Types, Purpose and Applications
Thermal Treatment in general:
Thermal treatment of metals is a vast subject by itself. Only a brief description of the most common thermal process and treatments will be given here. It is important to emphasize that improper execution of heat treatment can cause distortions and imperfections in the product that the material testing specialist needs to be aware of.
There a wide variety of thermal treatments used for metals. However, they ay be divided into two basic groups: (1) those that increase the strength of materials and (2) those that increase their ductility and toughness. As will be seen, it is rarely possible to obtain an increase in both properties simultaneously most of the time, an increase in strength and hardness is accompanied with a decrease in toughness and ductility.
Quenching and Martensite Formation in Steels
When steel is quenched (cooled rapidly) form the austenitizing temperature, an entirely different type of transformation occurs, the austenite transforms to martensite.Martensite is metastable and has the same composition as the austenite from which it forms, but instead of a face-centered cubic (FCC) structure it has a structure known as body-centered tetragonal (BCT).
A very significant characteristic of martensite is that it has a potential for very high hardness. Depending on the chemical composition of steel, the hardness can be as high as 65 HRC (Rockwell C). There is also a volumetric increase as austenite transforms to martensite. This volumetric increase is the source of distortion and other defects in the parts.
Normalizing and Annealing Process
The term Annealing originally was used by craftspeople who discovered the benefits of heating some materials to elevated temperatures, then cooling them slowly. The structural changes that occur during annealing are not the same for all materias.Annealing typically involves the following sequence: (1) heating the piece to a specific temperature range, (2) holding at that temperature for a period of time (soaking), and (3) cooling it slowly at a specific cooling rate. Annealing process are categorized into the following groups:
Process Anneal: (also called intermediate annealing) is used to remove strain hardening due to previous cold working and to restore ductility so that additional cold working can be performed on the product. For instance, after each pass of cold-rolled brass (or steel) between rollers, the sheets are reheated before their thickness is further reduced in the following step. This process is often done in an inert atmosphere to prevent oxidation. During reheating, the material is recrystallized, that is, new grains free of dislocations are formed. The temperature of recrystallization for cooper is in the range between 200ºC and 300ºC (392ºF and 572ºF).Annealing temperatures are slightly higher, between 260ºC and 650ºC (500ºF and 1202ºF), for cooper.
Selecting the temperature and the annealing time is an art in itself. Too high temperature and too long a duration of the process cause excessive grain growth, which makes material brittle In hot-working process, deformation and recrystallization occurs almost simultaneously so that the annealing between processing steps is not necessary.
Full Anneal: is normally used for steels with medium carbon content(0.35 to 0.65% carbon) to improve machinability and formability. Steel is first fully austenized, where it is heated 25ºC to 30ºC (77ºF to 86ºF) above the so-called Ac3 line, which separates the y-region from mixed y- and a-regions in the phase diagram, then cooled slowly in the furnace. Furnace cooling produces very coarse pearlite that has the softest microstructure possible for that steel. Typically, the product receives additional heat treatments after machining or forming to improve strength.
Normalizing: is a process used to homogenize alloy steels. Austenization is performed at approximately 50ºC to 60ºC (122ºF to 140ºF) above Ac3 temperature for that steel for a short time. The temperature is higher than that in the full. annealing process to accelerate the diffusion required for dissolution of the alloying elements (Ni, Cr, Mo, V and others) While avoiding the excessive grain growth that would occur at longer exposure to still higher temperatures. Steels are held at the normalizing temperatures for a sufficient period of time to effectively dissolve most of the alloying elements. Excessively long soaking times are avoided as they may cause grain growth, which will adversely affect the mechanical properties. Heating is followed by the circulated air cooling, which is faster than furnace cooling used in full annealing.
Stress Relief: Is a process designed to eliminate internal stress in the prt, referred to as residual stresses, which occur due to welding, forming or machining, or may be due to volumetric change, such as when quenching steel during hardening. These stress are particularly present in steels due to the expansion of iron as it transforms from FCC to BCC structure upon cooling.
Therefore, the material is under stress even when no external loads are applied, hence the term "residual", meaning "left over" stress. When the part is subsequently machined, the equilibrium will be disturbed and the part may distort and sometimes crack.
Residual Stress. Residual Stress are hidden hazards in design and manufacturing, and should be minimized. The most common method of residual stress mitigation is by heating to temperatures substantially below the austenite-to-ferrite transformation to allow some diffusion of atoms at short distances. This heating is then followed by slow cooling. Other methods include mechanical working of the part.
It is important to mention that residual stress can also be introduced by mechanical deformation in the plastic region, specially under nonuniform loading, such as in bending. In some cases, these stress can be eliminated. Product designers should be aware that the elimination of residual stress in a part is one of the most formidable problems in design. Nondestructive test specialist must recognize the significance of residual stress on component integrity, as well.