Heat treatment of steels gives different forms of modified micro-structures and modified desired physical, chemical and strength properties. Process of heat treatment does phase transformations during heating and cooling after changing a micro-structure in a solid state. These processes are mostly thermal only (Heating and cooling) and changes only micro-structure. The object is heated, held at that temperature and then cooled. Heating rate and more important is the cooling rate which controls the micro-structure and hence controls the physical, chemical and strength properties.
Processes of Heat Treatment
According to cooling rate, there are two main heat treatment processes:
1. Annealing – slow cooling rate (in air or within a furnace)
2. Quenching – fast cooling rate (in oil or in water)
Produces equilibrium structures according to the Fe-Carbon diagram. It relieves the internal stresses, increases the ductility and improves the machinability.
Gives non-equilibrium structures (martensite) and increases hardness, brittleness as well as strength.
TYPES OF ANNEALING
(i) Normalizing or Full Annealing
(iii) Stress relieving
This process increases ductility, toughness and strength and cooling is done in STILL AIR. In this, the object is heated to temperature is 30-50°C above A3 (8700C) in Austenite field. It is held at this temperature (called soaking temperature) for few hours. The soaking temperature depends on carbon content. After soaking the object, it is cooled in STILL AIR. This cooling produces small grain size structure. The small grain size structure improves both toughness and strength (especially yield strength). During normalizing, there is Allotropic transformation upon heating γ→α
Important: There is no change of grain size during cooling of Austenite in normalizing.
Spherodising increases ductility. The process is limited to cold worked objects of steels containing carbon > 0.5%. The object is heated up to lower critical temperature A1 (727°C). At this temperature, cold worked Ferrite will recrystallize and the iron carbide present in the form of Pearlite will appear as spheres or “balls ”. This change of carbides shape reduces the strength and hardness. Thus material becomes soft and ductile.
Quenching increases hardness, water quenching gives more hardness than air quenching. Soaking temperature is 30-50°C above A3 (8700C) or A1(7270C), then fast cooling (in water or oil) with critical cooling rate. The critical cooling rate is required to obtain non-equilibrium structure called marten site. During fast cooling Austenite cannot transform to Ferrite and Pearlite by atomic diffusion. The critical cooling rate is the slowest speed of quenching that will ensure maximum hardness (full Martensitic structure). Marten site is Tetragonal body centered cubic structure. Marten site is very hard and brittle. Marten site has a “needle-like”. Marten site is a distorted lattice structure. The speed of quenching, type of quenching medium used and the size of component affect the amount of Marten site formed. Quenching provides increased hardness.
There increases in toughness. Tempering is done after quenching. This process is carried out on hardened steels to remove the internal stresses and reduce brittleness created by quenching. In tempering, steel is heated to a temperature between 200 and 600°C depending upon the final properties desired. This heating removes carbon atoms to diffuse out of the distorted lattice structure associated with marten site, and thus relieve some of the internal stresses. Thus the hardness is reduced; the ductility and the toughness are increased. The higher is the tempering temperature, the greater will be the toughness to absorb bigger shocks.