HEA
REATING By Charlie O. Burke (EAA 18826) 7905 Renoir Drive Jacksonville, Florida
EAT TREATING IS AN art within itself, and may be H defined as a series of heating and cooling operations on metals in the solid state, the purpose of which is to alter the
properties so that the metal may be more useful. By heat treating, a metal can be made softer, more ductile, harder, stronger, etc., but no one operation can produce all of these characteristics; in fact, some properties are often improved at the expense of others. We will deal here only with the heat treatment of ferrous metals, wherein the most common operations are hardening, tempering, normalizing, annealing, and Casehardening.
make them more ductile, and refine their grain structures. Annealing consists of heating the metal to the proper temperature, soaking for the required length of time, and then cooling it back to room temperature. The rate at which the metal is cooled from the annealing temperature varies greatly. To produce maximum softness in steel, the metal must be cooled very slowly. This is accomplished by burying the hot part in sand, ashes, or some other substance that does not conduct heat readily, or by shutting off the furnace and allowing the furnace and part to cool together. NORMALIZING
HARDENING
For most steels, the hardening treatment consists of heating the steel to the correct temperature, soaking or holding for the required length of time, and then cooling it rapidly by plunging the steel into oil, water, or brine. Although most steels must be cooled rapidly for hardening, a few may be cooled in air from the hardening temperature. Hardening increases the hardness and strength of the steel but makes it less ductile. TEMPERING
After the hardening treatment, steel is often harder than necessary and is too brittle for most practical uses. In
addition, severe internal stresses are set up during the rapid cooling from the hardening temperature. To relieve the internal stresses and reduce brittleness, steel is tempered after being hardened.
Tempering consists of heating the steel to a certain temperature, holding the metal at that temperature for the required length of time, and then cooling it, usually in still air. The resultant strength, hardness, and ductility depend on the temperature to which the steel is heated during the tempering process.
Normalizing consists of heating the part to the proper temperature, holding it at that temperature until it is uniformly heated, and then removing it from the furnace and cooling it in still air. Steel parts are normalized to relieve the internal stresses set up by machining, forging, bending, or welding. CASEHARDENING
In Casehardening, the surface of the metal is changed chemically by introducing a high carbide or nitride content. When heat treated, the surface responds to hardening while the core toughens. The common forms of Casehardening are carburizing, cyaniding, and nitriding. Although each form of Casehardening differs somewhat in the method of treatment, we will briefly cover nitriding which is quite common in crankshaft processing. The parts to be nitrided are hardened and tempered before being nitrided. The parts are then placed in a nitriding furnace and heated to a temperature of approximately 1000 degrees F. With the parts at this temperature, ammonia gas is circulated within the specially constructed furnace chamber. The high temperature cracks the ammonia gas into nitrogen and hydrogen. The nitrogen reacts with the iron to form iron nitride. The iron nitride is
ANNEALING
In general, annealing is the opposite of hardening. Metals are annealed to relieve internal stresses, soften them, 42
AUGUST 1971
dispersed in minute particles at the surface and works inward, the depth of penetration depending on the length of the treatment.
