Unit 2

Heat Treatment of Iron and Steels

• To increase strength, hardness and wear resistance (bulk hardening, surface hardening)
• To increase ductility and softness (Tempering, Recrystallization Annealing)
• To increase toughness (Tempering, Recrystallization annealing)
• To obtain fine grain size (Recrystallization annealing, Full annealing, Normalizing)
• To remove internal stresses induced by differential deformation by cold working,
nonuniform cooling from high temperature during casting and welding (Stress relief
annealing)
• To improve machinability (Full annealing and Normalizing)
• To improve cutting properties of tool steels (Hardening and Tempering)
• To improve surface properties (surface hardening, high temperature resistanceObjective
• To improve surface properties (surface hardening, high temperature
resistanceprecipitation hardening, surface treatment)
• To improve electrical properties (Recrystallization, Tempering, Age hardening)
• To improve magnetic properties (Hardening, Phase transformation)
Different Types Of Steels, their Properties and Uses
 1. Annealing

• Annealing is a process involving heating and cooling, usually applied to produce

• The term also refers to treatments intended to alter mechanical or physical properties,
produce a definite microstructure, or remove gases.

• The temperature of the operation and the rate of cooling depend upon the material
being annealed and the purpose of the treatment.
a. Full Annealing

Full annealing is a softening process in which a steel is heated to a temperature above the
transformation range (Acs) and, after being held for a sufficient time at this temperature, is cooled
slowly to a temperature below the transformation range (Art). The steel is ordinarily allowed to cool
slowly in the furnace, although it may be removed and cooled in some medium such as mica, lime,
or ashes, that insures a slow rate of cooling. Since the transformation temperatures are affected by
the carbon content, it is apparent that the higher carbon steels can be fully annealed at lower
temperatures than the lower carbon steels.steels must be heated to above the line GOSK. The
temperature range normally used for full annealing is 25 to 50 deg F above this line (the upper
critical). The microstructures of the hypoeutectoid steel -> that result after full annealing are quite
similar to equilibrium conditions. Eutectoid and hypereutectoid steels frequently spheroidize
partially or completely on full annealing.
 Normalizing

• Normalizing is a process in which a steel is heated to a temperature above the Ac3 or the Acm
and then cooled in still air. The purpose of the treatment is to obliterate the effects of any
previous heat treatment (including the coarse-grained structure sometimesresulting from high
forging temperatures) or cold-working and to insure a homogeneous austenite on reheating for
hardening or full annealing. The resultant structures are pearlite or pearlite with excess ferrite
or cementite, depending upon the composition of the steel.

• They are different from the structures resulting after annealing in that, for steels of the same
carbon content in the hypo- or hypereutectoid ranges, there is less excess ferrite or cementite
and the pearlite is finer. These are the results of the more rapid cooling. Since the type of
structure, and, therefore, the mechanical properties, are affected by the rate of cooling,
considerable variations may occur in normalized steels because of differences in section
thickness of the shapes being normalized.
 Hardening

• Steels can be hardened by the simple expedient of heating to above the Ac3
transformation, holding long enough to insure the attainment of uniform temperature
and solution of carbon in the austenite, and then cooling rapidly (quenching).
Complete hardening depends on cooling so rapidly (fig. 9, A and B) that the austenite,
which otherwise would decompose on cooling through the Arlf is maintained to
relatively low temperatures. When this is accomplished, the austenite transforms to
martensite on cooling through the M,—Mf range. Rapid cooling is necessary only to
the extent of lowering the temperature of the steel to well below the nose of the S-
curve. Once this has been accomplished, slow cooling from then on, either in oil or in
air, is beneficial in avoiding distortion and cracking. Special treatments, such as time
quenching and martempering, are designed to bring about these conditions. As
martensite is quite brittle, steel is rarely used in the as-quenched condition, that is,
without tempering. The maximum hardness that can be obtained in completely
hardened low-alloy and plain carbon structural steels depends primarily on the carbon
content.
 Case Hardening

• Case hardening is a process of hardening a ferrous alloy so that the surface layer or case is
made substantially harder than the interior or core. The chemical composition of the surface
layer is altered during the treatment by the addition of carbon, nitrogen, or both. The most
frequently used case-hardening processes are carburizing, cyaniding, carbonitriding, and
nitriding.