Which stainless steel is non-magnetic and stays that way?
A non-magnetic requirement applies to the finished component, not just the bar it was cut from. Many austenitic stainless steels are non-magnetic when annealed but become slightly magnetic as soon as they are cold worked, and machining, forming and thread rolling all cold-work the surface. Fermonic®® 50 is the grade of choice here: a nitrogen-strengthened austenitic that work-hardens to high strength yet stays fully non-magnetic even after severe cold work, so a machined or formed part holds its low permeability.
What makes a component non-magnetic, not just the bar?
Because magnetism is picked up during forming, grade selection for sensors, sonar, mine countermeasures and instrumentation has to start from what stays non-magnetic in service, not what tests non-magnetic as raw bar. That is why standard austenitic grades are ruled out for magnetically sensitive work, and why Fermonic®® 50 and the copper alloys below are specified instead.
Which non-magnetic alloy for seawater or wetted parts?
Where a non-magnetic component is also immersed or splashed, the high-strength copper alloys Hiduron®® 130 and Hidurel®® 5, and the cupronickel Hiduron®® 191, combine very low magnetic permeability with seawater and crevice corrosion resistance. That makes them suitable for wetted sensor and instrument hardware where a stainless grade alone would not survive the seawater exposure.
Typical non-magnetic components
Sensor and instrument housings, sonar hardware, mine countermeasures fittings, structural fixtures and brackets, and fasteners for magnetically sensitive assemblies.