Heat Treatment Process
Heat treatment process is a series of operations involving the heating and cooling of metals in the solid state. Its purpose is to change a mechanical property or combination of mechanical properties so that the metal will be more useful, serviceable, and safe for definite purpose. By heat treating, a metal can be made harder, stronger, and more resistant to impact, Heat treatment process can also make a metal softer and more ductile. No one heat-treating operation can produce all of these characteristics. In fact, some properties are often improved at the expense of others. In being hardened, for example, a metal may become brittle.
The annealing procedure for aluminum alloys consists of heating the alloys to an elevated temperature, holding or soaking them at this temperature for a length of time depending upon the mass of the metal, and then cooling in still air. Annealing leaves the metal in the best condition for cold-working. However, when prolonged forming operations are involved, the metal will take on a condition known as ” mechanical hardness” and will resist further working. It may be necessary to anneal a part several times during the forming process to avoid cracking. Aluminum alloys should not be used in the annealed state for parts or fittings
Soft annealing is generally applied to steels, that contains carbon above 0.5%. These types of steel are usually too hard for forming (e.g. forging) or cutting after a hardening process, due to their microstructure, which consists of pearlite and cementite.
The aim of the soft annealing process is to form an even distribution of spheroidal carbides in the steel, which will make the material softer and tougher. Normally, increasing the size of the spheroids will increase the steel’s machinability
Normalizing involves heating the part above the upper critical temperature and then air cooling outside the furnace to relieve residual stresses in a gear blank and for dimensional stability. Normalizing is often considered from both a thermal and microstructural standpoint. In the thermal sense, normalizing is austenitizing followed by cooling in still or slightly agitated air or nitrogen. In a microstructural sense, normalizing produces a more homogenous structure. A normalized part is very machinable but harder than an annealed part. Normalizing also plays a significant role in the control of dimensional variation during carburizing.
Tempering. Any temperature under the lower critical temperature (Ac1) can be used for tempering, but it is the balance of hardness, strength, and toughness required in service that determines the final tempering temperature. Tempering in the range of 150 to 200°C (300 to 400″F) is common for gearing, producing a slight increase in toughness that is adequate for most applications requiring high strength and fatigue resistance where loading is primarily compressive, Double tempering is sometimes performed on gears to ensure completion of the tempering reaction and to promote stability of the resulting microstructure.
Water Quenching can be done by plunging the hot steel in water. The water adjacent to the hot steel vaporizes, and there is no direct contact of the water with the steel. This slows down cooling until the bubbles break and allow water contact with the hot steel. As the water contacts and boils, a great amount of heat is removed from the steel. With good agitation, bubbles can be prevented from sticking to the steel, and thereby prevent soft spots.
Water is a good rapid quenching medium, provided good agitation is done. However, water is corrosive with steel, and the rapid cooling can sometimes cause distortion or cracking.
Think about it, oil is less dense than water, less thermal mass for the same volume. It is not as good a conductor of heat as water so the heat is going to build up around the part raising the temperature of the quenchant even higher. It does not evaporate therefore cooling itself like water (it does, except that white ‘smoke’ is very near the flash point – something you do not want).
Commercial oil quenching tanks have water cooled heat exchanges to keep the oil temperature under control. High flashpoint oils are better than low. Many synthetic oils are non-flamable and make GREAT quenchants for the small shop.
Surface hardening is the process of hardening the surface of a metal, often a low carbon steel, by infusing elements into the material’s surface, forming a thin layer of a harder alloy. Case hardening is usually done after the part in question has been formed into its final shape, but can also be done to increase the hardening element content of bars to be used in a pattern welding or similar process. The term face hardening is also used to describe this technique, when discussing modern armour.