Heat Treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape. Heat treatment is sometimes done inadvertently due to manufacturing processes that either heat or cool the metal such as welding or forming.
Heat Treatment is often associated with increasing the strength of material, but it can also be used to alter certain manufacturability objectives such as improve machining, improve formability, restore ductility after a cold working operation. Thus it is a very enabling manufacturing process that can not only help other manufacturing process, but can also improve product performance by increasing strength or other desirable characteristics.
Steels are particularly suitable for heat treatment, since they respond well to heat treatment and the commercial use of steels exceeds that of any other material. Steels are heat treated for one of the following reasons:
Full Annealing is the process of slowly raising the temperature about 50 ºC (122 ºF) above the Austenitic temperature line A3 or line ACM in the case of Hypoeutectoid steels (steels with < 0.77% Carbon) and 50 ºC (122 ºF) into the Austenite-Cementite region in the case of Hypereutectoid steels (steels with > 0.77% Carbon). It is held at this temperature for sufficient time for all the material to transform into Austenite or Austenite-Cementite as the case may be. It is then slowly cooled.
The grain structure has coarse Pearlite with ferrite or Cementite (depending on whether hypo or hyper eutectoid). The steel becomes soft and ductile.
Process Annealing is used to treat work-hardened parts made out of low-Carbon steels (< 0.25% Carbon). This allows the parts to be soft enough to undergo further cold working without fracturing. Process annealing is done by raising the temperature to just below the Ferrite-Austenite region, line A1on the diagram. This temperature is about 727 ºC (1341 ºF) so heating it to about 700 ºC (1292 ºF) should suffice. This is held long enough to allow recrystallization of the ferrite phase, and then cooled in still air. Since the material stays in the same phase through out the process, the only change that occurs is the size, shape and distribution of the grain structure. This process is cheaper than either full annealing or normalizing since the material is not heated to a very high temperature or cooled in a furnace.
Stress Relief Anneal is used to reduce residual stresses in large castings, welded parts and cold-formed parts. Such parts tend to have stresses due to thermal cycling or work hardening. Parts are heated to temperatures of up to 600 - 650 ºC (1112 - 1202 ºF), and held for an extended time (about 1 hour or more) and then slowly cooled in still air.
Normalizing is the process of raising the temperature to over 60 º C (140 ºF), above line A3 or line ACM fully into the Austenite range. It is held at this temperature to fully convert the structure into Austenite, and then removed form the furnace and cooled at room temperature under natural convection. This results in a grain structure of fine Pearlite with excess of Ferrite or Cementite. The resulting material is soft; the degree of softness depends on the actual ambient conditions of cooling. This process is considerably cheaper than full annealing since there is not the added cost of controlled furnace cooling.
The main difference between full annealing and normalizing is that fully annealed parts are uniform in softness (and machinablilty) throughout the entire part; since the entire part is exposed to the controlled furnace cooling. In the case of the normalized part, depending on the part geometry, the cooling is non-uniform resulting in non-uniform material properties across the part. This may not be desirable if further machining is desired, since it makes the machining job somewhat unpredictable. In such a case it is better to do full annealing.
Hardening (Monitored & Calibrated) is a function of the Carbon content of the steel. Hardening of a steel requires a change in structure from the body-centered cubic structure found at room temperature to the face-centered cubic structure found in the Austenitic region. The steel is heated to Autenitic region. When suddenly quenched, the Martensite is formed. This is a very strong and brittle structure. When slowly quenched it would form Austenite and Pearlite which is a partly hard and partly soft structure. When the cooling rate is extremely slow then it would be mostly Pearlite which is extremely soft.
Hardenability, which is a measure of the depth of full hardness achieved, is related to the type and amount of alloying elements. Different alloys, which have the same amount of Carbon content, will achieve the same amount of maximum hardness; however, the depth of full hardness will vary with the different alloys. The reason to alloy steels is not to increase their strength, but increase their hardenability — the ease with which full hardness can be achieved throughout the material.
Usually when hot steel is quenched, most of the cooling happens at the surface, as does the hardening. This propagates into the depth of the material. Alloying helps in the hardening and by determining the right alloy one can achieve the desired properties for the particular application.
Such alloying also helps in reducing the need for a rapid quench cooling — thereby eliminate distortions and potential cracking. In addition, thick sections can be hardened fully.
Quenching (Air/Oil/Water) is the act of rapidly cooling the hot steel to harden the steel.
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.
Oil: Oil is used when a slower cooling rate is desired. Since oil has a very high boiling point, the transition from start of Martensite formation to the finish is slow and this reduces the likelihood of cracking. Oil quenching results in fumes, spills, and sometimes a fire hazard.
Quenches are usually done to room temperature. Most medium carbon steels and low alloy steels undergo transformation to 100% Martensite at room temperature. However, high carbon and high alloy steels have retained Austenite at room temperature. To eliminate retained Austenite, the quench temperature has to be lowered.
Machining induces stresses in parts. The bigger and more complex the part, the more the stresses. These stresses can cause distortions in the part long term. If the parts are clamped in service, then cracking could occur. Also hole locations can change causing them to go out of tolerance. For these reasons, stress relieving is often necessary.
Typically, the parts that benefit from stress relieving are large and complex weldments, castings with a lot of machining, parts with tight dimensional tolerances and machined parts that have had a lot of stock removal performed.
Stress Relieving is done by subjecting the parts to a temperature of about 75 ºC (165 ºF) below the transformation temperature,line A1 on the diagram, which is about 727 ºC (1340 ºF) of steel—thus stress relieving is done at about 650 ºC (1202 ºF) for about one hour or till the whole part reaches the temperature. This removes more than 90% of the internal stresses. Alloy steels are stress relieved at higher temperatures. After removing from the furnace, the parts are air cooled in still air.