How to make metal stronger by heat treating, alloying and strain hardening

Summary

Dr. Billy Wu's video explores methods to enhance material strength through processing techniques. It explains the significance of non-equilibrium conditions in manufacturing, such as cooling rates, which affect mechanical properties. The video outlines three primary strengthening approaches: solid solution hardening through alloying, strain hardening via plastic deformation, and precipitation hardening through heat treatments. It delves into how impurities, grain size, and precipitates impede dislocation movement, thereby increasing strength. The video also discusses steel's unique response to heat treatments, resulting in various structures like pearlite, bainite, and martensite, and uses time-temperature-transformation diagrams to illustrate the effects of cooling rates on microstructure formation.

Takeaways

• 🔨 The importance of material processing: Understanding non-equilibrium conditions in manufacturing can significantly affect a material's mechanical properties.
• 🗡️ Sword making example: Strengthening metal through quenching demonstrates the impact of cooling rates on material properties.
• 🔧 Gear example: Using selective heating and rapid cooling to achieve a balance between hardness, wear resistance, and ductility.
• 🛠️ Three material strengthening approaches: Solid solution hardening, strain hardening, and precipitation hardening.
• 🧬 Solid solution hardening: Alloying with impurities to impede dislocation movement, thereby increasing tensile and yield strength.
• 💎 Grain size's role: Smaller grains lead to increased strength due to more grain boundaries acting as barriers to dislocation movement.
• ⚖️ Hall-Petch equation: Quantifies the relationship between grain size and yield strength in materials.
• 🔨 Strain hardening: Increasing strength through plastic deformation at the cost of reduced ductility.
• 🌡️ Precipitation hardening: Forming a small, uniformly dispersed phase to impede dislocation movement and increase strength through heat treatment.
• 📈 TTT diagrams: Time-Temperature-Transformation diagrams help in understanding and mapping the influence of cooling rates on microstructures.
• 🛑 Overaging effect: Prolonged heat treatment can lead to grain growth, loss of strength improvements, and a return to equilibrium microstructure.

Q & A

What is the main focus of Dr. Billy Wu's video?

The video focuses on how to make materials stronger through various processing methods, following on from previous discussions on equilibrium phase diagrams and steels.

Why is it important to understand non-equilibrium conditions in material processing?

Understanding non-equilibrium conditions is crucial because manufacturing often occurs under such conditions, and factors like cooling rate can dramatically affect the mechanical properties of a material.

How does quenching a sword in water after forming it increase its strength?

Quenching in water rapidly cools the metal, which can increase its strength by altering its microstructure in a way that enhances hardness and wear resistance.

What is the purpose of selectively heating and cooling a gear to create a hard outer shell but ductile core?

This process is used to achieve a balance between the hardness and wear resistance of the gear's teeth and the ductility of its core, which is important for the gear's overall performance and durability.

What are the three high-level approaches for strengthening a material discussed in the video?

The three approaches are solid solution hardening, strain hardening (or cold working), and precipitation hardening.

How does solid solution hardening work to increase a material's strength?

Solid solution hardening involves alloying a material with an impurity, which introduces lattice strains that impede the movement of dislocations, thereby increasing the material's strength.

What is the significance of grain size in a metal's mechanical properties?

Grain size significantly influences a metal's mechanical properties because smaller grains increase the resistance to dislocation movement, making the material harder and stronger.

How does the Hall-Petch equation relate to the mechanical properties of a material?

The Hall-Petch equation quantifies the relationship between the yield strength of a material and its grain size, showing that a finer grain size leads to higher strength.

What is the trade-off associated with strain hardening or cold working?

While strain hardening increases a material's strength by increasing dislocation density and refining grain size, it often comes at the cost of decreased ductility, necessitating a balance between strength and ductility.

How does precipitation hardening differ from solid solution hardening and strain hardening?

Precipitation hardening involves forming a small and uniformly dispersed second phase within the material through specific heat treatment processes, which impedes dislocation movement and increases strength, unlike solid solution hardening and strain hardening which rely on alloying and plastic deformation, respectively.

What is the role of time-temperature-transformation (TTT) diagrams in understanding steel heat treatments?

TTT diagrams help to quantify and map out the influence of cooling rates and the resulting microstructures in steel, showing how different cooling rates lead to the formation of various non-equilibrium phases like pearlite, bainite, and martensite.

Why is it important to control the heat treatment process in precipitation hardening?

Proper control of the heat treatment process is essential in precipitation hardening to avoid overaging, where prolonged heating allows the second phase to grow excessively, leading to a loss of the strength improvements achieved through the formation of small precipitates.

How does the structure of steel change with different cooling rates during heat treatment?

The structure of steel changes significantly with cooling rates: slow cooling leads to coarse pearlite, faster cooling forms finer pearlite or bainite, and very rapid cooling results in the formation of martensite, which is hard but brittle.

Outlines

00:00

🛠️ Material Strengthening Through Processing

Dr. Billy Wu introduces the concept of enhancing material strength through processing methods. He emphasizes the importance of understanding non-equilibrium conditions in manufacturing, which can significantly affect a material's mechanical properties. Examples such as sword making and gear manufacturing illustrate how processes like quenching and induction heating can alter a material's hardness and ductility. The video outlines three primary methods for material strengthening: solid solution hardening, strain hardening, and precipitation hardening. The summary also touches on the significance of grain size and its impact on material strength, as well as the role of dislocations in crystalline materials.

05:07

🔩 The Mechanics of Material Strengthening

This paragraph delves into the specifics of how materials can be made stronger. It starts by explaining solid solution hardening, where an alloy is strengthened by the addition of impurities that impede dislocation movement. The discussion then moves to strain hardening, where plastic deformation increases the material's strength at the expense of ductility. The paragraph further explains that increased dislocation density and smaller grain size contribute to this strengthening effect. Precipitation hardening is introduced as a method to form a small, uniformly dispersed phase that impedes dislocation movement, thereby increasing strength. The process involves a two-stage heat treatment, with an initial solution heat treatment to create a supersaturated phase, followed by precipitation heat treatment to form the second phase. The risks of overaging, where prolonged heating leads to a loss of strengthening properties, are also highlighted.

10:13

🔨 Heat Treatments and Their Impact on Steel

The final paragraph focuses on the application of the discussed concepts to steel, a commonly used material. It revisits the eutectoid composition of steel and the formation of structures like pearlite from austenite phase. The paragraph explains how different cooling rates during heat treatments can lead to the formation of various non-equilibrium phases in steel, such as coarse pearlite, fine pearlite, bainite, and martensite. The formation of martensite is particularly emphasized due to its extreme hardness and brittleness, which can be mitigated through tempering to restore ductility. The time-temperature-transformation (TTT) diagram is introduced as a tool for understanding and quantifying the effects of cooling rates on microstructure formation in steel.

Keywords

💡Material Strengthening

Material strengthening refers to the process of enhancing the mechanical properties of materials, such as their hardness and tensile strength. In the video, Dr. Billy Wu discusses various methods to achieve this, including alloying, cold working, and heat treatments. The concept is central to the video's theme as it underpins the discussion on how processing techniques can significantly alter a material's strength.

💡Non-equilibrium Conditions

Non-equilibrium conditions describe situations where the manufacturing or processing of a material does not allow it to reach a state of equilibrium, often due to rapid changes in conditions like cooling rates. The video emphasizes the impact of non-equilibrium conditions on material properties, such as how rapid cooling can lead to the formation of martensite in steel, which is a very hard but brittle phase.

💡Quenching

Quenching is a heat treatment process where a material is rapidly cooled from a high temperature to room temperature, usually by immersing it in water or oil. In the context of the video, quenching is used to increase the strength of metals, such as in sword making, by creating a hard outer shell through rapid cooling.

💡Solid Solution Hardening

Solid solution hardening is a process where impurities are alloyed into a material to increase its strength. The video explains that the addition of impurities, either smaller or larger than the host solvent atoms, creates lattice strains that impede dislocation movement, thereby increasing the material's strength. An example given is the nickel-copper alloy.

💡Strain Hardening

Strain hardening, also known as cold working, involves plastically deforming a material to enhance its strength. The video describes how this process increases the dislocation density within the material, making it more difficult to deform and thus stronger. However, it also mentions the trade-off with decreased ductility.

💡Precipitation Hardening

Precipitation hardening is a heat treatment process that strengthens a material by forming a small and uniformly dispersed second phase within the primary phase. The video explains that this process involves a two-stage heat treatment: solution heat treatment to create a supersaturated single phase, followed by precipitation heat treatment to allow the formation of small dispersions of a second phase.

💡Dislocations

Dislocations are linear defects in a crystalline material that play a crucial role in its mechanical properties. The video discusses how the movement of dislocations is impeded by lattice strains introduced through solid solution hardening and by grain boundaries, which are barriers to dislocation movement. The impediment of dislocation movement is a key mechanism by which material strength is increased.

💡Grain Size

Grain size refers to the dimensions of individual grains in a polycrystalline material. The video explains that a fine-grained material tends to be harder and stronger because grain boundaries provide more resistance to dislocation movement. The script also mentions how processing conditions, such as annealing at different temperatures, can influence grain size.

💡Hall-Petch Equation

The Hall-Petch equation is a mathematical relationship that links the yield strength of a material to its grain size. The video describes how this equation helps to quantify the influence of grain size on a material's mechanical properties, with a finer grain size leading to higher strength due to increased resistance to dislocation movement.

💡Overaging

Overaging is a term used in the context of precipitation hardening to describe the loss of strengthening properties when a material is heated for too long. The video explains that overaging occurs when the second phase in the material continues to grow, eventually leading to an equilibrium microstructure and a loss of the strength improvements gained through precipitation hardening.

💡Time-Temperature-Transformation (TTT) Diagram

A TTT diagram is a graphical representation used to understand the kinetics of phase transformations in materials as a function of time and temperature. The video uses the TTT diagram for iron-carbon alloys to illustrate how different cooling rates lead to the formation of various non-equilibrium phases such as pearlite, bainite, and martensite, which have different mechanical properties.

Highlights

Importance of understanding non-equilibrium conditions in material processing for mechanical property enhancement.

Example of sword making to illustrate the impact of quenching on metal strength.

Different mechanical properties of materials can be achieved through selective heating and rapid cooling for applications like gears.

Introduction of three approaches to material strengthening: solid solution hardening, strain hardening, and precipitation hardening.

Solid solution hardening increases strength by alloying with impurities that impede dislocation movement.

Strain hardening or cold working enhances a material’s strength through plastic deformation.

Precipitation hardening forms a small, well-dispersed second phase to impede dislocation movement and increase strength.

Grain size significantly influences mechanical properties, with smaller grains leading to increased strength.

Hall-Petch equation quantifies the relationship between grain size and yield strength.

Strain hardening increases dislocation density and decreases grain size, leading to higher strength but reduced ductility.

Precipitation hardening involves a two-stage heat treatment process to create a supersaturated material and precipitates.

Silver-copper alloy example demonstrates the precipitation hardening process and its impact on dislocation movement.

Overaging effect in precipitation hardening can lead to loss of strength if the second phase grows too large.

Different forms of low alloy steels and their transformation under various heat treatments.

Tempering process to restore ductility to brittle martensite in steel.

Time-Temperature-Transformation (TTT) diagrams for understanding non-equilibrium structures formed at different cooling rates.

Summary of material property variations with composition and processing conditions for achieving desired mechanical properties.

Transcripts

Hi, I’m Dr. Billy Wu and in this video we’ll be  talking about how we can make materials stronger  

through the way we process them. This follows  on from 2 previous videos where we discussed  

equilibrium phase diagrams and steels, so  if you haven’t checked those out please do.

So first of all lets explore  why this is important.

In previous videos we explored how  the mechanical properties of materials  

vary depending on their composition and  resulting equilibrium microstructures. However,  

in many cases the manufacturing conditions are  under non-equilibrium conditions. Here factors  

such as the cooling rate dramatically affect the  resulting mechanical properties of a material.

One example is in sword making, where we might  want to increase the strength of the metal  

by quenching this in water after  we’ve heated and formed the sword.

In another example, we might have a gear, where  we want the teeth to have extremely high hardness  

and wear resistance but still have a ductile  core. Here, we might use approaches such as  

induction heating to selective heat a  region and then rapidly cool this to  

have a hard outer shell but ductile centre.  In both of these cases, we are looking at  

non-equilibrium processes and having a detailed  understanding of what is actually going on  

will help us to better select appropriate  materials and their manufacturing processes.

So, at a high level there are 3  approaches for strengthening a material,  

that we’ll cover in this video.

The first is solid solution hardening,  where we alloy a material with an impurity.

The second is strain hardening, or cold working,  

where we plastically deform a  material to enhance it’s strength.

And finally precipitation hardening, where we form  a 2nd well dispersed and small phase in a material  

through specific heat treatment processes.

So lets start with solid solution hardening.

Now in nearly all cases, high purity metals  are softer than their alloys and increasing  

the amount of impurities increases  their tensile and yield strength,  

which you can see in the  nickel-copper alloy example.

The reason for this increase in the strength  is due to the impurities in the material  

impeding the movement of dislocations.

In crystalline materials, a dislocation is a  linear defect in the material, and the ability for  

these dislocations to move is strongly correlated  to the mechanical properties of this material.

In terms of the type of impurities and their  impact, we can alloy in an element which is  

smaller than the host solvent atoms  to create a tensile lattice strain.

Alternatively, we can alloy the material with  a solute which is larger than the host element  

resulting in a compressive lattice strain. In  both of these cases, the lattice strain makes it  

more difficult for these dislocations to move,  which results in an increase in the strength.

Now, beyond atomic level influences, the size of  the individual grains in a metal also has a strong  

influence on the mechanical properties. Again  this is related to the mobility of dislocations  

within the material, as these dislocations have  to happen over these grain boundaries during  

plastic deformation. Therefore grain boundaries  act as barriers to the dislocation movement

A fine grained material, therefore tends to be  harder and stronger, as there is more resistance  

to dislocation movement. Processing conditions, in  particular, have a strong influence on the size of  

these grains. Here we can see for a metal alloy  which has been annealed (or heated) at 550 °C for  

1 hour, has smaller grains than the same material  which has been annealed for 1 hour at 650 °C,  

as the higher temperatures allow  for larger grains to develop.

So, clearly there is a relationship between  the grain size and the mechanical properties  

but how do we quantify this. One basic  approach is to use the Hall-Petch equation  

which relates the yield strength of a material  to a baseline constant yield stress value,  

sigma i, plus a contribution from a constant  k divided by the square root of the average  

grain size. This helps us to get a basic  understanding of the influence of grain size.

Now, previously we saw that we can control the  grain size through controlling the temperature  

but we can also do so by strain hardening,  or cold working, the material, whereby it  

becomes stronger through plastic deformation. This  effect can be seen with steel, brass and copper.

Here, we can define the degree of plastic  deformation by the amount of cold working,  

which we can take as the difference  in the cross sectional area before  

and after deforming a metal,  over the original area.

However, whilst the strength of a material  increases, this often comes at the cost  

of decreased ductility, so the balance  between the 2 properties needs to be made.

And in terms of what is going on, strain  hardening increases the dislocation density,  

therefore making it harder to deform the material  as there are more dislocations per unit volume  

and also the grains have become smaller  providing additional barriers to their movement.

Finally, we have precipitation hardening  as a means of strengthening a material.  

Here a small and uniformly dispersed phase is  formed in the original phase which again has  

the impact of impeding dislocation movement and  thus increasing the strength of the material.

This can be achieved by specific  heat treatment processes  

which allow for non-thermodynamic,  or meta-stable, structures to exist.

For precipitation hardening  this is achieved in 2 stages.

Solution heat treatment where  by a supersaturated single phase  

is created by quenching a material rapidly.

And a second precipitation heat treatment  phase where the material is reheated  

to allow for the formation of  small dispersions of a 2nd phase.

Now lets have a closer look at what’s actually  going on in this process. Here, lets take the  

example of a silver-copper alloy which has limited  solid solubility as shown in this phase diagram.

If we select a low silver composition  and heat the material up to T0  

which is in the single phase alpha region  we have a homogeneous single phase material.

Then if we rapidly cool the material from T0  to T1 we enter into the alpha plus beta region,  

however, because we’ve cooled the material  so fast, this doesn’t give enough time for  

atoms to diffuse to their thermodynamically  preferred position and as such we end up with  

a supersaturated alpha phase, where the beta  phase has not had time to precipitate out.

Now in the next stage we have the  precipitation heat treatment process.

Here, the supersaturated alpha is heated to T2  which is still within the alpha plus beta region,  

but at the higher temperature diffusion happens  faster and beta particles can start to form.

After holding the material at  T2 for a set amount of time,  

it is then cooled to lock in the structure.  At this point the cooling rates are less  

important but doing this faster allows for the  desired structure to be locked in. Now at this  

stage the materials microstructure consists of  grains of alpha with small precipitates of beta.  

Given that this is meta-stable, there  is often significant lattice strains  

and the small precipitates help to further impede  dislocation movement and thus increase strength.

However, if the heat treatment  process is not properly controlled,  

we can lose the beneficial  strengthening properties.

Now, we know strength and hardness  increase is a function of temperature  

and the time which controls the precipitation  of the highly dispersed and fine beta particles  

from the supersaturated alpha phase.

However, if the material is heated for too  long, the 2nd phase keeps growing such that  

the thermodynamic structure is achieved, leading  to a loss of the strength increase. This effect  

is called overaging, and can happen at  room temperature with some materials.

Now finally lets look different forms of  low alloy steels which are alloys of iron  

and carbon. We’re interested in this system  since steel is such as commonly used material.

A detailed summary of steel  was provided in another video  

but lets quickly revisit this here. The eutectoid  composition of steel at room temperature  

consists of a lamella like structure of soft  ferrite and hard cementite, we call pearlite  

which transformed from a gamma austenite phase.

At high temperatures and  relatively low carbon compositions,  

we have a single phase austinite material.

As we cool this material down, we enter  into a 2 phase ferrite and austinite phase.

And finally as we continue to cool, we enter into  a 2 phase ferrite plus cementite region, where the  

remaining austenite has transformed into pearlite,  which is held together by pro-eutectoid ferrite.

Here equilibrium transformations are  driven by the diffusion of atoms. However,  

if the rate of cooling is too fast, then the  carbon atoms don’t have enough time to reach  

their thermodynamically stable locations resulting  in the formation of other structures or phases

Now if we start to look at how different  heat treatments affect the structure of steel  

we see that we can form  various non-equilibrium phases.

When we do full annealing of the steel,  which is a very slow furnace cooling,  

this leads to the formation of coarse pearlite.

If we then cool this slightly faster  through a normalising processing  

which is a faster air cooling, we =still  form pearlite but with a finer structure.

Then if we cool even faster,  say with a forced air cooling,  

we form even finer structures of ferrite  and cementite which we call bainite.

Finally if we quench the material in  water, which cools it down very rapidly,  

giving no time for the carbon to  diffuse, we form a meta-stable FCC phase  

which we call martensite which is extremely  hard and brittle. In reality, martensite,  

is too brittle to be used in most applications,  so by tempering, or reheating the material,  

we can form tempered martensite to  restore some degree of ductility.

So evidently heat treatments involve  non-equilibrium processes and are used  

to alter the microstructure to  achieve the desired properties  

but how can we quantify the rates of cooling  and the resulting microstructures a bit more.

Well in this case we can use the  time-temperature-transformation diagram,  

or TTT plot.

Here we have temperature on the y-axis and  time on the x-axis. For the iron-carbon TTT  

plot we have a few features to note. First of  all we have the eutectoid temperature, where  

above this we have a stable austenite phase and  below this we have an unstable austenite phase.

We then have several contours which represent  the point at which pearlite starts to form  

and when this has finished to form 100% pearlite.

At the bottom, we also see the point  at which martensite starts to form.

Now, if we have a slow cooling process, which is  represented by a line with a shallow gradient, we  

can see we end up into the complete ferrite plus  cememtite region where we form coarse pearlite.

As we increase the rate of cooling,  we gradually form fine pearlite

And then bainite, which has extremely  thin ferrite and cementite regions.

And finally, if we quench the hot steel, we  rapidly cool the material such that it never  

enters into the pearlite region and instead only  forms martensite. Therefore, these TTT diagrams  

are useful for understanding the non-equilibrium  structures which form at different cooling rates.

So to summarise.

Material properties clearly vary significantly  with both composition and processing conditions.

In the case of solid solution hardening,  we alloy a material with impurities which  

introduces lattice strains which  impede dislocation movement.

Grain boundaries also hinder this dislocation  movement, and if we decrease the size of these  

grains through processes such as  strain hardening or cold working,  

we can decrease the grain size and increase the  dislocation density to increase the strength.

Thirdly, we can use precipitation hardening  to form a highly dispersed 2nd phase  

which again hinders dislocation movement. This  is achieved through a 2-stage heat treatment,  

whereby we first perform a solid solution heat  treatment to create a supersaturated material.  

Then we reheat the material to  form the 2nd phase of precipitates.  

However, if this heating process occurs for too  long, then the grains keep growing and eventually  

the equilibrium microstructure is achieved  again which loses the strength improvements.

In the case of steel, the strength  is very sensitive to heat treatments,  

where depending on the cooling rate,  structures including coarse and fine pearlite,  

bainite, martensite and  tempered martensite can form.

Finally, we looked at how we can quantify  and map out the influence of these cooling  

rates and resulting microstructures using  time-temperature-transformation plots.

So, thank you for listening and hopefully  this video has been a useful introduction  

into how we can make materials stronger, through  processes such as alloying, cold working and heat  

treatments. Again, this video follows on from  previous videos on equilibrium phase diagrams  

and steels so if any of the concepts  weren’t clear please do check them out.