Ni₃Al intermetallic particles nucleate and grow within the martensitic matrix, pinning dislocation movement and producing significant increases in hardness and yield strength. The aging temperature is the single variable that determines the strength-toughness balance of the final part.
Is 13-8 Mo PH (1.4534) magnetic?
Yes — 1.4534 / UNS S13800 is ferromagnetic in both the solution-annealed and aged conditions (magnetic permeability > 1). Ferromagnetism increases after aging as the martensitic transformation completes. This contrasts with austenitic stainless steels such as 304 and 316L, which are non-magnetic in the annealed condition. The ferromagnetic character should be considered in applications involving electromagnetic equipment or magnetic particle inspection (MPI) NDT procedures.
What is the maximum continuous service temperature for 1.4534?
The maximum recommended continuous service temperature is approximately 316°C (600°F) in the H1000 or H1050 conditions without significant mechanical property degradation. The alloy has good oxidation resistance to 593°C (1,100°F) for short-term or intermittent exposure. Prolonged exposure in the temperature range of 288–482°C can cause toughness embrittlement in some conditions. As a general rule, the peak in-service temperature should be kept at least 28°C (50°F) below the aging temperature used during heat treatment.
Is 1.4534 (UNS S13800) approved for NACE MR0175 sour service?
Yes. UNS S13800 is listed in NACE MR0175 / ISO 15156-3 as permitted for use in sour (H₂S-containing) petroleum production environments. The standard permits use in the H1150 condition at a maximum hardness of 36 HRC (352 HV10). An Engineering Critical Assessment (ECA) per ISO 15156-3 Annex B may qualify H1050 condition within defined H₂S partial pressure and chloride limits. Consult ISO 15156-3 Table B.3 directly for the specific designation, condition, and environmental limits applicable to your project. Weld metal and HAZ must also meet the hardness limit.
What is the difference between AMS 5629 and AMS 5864 for UNS S13800?
Both AMS specifications cover UNS S13800 for bar, wire, forgings, and rings in North American aerospace applications. AMS 5629 is the more established and broadly cited specification covering the full product form range and heat treatment conditions. AMS 5864 covers bar, ring, and forgings and may specify different minimum mechanical property requirements for specific conditions. Always confirm which AMS specification and condition code applies to your engineering drawing, and verify that the manufacturer's material certification explicitly references the correct specification and heat treatment condition.
What sizes and product forms are available from Jiangsu Liangyi in 1.4534?
Jiangsu Liangyi manufactures 1.4534 (X3CrNiMoAl13-8-2 / UNS S13800) in the following product forms: round bars up to 2,000 mm diameter; square, flat, and rectangular bars; seamless rolled rings up to 6,000 mm OD; hollow forgings, sleeves, and hubs; discs, plates, and blocks up to 3,000 mm diameter; shafts and bars up to 15 m length; and custom shapes to customer drawings. Single-piece weights range from 30 kg to 30,000 kg. EN 10204 Type 3.1 MTCs are supplied as standard; Type 3.2 is available upon request. See the 1.4534 open die forgings and seamless rolled rings page for full dimensions and to request a quotation.