Superalloys used for aerospace fastener require a combination of mechanical strength, notch toughness, elevated temperature capability and corrosion/oxidation resistance.
Typical superalloy requirements
Mechanical strength requirements might include tensile, shear, fatigue, creep and/or stress rupture strengths, depending on the application demands. Strength requirements have increased as new developments in engine design and other technologies have arisen.
The continual need for greater thrust output and better fuel efficiency has resulted in faster-spinning, hotter-running gas turbine engines. This, in turn, has created the need for alloys that can withstand higher stresses and temperatures, a trend that is expected to continue.
Another critical material property is the ability to resist corrosion at ambient and elevated temperatures, including general corrosion, crevice corrosion, stress corrosion, oxidation and sulfidation. For joints exposed to potentially corrosive environments, the fastener should be galvanically compatible with the joint. That is, the materials used should be relatively close on the galvanic series. If this is not practical, platings or coatings can be applied to the “noble” corrosion resistant fasteners to promote galvanic compatibility with structures assembled from more “active” materials, such as aluminum alloys.
It is important to understand that the properties of a superalloy are not merely the result of its composition. Material processing and fastener manufacturing play roles in achieving the desired combination of properties. Cold working, thermal treatments and the sequence of critical manufacturing operations significantly enhance performance.
Marine, space and medical applications are just a few of the environments that challenge the superalloys . Other applications are...
• Space Shuttle Orbiter & Solid Rocket Motors
• Aircraft Structure & Landing Gears
• Aircraft Gas Turbine Engines
• Gas Turbine Engines for Power Generation
• High-Performance Automotive Engines
• Marine—Ships, Submarines, Naval Aircraft
• Petrochemical Equipment
• Chemical Processing Equipment
• Medical X-Ray Imaging Equipment
• Cryogenic Uses
MP35N is an excellent superalloy invented by SPS. The details of this alloy are as follows.
MP35N Basics
The alloy's combination of high strength,outstanding corrosion resistance and excellent cryogenic properties meet the requirements for airframe and other Shuttle fastener applications.
Since its development, MP35N has been successfully used in applications outside of the aerospace industry. For instance, it is the highest strength material approved for structural components in aggressive, sour well environments (NACE specification MR0175 Sulfide Stress Cracking Resistant Metallic Materials for Oil Equipment).
MP35N material used for the manufacture of fasteners conforms to the requirements of the aerospace material specification AMS 5844.
Nominal Composition
Nickel 35%
Cobalt 35%
Chromium 20%
Molybdenum 10%
Metallurgical Strengthening Mechanisms
The prime strengthening mechanism in this alloy is the solid state phase transformation of part of the matrix from a face-centered cubic (fcc) crystal structure to a hexagonal close-packed (hcp) structure by cold working. This transformation occurs because of the high cobalt content in the alloy, and has been termed “the MULTIPHASE reaction.” The presence of two distinct crystal structures poses a barrier to the motion of dislocations and leads to pronounced strengthening. Subsequent age hardening acts to stabilize these two phases through the process of solute partitioning, which contributes to further strengthening
MP35N Properties Profile
• High strength coupled with excellent ductility and toughness
• Most corrosion resistant fastener alloy available...resists general corrosion,
stress-corrosion cracking and crevice corrosion
• Maximum operating temperature of 750°F
• Highest fatigue strength of corrosion resistant fastener materials
• Shear strength of 145 ksi minimum has been the highest of the corrosion
resistant fastener materials for many years.
MP35N Corrosion Resistance
MP35N is often selected as a fastener alloy for its ability to resist stress corrosion cracking.
In laboratory tests, MP35N alloy samples were loaded to 90% of their proportional limit and alternately immersion tested in 3% salt solution for 10 minutes and in moving air for 50 minutes. After 2500 hours, no evidence of general corrosion or cracking appeared.
The table below compares the corrosion rate, in mils per year, of MP35N and three other corrosion resistant alloys in an accelerated corrosion test at 120°F. For all three chloride containing environments, the MP35N specimens exhibited no discernible surface or crevice corrosion.
Crevice corrosion tests were performed on MP35N, MP159,13-8Mo and Custom 455 alloy bolts in a 10% ferric chloride solution. Several bands of electroplater's tape were applied to the bolts to promote the conditions necessary for crevice corrosion. Following 6 hours of exposure to the solution, Multiphase alloy bolts exhibited no visible evidence of corrosion while the other alloy bolts experienced extensive attack.
MP35N Fastener Applications
• Airframes
• Space Shuttle structure
• Tie rods
• Petrochemical equipment
• Cryogenic uses
• Marine equipment
• Submersibles
• Racing engine connecting rod bolts
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