All substances, whether in the form of liquid, solid, or gas, will respond in some manner to an applied magnetic field, although in varying degrees. The magnetic field can be produced by an electric current or it may be the flux from either a permanent magnet or an electromagnet.
Ferromagnetism is the magnetic property of greatest interest in the context of welding metallurgy, because this particular magnetic behavior is frequently involved in welding operations.
Ferromagnetic Materials. Of all the elements in the periodic table only three, iron, cobalt, and nickel, are ferromagnetic at room temperature. However, ferromagnetic alloys can be formulated using various metallic elements which individually are not ferromagnetic. Alnico is an example of an Al-Ni-Co-Cu-Fe alloy used to make permanent magnets, although individually some of the elements of the magnet are not ferromagnetic. Ferromagnetic materials are divided into two classifications: magnetically soft materials,
and hard or permanent magnet materials.
Magnetically Soft Materials. Soft ferromagnetic materials are easy to magnetize, but retain little or none of the induced magnetism when the magnetizing force is removed. Magnetically soft materials made in large quantities include high-purity iron, silicon steels, iron-nickel alloys, iron-cobalt alloys, and ferrites.
Permanent (Hard) Magnet Materials. Hard ferromagnetic materials are difficult to magnetize, but they retain a significant degree of magnetization when the applied magnetic force is removed. Permanent magnet materials include both plain high-carbon steels and high-carbon alloy steels, magnet alloys that have useful magnetic properties from the combination of specific elements but which are virtually free of carbon, and metallic oxides that possess unique magnetic properties that make them commercially important.
Martensitic alloys are the best known and oldest of permanent magnet materials. The optimum magnetic properties result from untempered martensite in plain high-carbon steels (0.8 to 1.0percent carbon). Permanent magnet alloy materials include iron-chromium- carbon, and cobalt magnet steel.
Alnico types are probably the most popular of permanent magnet steels. There are a number of Alnico alloys, with a typical alloy containing 12A1-28Ni-5Co. Some alloys have copper and titanium contents. All these alloys are hard, brittle, and unmachinable, so they must be cast or finish-ground to shapc.