Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 5M

Year: Dec 2015

Almost all alloying elements(except,Al,Co,Si) increases the stability of supercooled austenite and retard both proeutectoid and the pearlitic reaction and then shift TTT curves of start to finish to right or higher timing.This is due to i)low rate of diffusion of alloying elements in austenite as they are substitutional elements,ii)reduced rate of diffusion of carbon as carbide forming elements strongly hold them.iii)Alloyed solute reduce the rate of allotropic change,i.e.γ→α,by solute drag effect on γ→α interface boundary.Additionally those elements (Ni,Mn,Ru,Rh,Pd,Os,Ir,Pt,Cu,Zn,Au) that expand or stabilize austenite,depress the position of TTT curves to lower temperature.In contrast elements (Be,P,Ti,V,Mo,Cr,B,Ta,Nb,Zr) that favour the ferrite phase can raise the eutectoid temperature and TTT curves move upward to higher temperature. However Al,Co,andSi increase rate of nucleation and growth of both ferrite or pearlite and therefore shift TTT diagram to left. In addition under the complex diffusional effect of various alloying element the simple C shape behavior of TTT diagram get modified and various regions of transformation get clearly separated. There are separate pearlitic C curves, ferritic and bainitic C curves and shape of each of them are distinct and different.

The effect of alloying elements is less pronounced in bainitic region as the diffusion of only carbon takes place (either to neighbouring austenite or within ferrite) in a very short time (with in a few second) after supersaturated ferrite formation by shear during bainitic transformation and there is no need for redistribution of mostly substitutional alloying elements.Therefore bainitic region moves less to higher timingin comparison to proeutectoid/pearlitic region. Addition of alloying elements lead to a greater separation of there actions and result separate C-curves for pearlitic and bainitic regions