An investigation into the selection of a suitable material was conducted for locking bars located on a marine vessel. The locking bars are responsible for holding a platform of the rig in place, and the entire load of the rig is supported on a minimum of four locking bars at one time. The bars (Figure 1) are currently made out of ROQ tuf AM700 material, a high strength low alloy steel with a tensile strength of 780 to 900 MPa.
After only one year in service they showed severe signs of general corrosion, surface wear and some plastic deformation on the surface where the bars rest on the dog rack. Any one of these observations could have led to failure of the part.
The maximum load on each leg was calculated to be 746kN. The following stresses were also determined and compared with the mechanical properties of ROQ tuf:
- Bearing stress on locking bar (σ Bearing) = 373MPa
- Shear stress on locking bar (τ) = 108 MPa
- Bending strength at the middle of the dog rack width (σ Bending) = 405MPa
Based on the yield and tensile strength of ROQ tuf no deformation should have occurred as the stresses are below the strength values and the number of cycles for high cycle fatigue has yet to be reached.
Figure 1: Locking Bar
The following material requirements were established as displayed in Table 1.
|Max bending stress w/o wave & wind loads||405 MPa|
|Min tensile strength||780 MPa based on ROQ tuf|
|Fatigue strength/Endurance limit||410 MPa (base on max cyclic stress)|
Table 1 – Materials requirements
Using the above criteria, constraints were applied to the CES material selector database to find a suitable material. 48 materials passed all the requirements, of which only three were steels. The steels which passed were different grades of 17-4PH martensitic stainless steel. 17-4PH is a martensitic precipitation hardenable chromium-nickel-copper steel with high strength and toughness and adequate level of resistance to atmospheric corrosion.
The following was concluded for the material selection as recommended by One Eighty materials testing:
- AISI 4340 (in normalized condition) or EN24W were selected as the most suitable materials for the application with a thermal spray coating to resist corrosion.
- Both these materials have superior fatigue resistance and good tensile properties and resistance to fast brittle fracture.
- A Nickel-Chrome thermal spray coating with a minimum thickness of 150µm was required for the wear surfaces.
- The rest of the locking bar can be coated with a thermal spray Aluminium (TSA) or Zinc-Aluminium with a 200 μm thickness.
- The risk factor associated with fracture or failure of the coating must be considered and hence a routine maintenance schedule should be followed.