How to make metal stronger by heat treating, alloying and strain hardening
Summary
TLDRDr. 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
🛠️ 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.
🔩 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.
🔨 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.
Mindmap
Keywords
💡Material Strengthening
💡Non-equilibrium Conditions
💡Quenching
💡Solid Solution Hardening
💡Strain Hardening
💡Precipitation Hardening
💡Dislocations
💡Grain Size
💡Hall-Petch Equation
💡Overaging
💡Time-Temperature-Transformation (TTT) Diagram
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.
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