Steel Metallurgy - Principles of Metallurgy

Summary

This video script delves into the composition and properties of steel, highlighting the role of alloying elements like carbon, manganese, and chromium in affecting strength, hardness, and other characteristics. It also discusses the significance of the Iron Carbon Equilibrium Diagram, Continuous Cooling Transformation, and Time Temperature Transformation diagrams in understanding steel's behavior during solidification and heat treatment.

Takeaways

• 🔨 Steel is primarily composed of iron with up to 1% carbon and other alloying elements, which can make up to 50% in some steels.
• 🔧 Alloying additions in steel, such as carbon, silicon, manganese, and others, significantly affect properties like strength, hardness, toughness, ductility, and more.
• 🌡️ The Iron Carbon Equilibrium Diagram is crucial for understanding steel structures at different temperatures and carbon contents, influencing the behavior of complex steel alloys.
• 🌡️ The A1 and A3 temperatures are key points in steel transformation, marking the transition from austenite to pearlite and from ferrite to austenite, respectively.
• 🧪 Alloying elements like manganese, chromium, and molybdenum enhance hardenability, allowing for deeper hardening throughout the steel's thickness.
• 🌡️ The Continuous Cooling Transformation (CCT) and Time Temperature Transformation (TTT) diagrams are essential tools in metallurgy for determining steel structures at various cooling rates.
• 🔬 The structure of steel can vary from pearlite with ferrite or cementite (soft), to martensite (hard), to bainite (intermediate), depending on the cooling process.
• 🚀 Carbon is a key element in steel, affecting both strength and hardness, and is essential for the formation of microstructures.
• 🔍 The hardenability of steel is distinct from hardness, with hardenability describing the depth of hardening achievable in the steel, influenced by alloying elements.
• 🛠️ Strengthening mechanisms in steel, such as grain size reduction, cold working, solid solution strengthening, and dispersion or precipitation strengthening, work to impede dislocation movement, thereby increasing strength.

Q & A

What is steel primarily composed of?

Steel is primarily composed of iron with up to 1% carbon, plus other alloying additions which generally total less than 5%, but can be as great as 50% in some steels.

How do alloying additions affect the properties of steel?

Alloying additions can affect various properties of steel such as strength, hardness, toughness, ductility, fatigue, formability, machinability, weldability, and corrosion resistance, each in a different way depending on the type and amount added.

What is the role of carbon in steel?

Carbon is the most important element for strength and hardness in steel. As the level of carbon increases, the tensile strength and hardness also increase. It is essential for the formation of microstructures.

Why is phosphorus usually considered an impurity in steel?

Phosphorus is considered an impurity because it significantly reduces the toughness and ductility of steel. However, it can be used as a solid solution strengthener.

What is the impact of sulphur on steel properties?

Sulphur is generally an impurity that reduces the ductility, toughness, and weldability of steel. It can form iron sulphide, which can cause the steel to break up during hot working, a phenomenon known as hot shortness.

How does manganese affect steel?

Manganese has a great effect on hardenability and can strengthen steel through solid solution strengthening. It also combines with sulphur to prevent hot shortness.

What is the significance of chromium in steel?

Chromium increases the hardenability of steel and forms very stable carbides with carbon, which are excellent for wear and abrasion resistance. It is used in high levels in stainless steels for corrosion resistance by creating a protective oxide film on the surface.

Why is molybdenum added to steel?

Molybdenum increases the hardenability of steels. When combined with chromium and nickel, it has a strong multiplicative effect on hardenability and is used in both alloy and stainless steels.

What is the role of nickel in steel?

Nickel increases the hardenability of steel and can increase the toughness of steels, particularly at low temperatures. It is also present in large amounts in stainless steels.

How does the Iron Carbon Equilibrium Diagram help in understanding steel?

The Iron Carbon Equilibrium Diagram is used to understand what structures will be formed at what temperatures and carbon contents, and to calculate how much liquid and solid will be present at a given temperature. It helps in understanding the behavior of complex steel alloys.

What are the differences between the Continuous Cooling Transformation (CCT) diagram and the Time Temperature Transformation (TTT) diagram?

The CCT diagram shows the structures achievable by continuously cooling from the austenitization temperature at a constant rate, while the TTT diagram shows how long it would take for a structure to be achieved by holding at a given temperature. Both diagrams help in selecting the optimum steel and process parameters for achieving a given set of properties.

How does hardenability relate to the depth of hardness achieved in steel?

Hardenability describes how deep into the steel hardening can be achieved. A steel with high hardenability will have the same hardness throughout the thickness of the product, which is important for large components.

What are the different strengthening mechanisms in steel?

Strengthening mechanisms in steel include reducing grain size, cold working, solid solution strengthening, and dispersion or precipitation strengthening. These mechanisms work by reducing the ability of dislocations to move through the steel.

Outlines

00:00

🔩 Fundamentals of Steel Composition and Properties

This paragraph introduces the basics of steel metallurgy, explaining that steel is an alloy primarily composed of iron with up to 1% carbon and other alloying elements. It highlights the role of alloying additions in determining steel's properties such as strength, hardness, toughness, ductility, fatigue, formability, machinability, weldability, and corrosion resistance. The paragraph lists various elements that can be added to steel, including carbon, silicon, manganese, and others, detailing their specific effects on steel properties. For instance, carbon is crucial for strength and hardness, while manganese enhances hardenability. The paragraph also discusses impurities like phosphorus and sulfur, which can reduce steel's toughness and ductility but can be used in certain applications to improve machinability.

05:01

🌡️ Phase Transformations and Alloying Elements in Steel

This section delves into the phase transformations that occur in steel during solidification and cooling, using the Iron Carbon Equilibrium Diagram to illustrate the process. It explains the significance of temperatures A1 and A3, and the formation of different structures based on carbon content, such as ferrite, pearlite, and cementite. The paragraph further discusses the effects of alloying elements like aluminum, niobium, vanadium, boron, and nitrogen on steel properties, particularly their roles in strengthening mechanisms and grain growth restriction. It also touches on the concept of hardenability and how it is influenced by the alloying elements, as well as the importance of understanding phase transformations for selecting appropriate steel types and processing parameters.

10:05

📉 Understanding Hardenability and Strengthening Mechanisms

This paragraph focuses on the concept of hardenability in steel, which is the ability to achieve a hardened structure throughout the material. It clarifies the difference between hardenability and hardness, with the former referring to the depth of hardening and the latter to the microstructure resulting from cooling. The paragraph explains how elements like manganese, molybdenum, chromium, nickel, and boron can enhance hardenability, allowing for a harder structure at slower cooling rates. It also introduces the continuous cooling transformation (CCT) and time-temperature transformation (TTT) diagrams, which are essential tools for determining steel structures at various cooling rates. The paragraph concludes with an overview of strengthening mechanisms in metals, including grain size reduction, cold working, solid solution strengthening, and dispersion or precipitation strengthening, which all serve to impede dislocation movement and increase metal strength.

Keywords

💡Steel

Steel is an alloy primarily composed of iron and carbon, with additional alloying elements that can be added to alter its properties. It is the most widely used metal due to its strength, durability, and versatility. In the video, steel's composition and properties are central to understanding its applications and the various processes involved in its production and modification.

💡Alloying Additions

Alloying additions refer to the elements added to steel to change its properties, such as strength, hardness, and corrosion resistance. The script mentions that these additions can be up to 50% in some steels and include elements like carbon, silicon, and chromium. These additions are crucial for tailoring steel to specific applications.

💡Carbon

Carbon is the most important element in steel for increasing strength and hardness. As the carbon content increases, so does the steel's tensile strength. The script explains that carbon is essential for the formation of microstructures and plays a significant role in the Iron Carbon Equilibrium Diagram.

💡Hardenability

Hardenability is a key property of steel that describes its ability to harden throughout its depth when cooled. The script mentions that elements like manganese, molybdenum, and chromium increase hardenability, allowing for a deeper and more uniform hard structure in the steel, which is particularly important for large components.

💡Strengthening Mechanisms

Strengthening mechanisms are processes that increase the strength of steel by limiting the movement of dislocations within the metal's structure. The script discusses several mechanisms, including grain size reduction, cold working, solid solution strengthening, and dispersion or precipitation strengthening, all of which contribute to steel's overall strength and performance.

💡Iron Carbon Equilibrium Diagram

The Iron Carbon Equilibrium Diagram is a critical tool in metallurgy that illustrates the phases of iron and carbon at different temperatures and compositions. The script uses this diagram to explain the formation of different structures in steel, such as austenite, ferrite, and pearlite, which are fundamental to understanding steel's properties.

💡Continuous Cooling Transformation (CCT) Diagram

The CCT diagram is used to determine the structures that form in steel at various cooling rates. The script explains that this diagram is essential for engineering applications and helps in selecting the appropriate steel and process parameters to achieve desired properties.

💡Time Temperature Transformation (TTT) Diagram

The TTT diagram represents the time it takes for a specific structure to form in steel when held at a certain temperature. The script mentions that this diagram is used in conjunction with the CCT diagram and is primarily used in heat treatment processes to achieve specific steel properties.

💡Ferrite

Ferrite, also known as alpha-iron, is a structure in steel that forms at lower temperatures and has a body-centered cubic crystal structure. The script discusses how the amount of ferrite increases as the steel cools and how it combines with cementite to form pearlite, which affects the steel's hardness.

💡Pearlite

Pearlite is a microstructure in steel composed of alternating layers of ferrite and cementite. The script explains that pearlite forms when the remaining austenite in steel cools and cannot accommodate all the carbon, resulting in a mixture of ferrite and cementite. Pearlite is softer than other structures like martensite.

💡Martensite

Martensite is a very hard microstructure in steel that forms rapidly from austenite when cooled quickly. The script notes that the formation of martensite is related to the steel's hardenability, and it represents the hardest structure that can be achieved in steel, contributing significantly to its strength.

Highlights

Steel is primarily iron with up to 1% carbon and other alloying additions.

Alloying additions in steel can affect properties like strength, hardness, toughness, ductility, fatigue, formability, machinability, weldability, and corrosion resistance.

Carbon is the most important element for strength and hardness in steel.

Phosphorus and sulphur are often considered impurities in steel, reducing toughness and ductility.

Manganese enhances hardenability and prevents hot shortness in steel.

Chromium increases hardenability and provides corrosion resistance in stainless steels.

Molybdenum, when combined with chromium and nickel, has a strong multiplicative effect on steel hardenability.

Nickel increases the hardenability and toughness of steels, especially at low temperatures.

Silicon is used for deoxidation and improving the fluidity of steel during casting.

Aluminium deoxidises steel and restricts grain growth by forming nitrides.

Niobium increases yield strength, tensile strength, and toughness in steel.

Vanadium enhances hardenability and toughness by restricting grain growth.

Boron significantly increases the hardenability of steels and enhances the effect of other alloying elements.

Nitrogen forms nitrides with other elements, increasing hardness and tensile strength but reducing toughness and ductility.

The Iron Carbon Equilibrium Diagram is crucial for understanding steel structures and phase transformations.

Continuous Cooling Transformation (CCT) and Time Temperature Transformation (TTT) diagrams help determine steel structures at various cooling rates.

Hardenability describes how deep into the steel hardening can be achieved and is influenced by elements like manganese, molybdenum, chromium, nickel, and boron.

Strengthening mechanisms in steel include grain size reduction, cold working, solid solution strengthening, and dispersion or precipitation strengthening.

Transcripts

Steel Metallurgy. Steel is the widest used metal, but what constitutes

a steel? How can we affect the properties? And what happens during the solidification

of steel? These are a few key insights that we will

try to uncover in this module. Steel is primarily iron with up to 1% carbon,

plus other alloying additions. In the majority of steels this alloying addition generally

totals less than 5%, but in some steels this can be as great as 50%.

A steel composition can be thought of as a recipe; different amounts of each ingredient

make up your final product. In steel these ingredients are known as alloying additions;

each addition affects the properties of the steel in a different way. Depending on the

amount and type of alloying additions added we can affect the following properties in

a different way:

Strength –the ability to withstand load in tension.

Hardness – the ability to resist plastic deformation, usually via penetration. Also

described as resistance to scratching or abrasion. Toughness –the ability to absorb energy.

Ductility – the ability to deform without fracture.

Fatigue – the weakening of metal caused by repeatedly applied load.

Formability – the ease in which metal can be moulded into the final product.

Machinability – the ease in which metal can be processed into the final product with

cutting tools. Weldability – the ease in which metal can

be joined. Corrosion – the ability to withstand chemical

reaction through oxidation. To effect the steel in this manner a wide

variety of elements can be added, these include: Carbon, Silicon, Manganese, Phosphorus, Sulphur,

Chromium, Molybdenum, Nickel, Aluminium, Niobium – also called columbium, Titanium, Vanadium,

Copper, Boron, Nitrogen, Tungsten, Cobalt. Carbon – carbon is the most important element

for strength and hardness; as the level of carbon is increased the tensile strength and

hardness also increases. Carbon is a cheap way of increasing the strength and it is essential

for the formation of microstructures.

Phosphorus – phosphorus is usually classed as an impurity as it significantly reduces

the toughness and ductility of a steel, but it can be used as a solid solution strengthener,

this is explained later in this module.

Sulphur – sulphur is usually classed as an impurity as it reduces the steels ductility,

toughness and weldability. Sulphur can form with iron to produce a low melting point impurity

called iron sulphide, this can collect along grain boundaries and cause the steel to break

up during hot working, this is called hot shortness. Sulphur is sometimes added to steel

to aid machinability.

Manganese – manganese has a great effect on hardenability and can be found in most

commercial steels. Manganese can strengthen the steel through solid solution strengthening,

which is explained later in this module. It also combines with Sulphur to prevent hot

shortness.

Chromium – Chromium increases the hardenability of steel, it can join together with carbon

to form very stable carbides which are excellent for wear and abrasion resistance. It is used

in high levels in stainless steels for corrosion resistance, it does this by creating a protective

oxide film on the surface of the stainless steel.

Molybdenum – Molybdenum also increases the hardenability of steels. When combined with

chromium and nickel it has a strong multiplicative effect on hardenability. It is both in alloy

and stainless steels. Nickel – Nickel as with chromium and molybdenum,

increases the hardenability of steel and can increase the toughness of steels, particularly

at low temperatures. Again it is present in large amounts in stainless steels.

Silicon – Silicon is mainly used to remove oxygen from steel in a process called deoxidation.

The removal of oxygen in steel is important as oxygen can form voids in steel known as

blow holes and porosity. Oxygen can also combine with other elements to form brittle particles

(known as oxides). Silicon can be used to increase the fluidity when casting steels.

Aluminium – Aluminium is primarily used to deoxidise the steel. It can combine with

nitrogen to form Nitrides which can restrict grain growth.

Niobium – Niobium, also called columbium, in small amounts can increase yield strength,

tensile strength and toughness.

Vanadium – Vanadium is used to increase the hardenability and toughness of steels

through it’s ability to restrict grain growth.

Boron – Boron can significantly increase the hardenability of steels and can enhance

the effect of other alloying elements.

Nitrogen – Nitrogen is often added in combination with other elements to form nitrides. These

nitrides increase the hardness and tensile strength, but at the expense of toughness

and ductility. When we add alloying elements they do not

always work in isolation, sometimes the elements work in conjunction and can cause a multiplication

effect that would not be expected from the sum off the individual additions. For example,

both chromium and molybdenum may be added individually to a steel in order to strengthen

it, but a small amount of molybdenum used in conjunction with chromium will result in

a much greater strengthening effect than using one of the elements alone.

The addition of carbon to iron is probably the most important addition in cast irons

and steels, this makes a diagram called ‘The Iron Carbon Equilibrium Diagram’ very useful.

Equilibrium means that enough time has been allowed on heating and cooling for any reactions

to fully complete.

Many of the basic features of this diagram influence the behaviour of the most complex

steel alloys. The diagram is used to understand what structures

will be formed at what temperatures and at what carbon contents. We can also see at what

temperature different compositions melt and we can calculate how much liquid and solid

will be present at a given temperature, and can see when a steel will be fully solid.

We can also calculate how much of each structure or phase will be present at a given temperature.

Script On the diagram there are several points of

interest. • A1, the temperature at which austenite

turns to pearlite. Below this temperature austenite (gamma-iron) does not exists, this

is 723°C. • A3, the temperature when Ferrite (alpha-iron)

transforms to Austenite (gamma-iron). For pure iron this occurs at 910°C, but lowers

in temperature along the line to 0.8%Carbon then it increases in temperature up to 2%

carbon. • Liquidus temperature, the temperature

at which steel of a given composition fully turns to a liquid.

Below 723°C we can see Fe3C which is iron-carbide, this is called Cementite which is a ceramic

compound of iron and carbon. Steels with less than 0.8% carbon consist

of a structure of ferrite and pearlite, pearlite is a structure that consists of iron-carbide

(cementite) and ferrite (alpha-iron) in parallel laths. Above 0.8% Carbon, cementite and pearlite

are primary constituents. If we take an example of a 0.3% carbon steel,

the steel is molten until we cool to 1510°C, at this point the liquid iron starts to solidify

into delta iron, form 1510°C to 1495°C the amount of delta iron increases while the amount

of liquid decreases, at 1495°C the Body Centred Cubic delta iron transforms to Face Centred

Cubic austenite. As we continue to cool to 1454°C the amount of austenite increases

and the liquid decreases until we have a fully solid austenitic structure.

As we decrease in temperature further the structure remains austenite until we hit 820°C

where it starts to form Body Centred Cubic ferrite, from 820°C to 723°C the amount

of austenite decreases and the amount of ferrite increases until the remainder of the austenite

will transform, but as austenite has a higher solubility for carbon than ferrite the ferrite

that forms will not be able to accommodate all the carbon that was contained in the austenite

and thus the remaining austenite will form a mixture of ferrite (alpha-iron) and iron-carbide

(cementite), this structure is known as pearlite. Here we can see some examples of different

carbon contents and the structures produced. Low carbon, structure consists primarily of

ferrite with small grains of pearlite. Medium carbon, structure consists primarily

of pearlite with a small percentage of ferrite High carbon, at 0.8% carbon the structure

consist pearlite. Above 0.8% carbon the structure consists of

pearlite and cementite. In addition to the iron carbon equilibrium

diagram there are two other diagrams that are used extensively in metallurgy, these

are the CCT (continuous cooling transformation) diagram and the TTT (time temperature transformation)

diagram. We briefly outline these in this module and will go into these in a greater

depth in a following module on heat treatment, where these are primarily used.

While the Iron carbon diagram describes the structures of steel under equilibrium conditions,

where enough time has been allowed on heating and cooling for any reactions to fully complete,

both the continuous cooling transformation and time temperature transformation diagrams

allow determination of structures at various cooling rates, from slow to very fast. Both

these diagrams are helpful in selecting the optimum steel and process parameters to achieve

a given set of properties.

Continuous cooling transformation diagrams are generally more appropriate for engineering

applications. The diagrams show the structures that are achievable continuously cooling from

the austenatisation temperature at a constant rate. These diagrams often show the structure

that can be achieved at the centre of different size bars for cooling in water, oil and air.

Some diagrams also have the hardness that is achieved from this structure.

Time temperature transformation diagrams show how long it would take for a structure to

be achieved by holding at a given temperature. This diagram allows you to plot varying cooling

rates and show the structure that would be achieved.

Hardenability. In metallurgy the hardenability of a steel

is a key parameter and when we talk about hardenability in steels we are often describing

how deep into the steel we can achieve hardening. If a steel is described as having a low hardenability

this will mean that the steel will produce a shallower depth of hardness. When a steel

has a high hardenability it will be the same hardness throughout the thickness of the product,

highly hardenable steels are more important in large components. Hardenability is not

to be mistaken for hardness, when describing the hardness we are often looking at the microstructure

achieved during cooling. For a given steel it can be assumed that the quicker the cooling

rate the greater chance of achieving a harder structure and if that steel has a high hardenability

this hard structure will be present deeper into the thickness.

We can increase the hardenability of a steel by adding elements like Manganese, Molybdenum,

Chromium, Nickel and Boron. When we add these elements it increases the hardenability and

this will enable us to achieve a harder structure deeper into the thickness. The ability of

the hardness to go deeper into the thickness is because it is easier to achieve a harder

structure at a slower cooling rate. The structure of a steel can be pearlite with

ferrite or cementite which is the softest structure, martensite which is the hardest

structure and bainite which is in between. Looking at the continuous cooling transformation

diagram, if a steel has a low hardenability, ferrite and perlite transformation will be

shown in the upper left hand side of the diagram, meaning it will be difficult or even impossible

to form martensite. If a steel has high hardenability, transformation to martensite will be shown

at the bottom right hand side of the diagram, meaning that a steel will fully transform

to martensite over a large range of thicknesses. Strengthening mechanisms.

In the ‘Introduction to Materials’ Module we talked about dislocations being present

in metals, these dislocations reduce the strength of the metal. The principle of strengthening

mechanisms is to reduce the ability of these dislocations to move through the metal, this

can be achieved by the reduction in grain size, cold working, solid solution strengthening

and dispersion or precipitation strengthening, these strengthening mechanisms can be applied

individually or in combination.

In metal it is estimated that there are 10 million to 1 billion dislocations per cm2

and each dislocation has a strain field associated with it.

With Grain Size, the Grains can interact with the dislocations preventing further movement

of them. If we reduce the grain size we can increase the number of grains interacting

with the dislocations, preventing their movement and thus strengthening the metal.

Cold work introduces a large amount of strain into the metal, this strain interacts with

the dislocations strain field, impeding the movement of the dislocations.

Solid solution strengthening is applied when we add other chemical elements to a metal.

As discussed in the ‘Introduction to Materials’ module the elements added can either fall

between the atoms of the bulk material or replace the atoms. Within steel, Carbon atoms

falling between Iron atoms and Nickel atoms replace iron atoms. This will either be called

interstitial or substitutional solid solution strengthening and will cause distortion in

the atomic structure, this distortion interacts with the dislocations, preventing the dislocation

movement and strengthening the steel. Dispersion or precipitation strengthening

is highly related to the structure of the metal and takes place when a phase is finely

precipitated through a softer matrix. This hard precipitate acts as a barrier to dislocation

movement, the precipitates can also produce a strain field that interacts with the dislocations

strain field. So in Summary

• We have the ability to tailor the properties of a steel by adding different alloying elements

• Alloying elements do not always work in isolation, sometimes they cause a multiplication

effect. • Carbon to iron is probably the most important

addition to cast irons and steels. • The Iron Carbon Equilibrium Diagram, Continuous

Cooling Transformation diagram and Time Temperature Transformation diagram are the most widely

used diagrams in steel metallurgy. • Continuous cooling transformation and

time temperature transformation diagrams allow determination of structures at a variation

of cooling rates. • Hardenability describes how deep into

the steel we can achieve hardening. • Hardenability is not to be mistaken for

hardness, when describing the hardness we are often looking at the structure achieved

from cooling. • The structure of a steel can be pearlite

with ferrite or cementite which is the softest structure, martensite which is the hardest

structure and bainite which is in between. • Dislocations reduce the strength of the

metal. The principle of strengthening mechanisms is to reduce the ability of these dislocations

to move through the steel. • Strengthening can be achieved by the reduction

in grain size, cold working, solid solution strengthening and dispersion or precipitation

strengthening.