Heat Treatment of Metals
HEAT TREATMENT OF METALS
Heat treatment is any one of a number of controlled heating and cooling operations used to bring about a desired change in the physical properties of a metal. Its purpose is to improve
the structural and physical properties for some particular use or for future work of the metal. There are five basic heat treating processes: hardening, case hardening, annealing,
normalizing, and tempering. Although each of these processes bring about different results in metal, all of them involve three basic steps: heating, soaking, and cooling.
HEATING
Heating is the first step in a heat-treating process. Many alloys change structure when they are heated to specific temperatures. The structure of an alloy at room temperature
can be either a mechanical mixture, a solid solution, or a combination solid solution and mechanical mixture.
A mechanical mixture can be compared to concrete. Just as the.sand and gravel are visible and held in place by the cement. The elements and compounds in a mechanical
mixture are clearly visible and are held together by a matrix of base metal. A solid solution is when two or more metals are absorbed, one into the other, and form a solution. When an
alloy is in the form of a solid solution, the elements and compounds forming the metal are absorbed into each other in much the same way that salt is dissolved in a glass of water.
The separate elements forming the metal cannot be identified even under a microscope. A metal in the form of a mechanical mixture at room temperature often goes into a solid solution or
a partial solution when it is heated. Changing the chemical composition in this way brings about certain predictable changes in grain size and structure. This leads to the second
step in the heat treating process: soaking
SOAKING
Once a metal part has been heated to the temperature at which desired changes in its structure will take place, it must
remain at that temperature until the entire part has been evenly heated throughout. This is known as soaking. The more mass the part has, the longer it must be soaked.
COOLING
After the part has been properly soaked, the third step is to cool it. Here again, the structure may change from one chemical composition to another, it may stay the same, or it
may revert to its original form. For example, a metal that is a solid solution after heating may stay the same during cooling, change to a mechanical mixture, or change to a
combination of the two, depending on the type of metal and the rate of cooling. All of these changes are predictable. For that reason, many metals can be made to conform to
specific structures in order to increase their hardness, toughness, ductility, tensile strength, and so forth.
HEAT TREATMENT OF FERROUS METALS
All heat-treating operations involve the heating and cooling of metals, The common forms of heat treatment for ferrous metals are hardening, tempering, annealing,
normalizing, and case hardening.
HARDENING
A ferrous metal is normally hardened by heating the metal to the required temperature and then cooling it rapidly by plunging the hot metal into a quenching medium, such as oil,
water, or brine. Most steels must be cooled rapidly to harden them. The hardening process increases the hardness and strength of metal, but also increases its brittleness.
Age Hardening
Age hardening is a heat-treatment process used to strengthen metal alloys. Unlike ordinary tempering, alloys must be kept at elevated temperature for hours, or "aged," to allow precipitation to take place.
Age hardening creates changes in physical and mechanical properties by producing fine particles of a precipitate phase, which impede the movement of dislocations, or defects in a crystal's lattice. Dislocations serve to harden the material.
TEMPERING
Steel is usually harder than necessary and too brittle for practical use after being hardened. Severe internal stresses are set up during the rapid cooling of the metal. Steel is tempered
after being hardened to relieve the internal stresses and reduce its brittleness. Tempering consists of heating the metal to a specified temperature and then permitting the metal to
cool. The rate of cooling usually has no effect on the metal structure during tempering. Therefore, the metal is usually permitted to cool in still air. Temperatures used for tempering
are normally much lower than the hardening temperatures. The higher the tempering temperature used, the softer the metal becomes. High-speed steel is one of the few metals that
becomes harder instead of softer after it is tempered.
Metals are annealed to relieve internal stresses, soften them, make them more ductile, and refine their grain structures. Metal is annealed by heating it to a prescribed temperature,
holding it at that temperature for the required time, and then cooling it back to room temperature. The rate at which metal is cooled from the annealing temperature varies greatly.
Steel must be cooled very slowly to produce maximum softness, This can be done by burying the hot part in sand, ashes, or some other substance that does not conduct heat readily
(packing), or by shutting off the furnace and allowing the furnace and part to cool together (furnace cooling).
ANNEALING AND SPHEROIDIZING TEMPERATURES
Hypereutectoid steels consist of pearlite and cementite. The cementite forms a brittle network around the pearlite. This presents difficulty in machining the hypereutectoid steels. To improve
the machinability of the annealed hypereutectoid steel spheroidize annealing is applied. This process will produce a spheroidal or globular form of a carbide in a ferritic matrix which makes
the machining easy. Prolonged time at the elevated temperature will completely break up the pearlitic structure and cementite network. The structure is called spheroidite. This structure is
desirable when minimum hardness, maximum ductility and maximum machinability are required.
Ferrous metals are normalized to relieve the internal stresses produced by machining, forging, or welding. Normalized steels are harder and stronger than annealed steels. Steel is
much tougher in the normalised condition than in any other condition. Parts that will be subjected to impact and parts that require maximum toughness and resistance to external
stresses are usually normalized. Normalizing prior to hardening is beneficial in obtaining the desired hardness, provided the hardening operation is performed correctly. Low carbon
steels do not usually require normalizing, but no harmful effects result if these steels are normalized. Normalising is achieved by heating the metal to a specified temperature (which
is higher than either the hardening or annealing temperatures), soaking the metal until it is uniformly heated, and cooling it in still air.
CASE HARDENING
Case hardening is an ideal heat treatment for parts which require a wear-resistant surface and a tough core, such as gears, cams, cylinder sleeves, and so forth. The most common
case-hardening processes are carburising and nitriding. During the case-hardening process, a low-carbon steel (either straight carbon steel or low-carbon alloy steel) is heated to a
specific temperature in the presence of a material (solid, liquid, or gas - Carbon or Nitrogen rich gas) which decomposes and deposits more carbon into the surface of a steel. Then, when the part is cooled
rapidly, the outer surface or case becomes hard, leaving the, inside of the piece soft but very tough.