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

Summary

Highlights

• Dr. Billy Wu discusses the influence of non-equilibrium manufacturing conditions on metal strength. 🎥
• Solid solution hardening increases strength by alloying metals with impurities. 🚀
• Discusses how strain hardening enhances metal strength through plastic deformation. 🔩
• Explains precipitation hardening using a two-step heat treatment process. 🔥
• Illustrates significance of cooling rates on steel microstructures using TTT plots. 📊
• Martensite formation from rapid cooling creates a hard, yet brittle, structure. 🧊
• Importance of controlling heat treatment to prevent over-aging and loss of strength. 🕰️

Key Takeaways

• Grain size influences material strength; finer grains offer more resistance to dislocation movement. 🌾
• Solid solution hardening uses impurities to create lattice strains, boosting material strength. 💪
• Precipitation hardening involves two heat treatment stages to form a second phase, enhancing strength. ⚙️
• Strain hardening, or cold working, increases dislocation density, making metals harder. 🛠️
• Cooling rates impact microstructure and strength in steel, illustrated by TTT diagrams. 📉
• Over-aging can occur if the heat treatment is improperly timed, reducing material strength. ⏰
• Martensite, a phase formed through rapid cooling, is extremely hard but often brittle. ❄️

Overview

Chapters

• 00:00 - 00:30: Introduction to Material Strengthening Dr. Billy Will introduces the topic of material strengthening in this chapter. He emphasizes the importance of understanding the relationship between processing methods and the enhancement of material strength. This chapter builds on previous discussions about equilibrium phase diagrams and steels, suggesting prior knowledge in these areas is helpful. The focus is set on how the mechanical properties of materials can be influenced by their composition.
• 00:30 - 01:30: Non-Equilibrium Manufacturing Conditions The chapter discusses non-equilibrium manufacturing conditions and their impact on microstructures and mechanical properties of materials. It highlights the importance of factors like cooling rates, which can significantly alter the material's properties. An example given is sword making, where quenching metal in water after heating increases its strength. Another example mentioned involves manufacturing gears, similarly affected by these conditions.
• 01:30 - 02:30: Approaches to Strengthening Materials This chapter discusses various approaches to strengthen materials, focusing on achieving high hardness and wear resistance while maintaining a ductile core. One method mentioned is induction heating, which selectively heats a region and then rapidly cools it, creating a hard outer shell with a ductile center. The importance of understanding non-equilibrium processes to select suitable materials is emphasized.
• 02:30 - 03:30: Solid Solution Hardening The chapter introduces the concept of solid solution hardening as a method to strengthen materials. It is one of three approaches discussed, including strain hardening and precipitation hardening. Solid solution hardening involves alloying a material with an impurity to enhance its strength.
• 03:30 - 05:30: Grain Size and Mechanical Properties The chapter discusses the relationship between grain size and mechanical properties, specifically focusing on solid solution hardening. It explains that pure metals tend to be softer compared to their alloy counterparts. By increasing impurities, tensile and yield strengths are enhanced. This is exemplified through the nickel-copper alloy, where impurities hinder dislocation movement, thus improving strength.
• 05:30 - 06:30: Strain Hardening or Cold Working In crystalline materials, a dislocation is a linear defect that influences mechanical properties. The movement of these dislocations is affected by the type of impurities present. By adding an element smaller than the host solvent atoms, a tensile lattice strain is created. Alternatively, the material can be alloyed with a solute to induce strain hardening or cold working.
• 06:30 - 09:30: Precipitation Hardening This chapter delves into the concept of precipitation hardening, a method used to increase the strength of alloys. It starts by explaining how the introduction of atoms that are larger than the host element leads to compressive lattice strains. These strains hinder the movement of dislocations, thereby enhancing the material's strength. Furthermore, the chapter highlights the role of grain size in influencing the mechanical properties of metals, tying it back to dislocation mobility within the material.
• 09:30 - 13:30: Heat Treatment in Low Alloy Steels The chapter titled 'Heat Treatment in Low Alloy Steels' discusses the role of grain boundaries during plastic deformation. Grain boundaries serve as obstacles to dislocation movement, resulting in fine-grained materials being harder and stronger due to increased resistance. The processing conditions significantly impact the size of the grains. An example is provided, showing a metal alloy that has been annealed or heated to illustrate these effects.
• 13:30 - 16:30: Time Temperature Transformation Diagrams (TTT) The chapter discusses the concept of Time Temperature Transformation Diagrams (TTT). It explains how the temperature and the duration of heat treatment impact the grain size of a material. Specifically, heating a material to 550 degrees Celsius for one hour results in smaller grains compared to heating the same material to 650 degrees Celsius for the same time period, due to the higher temperature allowing larger grains to form. This indicates a clear relationship between the grain size and the mechanical properties of the material. The chapter suggests that one way to quantify this relationship is using the Hall-Petch equation, which connects the yield strength of a material to a baseline constant yield.
• 16:30 - 18:00: Summary and Conclusion In this chapter, the influence of grain size on stress values is discussed. It highlights how stress value (sigma) is influenced by a constant (k) divided by the square root of the average grain size, offering a fundamental understanding of grain size impact. It mentions that apart from grain size, there are other influencing factors as well. The chapter also reviews methods for controlling grain size, emphasizing the roles of processing temperature, strain hardening, and cold working in material strengthening.

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

• 00:00 - 00:30 hi i'm dr billy will 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 two previous videos where we discussed equilibrium phase diagrams and steels so if you haven't checked these out please do so first of all let's explore why this is important in previous videos we explored how the mechanical properties of materials vary depending on their composition and
• 00:30 - 01:00 resulting equilibrium microstructures however in many cases the manufacturing conditions are actually 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 repeat it and form the sword in another example we might have a gear
• 01:00 - 01:30 where we want the teeth to have extremely high hardness and wear resistance but have a ductile core here we might use approaches such as induction heating to selectively heat a region and then rapidly call this to have a hard outer shell but also doctoral center in both of these cases though we're looking at non-equilibrium processes and having a detailed understanding of what's actually going on will help us to better select appropriate materials and their
• 01:30 - 02:00 manufacturing processes so at a high level there are free 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 its strength and finally precipitation hardening where we form a second well-dispersed and small phase in a material
• 02:00 - 02:30 through specific heat treatment processes so let's start with solid solution hardening now in nearly all cases high purity metals are softer than the alloys and increasing the amount of impurities increases their tensile and yield strengths which you can see in the nickel copper alloy example the reason for this increase in strength is due to the impurities in the material impeding the movement of dislocations
• 02:30 - 03:00 in crystalline materials a dislocation is a linear defect in the material and the ability for these dislocations to move is strongly correlated to its mechanical properties in terms of the type of impurities and their impacts 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
• 03:00 - 03:30 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 increase in the strength now beyond atomic level influences the size of 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
• 03:30 - 04:00 as these dislocations have to happen over these grain boundaries during plastic deformation therefore grain boundaries act as a barrier to dislocation movement a fine-grained material therefore tends to be harder and stronger as there's 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
• 04:00 - 04:30 at 550 degrees for one hour has smaller grains than the same material which has been annealed for one hour at 650 degrees as the higher temperatures allow for larger grains to develop so clearly there's a relationship between the grain size and the mechanical properties but how do we quantify this one basic approach is to use the whole patch equation which relates the yield strength of a material to a baseline constant yield
• 04:30 - 05:00 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 but there's many other things going on now previously we saw that we can control the grain size through controlling the processing temperature but we can also do this by strain hardening or cold working the material whereby it becomes stronger through
• 05:00 - 05:30 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 take as the difference between the cross-sectional area before and after deforming the metal over the original area however whilst the strength of a material increases this often comes at a cost of decreased ductility so a balance between these two
• 05:30 - 06:00 properties needs to be made in terms of what's going on strain hardening increases the dislocation density therefore making it harder to deform the material as there's 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
• 06:00 - 06:30 again has the impact of impeding dislocation movement and therefore increasing the strength of the material this can be achieved by specific heat treatment processes which allow for non-thermodynamic or metastable structures to exist for precipitation hardening this is achieved in two stages solution heat treatment where a supersaturated single phase is created by quenching a material rapidly
• 06:30 - 07:00 and a precipitation heat treatment phase where the material is reheated to allow for the formation of small dispersions of a second phase now let's have a closer look at what's actually going on in this process here let's take the example of a silver copper alloy which has limited solid solubility as shown in the phase diagram if we select a low silver composition and heat the material up to a temperature of t0
• 07:00 - 07:30 which is the single phase alpha region we have a homogeneous and single phase material if we then rapidly call the material from t 0 to t 1 we enter into the alpha plus beta region however because we've cooled the material so fast this doesn't give enough time for the atoms to diffuse to their thermodynamically preferred position and as such we end up with a supersaturated alpha phase where the beta phase hasn't had time to
• 07:30 - 08:00 precipitate out yet now in the next stage we have the precipitation heat treatment process here the supersaturated alpha is heated to t2 which is still in the alpha plus beta region but at the higher temperature diffusion happens faster and the beta particles can start to form more rapidly after holding the material at t2 for a set amount of time
• 08:00 - 08:30 it's then called to lock in the structure and at this point the cooling rates are less important but doing so 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 and given that this is a metastable state there's often significant lattice strains and the small precipitates help to further impede dislocation movement and therefore this increases the
• 08:30 - 09:00 strength of the material 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 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 second phase keeps growing such that the thermodynamic structure is
• 09:00 - 09:30 achieved leading to a loss of the strength increase this effect is called over aging and can happen at room temperature with some materials now finally let's look at different forms of low alloy steels which are alloys of iron and carbon we're interested in this system since steel is such a commonly used material a detailed summary of steel was provided in another video but let's quickly revisit this here the eutectoid composition of steel at room
• 09:30 - 10:00 temperature consists of lamellar-like structures of soft ferrite and hard cementite which we call perlite which transformed from a gamma austenite phase at high temperatures and relatively low carbon compositions we have a single phase of austenite as we call the material down we enter into a two phase austenite and ferrite region
• 10:00 - 10:30 and finally as we continue to call we enter into a two-phase ferrite plus cementite region where the remaining austenite has transformed into perlite 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
• 10:30 - 11:00 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 perlite if we then cool it slightly faster through a normalizing process which is faster air cooling we still form perlite but with a finer structure then if we call even faster say with forced
• 11:00 - 11:30 air cooling we form an even finer structure of ferrite and cementite which we call bainite finally if we quench the material in water which calls it down very rapidly giving no time for the carbon to diffuse we form a metastable face centered cubic phase which we call martensite which is extremely hard and brittle now in reality martensite is too brittle to be used in most applications so by tempering or reheating the material
• 11:30 - 12:00 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 do we quantify the rates of cooling and the resultant microstructures a bit more well in this case we can use the time temperature transformation diagram or ttt plot here
• 12:00 - 12:30 we have temperature on the y-axis and time on the x-axis now for the iron carbon ttt plot we have a few features to note first of all we have the eutectoid temperature where above this line we have a stable austenite phase and below the line we have a unstable austenite phase then have several contours which represent the point at which perlite starts to form and when this is finished to have 100
• 12:30 - 13:00 per light 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 cementite region where we form coarse perlite as we increase the rate of cooling we gradually form fine perlite and then bainite which has extremely thin ferrite and cementite regions
• 13:00 - 13:30 and finally if we quench the hot steel we rapidly cool the material such that it never enters into the perlite region and instead forms martensite therefore these ttt diagrams are useful for understanding the non-equilibrium structures which form from different cooling rates so to summarize material properties clearly vary significantly with both composition and processing conditions in the case of solid solution hardening
• 13:30 - 14:00 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 second phase
• 14:00 - 14:30 which again hinders dislocation movement this is achieved through a two-stage heat treatment process whereby we first perform a solid solution heat treatment to create a supersaturated material then we reheat this material to form the second 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 and therefore we lose
• 14:30 - 15:00 the strength improvements of the small particulates in the case of steel the strength is very sensitive to the heat treatment process where depending on the cooling rate structures such as coarse and fine perlite bainite martensite and tempered martensite can form and finally we looked at how we can quantify and map out the influence of these cooling rates and result in microstructures using the time temperature transformation plots or ttt diagrams
• 15:00 - 15:30 so thank you for listening and hopefully this video has been a useful introduction into how we can make material 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 these concepts weren't clear please do check them out