MAE378-Lecture 08

Recrystallization Recrystallization

________ is induced after a material has been cold worked to reduce strain hardening effects Induces a change in size, shape, and distribution.

Hypereutectoid steel

________ has between 0.8 % and 2 % of C, composed by pearlite and cementite.

A1

Normalizing vs. annealing In normalizing, cooling will be different in different locations Properties will vary between the surface and interior in normalized steel Lower cost of normalizing is justified if uniform properties is not critical Stress- relief anneal Reduces residual stresses in casting, welded assemblies, and cold- formed products Materials are heated to temperature below ________, held for period of time and then slow cooled.

CCT

________ and TTT diagrams showed how much austenite transform in microstructure when you have different cooling rate.

Cementite

________: is an intermetallic compound of iron and carbon (6.67 % C and 93.3 % Fe)

Fe3C

Note: Martensite is formed by the rapid cooling (quenching) of the austenite form of iron at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite (________)

fcc

Because of ________ structure has more interstitial positions, the solid solubility of austenite is much higher than ferrite.

BCC

Ferrite: Is a solid solution of limited amounts of carbon in iron with a body- centered cubic (________) crystal structure Austenite: Within a certain temperature range iron undergoes polymorphic transformation from ________ to fcc becoming gamma iron (austenite)

Martensite

________: When Austenite cool down very rapidly, carbon atoms do not have enough time to form cementite.

material of components

Product are usually martensite or pearlite depending on the cooling media as well as the ________.

α ferrite

It is composed by pearlite and ________.

lamellar structure

Note: Pearlite is a two- phased, ________ composed by alternating layers of cementite and α ferrite.

Water

________ is an effective quenching medium because of its high heat of vaporization and relatively high boiling point b.

Hypoeutectoid steel

________ has less than 0.8 % of C in its composition.

term hardenability

The ________ should not be confused with hardness, which is resistance of a materials to indentation or scratching.

Processing heat treatments for steel c. Heat treatments for non-ferrous materials d. Strengthening heat treatment for

 Steel  Non-ferrous materials Iron-Carbon Equilibrium Phase Diagram Commercial pure Iron contains 0.008% C, Steels up to 2.11% C and cast irons up to 6.67% C

Cementite

is an intermetallic compound of iron and carbon (6.67% C and 93.3% Fe)

Note

Pearlite is a two-phased, lamellar structure composed by alternating layers of cementite and α ferrite

Note

Martensite is formed by the rapid cooling (quenching) of the austenite form of iron at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite (Fe3C)

Processing heat treatment for steel Annealing Annealing is a common heat treatment process may be used to

Reduce strength and hardness Removes residual stresses Improves toughness Restores ductility Refines grain size Full Annealing Hypoeutectoid steels are heated to convert the grain structure to homogenous single-phase austenite, then control cooled The cooling results in coarse pearlite with excess ferrite Results in soft and ductile steel  Hypereutectoid steels undergo a similar process but the structure will be coarse pearlite with excess cementite (cooling slowly)

Note

If materials were to be slow cooled from all austenite region, a continues network of cementite may form in the grain boundary make the material brittle

Note

While full anneal produce the softest and weakest properties

Note

In TTT diagrams, after cooling to a transformation temp, you keep the temp constant until the transformation of austenite to the required transformation product is complete

Note

In CCT diagrams, there is continues cooling ( no holding of temp)

Martensite

When Austenite cool down very rapidly, carbon atoms do not have enough time to form cementite

Structures resulting from transformation at various temperatures Note

The highest hardness of the pearlite phase is 400 HB whereas for Martensite can achieve to 700 HB

Note

A round test bar 100 mm long is austnized (100 %) and then quenched directly at one end with a steam of water at 24°C

Iron Carbon Content

.008% Carbon

Steel Carbon Content

up to 2.11% Carbon

Cast Iron Carbon Content

up to 6.67% Carbon

Ferrite

phase of iron composed of solid solution of limited amounts of carbon in iron with a body-centered cubic (BCC) crystal structure

Austenite

phase of iron after undergoing polymorphic transformation from bcc to fcc becoming gamma iron

Cementite

phase of iron composed of intermetallic compound of iron and carbon
very hard and brittle

Hypoeutectoid

steel has less than 0.8% of C in its composition
composed of pearlite and alpha-ferrite

Hypereutectoid

steel has between 0.8% and 2% C
composed of pearlite and cementite

Pearlite

phase of iron two-phased, lamellar structure composed by alternating layers of cementite and alpha ferrite

Martensite

phase of iron formed by rapid cooling (quenching) of austenite so that carbon atoms cannot diffuse out of the crystal structure

Heat Treatment

expression to describe the controlled heating and cooling of materials for purpose of alternating their structure and properties

Annealing

common heat treatment process used to
-reduce strength and hardness
-remove residual stresses
-improve toughness
-restore ductility
-refine grain size
Heating below the lower critical temperature

Full Annealing with Hypoeutectoid

steels are heated to convert grain structure to homogenous single-phase austenite, then control cooled to get coarse pearlite with excess ferrite
results in soft and ductile steel

Full Annealing with Hypereutetoid

steels are heated to convert grain structure to and then control cooled to get coarse pearlite with excess cementite

Full Annealing

Heating 30-50°C above critical temperature, maintaining for a specific period of time, and then slowly cooling down the inside of the furnace
Time consuming and takes a lot of energy

Normalizing

heating steel to a temp higher than annealing
Aim is t obtain ferrite-pearlite microstructure
More effective than annealing
Cooled in air
Results in different properties between interior and exterior of the steel

Stress Relief Anneal

reduces residual stresses from casting, welded assemblies, and cold formed products.
Heated to below A1 and then time cooled

Spherodizing Anneal

Applicable to steels with more than 0.5% C
Parts heated to below A1
Cementite changes from lamellar formation to alpha ferrite matrix with particles of spheroidal cementite

Recrystallization

induced after a material has been cold worked to reduce strain hardening effects.
induces change of size, shape, and distribution

Hardenability

capability of an alloy to be hardened by heat treatment

Hardenability Depends On

carbon content
alloying elements present
cooling rate
austenite grain size and transformation to pearlite

Time-Temperature-Transformation (TTT) Diagram

indicates a specific transformation start end end
shows percentage of transformation of austenite at a particular temp
Goal is to obtain type and portion in the curve to obtain specific propertied

Continuous Cooling Transformation (CCT) Diagram

used to represent which types of phase changes will occur in a material as it is cooled at different rates
These diagrams have no holding temp (continuous cooling)

A in CCT Diagram

Austenite

B in CCT Diagram

Bainite

M in CCT Diagram

Martensite

P in CCT Diagram

Pearlite

Jominy Test

Round bar splashed with water on one end that provides different cooling rates along the bar. Gives difference in properties for different cooling rates from one test

Quenching

submerging hot material in a liquid in order to induce rapid cooling

Stages of Quenching

Formation of the Vapor Jacket
Nucleate boiling phase
Conduction and convection

Formation of the Vapor Jacket

thin gaseous layer between the metal and the liquid during cooling

Nucleate Boiling Phase

Produces rapid rates of cooling down to the boiling point of the quenchant

Conduction and Convection

Slower cooling from the boiling point to room temperature

Types of Quenchant (Fastest to Slowest)

Water
Brine
Water based polymer
Oil
Molten salt baths
High pressure steam quenching