Heat Treatment of Metals

Heat-Treatment  Heat treatment is a method used to alter the physical, and sometimes chemical properties of a material. The most common application is metallurgical  It involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material It applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally Generally, heat treatment uses phase transformation during heating and cooling to change a microstructure in a solid state.

Types of Heat-Treatment (Steel)  Annealing  Tempering, and Quenching  Precipitation hardening  Case hardening

Annealing A heat treatment process in which a metal is exposed to an elevated temperature for an extended time period and then slowly cooled. Purpose: Relieve stresses of cold working Increase softness, ductility and toughness Produce specific microstructure

Annealing Three Stages of Annealing 1. Heating to a desired temperature 2. Holding or soaking at that temperature 3. Cooling usually to room temperature Note: Time in above procedures is important - During heating and cooling temp gradients exit b/w inside and outside portions of part. If rate of temp change is tool high, temp gradients will induce internal stress in part and hence cracking 2   1 3 T T α+Fe3C α+Fe3C Time

Types of Annealing Stress-Relief Annealing (or Stress-relieving) Normalizing Full Annealing Spheroidizing Annealing (or Spheroidizing ) Isothermal Annealing

Iron-C Phase Diagram A B

Temp Ranges in Fe-C Phase Diagram A1. Lower critical Temp A3. Upper critical Temp for Hypo- eutectoid steels Acm. Upper critical Temp for Hyper- eutectoid steels    Fe3C T    Eutectoid α+Fe3C

Temp Ranges for Annealing Processes

1. Stress-Relief Annealing  It is an annealing process below the transformation temperature A1, with subsequent slow cooling, the aim of which is to reduce the internal residual stresses in a workpiece without intentionally changing its structure and mechanical properties

1. Stress-Relief Annealing  For plain carbon and low-alloy steels the temperature to which the specimen is heated is usually between 450 and 650˚C, whereas for hot-working tool steels and high-speed steels it is between 600 and 750˚C  This treatment will not cause any phase changes, but recovery & recrystallization may take place.  Machining allowance sufficient to compensate for any warping/distrotion resulting from stress relieving should be provided

Causes of Residual Stresses Mechanical factors (e.g., cold-working during metal forming/machining) Thermal factors (e.g., thermal stresses caused by temperature gradients within the work-piece during heating or cooling) Metallurgical factors (e.g., phase transformation upon cooling wherein parent and product phases have different densities - In the heat treatment of metals, quenching or rapid cooling is the cause of the greatest residual stresses

Stress Relief Annealing – Temperature & Time Vs Stresses  Higher temperatures and longer times of annealing bring residual stresses to lower levels  All kinds of times (heating time, soaking time, cooling time)

Stress Relief Annealing – Cooling Rate Vs Stresses  The residual stress level after stress-relief annealing will be maintained only if the cool down from the annealing temperature is controlled and slow enough that no new internal stresses arise.  New stresses that may be induced during cooling depend on: Cooling rate Cross-sectional size of the work- piece, and (3)Composition of the steel

2. Normalizing  A heat treatment process consisting of austenitizing at temperatures of 50–80˚C above upper critical temperature (A1 , Acm) followed by slow cooling (usually in air)  The aim of which is to obtain a fine- grained, uniformly distributed, ferrite– pearlite structure  Normalizing is applied mainly to unalloyed and low-alloy hypo-eutectoid steels  For hypereutectoid steels the austenitizing temperature is 50–80˚C above the ACm transformation temperature

Normalizing – Heating and Cooling Purpose of soaking: To allow metal to attain uniform temp All the austenite A3 transform into pearlite, especially for hyper-eutectoid A1 compositions

Normalizing – Austenitizing Temperature Range Depend on composition Increase in C % reduces temp for hypo-eutectoid steels Increase in C % increases temp for hypo-eutectoid steels

Effect of Normalizing on Grain Size  Normalizing refines (reduces) the grains of a steel that have become coarse (long and irregular) as a result of heavy deformations as in forging or in rolling  The fine grains have higher toughness than coarse grains, Steel with 0.5% C

Normalizing after Rolling  After hot rolling, the structure of steel is usually oriented in the rolling direction  To remove the oriented structure and obtain the uniform mechanical properties in all directions, a normalizing annealing has to be performed

Normalizing after Forging  After forging at high temperatures, especially with work-pieces that vary widely in cross sectional size, because of the different rates of cooling from the forging temperature, a heterogeneous structure is obtained that can be made uniform by normalizing  Normalizing is also done to improve machinability of low-c steels

Normalizing – Holding Time  Holding time at austenitizing temperature may be calculated using the empirical formula: t = 60 + D where t is the holding time (min) and D is the maximum diameter of the workpiece (mm).

3. Full Annealing For compositions less than eutectoid, the metal is heated above A3 line to form austenite For compositions larger than eutectoid, the metal is heated above A1 line to form austenite and Fe3C Cooled slowly in a furnace instead in air as in Normalizing. Furnace is switched off, both metal and furnace cool at the same rate Microstructure outcome: Coarse Pearlite. In Normalizing, structure? Structure is relatively softer than that in Normalizing Full annealing is normally used when material needs to be deformed further. Usually applied for low and medium C steel

4. Spheroidizing Annealing  It is also called as Soft Annealing  Any process of heating and cooling steel that produces a rounded or globular form of carbide (Fe3C)  It is an annealing process at temperatures close below or close above the A1 temperature, with subsequent slow cooling  Used for Medium & High C-Steels - Spheroidite can form at lower temperatures but the time needed drastically increases, as this is a diffusion-controlled process.   Fe3C   Fe3C

Spheroidizing: How to Perform  By heating alloy at a temp just below A1 (700C). If pre-cursor structure is pearlite, process time will range b/w 15 & 25Hrs  Heating alloy just above A1 line and then either cooling very slowly in the furnace or holding at a Temp just below A1  Heating & cooling alternatively within ±50C of the A1 line.

Spheroidizing - Purpose  The aim is to produce a soft structure by changing all hard micro-constituents like pearlite, bainite, and martensite (especially in steels with carbon contents above 0.5% and in tool steels) into a structure of spheroidized carbides in a ferritic matrix a medium-carbon low-alloy steel after soft annealing at 720C; a high-speed steel soft annealed at 820C.

Spheroidizing - Uses ductile form of steel  Such a soft structure is required for good machinability of steels having more than 0.6%C and for all cold-working processes that include plastic deformation.  Spheroidite steel is the softest and most ductile form of steel

5. Isothermal Annealing High proportion of ferrite  Spheroidizing is more useful for improving machinability of high C steel than that of low and medium C steels.  In fact, spherodized low and medium C steels become over soft for machining and give long shavings which accumulate on tool cutting edge and produce poor surface.  Hypoeutectoid low-carbon steels as well as medium-carbon structural steels are often isothermally annealed, for best machinability  An isothermally annealed structure should have the following characteristics: High proportion of ferrite Uniformly distributed pearlite grains Fine lamellar pearlite grains

Process – Isothermal Annealing  Austenitizing followed by a fast cooling to the temperature range of pearlite formation (usually about 650˚C.)  Holding at this temperature until the complete transformation of pearlite  and cooling to room temperature at an arbitrary cooling rate ?    Fe3C