Often referred to as Alloy 500 or Monel 500, Monel K-500 is a harder variation of Monel 400.
While not as readily available in as large of range of fasteners off-the-self as Monel 400, Monel K-500 is still a widely supplied material for several fasteners, including Hex Bolts, Socket Head Cap Screws, Studs, Hex Nut, Flat Washers and Lock Washers.
The added hardness of K-500 makes it commonly used in pumps, joints, and other applications that require an amount of wear resistance.
Specs:
QQ-N-286
UNS N05500