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1. Introduction
Precipitation-Hardening (PH) 13-8Mo (also known as UNS S13800, AMS 5629/5864, ASTM A564) is a premium martensitic precipitation-hardening stainless steel, representing one of the highest-performance grades in its category. It is widely recognized for its exceptional combination of ultra-high strength, outstanding fracture toughness, and superior resistance to stress corrosion cracking (SCC) in marine and industrial environments.
13-8 Mo stainless steel has good fabrication characteristics and can be age-hardened by a single low-temperature treatment. Cold working prior to ageing increases the hardening, particularly at lower ageing temperatures. It has been used for valve parts, fittings, cold-headed and machined fasteners, shafts, landing gear parts, pins, lock washers, aircraft components and nuclear reactor components, as well as in petrochemical applications requiring resistance to stress-corrosion cracking. This alloy is generally considered where high strength, toughness, corrosion resistance and resistance to stress-corrosion cracking are required in a steel showing minimal directionality in properties.
2. Core Chemical Composition
The alloy’s properties are derived from its precisely balanced composition:
| Element | Content Range (wt%) |
| Chromium (Cr) | 12.25 – 13.25 |
| Nickel (Ni) | 7.50 – 8.50 |
| Molybdenum (Mo) | 2.00 – 2.50 |
| Aluminum (Al) | 0.90 – 1.35 |
| Carbon (C) | ≤ 0.05 |
| Manganese (Mn) | ≤ 1.00 |
| Silicon (Si) | ≤ 1.00 |
| Phosphorus (P) | ≤ 0.010 |
| Sulfur (S) | ≤ 0.008 |
Key roles of alloying elements:
- Chromium: Forms a protective passive film for corrosion resistance and stabilizes the martensitic matrix.
- Nickel: Enhances toughness and provides the base for precipitation-hardening phases like Ni₃Al.
- Molybdenum: Boosts resistance to pitting, crevice corrosion, and chloride-induced SCC.
- Aluminum: Enables precipitation hardening, forming nano-sized intermetallic precipitates that dramatically increase strength.
3.Mechanical Properties (Typical, Condition H1150M)
| Property | Value |
| Tensile Strength | 1000 – 1100 MPa (145 – 160 ksi) |
| Yield Strength (0.2% offset) | 830 – 900 MPa (120 – 130 ksi) |
| Elongation | ≥ 15% |
| Reduction of Area | ≥ 60% |
| Hardness (HRC) | 32 – 36 |
| Fracture Toughness (KIC) | ≥ 100 MPa·√m |
Properties can be tailored via heat treatment (e.g., H900, H950, H1000, H1150) to meet specific strength-toughness tradeoffs.
4.Typical Mechanical Properties
| Material | Condition | Ultimate Tensile Strength (ksi) | 0.2 % Yield Strength (ksi) | Elongation % in 2D | % Reduction of Area | Rockwell C Hardness |
| Alloy 13-8 Sheet AMS 5864 | Cond A | – | – | – | – | 38 max |
| Alloy 13-8 Sheet AMS 5864 | H950 | 220 | 205 | 10 | 45 | 45 |
| Alloy 13-8 Sheet AMS 5864 | H1000 | 205 | 190 | 10 | 50 | 43 |
| Alloy 13-8 Sheet AMS 5864 | H1025 | 185 | 175 | 11 | 50 | 41 |
| Alloy 13-8 Sheet AMS 5864 | H1050 | 175 | 165 | 12 | 50 | 40 |
| Alloy 13-8 Sheet AMS 5864 | H1100 | 150 | 135 | 14 | 50 | 34 |
| Alloy 13-8 Sheet AMS 5864 | H1150 | 135 | 90 | 14 | 50 | 30 |
| Material | Condition | Ultimate Tensile Strength (ksi) | 0.2 % Yield Strength (ksi) | Elongation % in 2D | % Reduction of Area | Rockwell C Hardness |
| Alloy 13-8 Bar AMS 5629 | Cond A | – | – | – | – | 363 HB Max |
| Alloy 13-8 Bar AMS 5629 | H950 | 220 | 205 | 10 | 45 | 45 |
| Alloy 13-8 Bar AMS 5629 | H1000 | 205 | 190 | 10 | 50 | 43 |
| Alloy 13-8 Bar AMS 5629 | H1025 | 185 | 175 | 11 | 50 | 41 |
| Alloy 13-8 Bar AMS 5629 | H1050 | 175 | 165 | 12 | 50 | 40 |
| Alloy 13-8 Bar AMS 5629 | H1100 | 150 | 135 | 14 | 50 | 34 |
| Alloy 13-8 Bar AMS 5629 | H1150 | 135 | 90 | 14 | 50 | 30 |
5. Key Advantages
- Ultra-High Strength-to-Weight Ratio: Achieves tensile strengths exceeding 1100 MPa, outperforming conventional stainless steels.
- Superior Fracture Toughness: Maintains high toughness even at cryogenic temperatures, making it ideal for critical structural components.
- Excellent Corrosion Resistance: Resists pitting, crevice corrosion, and stress corrosion cracking in chloride-rich environments (e.g., seawater, offshore platforms).
- Uniform Mechanical Properties: Produced via VIM+VAR (Vacuum Induction Melting + Vacuum Arc Remelting) for ultra-clean ingots, ensuring consistent longitudinal and transverse properties.
- Good Fabricability: Can be machined, welded, and formed in the solution-annealed state, then hardened via low-temperature aging (480–510°C).
6.Use at Elevated Temperatures
PH 13-8 Mo stainless steel has excellent oxidation resistance up to approximately 1100°F. However, long-term exposure to temperatures between 600 and 900°F (288 and 482°C) can result in reduced toughness in precipitation-hardenable stainless steels. In some cases, the reduction in toughness can be minimized by using higher aging temperatures. Short exposures to elevated temperatures are acceptable, provided that the maximum temperature is at least 50°F (28°C) lower than the aging temperature.
7. Heat Treatment Process
- Solution Annealing: 925°C (1700°F) for 15–30 minutes, followed by air or oil cooling to -75°C (-100°F) to ensure full martensitic transformation.
- Aging (Precipitation Hardening): 480–510°C (900–950°F) for 4 hours, air-cooled. Higher aging temperatures (e.g., 595°C/1100°F) produce slightly lower strength but improved toughness.
8.Corrosion Resistance
In condition H950, PH13-8Mo stainless steel exhibits a similar level of rust resistance to Type 304 stainless steel in a 5 weight percent salt fog environment. In strongly oxidising and reducing acids, as well as in atmospheric exposure, the general corrosion resistance of PH 13-8 Mo stainless steel is similar to that of Type 304. As with other precipitation-hardening stainless steels, the alloy’s level of general corrosion resistance is greatest in the fully hardened condition, decreasing slightly with increased ageing temperature. Numerous tests simulating a marine environment have demonstrated that the alloy exhibits a high level of resistance to stress-corrosion cracking in both the wrought and welded conditions. To achieve the best possible resistance to stress-corrosion cracking, it is recommended that the alloy is aged at a minimum temperature of 1000°F (538°C).
To optimise corrosion resistance, surfaces must be free of scale and foreign particles, and finished parts should be passivated.
9. Typical Applications
- Aerospace: Landing gear components, turbine engine parts, and structural fasteners requiring high strength and toughness.
- Oil & Gas: Subsea equipment, valve components, and drill string parts exposed to high pressure and corrosive fluids.
- Marine Engineering: Propeller shafts, rudders, and offshore platform components.
- Defense: Missile components, armor plating, and high-stress weapon parts.
- Medical: Surgical instruments and implants requiring biocompatibility and high strength.
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