Nitriding steels are alloy steels (low- and medium-carbon steels with combinations of chromium and aluminum or nickel, chromium, and aluminum) designed particularly for optimum results when they are subjected to the nitriding operation. The composition is such that the required microstructure for optimum nitriding is produced after heat treatment. Nitrided parts made from nitriding steels have extremely high surface hardnesses of about 92 to 95 Rockwell N scale, wear resistance, and resistance to certain types of corrosion.
Processing and Applications
Nitriding consists of exposing steel parts to gaseous ammonia at about 538°C to form metallic nitrides at the surface. The hardest coatings are obtained with aluminum-bearing steels. Nitriding of stainless steel is known as Malcomizing. After nitriding, these steels have extremely high surface hardnesses of about 92 to 95 Rockwell N. The nitride layer also has considerable resistance to corrosion from alkalies, the atmosphere, crude oil, natural gas, combustion products, tap water, and still salt water. Nitrided parts usually grow about 0.003 to 0.005 cm during nitriding. The growth can be removed by grinding or lapping, which also removes the brittle surface layer. Most uses of nitrided steels are based on resistance to wear. The steels can also be used at temperatures as high as 538°C for long periods without softening. The slick, hard, and tough nitrided surface also resists seizing, galling, and spalling. Typical applications are cylinder liners and barrels for aircraft engines, bushings, shafts, spindles and thread guides, cams, rolls, piston pins, rubber and paper-mill product rolls, special oil tool equipment, bearings, rollers, etc.
Fabrication
The fabrication characteristics of nitriding steels are basically the same as those of other steels of similar alloy content. They can be drilled, broached, tapped, milled, sawed, or ground. Light feeds and depth of cuts are recommended. Welding is done with rod or wire of similar composition. Flash welding is permissible.
If very heavy cuts are involved, they usually are made prior to heat treatment. Normal machining is done on heat-treated material, and is followed by a stress-relieving treatment of not less than 37.8°C above the nitriding temperature, before finish machining or grinding. It is essential that in machining, sufficient removal be allowed to remove all decarburiza-tion from the surface prior to nitriding. The surface also must be clean and free of any surface contamination.
Because nitriding is a low-temperature treatment, little or no warpage is encountered. If it is necessary to straighten because of residual stress, the part should be heated to 538 to 593°C to prevent surface cracking.
Nitrided parts normally grow about 0.03 to 0.05 mm during nitriding. This may be removed by grinding or lapping. This also has the advantage of removing a brittle layer on the surface and exposes a slightly harder layer immediately beneath it. This operation, however, will reduce corrosion resistance to a large degree.
Nitriding steels are available in all standard steel forms. They can be purchased heat-treated or annealed to desired physical properties.
Most uses of nitriding steels are based on resistance to wear. An outstanding property is that these steels can be heated to as high as 538°C for long periods without softening.
The slick, hard surface produced also makes it ideal to prevent seizing, galling, and spalling, and it is not readily attacked by combustion products.
