Spheroidizing - Mechanism

� The solubility of carbon in ferrite, which is very low at room temperature (0.02% C), increases considerably up to the Ac1

temperature

� At temperatures close to Ac1, the diffusion of carbon, iron, and alloying atoms is so great that it is possible to change the structure in the direction of minimizing its energy content

Spheroidizing - Process� Prolonged heating at a temperature just bel

ow the lower critical temperature, usually followed by relatively slow cooling

� In the case of small objects of high C steels,the spheroidizing result is achieved more ra

pidly by prolonged heating to temperaturesalternately within and slightly below the criticaltemperature range

� Tool steel is generally spheroidized by heatingto a temperature of 749°-804°C and higher formany alloy tool steels, holding at heat from 1 to4 hours, and cooling slowly in the furnace

CASE HARDENING

�Case hardening or surface hardening is the process of hardening the surface of a metal, often a low carbon steel, by infusing elements into the material's surface, forming a thin layer of a harder alloy.

�Case hardening is usually done after the part in question has been formed into its final shape

Case-Hardening - Processes

� Flame/Induction Hardening

�Carburizing

� Nitriding

�Cyaniding

�Carbonitriding

Flame and induction hardening

� Flame or induction hardening are processes in which the surface of the steel is heated to high temperatures (by direct application of a flame, or by induction heating) then cooled rapidly, generally using water

� This creates a case of martensite on the surface.

� A carbon content of 0.4–0.6 wt% C is needed for this type of hardening

� Application Examples -> Lock shackle and Gears