This is the second section on the role of alloying element in steel. In this section, we are going to consider what kind of alloying elements are used in the steel products in practice and what kind of effects are expected from the alloying addition. The general role of alloying element can be classified into three categories The first one is to improve the mechanical property such as strength and ductility and toughness and it is led by the ability of alloying element, generating solid solution hardening, or forming fine particles to cause precipitation hardening. Some alloying elements are known to change the fracture mode from brittle manner to ductile fracture which often improves the toughness of steel product, particularly, at low temperature. Second role of alloying element is controlling the phase transformation behaviour. As I mentioned in the previous class, the most important phase transformation in steel will be the austenite decomposition. The presence of specific alloying elements in steel affects the transformation kinetics of austenite decomposition as well as the thermodynamic stability of the constituent phases. The hardenability or recrystallization behavior of austenite can be controlled with specific alloying elements which manipulates the microstructure as we intended for the improvement of the steel products. Finally, the alloying element are added to obtain specific functions in steel products. Increasing corrosion or oxidation resistance, improving magnetic properties are representative ones. This slide summarizes representative alloying elements generally used in steel. The alloying elements in gray color are often used to increase the strength of the steel by solid solution hardening effect. Manganese, carbon, silicon, or nickel are representative alloying elements. The alloying elements in blue color have higher tendency to form a compound with carbon which is called carbide. The fine particles of carbide can either increasing the strength by precipitation hardening, or control the microstructure by acting as an obstacle to the motion of interface or dislocation. The alloying element indicated with broken line are known to affect the phase transformation kinetics, in particular, the decomposition of austenite. It provides us more effective way to control the microstructure by various kind of heat treatments, eventually giving us a chance to create high-performance steel products. The role of nickel is somewhat special in steel. Nickel is an effective element in improving toughness at lower temperature. Besides, nickel can stabilize the austenite, so some stainless steels contain considerable amount of nickel, making the austenite microstructure stable even at room temperature. In application of alloying element one thing we have to remember is that one element does not necessarily has a single effect. For instance, when we add chromium in steel it will increase the oxidation resistance but at the same time it increased hardenability and also affect the precipitation behavior. Therefore, for optimal alloy design, those kinds of side effects and also the cross effect between alloying elements should be precisely taken into account. When we add alloying element in iron and steel, it will intrinsically affect the free energy of the constituent phases and any change in the free energy of constituent phases eventually brings about subsequent changes in the equilibrium phase diagram. Even without detailed knowledge on the thermodynamic properties, a glance at the binary phase diagram let us know the influence of the alloying element on the phase stability. When you look into the change of phase diagram as the increase of alloying elements, their effect can be categorized into two type. One is that expanding the austenite stable region and the other is that increasing the ferrite stable domain. Alloying elements like manganese, nickel, carbon and nitrogen is representative alloying element expanding the austenite stable region. These elements actually increase the thermodynamic stability of austenite, so they are called as austenite stabilizer. On the contrary, the elements such as silicon, aluminum, or phosphorus increase the ferrite stable domain as their content increase. Those elements are likely to increase the stability of ferrite so they are called as ferrite stabilizer. The more important effect of alloying elements on the constituent phase is coming from their influence on the rate of phase change. Let's consider an iron-carbon binary alloy of which carbon concentration is around 0.77, corresponding this point. At the initial condition the alloy is kept at temperature T1, let's say 1000 degree Celsius. And then we rapidly cool down the sample to T2, let's say 600 degree Celsius. At very first, the austenite can exist even at T2 but it is unstable at T2 so the pearlite start form at the grain boundary of the austenite and it will eventually cover the entire microstructure. Even though the equilibrium phase diagram let us know that the equilibrium phase at temperature T1 is austenite and it will transform into pearlite when we decrease the temperature to T2, it does not contain any information on the time of onset of the transformation or how long it will take to transform into pearlite. The character of time-dependent progress of phase transformation is captured in the time-temperature-transformation diagram shown in this figure. The time temperature transformation diagram for specific chemical composition contains the information on the time-dependent microstructure evolution related to the austenite decomposition. For example, at 600 degrees Celsius, the diagram tell us that the pearlite transform starts in two second and it will complete in 10 second. It will also indicate when we lower the transformation temperature down to let's say 500 degrees Celsius then the product phase will be bainite, not pearlite. Because the time-temperature-transformation diagram exhibits the character of time dependent austenite decomposition, naturally, it contains the information on the evolution of non-equilibrium microstructure. We can investigate the influence of alloying element on the rate of phase change with the time temperature transformation diagram. However, in practice, we encounter the transformation behavior during continuous cooling from austenitization temperature more often. Here let's consider the alloying effect on the rate of phase change in the frame of another diagram showing the progress of phase transformation for a given cooling rate from austenitization temperature. It is called as continuous cooling transformation diagram. In constructing continuous cooling transformation diagram, we consider the initiation and the completion of phase transformation for specific cooling rate. For instance, this continuous cooling transformation diagram illustrates the initiation and the completion of the pearlite transformation by this point with this specific cooling rate. For the steel composition in this continuous cooling transformation diagram, it tells us that we will obtain pearlite microstructure when the cooling rate is slow enough. But if we increase the cooling rate to this critical level, the pearlite transformation does not happen and the austenite will transform into martensite around 200 degrees Celsius. The implication of the effect of alloying element on the rate of phase change is that the addition of alloying element will shift the transformation curve in the continuous cooling transformation diagram. For instance, if we increase the content of alloying element such as manganese or chromium, it is likely to shift the pearlite transformation curve to longer time side as shown in this right diagram which means the austenite decomposition into pearlite is retarded by the addition of alloying element. With the influence of alloying elements on the rate of phase change, now we can obtain the martensite with rather slower cooling rate, for example with this cooling rate, which cannot be possible in the this original alloy. The influence of alloying elements on the kinetics on the austenite decomposition let us control the microstructure of steel more effectively with practically available heat treatment condition. This figure summarized the effect of various alloying elements on the transformation kinetics of austenite decomposition. It is interesting to see that most of the alloying elements retard the ferrite, pearlite and bainite transformation and decrease the martensite starting temperature. This figure suggests that we can obtain the martensite microstructure even at relatively slow cooling rate as long as the steel contains proper amount of alloying elements such as carbon, manganese, chromium and so on. Then let's consider cooling of steel sample having large dimension. The surface of the sample contacting with the cooling medium is likely to be cooled down quickly but the cooling rate of the sample interior will not be that fast. This situation causes different cooling pattern depending on the distance from the sample surface and it possibly generates inhomogeneous microstructure and mechanical properties. That is because the interior of the sample does not have a cooling rate enough to avoid any high temperature transformation product such as ferrite or pearlite but the surface layer has higher chance to transform into martensite because of high cooling rate. This inhomogeneity in microstructure and mechanical properties can be alleviated by addition of proper alloying element which can retard the transformation kinetics, so that the formation of ferrite or pearlite is effectively suppressed even at lower cooling rate. These two graphs compare the actual hardness distribution of two steels across the diameter of the sample after cooling in water. You can see clearly that the steel containing chromium has higher hardness inside of the sample which is led by suppression of the formation of ferrite or pearlite by the retardation of the transformation kinetics.
