Title of Assignment Process Annealing versus Spheroidising Anneals

Title of Assignment
Process Annealing versus Spheroidising Anneals: Their Characteristics, Precautions
and Limitations
Programme Title
Course Code and Title
Leng Keng Hui
Date of Submission
Marks Allocation
Introduction /10
Content                         /50
Conclusion /15
Reference                 / 5
Total Marks /100

Process Annealing versus Spheroidising Annealing: Their Characteristics, Precautions, and Limitations.

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Heat treatment is defined as heating a metal to a specified temperature, keeping it at that temperature for some time followed by cooling at a specified rate. It is a tool to get required microstructure and properties in the metal. Heat treatment is controlled heating and cooling basically. The basic steps of heat treatment are: Heating ? Soaking ? Cooling. There are different types of heat treatments: Annealing, Normalizing, Hardening, Tempering, and Precipitation Hardening. The figure 1 below shows different heat treatment temperature ranges for plain carbon steel. Therefore, process annealing and spheroidizing anneal is discussing as below.

Figure 1: Different heat treatment temperature range for plain carbon steel.

Process Annealing
The product after cold working results in strain hardening. Which means the dislocations are increased by cold work. Cold worked metals become brittle, reheating, which increase ductility results in recovery, recrystallization and grain growth. This process is called process annealing. Recrystallization annealing process consists of heating a steel component below A1 temperature for example, at temperature between 625? and 675? (recrystallization temperature range of steel), holding at this temperature and subsequent cooling. This makes the steel easier to form. This heat treatment is commonly applied in the sheet and wire industries. Slow cooling is not essential for process annealing, since any cooling rate from temperatures below A1 will result in the same microstructure and hardness. The objective of process annealing is to restore ductility of the cold worked metal. Figure 2 shows the changes of material properties during process annealing.

Figure 2: Change of material properties during process annealing.

There are three phases of process annealing, which are recovery, recrystallization and grain growth. During recovery phase, relief of some of the internal strain energy of a previously cold worked material, often a small drop in hardness, rearrangement of dislocation to form sub grains. However, overall grain shape and orientation remain unchanged. During recrystallization phase, replacement of cold-worked grains by new ones. There are new orientations, a new grain size and a new grain shape. Recrystallization causes the major hardness decrease. During grain growth phase, growth of recrystallized grains at the expense of other recrystallized grains.

At the start, recovery occurs in which there is a change in the stored energy without any obvious change in the optical microstructure. In addition, physical properties such as electrical and thermal conductivities are recovered to their precold-worked states. However, recovery has a negligible effect on the strength and hardness of the material, and the microstructure is unchanged.
Recovery associated with a simple form of plastic deformation is probably a matter of annihilating excess dislocations. Such annihilation can occur by the coming together of dislocation segments of opposite sign (that is, negative edges with positive edges and left-hand screws with right-hand screws). Another recovery process is polygonization. Dislocations become mobile at a higher temperature, eliminate and rearrange to give polygonisation. The misorientation ? between grains can be described in terms of dislocations.  Inserting an edge dislocation of Burgers vector b is like forcing a wedge into the lattice, so that each dislocation is associated with a small change in the orientation of the lattice on either side of the extra half plane. Figure 3 shows realignment of dislocation for the polygonisation.

Figure 3: Realignment of dislocation for polygonization.
This follows recovery during annealing of cold worked material. Driving force is stored energy during cold work. It involves replacement of cold-worked structure by a new set of strain-free, approximately equi-axed grains to replace all the deformed crystals. This process occurs above recrystallization temperature which is defined as the temperature at which about 1/3 to 1?2 of the melting temperature for pure material but is not a constant. Its value is affected by the amount of plastic deformation prior to heating and composition.

A minimum amount of deformation is needed to cause recrystallization. Smaller the degree of deformation, need higher recrystallization temperature. The finer is the initial grain size; lower will be the recrystallization temperature. The larger the initial grain size, the greater degree of deformation is required to produce an equivalent recrystallization temperature. Greater the degree of deformation and lower the annealing temperature, the smaller will be the recrystallized grain size. The higher is the temperature of cold working, the less is the strain energy stored and thus recrystallization temperature is correspondingly higher. The recrystallization rate increases exponentially with temperature.
Grain growth
Grain growth follows complete crystallization if the material is left at elevated temperatures. Grain growth does not need to be preceded by recovery and recrystallization; it may occur in all polycrystalline materials. Incorporation of impurity atoms and insoluble second phase particles are effective in retarding grain growth. Grain growth proceeds more rapidly as temperature increase. Large grains grow at the expense of small ones. Grain boundary area reduce by a process of grain boundary migration, the driving force for grain growth is the release of boundary surface energy, reduction of grain boundary area is shown as figure 4. The average grain size increase with time and depends on temperature too as shown in figure 5.

Figure 4: Reduction of grain boundary area.

Figure 5: Average grain size with time and temperature.

The driving forces for recovery are a decrease in vacancy concentration, energy reduction accompanying the realignment of dislocations into subgrain boundaries, and a reduction of stored elastic energy as stresses are relieved. During recovery subgrains formation, residual stresses are relieved and there is a recovery of most of the original electrical conductivity. There may be some reduction of hardness. The driving force for recrystallization is the reduction of energy as old grains with many dislocations are replaced by new grains with far fewer dislocations. New crystallographic textures are formed. There is a major drop in hardness. The driving force for grain growth is the reduction of grain boundary surface area. New textures are formed. Growth is limited by inclusions. The microstructure of a metal is changed by cold working and process annealing as shown in figure 6.

Figure 6: Changes of microstructure by cold working and process annealing.

Figure 6 (a) shows if the starting metal has already been annealed it will have a comparatively low dislocation density. Figure 6 (b) shows cold working greatly increases the dislocation density. Figure 6(c) shows process annealing leads initially to recovery which is dislocations move to low-energy positions. Figure 6(d) shows new grains nucleate and grow during further energy applied. Figure 6(e) shows the fully recrystallized metal consists entirely of new unreformed grains.
Spheroidizing Anneal
Spheroidizing anneal is also called as soft annealing. Spheroidizing anneal is one of the variant of the annealing process that produces typical microstructure consisting of the globules (spheroid) of cementite or carbides in the matrix of ferrite. The following methods are used for spheroidizing anneal. Holding at just below A1:Holding the steel component at just below the lower critical temperature (A1) transforms the pearlite to globular cementite particles. But this process is very slow and requires more time for obtaining spheroidized structure as shown in Figure 7.
Thermal cycling around A1: In this method, the thermal cycling in the narrow temperature range around A1 transforms cementite lamellae from pearlite to spheroidal. During heating above A1, cementite or carbides try to dissolve and during cooling they try to re-form. This repeated action spheroidises the carbide particles. Spheroidised structures are softer than the fully annealed structures and have excellent machinability by changing all hard constituents like pearlite, bainite, and cementite (especially in steels with carbon contents above 0.5% and in tool steels) into a structure of spheroidized carbides in a ferritic matrix. This heat treatment is utilized to high carbon and air hardened alloy steels to soften them and to increase machinability, and to reduce the decarburization while hardening of thin sections such as safety razor blades and needles.

Figure 7: Process of spheroidizing anneal.

The basic process can be carry out by heating the product to a temperature just below the Ferrite-Austenite line (line A1, 727 ºC). Hold the temperature for a prolonged time, slowly cooling. The physical mechanism of spheroidizing anneal is based on the coagulation of cementite particles within the ferrite matrix, for which the diffusion of carbon is decisive. Globular cementite within the ferritic matrix is the structure having the lowest energy content of all structures in the iron–carbon system. The carbon diffusion depends on temperature and time.
As mention above the time taken to progress spheroidizing anneal consume longer time as compare with other annealing process so, to reduce the time of this process may apply isothermal spherification annealing process. After steel heat insulation, along with the furnace cooled to slightly below the Ar1 temperature isothermal (usually in Ar1 below 10-30?). After isothermal with the slow cooling furnace to about 500? then take out for air cooling. The advantages of it are short period, uniform spheroidization and easy quality control.

Process Annealing Spheroidizing Anneal
Usually process time 1hour 24hours
Temperature used for carry out the process 600-650? Just under A1
Application Sometimes used to selectively heat treat localized cold worked area.

Used in production of steel wire, nails etc. Sometimes buy steel in spheroidized condition for good dimensioning on machining and then heat treat later.

Cooling time Cooling in still air up to room temperature. Very slow cooling up to room temperature
Holding style Isothermal holding Isothermal holding
Microstructure Globular carbides New and free stress grain
Type of steel Hypoeutectoid steel Hypereutectoid steel
Material properties Ductility increase, hardness decrease, toughness increase Ductility increase, hardness decrease, toughness increase.

Energy consumption Lower energy consumed Higher energy consumed
Table 1: Similarly and difference of process annealing and spheroidizing anneal.

In the conclusion, the process, characteristic, precaution and recommendation of process annealing and spheroidising anneal are discussing as above. The aim to applied process annealing is to negate effect of cold working by recovery or recrystallization; the aim of spheroidizing anneal is to make very soft steels for good machining. Table 1 shows some differences and similarly of process annealing and spheroidizing anneal. Process annealing need shorter time and lower temperature than spheroidizing anneal. Process annealing usually carry out with hypoeutectoid steel but spheroidizing anneal carry out with hypereutectoid steel. Both processes use to achieve higher ductility, higher toughness and lower brittleness. Spheroidizing anneal may consumed more energy than process annealing because the longer time and higher temperature are used.
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