In the previous section, we consider the role of alloying elements regarding to the phase stability and the rate of phase transformation. Here in the final section of this lecture, we are going to think about the hardening effect, led by alloying element. There are two representative hardening effects generated by the alloying elements; one is solid solution hardening and the other is the precipitation hardening. The solid solution hardening increase the strength of the materials by solute atoms. The presence of the solute atom in the solvent matrix is likely to distort the lattice structure due to the difference in the atomic size. Of course the lattice distortion is not the only effect of solute atom. And it may change the modulus of material locally and may generate the short-range ordering. Whatever effect the solute atom generates, it is likely to interact with the dislocation in the material, acting as effective obstacles for the dislocation motion which in turns out to increase the strength of the material. This graph compares the ability of various alloying elements to generate the solid solution hardening effect in iron and steel. Boron, carbon, nitrogen, and phosphorus are the most powerful alloying element regarding to the solid solution hardening. But you have to remember that the solubility of boron, carbon, nitrogen is quite limited in BCC iron and phosphorus is to cause a severe embrittlement in iron. Therefore silicon, manganese, and copper are most frequently used in practice to expect the solid solution hardening in steel. The precipitation of fine particles in the matrix phase is also effective in increasing the strengths of materials. In making fine precipitates the steel matrix, alloying elements which have a propensity to form carbides are often used. As I mentioned in the previous section, titanium, niobium, beryllium, chromium molybdenum, and tungsten are typical other element for this purpose. This is a transmission electro micrograph showing a nano-sized titanium carbide particles in actual precipitate-hardened steel. To apply the precipitation hardening in more effective way, not only addition of proper element but also control of the size and distribution of precipitate is very important. It is because the interaction between precipitate and dislocation is strongly affected by the size and distribution of the precipitate. When the precipitate size is small and thus the spacing between the precipitate is small and the interface between the precipitate and matrix is coherent, then the dislocation is likely to cut the particle during the glide on the slip plane. In this case, more strengthening is expected with larger precipitate. However when the size of precipitate is too large to main to maintain the coherency at interface, the dislocation have to bypass the precipitate during the glide. In this situation, the stress for the dislocation bypass decreases with increasing precipitation size, therefore more strengthening is expected with a smaller precipitate. By counterbalancing of this two types of interaction, the maximum precipitation hardening effect is expected at certain critical size of precipitate, as shown here, which we have to come up with for making full use of the precipitation hardening. While the main purpose of precipitation of carbide in steel is a strengthening effect, it is also applied for microstructure control, in particular, the control of austenite microstructure in high-temperature process. Hot rolling is one of the typical high temperature process in making steel product like plate and sheet. During the hot rolling, the austenite is deformed to have elongated grains with many deformation substructures. Because the deformed substructures have higher stored energy, the restoration process which is called as recrystallization occurs to reduce the stored energy by nucleating and growth of new grains. However sometimes you would like to keep the deformed austenite grains until they transform into low-temperature microstructure. It is because the deformation substructure in austenite is beneficial in obtaining very fine final microstructure after the austenite decomposition. One of the way to keep the deformed structure in austenite is conducting hot rolling at lower temperature where the recrystallization is difficult to occur. However the hot rolling at lower temperature is not that easy because the workability is deteriorated as we decrease the temperature. In this situation the precipitation of certain carbide is very useful in suppressing the occurrence of the recrystallization at high temperature. The niobium, titanium, and vanadium carbide are known to be very effective in increasing the temperature for the occurrence of recrystallization as shown in this figure. This is because the precipitation of those carbides on the dislocation makes the restoration process sluggish and thus the recrystallization is difficult to happen. Therefore by proper addition of those carbide forming elements, we can keep the deformation structure of austenite even at high temperature giving fine microstructure after the decomposition of austenite. These alloying elements to control the microstructure of austenite and to obtain the precipitation hardening at the same time are called as micro-alloying element which has been essential concept for the development of high strength low alloy steels.
