Material Properties: Alloying, Heat Treatment, Mechanical Working and Recrystallization!
Method # 1. Alloying:
Alloy steels are broadly classified into two categories:
(i) Low Alloy Steels
(ii) High Alloy Steels
In low alloy steels, the total content of the alloying elements, such as Cr, Ni, Mo, V, and Mn, is kept within 5%. Each alloying element imparts a specific property to the original material.
Method # 2. Heat Treatment:
Control of material properties can also be achieved without the addition of other elements. This is done by subjecting the material to a controlled cycle of heating and cooling. To illustrate this, let us take a simple example where austenite steel (above 723°C) is cooled at different rates. Figure 1.29 shows the various resulting structures along with a few mechanical properties.
It is thus obvious that by changing only the rate of cooling, different phases can be achieved. The information on the change of phase with the cooling rate can be conveniently displayed with the help of a time-temperature-transformation diagram (commonly known as the TTT diagram).
In such a diagram, the temperature is plotted along the vertical axis (using a linear scale), whereas the abscissa represents the time on a logarithmic scale. The TTT diagram for carbon steel is given in Fig. 1.30. When austenite is brought to a temperature θ1 from θ0 (in essentially zero time) and thereafter held at θ1 the transformation to pearlite begins after a lapse of time t1, as shown by the point A in the figure.
Such a transformation, taking place at a constant temperature, is known as an isothermal transformation. The point B indicates a time t2, after which the transformation is complete. In Fig. 1.30, the transformations corresponding to other temperatures, viz., θ2 and θ3 are also shown. At about 600°C, the transformation starts after a minimum lapse of time, and this part of the diagram is called the nose.
Below this temperature, austenite transforms into bainite which is an intimate mixture of ferrite and cementite (cementite exists in the form of tiny spheroids). Bainite cannot be produced by continuous cooling.
When the temperature of isothermal transformation is decreased (above 600°C), the time required for the transformation reduces. This results in a finer grain structure as less time is available for the growth of new nuclei.
It may be noted that the curve, indicating the beginning of the transformation, does not exist below about 220°C. Below this temperature, austenite instantaneously starts transforming into martensite.
A TTT diagram is quantitatively valid only when the transformations are isothermal. In practical situations of heat treatment where a continuous cooling is involved, a modified TTT diagram (see Fig. 1.31) is used. The use of such a diagram can be explained as follows. If the cooling rate is very high (as shown by cooling curve 1), the entire austenite is transformed into martensite because the cooling curve does not enter the pearlite region.
With a moderate cooling rate (depicted by cooling curve 2), a portion of austenite is transformed into pearlite and the rest of it into martensite. The percentage of pearlite depends on the point of intersection of the cooling curve and the line AB (Fig. 1.31). When this point is nearer^, lesser pearlite is produced. So, when the desired structure is prescribed, steel should be heated beyond 723°C (where it is austenitic) and then cooled in a manner dictated by the TTT diagram.
We shall now briefly discuss some common heat treatment processes.
This refers to secondary heating of martensite obtained by a rapid cooling of austenite (Fig. 1.32). during this process, no change of phase takes place because the temperature is never raised beyond the lower critical temperature (723oC) this process hardens the steel with reduction in strength, also, it adds to the toughness and ductility. The different structures, indicated in Fig. 1.32, result from dispersion of carbides.
It includes, among others, full annealing, stress relieving, and process annealing.
The general purposes served by annealing are –
(a) Alteration of ductility and toughness,
(b) Induction of softness,
(c) Refinement of grain structure, and
(d) Removal of gases and stresses.
Full annealing consists in heating to a suitable (beyond the critical) temperature, maintaining this temperature for a definite period of time (to allow complete transformation into austenite) followed by slow cooling.
When only the ductility is increased, the process is called annealing, where the maximum temperature is lower than the critical temperature.
Stress relieving is similar to process annealing and the temperature does not go beyond the critical value. At higher temperatures, the lattice atoms move to rearrange themselves, relieving the internal stresses. Also, no change in the micro structure takes place.
This process is very similar to annealing. Here, the specimen is heated beyond the upper critical temperature and is cooled in still air rather than in the furnace. Therefore, the rate of cooling gets increased and this, in turn, results in slight hardening as well as loss of ductility unlike in annealing. This process improves strength and machinability.
This is a special type of tempering where the specimen is reheated to just below the lower critical temperature. By this process, the carbide in the steel is transformed into a globular form. This makes the steel relatively soft, machinable, and suitable for subsequent hardening treatment.
v. Case Hardening:
For low carbon steels, the nose of the TTT diagram suggests that no practical cooling rate can achieve direct transformation of austenite into martensite. One way to solve this problem is to shift the nose to the right through alloying. However, as hardness is normally required only at the surface of the specimen, alloying of the whole specimen is not necessary.
There are various treatments to impart surface hardness, such as- (I) carburizing, (II) nitriding, and (III) cyanide hardening. A process using any of these treatments is called case hardening.
(I) In carburizing, the specimen is heated beyond the upper critical temperature in a sealed container having the atmosphere of carbon. The heating is continued for 4-10 hours depending on the depth of penetration required. As a result, carbon diffuses into the surface layer, making the specimen harder.
(II) Nitrogen, in place of carbon, can also be used as a hardening agent. Here, the ammonia atmosphere is used. Also, the temperature required is about 1/2-1/3 of that in carburizing, but the heating is continued for a period almost twice that in carburizing. Normally, alloy steels containing chromium, vanadium, molybdenum are subjected to nitriding.
(III) Using the sodium cyanide atmosphere, both carbon and nitrogen can be effectively employed for imparting hardness. During heating, the nitrides are formed, whereas during subsequent quenching, the carbides are formed. Since heat is dissipated through the surface of a specimen, it is possible to achieve a high cooling rate if the heat content of the specimen is confined to the surface layer.
Higher hardness is normally required at the surface. So, if the surface layer is quickly heated to a suitable high temperature (keeping the core temperature unaffected) and then rapidly quenched, the austenite at the surface gets hardened. The two methods generally used for doing this are (a) flame heating and (b) induction heating. Such surface hardening treatments are normally done on steels containing 0.35% or more carbon.
(a) Flame Hardening:
In this, an oxyacetylene flame is moved over the specimen followed by a quenching spray (Fig. 1.33). The velocity of the flame and the material property determine the depth of the layer being hardened. Flame hardening does not produce any sharp boundary between the hardened layer and the core; as a result, there is no danger of the surface layer chipping out. The flame should be kept at a sufficient distance from the sharp corners to avoid overheating.
(b) Induction Hardening:
Here, the heating is done by placing the specimen in a high frequency magnetic field. The depth of penetration decreases as the frequency increases. So, the surface hardening of thin-walled sections requires high frequency.
Age Hardening and Precipitation Hardening:
This process of hardening is applicable only for those alloys that exist as a two-phase material at the room temperature and can be heated up to a single phase. The phase diagram of one such alloy is shown in Fig. 1.34. Assuming that the composition is 3% Cu and 97% Al, the alloy exists as a two-phase material (α + β) below a temperature θ1.
However, between θ1 and θ 2, it exists as a single-phase solid solution (α). A solution heat treatment process consists in heating the alloy to a temperature between θ1 and θ3. Also, a sufficient time is given at this temperature for the material to homogenize. A subsequent rapid quenching does not allow all the β-phase to separate out. Thus, the solution becomes supersaturated.
This supersaturated β-phase precipitates slowly, the rate being dependent on the final temperature after quenching. The precipitation takes place at the grain boundaries and crystallographic planes, making the slippage of atomic layers more difficult. Thus, the alloy becomes harder and stronger.
If the precipitation takes place at the room temperature, a longer time is necessary for the completion of precipitation, and this process is referred to as age hardening. On the other hand, if the precipitation rate is increased by quenching the specimen to a temperature higher than the room temperature, the process then is called precipitation hardening.
Depending on the precipitation temperature, the hardness (at the room temperature), instead of increasing continuously, may attain a maximum before it starts decreasing.
The optimal properties are very sensitive to both the temperature (at which the precipitation takes place) and the time elapsed after quenching. The nature of the variation of hardness with these two parameters is shown in Fig. 1.35.
Method # 3. Mechanical Working and Recrystallization:
The mechanical properties, e.g., strength and hardness, of a polycrystalline material are also governed by the grain size of the material. Figure 1.36 shows the nature of variation of two mechanical properties with the average grain size.
The grain size can be controlled by mechanical working and recrystallization. As we have already noted, the effective size of a grain is decided by the volume to surface area ratio. Figure 1.37 shows how the grains are deformed by various mechanical working processes.
Here, we can easily see that the effective grain size is reduced because the surface area of each grain increases, whereas the volume remains the same.
It is possible to restore the original grain geometry by heating the material up to a temperature where the new fine grains are formed. This process is called recrystallization and the temperature needed to achieve this is known as the recrystallization temperature. If the material is kept for a considerable period of time at this temperature, the newly formed fine grains grow in size.
On the other hand, if the material is cooled quickly, the grain size remains small. It should be noted that in the mechanical working processes, the grains are deformed, whereas in recrystallization, new grains are formed and, what is more, their size can be controlled. Normally, the recrystallization temperature of a metal is about 40% of its melting temperature in the absolute scale but this temperature depends also on the amount of prior cold work.
Figure 1.38a shows the variation of mechanical properties with the amount of mechanical working. Apart from changing the mechanical properties, the amount of cold work also governs the recrystallized grain size (see Fig. 1.38b).