HEAT TREATMENT

Intro & Annealing

Introduction A combination of heating and cooling operations, timed

and applied to metal or alloy in the solid state in a way that will produce desired properties ASM Metals Handbook

Modification of microstructure through a thermal process in order to get certain properties

Introduction

General procedure: heating holding cooling

heat

ing

Holding (time)

cooling

Introduction

Annealing Hardening Tempering Surface hardening Do not forget about I-T and C-T diagram More about steels

Introduction

Annealing

A heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled

Purposes:- Relieve stresses- Increase softness, ductility, and toughness- Produce a specific microstructure

Annealing

Stages: heating to desired temperature, holding or soaking, then cooling

Time and temperature dependence;- Internal stress- Sufficient time for transformation- Diffusion

Annealing

Process annealing Stress relief annealing Normalizing Full annealing Spheroidizing Homogenizing

Range of Annealing Temperature

Plain carbon steels More & detail process in heat treaters

guide handbook

Process annealing Heat treatment that is used to negate the effects of

cold workthat is, to soften and increase the ductility of a previously strain-hardened metal

It is commonly utilized during fabrication procedures that require extensive plastic deformation, to allow a continuation of deformation without fracture or excessive energy consumption

Recovery and recrystallization processes are allowed to occur fine grained microstructure heat treatment stop before grain growth

Process annealing Surface oxidation or scaling may be prevented or

minimized by annealing at a relatively low temperature (but above the recrystallizationtemperature) or in a nonoxidizing atmosphere

Some people call it recrystallization annealing Common: 1 hour at 600-650 C Yield strength and tensile strength drastically

reduced Commonly used in the production of steel wires,

nails, etc

Process annealing

Stress relief annealing Heat treatment that is used to eliminate internal or

residual stresses in metallic components Internal stress distortion and warpage Source of internal stress:

- plastic deformation processes such as machining and grinding - nonuniform cooling of a piece that was processed or fabricated at an elevated temperature, such as a weld or a casting- a phase transformation that is induced upon cooling wherein parent and product phases have different densities

Stress relief annealing

General process: heating to the recommended temperature, held there long enough to attain a uniform temperature, and finally cooled to room temperature in air

Annealing temperature relative low, up to 678 C effects of cold working and other heat treatments are not affected

Full annealing Heat treatment that is used to fully soften a carbon

steel, such that the steel is in its most ductile state Often utilized in low- and medium carbon steels that

will be machined or will experience extensive plastic deformation during a forming operation.

Process: heating to about 50 C above A3 line for hypoeutectoid steel, and 50 C above A1 for hypereutectoid steel austenitizing holding furnace cooled

Full annealing

The microstructural product of this anneal is coarse pearlite (in addition to proeutectoidphase)

Time consuming Yields a microstructure having small grains

and a uniform grain structure Typical cooling rate 1 C/min

Normalizing Heat treatment that is used to refine the

grains (i.e., to decrease the average grain size) and produce a more uniform and desirable size distribution; fine-grained pearlitic steels are tougher than coarse-grained ones

Process: heating to about 55 C above A3 line for hypoeutectoid steel, and 55 C above Acmline for hypereutectoid steel austenitizing holding air cooled

Normalizing

Cooling rate 5-10 C/min Produce fine pearlite structurehigher

strength, hardness than product of full annealing

Full annealing vs Normalizing

C-T diagram shows the difference in the cooling rate and final structure

Full annealing vs Normalizing

Microstructure mechanical properties

Spheroidizing Heat treatment that is used to develop spheroidite

structure Spheroidized steels have a maximum softness and

ductility and are easily machined or deformed improving machinability

Common for medium and high carbon steels Coalescence of the Fe3C to form the spheroid particles

during spheroidizing To some degree, the rate at which spheroidite forms

depends on prior microstructure. For example, it is slowest for pearlite, and the finer the pearlite, the more rapid the rate. Also, prior cold work increases the spheroidizing reaction rate.

SpheroidizingMethods: Heating the alloy at a temperature just below the

eutectoid or at about 700 C in the +Fe3C region of the phase diagram. If the precursor microstructure contains pearlite, spheroidizing times will ordinarily range between 15 and 25 h.

Heating to a temperature just above the eutectoid temperature, and then either cooling very slowly in the furnace, or holding at a temperature just below the eutectoid temperature.

Heating and cooling alternately within about 50 C of the A1 line

Spheroidizing

Homogenizing

Heat treatment that is used to eliminate the effect of segregation

Time consuming Requires annealing after homogenizing

Mechanical properties after annealing

Difference of microstructure leads to difference in mechanical properties