MEC281 MATERIALS SCIENCE

CHAPTER 3 THERMAL PROCESSING OF METALS

Rasdi bin DeramanFakulti Kejuruteraan Mekanikal

UiTM Pulau Pinang

3.1 INTRODUCTION

Heat Treatment is the controlled heating and cooling of metals to improve their physical and mechanical properties without changing the product shape.

Heat Treatment is often associated with increasing the strength of material, but it can also be used to alter certain manufacturability objectives such as improve machining, improve formability, restore ductility after a cold working operation.

Thus it is a very enabling manufacturing process that can not only help other manufacturing process, but can also improve product performance by increasing strength or other desirable characteristics.

3.1 INTRODUCTION (Cont…..)

Phase Transformations in Metals• Heat Treatment

(time and temp.)

• Nucleation of the new phase - formation of stable nuclei of the new phase. Nuclei are often formed at grain boundaries and other defects.Growth of new phase at the expense of the original phase.

Microstructure Mech. Properties

Most phase transformations involve change in composition redistribution of atoms via diffusion is required. The process of phase transformation involves:

Heat treatment of plain carbon steels.

• Heating and cooling properties of steels vary mechanical properties.

• Martensite: Metastable phase consisting of super saturated solid solution of C in

BC Tetragonal iron.

• Caused by rapid cooling of austenitic steel into room temperature (quenching).

Ms temperature of martensite start.

Mf temperature of martensite finish.

Microstructure of Fe – C in Martensites• Lath martensite: Less than 0.6% C and consists of

domains of lathe of different orientation. • Plate martensite: More than 0.6% C and have fine

structure of parallel twins.

Lath type

Plate type

Martensite (Cont..)

• Transfer to martensite is diffusionless.• No change of relative position of carbon atoms

after transformation.

• Strength and hardness increases with carbon content.• Strength is due to high dislocation concentration and interstitial solid solution strengthening.

TTT Diagrams

The family of S-shaped curves at different temp. are used to construct the TTT diagrams.

The TTT diagrams are for the isothermal (constant T) transformations (material is cooled quickly to a given temp. before the transformation occurs, and then keep it at that temp.). At low temp., the transformation occurs sooner (it is controlled by the rate of nucleation) and grain growth (that is controlled by diffusion) is reduced.

Phase Transformations in TTT Diagram including Martensite

TTT Diagrams (cont…)

Slow diffusion at low temperatures leads to fine-grained microstructure with thin-layered structure of pearlite (fine pearlite).

At higher temperatures, high diffusion rates allow for larger grain growth and formation of thick layered structure of pearlite (coarse pearlite).

At compositions other than eutectoid, a proeutectoid phase (ferrite or cementite) coexist with pearlite.

Additional curves for proeutectoid transformation must be included on TTT diagrams.

TTT Diagrams (cont…)

Critical Cooling Rate is defined as the lowest cooling rate which produces 100% Martensite while minimizing the internal stresses and distortions.

TTT Diagram and microstructures obtained by

different types of cooling rates

Phase Transformations in MetalsIsothermal Transformation ( TTT)

Diagrams

The absolute layer thickness depends on the temperature of the transformation. The higher the temperature, the thicker the layers.

The thickness of the ferrite and cementite layers in pearlite is ~ 8:1.

Phase Transformations in MetalsTime –temp. path – microstructure

Example: • Given in TTT

diagram, describe what transformations happen in:

a. Path 1 (Red line)

b. Path 2 (Green line)

c. Path 3 (Blue line)

d. Path 4 (Orange line)

Ans: a) 50% martensite+ 50% austenite. b) 100% martensitic c) 50% martensite + 50% bainite d) fine pearlite

Cold WorkA material is considered to be cold worked if its

grains are in a distorted condition after plastic deformation is completed. All the properties of a metal that are dependent on the lattice structure are affected by plastic deformation or cold working. The following properties are affected by cold work significantly:

• Tensile Strength• Hardness• Ductility • Yield Strength

Figure shows the effect of cold working on tensile strength, hardness, ductility and grain size. (The curve below ductility represents the change in grain size)

TYPES OF HEAT TREATMENT • Four basic types of heat treatment are

used today. They are annealing, normalizing, hardening, and tempering.

ANNEALING

A number of process heat-treating operations are classified under the general term of annealing. These may be employed to

• reduce hardness• remove residual stresses• improve toughness• restore ductility• refine grain size

Full annealing is the process by which the distorted cold worked lattice structure is changed back to one which is strain free through the application of heat. This process is carried out entirely in the solid state and is usually followed by slow cooling in the furnace from the desired temperature.

• The annealing process may be divided into three stages:1. Recovery2. Recrystallization3. Grain growth

• The principal effect of recovery is the relief of internal stresses due to cold working.

• When the load which causes plastic deformation is released, all the elastic deformation does not disappear. This is due to the different orientation of crystals, which will not allow some of them to move back when the load is released.

• Commercially, this low temp. treatment in the recovery range is known as stress relief annealing or process annealing.

Recovery

Recrystallization • Recrystallization takes place by a combination of

nucleation of strain free grains and the growth of these nuclei to absorb the entire cold worked material.

• The term recrystallization temperature refers to the approximate temperature at which a highly cold worked material completely recrystallizes in one hour.

Pure metals have low recrystallization temperatures as compared with alloys. Zinc, tin and lead have recrystallization temperatures below room temperature. This means that these metals cannot be cold worked at room temperature since they crystallize spontaneously, reforming a strain free structure.

The recrystallization temperatures of several metals and alloys are listed in table below.

Material Recrystallization Temperature (oF)

Copper ( 99.99 %) 250

Copper ( 5 % Zinc) 600

Aluminum (99.99 %) 175

Aluminum alloys 600

Low carbon steel 1000

Zinc 50

Tin 25

Lead 25

Grain Growth

• In this stage the tensile strength and hardness continue to decrease but at a much less rate than the recrystallization stage. The major change observed during this stage is the growth of the grain boundaries and reaching the original grain size as shown in figure below.

Effect of annealing on tensile strength, hardness, ductility and grain size.

Normalizing• Normalizing is a type of heat treatment applicable

to ferrous metals only. It differs from annealing in that the metal is heated to a higher temperature (austenite region) and then removed from the furnace for air cooling.

• Makes grain structure uniform, Increases strength.

• The purpose of normalizing is to remove the internal stresses induced by heat treating, welding, casting, forging, forming, or machining.

• Normalized steels are harder and stronger than annealed steels. In the normalized condition, steel is much tougher  than  in  any  other  

structural  condition.

Hardening• The hardening treatment for most

steels consists of heating the steel to a set temperature and then cooling it rapidly by plunging it into oil, water, or brine. Most steels require rapid cooling (quenching) for hardening but a few can be air-cooled with the same results.

• Hardening increases the hardness and strength of the steel, but makes it less ductile. Generally, the harder the steel, the more brittle it becomes. To remove some of the brittleness, it should temper the steel after hardening.

Tempering:• Tempering is essential after most

hardening operations to restore some toughness to the structure.

• Hardened steel can be tempered by re-heating it to a certain temperature below eutectoid temperature, and then cooling slowly. It is frequently performed as an integral part of the cycle in a seal quench furnace, with the parts fully protected against oxidation and decarburisation throughout the process.

Tempering (cont…)

•Generally tempering is conducted in the temp. range 150 to 700°C, depending on the type of steel and is time dependent as the microstructural changes occur relatively slowly.

Cross-section of gear teeth showing induction-hardened surfaces.

Isothermal decomposition of Austenite (Cont..)

• Upper Bainite Between 5500C and 3500C

• Lower Bainite Between 3500C and 2500C

Upper Bainite Lower Bainite

Martensite

Spheroidite

Effects of Tempering• Hardness decreases as temperature increases above

2000C

• This is due to diffusionof carbon atoms from interstitial sites to iron carbide precipitates.

Hardenability• Hardenability is used to describe the ability of an alloy to be hardened by the formation of martensite induced by quenching process.

The hardenability of steel depends primarily on:

# The composition of the steel.# The austenitic grain size.# The structure of the steel

before quenching.

Hardenability (cont…)

• Hardenability can be measured by using Jominy end quench test:

* Cylindrical bar (1” dia. And 4” length with 1/16”flange

at one end is austenitized and one end is quenched.

* Rockwell C hardness is measured up to 2.5” from quenched end.

24°C water

specimen (heated to phase field)

flat ground

4”

1”

Figure (a) End-quench test and cooling rate. (b) Hardenability curves for five different steels, as obtained from the end-quench test.

Small variations in composition can change the shape of these curves. Each curve is actually a band, and its exact determination is important in the heat treatment of metals, for better control of properties.

Hardenability (cont..)

Hardness versus distance from thequenched end.

Distance from quenched end.

Har

dn

ess,

HR

C

Hardenability (cont…)

• The cooling rate varies with position.

Why hardness changes with position?

Cooling rate (°C/s)

Hard

ness

, H

RC

20

40

60

100 20 30 40 50Distance from quenched end (mm)

210100 3

4140

8640

5140

1040

50

80

100

%M4340

Hardenability curves shown: The 4340 (1.85 Ni, 0.8 Cr & 0.25 Mo) alloy steel has exceptionally high hardenability and can be quenched to a hardness of RC=50 at 2” from the quenched end of a Jominy Test bar; 4140 (1% Cr & 0.2% Mo); 8640 (0.55% Ni, 0.5%Cr & 0.2% Mo); 5140 (0.85% Cr); 1040 (unalloyed steel)

• Effect of quenching medium:

Mediumairoil

water

Severity of Quenchsmall

moderatelarge

Hardnesssmall

moderatelarge

• Effect of geometry: When surface-to-volume ratio increases: --cooling rate increases --hardness increasesPosition

centersurface

Cooling ratesmalllarge

Hardnesssmalllarge

QUENCHING MEDIUM & GEOMETRY

Forging

Drawing

Rolling Extrusio

n

METAL DEFORMATION PROCESS

End of the Chapter 3

Based on the given data below, 1. Name the heat treatment process for each sample.2. Differentiate the microstructure and properties of each

steel after the heat treatment process.

0.8 wt % Carbon

steel

Heating Temp. (oC)

Soaking Time (min)

Cooling medium

A 860 50 Oil

B 860 50 Furnace

C 860 50 Soaking in water and reheat