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Modern Steel Products (2015) - lecture 5

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Automatisierte Medienanalyse

Erkannte Entitäten
it's a few things about where we had stopped by last week so we were talking about the following elements and on to
which the we've talked about chromium and Molly molybdenum and which is far were busy discussing the impact of war on me and at shown you this examples anomaly on the server these are with the goal the primary the beyond an Austinite grain boundaries so this is where you would expect for right to nuclear data this is an example here where the Steelers bore on Friday new interrupted the creation by quenching the steel so the UN transformed Austinite is present as Martin site and you can see very nicely C the the far-right having nuclear at the grain boundaries but if you add on you can see the same brains yes a grain boundaries rather and there's absolutely no fair right on the screen bound so do you more on present at grain boundaries is very strongly enriched there and and prevents nuclear Asian affairs you get as a consequence when you look at for insisted on this DTT diagram here you see here Top heart Of the t t diagram is related to variety formation you know that the lower part of the related to other decomposition reactions that marked the site and in the day-night formation but the top part here than in the same stealing now added boron you can see that the transformation is retarded and that's it as a consequence of this boron edition can't the grew and the never forget that if you look at it want to TT diagrams yes they're not only a function of the composition has they will be different also depending on the Mike rastructure there 2 important Microsoft trucks roll features that he Austinite can have the Fourier transforms it can have a smaller grain size over or a very large grain size that will in fact the transformation behavior also and another parameter that is of importance Michael structural parameters is whether or not the Austinite this deformed because you can transform can make far-right from they re crystallized Austinite can all make far-right from the form plastic lid to foreign the Austin for instance you are the former material in a rolling mill yes and it's cooling down and it's I am a cooling down faster than it can be crystallized all the you will for the far-right inside this deformed are Austin that's 1st have a look at the grain boundary the effect of the grain boundary so I knew I already told you that the premier here that's about when you have a Austinite too far-right decomposition the varied forms at the
boundaries yes nuclear patient yet but now imagine that you have the same steel but now the grain boundary density is much higher for instance said of having this just this 1 boundary I have no 4 times as many boundaries just stuck these pictures together and reduce the size but I basically then here is reduced the size of the grains right but you can see here obviously that I will form a lot more nuclei and this and that I will have a faster that I must have a faster transformation just purely on the basis of the smaller greater so if the grain size of Austin visited the mike rastructure 1 is smaller than the grain size of Mike restricted to and the timing To achieve a certain about it added that there's a certain the truck amount of transformation from performances this time here to achieve a certain amount of transformation sources said that the 2 dotted line and the full line that are for the same amount of transformation and if it takes me T to star seconds to transformed the MicroStrategy with the larger grain size and it will take me Tier 1 Starr seconds to deformed the to transform the same amount of Austinite with a smaller grain size and the relation will be that she want is equal to it do you want divided by D 2 times teacher and and that's obviously smaller than t to stop and that the smaller the grain size the faster the transformation has so you're TGT diagram your CGT the CCT diagrams are always for a specific grain size and if your brain size smaller transformation can be lost lost faster the same thing when the formation would defamation these seeker of say we are in a TGT diagram the seeker for transformation without defamation is here this would be the princess and cooling at this particularly rates now then the point where the transformation start as you know it's called the eight-part read temperature rights and this is the archery for that particular cooling rate was if I do but transformation from a deformed Austinite the strength of the transformation will be will start earlier yes and at higher temperatures so why is that although 2 in facts 1 of them is the kind that take effect and the other 1 is a thermodynamic effects 1st of all what is the thermodynamic effect when when you have a phase and it's deformed its energy is increased so it's less stable higher energy less stable so I will have a higher the driving force for the transformation that's that's it's a general thermodynamic factor the other factor is the kinetic factor when I have deferred to the material might restricted their lots more no creation sites for the transformation and he still effects together increase the rate of transformation so that I can say I haven't did the CC the curfew will move to the left will get faster because I have a higher nuclear nation rate it's like having a smaller grain size if you want to write more nuclei In the Microsoft rapture and then it also increases the the transformation starts earlier because I have a higher free energy for the deformed Austin we can so that let me show you an example here so of the strained transformation In a
strained Austinite so if the US tonight is not strange we get the grain boundaries for Austinite and at these grain boundaries you form of variety and now we look at the same MicroStrategy we we deformed material 12 per cent 40 per cent 60 per cent yes and then we quench this and we see in them look and Mike rastructure and what do we see is that now and SU deformed that did it Austinite you can see that inside the grain to now start to see verite nuclei here you can also see a grain and you could see that inside the grain you for verite nuclei and this is a heavily deformed material inside the the grain you can see a lot very fine additional variety nuclei so you get a much faster transformation and because you have so much so many the far-right nuclei you will have reduced the grain size very effectively and this is this idea 2 transport From the forint strained Austinite is the key to Termo mechanical processes OK so so let's just wrapped up around the review of our of the yellowing elements in stealing general so there are a lot of the dispute is that you don't have to slide a man of sausages made of his I noticed that was but it's not a fundamentally so quick we generally defied the following elements into far-right stabilizers or Austinite stabilize and that's basically on how does the a binary irony elements of the look like France on chrome ICU that the Austinite stability range incidents the temperature uh which the Austinite is stable that is the food the contract alright uneven form indicators of this the chromium former close to Gamal loop that so I call these elements that do this that's reduced the Stabile at the range of Austinite call them far-right stabilizes and you can see here that at 15 per cent of chromium yes you don't even former Austinite anymore right itself obviously chromium is a strong right stabilized but you have elements into the refers instead of contracting the stability range that expanded conferences here at 25 per cent of nickel I don't have any far-right anymore so these elements we call Austinite stabilizes they create an expanded and damage stability Rangers holding open the and so so that is thermodynamic stability so that the elements also have an impact on the t t t diagram and this is better an example here of what do some of them are we explain like boron I will will push back chromium will reduce the the transformation kinetics will see laid a little bit later on when we talk about money albeit that it was in the other direction because now you will be losses to stabilize the former Austinite molybdenum also but suppresses the the perlite and its formation was of reality elements depressed the EMS temperature that that is to do the explanation to this diagram here there so this this you do have a new note accepted it because it's so small on the screen I've just New I have separated the rights stabilizing elements and the Austinite stabilizing element discussion so I was so when you look at the influence of Allied elements you have to do look at different facets 1 element just doesn't do just 1 thing yes uh usually it doesn't multiple thinks of you In general the 1st thing you have to do is a look at the phase diagram is at an element that there is despair right stabilizer Austinite stabilizes and the result in the closed gamma field or contracted gamma field then it's a ferret stabilizer most of the far-right stabilizing elements Our substitution all elements With the exception of boron which is intercession the head because the of these verite stabilizing element reduce the Austinite stability range it means that the 80 Otfried temperature will increase as you add these elements now the important thing is what is the effect on the Austinite decomposition of these elements do they form Carbide's or nitrites in his or important points and then asserted that elements of our important to note a few things so about the but Austinite the decomposition most elements even tho they are affair right stabilized yes when you add them to steal they will slow down the transformation reaction yet so you'd think while at ad for instance aluminum is a strong for right stabilizer if you add aluminum to your steal it doesn't mean that if you will for verite passes the why is that while there the number of the facts for instance that element that particularly element can have an influence on the carbon diffuser fitted they can slow down the carbon diffuser fitted why would that have an effect while remember when we're making In steals when we're making variety From other Austinite the carbon is partition between those 2 phases the ,comma has to leave the the far-right and go into the Boston why does he do that because the solubility of carbon In Boston solo so whenever an element reduces the deficit of carbon it also slows down the decomposition reaction that's 1 thing the 2nd 1 Is that the elements themselves yes but rearrange themselves at the boundaries at the phase boundary so but you can have elements that enrich all are depleted at the boundary for instance silicon aluminum chromium do not really partition change places during the transformation but they have a tendency to the IDA depleted or reached at the interface of a need to rearrange themselves so that will slow down the kinetics of the reaction
and finally there are elements that also do not petition but have a tendency to stick around the phase boundary that they moved with phase boundary and that is in effect it's called solutes drag effect so for instance elements such as niobium and I'm taking him to a lesser extent vanadium accumulate active phase boundary and then it will slow down the the transformation very many of the far-right stabilizers are strong carbide form so that that will also have an impact on the on the on the transformation has an on the mike rastructure you get so it's a problem vanadium molybdenum tungsten Titaniums Niobe Human Services colonial there are strong carbide formers so the 1st ones you have 1st I decided Titaniums Carbide types niobium carbide types they don't there more stable than Simone ties so as soon as you add a even a very little bit of them they will bind carbon faster and more effectively than IRA yes they will always get former 1st before you for any seem price stability you have other types of Carbide just slightly less but stable yes and chrome vanadium and Molly if the concentration of these elements is not high enough then they will dissolve and and rich in the seaman died manganese chrome and of molybdenum at low content low concentrations are typically enriched in Simonton so if you add say 1 per cent or something on less than 1 per cent of chromium to a steel "quotation mark low-carbon steel you will not find chrome carbide this as you need to add 4 or 5 per cent of Crowe to form prone Carbide well industrial practice yes but is very complicated is more complicated than when I just said very often the Carbide's that you form are not pure carbon cobbled nitrites and so that means is like a mixture of where you get the carbon and nitrogen forming the compounds with but these elements grown right these these Carbide's In Carbon I tried the precipitate very strongly In fairer and in particular during the transformation From Austin 4 why is that because of the solubility Of these compounds in Farah but there highly sought more soluble in Austinite and when you do the transformation to far-right the solubility is much less will talk about this in a moment so that precipitated form a lot of small precipitous uh nitrites are also important but not fundamentally because we tend not to Alloy deals with nitrogen too much but there are certain deals where we will be doing this but usually are nitrogen content and steals is very low because they give that they give rise to aging heating phenomena so changing of with his aging that's changing mechanical properties as a function of time something we don't like it because you can deliver material with certain properties the material gets stored and then when it gets used it has to have the same properties than at the time you sold us on the materials that contain nitrogen however is sensitive to Beijing to the properties a mechanical properties will change after 6 months of storage but you don't want this and so we keep that's the reason why we keep the nitrogen levels very low of molybdenum and tungsten form Carbide's exclusively they don't form of nitrites and and the other elements grown vanadium you and you'll be a for both the conform nitrites and cobble nitrites and don't forget those certain things we we said about them elements that silicon is an element that will I suppress Carbide formation yes it will also suppress formation of perlite to a certain extent but it promotes the formation of far-right him chromium suppresses Bay nite and far-right transformation and promotes for light and molybdenum is very much an element we add whenever we want to promote day-night transformation and so it's process perlite and variety transformation indeed Austinite stabilizers are will we see a lot less not that many strong Austinite stabilizes being carbon steel I again what do they do with face grabbed expand the day Austinite field usually they are substitution elements manganese is the main 1 or the other ones nickel cobalt and copper but we don't use them very often so it's basically manganese and then to the smaller much smaller extends nickel and and cobalt only for special applications and Ed however carbon and nitrogen are Austinite stabilizes the A 3 temperature decreases as we add them because you expand the field so what do they do to the Austinite decomposition again always retarded why is it always retarded while simply because we stabilize the Austinite so there is a free energy effects you increases the about the stability of the Austinite and of course but the Austin nights that followed Austinite stabilizing elements also need to partition during transformations so that has an impact on the other transformation cobalt is not petitioners this some information you have and these Austinite stabilizes not Carbide formers except of course for carbon that required of this is this is of course not Jesus is the most nights but I think it's OK you that you can make East doesn't really former Carbide it's usually enriched in this human tide however and the same with Nichols 6 nickel is not that it doesn't former Carbide and is also not enriched in the car so it's ever been they don't tend to form nitrites is pretty pretty simple and that the Austinite stabilizer well basically it boils down to To manganese in for the low-carbon steels good so that the elements of and that we've discussed up to now we kind of assumed they were in
solution right so that if their carbon that convenience in subsidies interstitial solution of manganese it's in substitution of solution so but as I add 2 more for some reason and mortality elements for instance to achieve higher strength yes but there will come a point where this solution as supersaturated we are beyond the solubility limit and so you can have the prospectus Our precipitates a conforming grain boundaries or they can form inside the grades that can be entered granular so let's talk a little bit about this there are many of precipitates and steal the very important we try to control them and that's 1 of the things we do with it ,comma composition is controlling the precipitates here you see a Titaniums nitrite precipitate and you see something has grown on this tightening you might try precipitated manganese sulfide has formed in this particular case In it these precipitates can be very tiny conferences this is awesome niobium carbide precipitates the
following and if we we if you look at them at very high magnification and you use you could see that it's taken me very complexes like this very tiny citing United tried precipitate and on top of this b cap like that you'll be Carbide has precipitated has used this time titanium nitrite as a hedgerow genius the nuclear nations side so those the the main precipitates up
before we do we have continued there we make the difference between inclusions and precipitates they're not deciding to when that the precipitated this is the inclusions is not the person that enclosure is larger oxides of certified particles yes and that the results from steelmaking results from the makers for instance can be alumina particles but in precipitation we usually have Carbide's we talk about Carbide's or nitrites also sulfides and they tend to be small and they're made during this the processing of new steel the solids is solid processing of and they take up this year so that the various small these ones tend to be very lot In comparison so all of these elements there precipitates in in carbon steels them most important ones are Carbide's and and so if if you look at b I periodic table yes so this would be the irony here for our forms the number of Carbide's in the thermodynamics stable carbide is this seaman .period and we also for but what we call a transition carbide if at low temperature but the 1 at low temperature you precipitate In Carbide's out of solid solution supersaturated solid solution affair you don't form seamen tough new forms a transition Carver which recall epsilon car by or the 2 Carbide for to see this these are transition Carbide's if you wait long enough yes they will eventually disappear and be replaced by seamen time but in many steals in particular are those that are about he treated us for short times at of mold temperatures you form formed these kinds of transition carbide it in principle edge I said in there just a few minutes ago that if there were no manganese Carbide's yes you can you can make manganese Carbide but you never see them in steals because there not stable enough so as we move so to do the left-hand side of periodic table and removed from chrome and vanadium type in the name of fighting in what we do we go from less stable Carbide's too very stable Carver In particular ,comma such as Itanium Carbide vanadium carbide and now you'll be in Carver and these are very stable Carbide's and they don't form mixed carbide such as the where they're mixed with the irony to form seem ties but it has also indicated the main types of Carbide's state that you observed In steals the the 4 grown for instance the the type of carbide that you get falls as you change the composition of you steal them so that the you can see here that uh as you the increases the that she should be as you work you can't see it here but as you increase the that chrome content In in a steel that contains both Irish and carbon and chromium of the type of a carbide that you get is different you will you will go from from from 23 years carbon sinks to chrome 7 common so here in the reissue here is about 4 the 4 crawl there's the issue here is about to here and there these these types of carbohydrates as far as you go from a softer Carbide 2 very hard Carver I right in these compounds by where called hate compounds this was the main thing I want to so again this is again examples of what happens here in steals is dead as you go In this table from left to right you go from elements which which have no the carbide forming tendencies of silicon Silicon again forms Carbide silicon carbide very well known Carbide but in steals you don't see silicon carbide and cobalt and nickel gold for Carbide's in general so as you move to tungsten and molybdenum and vanadium run by you go to Strong carbide forwards Nunez an elements such as pro-morals Avery strong carbide formers the manganese is intermediate Carbide former but have you can see that and that as you go from left to right the the ratio all of the that particular element in this semen tied to the cops did that element in solution increases so it these elements a Carbide forming elements will always have a tendency to partition to be To do
this even tho know and that is important against because but that has an impact on the the composition of your car by the composition of the steel fence for instance is an example here about partitioning of chrome and manganese between far-right and Simone tight in EU tech toyed steel so so this is the partitioning coefficients years that normally if there is no partitioning of prone in the steel and the chrome and seaman tight is the same this is like you have tight last year's so the 2 Alpha Beta this is the distance this is the chrome content if you so you do the transformation at 700 deg C and when you look at the beginning of the transformation how much chrome is there in this type how much is there in the far right and the ratio is 1 but if you wait long enough you see that the krone after 1 hour to hour 10 hours is 15 times he has to the probe has now enraged In the same entirely yes well With time yes and then can reach very high levels and this is what that means that of course this partitioning will I have an impact on the properties of your of your of few around part like that same here for I'll make it needs this you basis as a function of time that the particular temperature you see I see the Austinite content increasing and then it's stable at 6 if I increase the temperature just 6 72 again at the beginning of the transformation there is no partitioning has and if you wait long enough you have an enrichment of about 10 times of the manganese in the seamen died about why would this be important as well for instance remember that's when we have perlite maybe this where the steals are prolific steals me maybe we want to make ball bearings with the dance with this steel and so on we need to know make several you die this paradise this human tide the dyes to seamen died in the polite when you have this partitioning yes it means that you have to rearrange also not only the carbon and me not only the carbon has to we we distributed but also the carbide added the chromium or the manganese so that slows down these steroid ideation reaction considerably this wouldn't this this summer it has this petitioning had does have practical impact look right but let's say a few words know about the precipitation of the how do we described as In so it's just this is just a little bit of filed into introduction don't worry too much about the matter but when you former precipitate inside that was this precipitate an ax but for as it can be for instance say aluminum yes last start of acts of nitrogen aluminum plus nitrogen forming aluminum nitrite but the debt that is the famous precipitate or Titaniums or let's say 90 Obion because we will have that example in moment my albeit plus carbon yes forming nite you'll be empowered by so that is the wallet to you it looks like a chemical reaction that actually is a precipitation reactions than aluminum in solution plus nitrogen in solutions forms an aluminum nitrite particles yes by albeit in solution carbon and solutions once salient carbide article it's clear right now the meaning of this doesn't mean that I take it 1 mole of of pure Niobe young and 1 mole of pure nitrogen and I make a reaction it doesn't mean that it means I have aluminum in solution in steel and have nitrogen in solution and steel and they former aide precipitate "quotation mark when they former precipitate well when the concentration is too high so so With this reaction presentation I I have I can have a free energy In this like any chemical reaction product phase -minus Starting phases the free energy of the starting phases and so that looks like best so we get it Standard values plus party natural log also the activity of my precipitate divided by product of the activity of these compounds nailed human carbon in solution yes OK when this reaction at equilibrium is equilibrium we we have melted cheese hero and so I can rewrite In this way there this here so but you know that the activity of a solid pure solid compound is what was and the we will assume that the activity of can be written as the concentration of at yes save for x concentration of effects and and this these this factor here I consider that to be a constant this calls the primary of that I can rewrite this this equation here it's something like that and and this is basically a solubility equation the the form of
this equation is that so so this log 10 times X is a divide by must so I can rewrite the spam times next year's 10 to the power of T plus being this and so I can this is this looks like this in In a graphically so if I put here 2 m is the concentration of that element in the steel and X is the concentration of that element x in the steel then this year looks like looks like this can ,comma that looks like this and of course there are different currents 4 different temperatures you wanted to 3 different temperatures giving them because you can see a and B are constants with tea is a variable that so I get in and out usually it's a section that as the temperature decreases 1 is smaller than T 2 and T 3 G 2 is smaller than 3 years of this line moves down and so on this is an example here for niobium carbide and so you have an idea will be last Percent of niobium and here carbon and this is this equation Mario times carbon content is equal to 80 "quotation mark plus city plus a over tea and you see here 8 of is minus 6 thousand 707 and so and temperatures usually encounter to you get less and you see at different temperatures this curve and as temperature goes down this curve comes closer to the axis right whenever we have a steel theirs we have a steal at a certain temperature and we have a certain composition so certain composition so I need composition here I don't know where have no be in carbon and this is this equation the solubility line at a certain temperature so any any composition steel composition that I can put in this In this graph because this is the justice of the mass per cent of carbon and here's mass per cent of been going from 0 to whatever value now you know what's important areas that let's say we have a piece of steel yes and we have niobium atoms only them on the larger and carbon atoms and by simplifying the picture I have 3 niobium atoms yes and no 6 carbon atoms right now I reducing the temperature and reducing the temperature and 1 moment yes but I'm passing what's called the solubility temperature so what does that mean I'm starting to make 9 William Cobb because I reached the suit the saturation point it's like having sugar right with a lot of sugar and very hot coffee yes it will you can dissolve a lot you reduce the temperature of the sugar if you have added a lot of sugar in your coffee will start to crystallize out there so so you start to form niobium carbide so this afternoon yes this action will form Malian cotton this when you reach the ability curve the solubility temperature you do not not Everything precipitates just a fraction will precipitate so that 1 temperature you form this afternoon and this carbon 4 the compound as the other ones OK they don't have to precipitate because the concentration in the solid has decreased when you formed as in that at the next temperature the 2nd item has to go out of solution at a lower temperature solid ability decreases further and I get the 3rd Adam Adams has etc and of course at every point I'm left to with some excess carbon atoms who do not have to precipitate yes OK can so what is important here what is that the way the concentration changes that's the way the concentration changes is controlled by the stoop yummy treat of the compound so the composition of your steel the name of human carbon content of his steel will always will soon so as to when you precipitate status this is the starting of I composition that as as you precipitate that the composition of the steel moves along the Nile them common composition and all the steel moves along this documentary collide just because you always state 1 carbon atom and 1 nitrogen atoms of carbon atoms and 1 ideal amount of out of solution yes come so how do we use this in practice so we needed this documentary clients and we need a solubility line that would would would assist solubility line means with respect to the composition has I think it will let let me just show a simple example or saying so I have this is this is basically the same graph is not that this the same as but now this is the
solubility line at 12 15 and this is that this life here and and I've indicated that the on above this line is red and below this line is blue right is not difficult up to now what does this mean if my composition is here would say I have 1 .period always sex per cent of carbon and . all 0 the point 25 per cent of now you'll be in this deal yes so I can put the composition here and what would its indirect the region yes the Comets Let's region means partially in solution that means the 90 albeit carbide is partially in solutions this the composition had been somewhere below this line the blue in the blue region has was fully in solutions so there solubility life that you calculate basically tells you you composition is below that line yes Everything is in solution composition is above that line some precipitation will occur yeah and how do you determine How much is it but only the precipitated well too From explain it here so again so we have for instance that scenario women carbon here is that we have a composition that's above this line has at this temperature of course that you know this temperature will change that let's just assume that at 1400 deg C the line goes through this point yes then this point is in the blue region yes OK so then it's fully in solution but as I dropped the temperature it will become partially sort and how does it work well I told you but the composition moves along this documentary clients and all of Of so specific line which is related to the composition of the compound that means that every time I precipitated 1 item of niobium 1 atom of carbon out of solution has ended up said the temperatures 1250 so I move along this and this is the new composition of the solids of the of the of the year in this particular case and if the Austin there 10 the composition of the steel hasn't changed that it just within the steel the nite be has formed and carbide some of it is still in solution right so when you reach this point you don't know the composition doesn't decrease anymore because it's that's that composition isn't can be and solution when you reach the soluble so this much of denial this much of the Nile River has formed niobium carbide you calculate this documentary client of his need to go back for that's very simple it's just basically the ratio it's a line going through your composition the ratio is a molecular weight of Niall Bureau Over the molecular weight they will be Over the molecular weight of carbon 10 get so this is an example here now so at but this would be the composition here
the starting steel composition so at 12 50 this is the amount of niobium that this will be present at the opening Carbide if I reduce the temperature further to 11 100 then the composition of the solid of the Austinite it continues to decrease and now I've the amount of niobium that has formed is known as niobium carbide is it has increased to this value here and of course yeah if this is this is the total amount of niobium in total this is the amount of niobium president Aznar you Union Carbide well then this is what this is the nite you'll be at present as In solution this is the amount of money albeit in solution To that's very useful on to have the solubility lines this and these are available for calculations this is for instance for a number of Carbide's In the far-right and Austinite and these are these parameters a and B which allow you to basically calculates the solubility Kerr and this is for tonight and so what does this mean for instance for instance this is a steel more . 0 4 % many albeit . 0 8 per cent of carbon unit and we look at the amount of now you will be aware that is in solution as a function of temperature well so you have the awkward 0 4 yes nailed and so as I go from high temperature too low temperature nothing happens and then when I reached 11 50 that is the solubility lines and from that point onwards the denial In solution decreases yes because I form my Union Carbide precipitately now I've indicated here this is a temperature range 1200 to 13 hundred little over 31 that temperature range is a typical temperature at which we reheat no carbon steel we reheat them at this particular temperature because at that temperature yes we this solved all the precipitates this and why do we do this because we want to control the way they precipitate afterward when control waited precipitate the size and the distribution and in particular this particular steel we want to make sure that now you'll be is in solution because it allows me to do will mechanical processing of steel can go good under let's start with that the number of other the number of important points we want to make about the relation between alloy eating and and strengthening yes and instead strength properties do we don't again we don't go into details here but it's important that I the you understand that we can use alloy too control transformations and change the MicroStrategy but why do we do this kind of well you 1 of the reasons we we change Microsoft tractors to change a mechanical properties and so on and here again the composition comes into play but in a slightly different in slightly different ways and so let's let's discuss the main points about the strengthening in relation to composition and microscopes so steal some very important materials extremely widely used 1 of the reasons is 1 of the
technical reasons for that is we have a huge spread of properties in terms of struck and so you do can have very very soft materials which go almost as low as I 100 to 200 makeup Pascal said it's relatively soft years and you can go all the way up to commercially you can buy it for 3 to 4 bigger Pascal Steve and that's a huge the breath of properties Prince and that's achieved by Mike rastructure control and true composition and processes right so so but what makes the strength in steel so so we need to say something
about formation of 1st so when you are young deformed steel gross copper clear you don't see much even if you do any out look at the Microsoft rupture with in optical microscopes Coke is not really much to see which which will tell you what's going on up to allow the defamation and and and wipe his defamation is in certain cases very difficult and in other cases using certain skills and you need to look into the structure of the grain when transmission electron microscope P for instance and you can see in these uh at that level of the magnification and resolution you can see these black clients and these are called dislocations in us and we will think of these dislocations asked but basically lanyard crystal defects of their linear because airlines sneers end and and will think about them for the sake of this particular courses having the following structure yes if you look into they did this crystal structure here is simple cubic crystal structure you can see that In these this red row of atoms and stops here this and and basically that this isn't like an extra half plane of atoms that's inserted in the laughter as for this type of the facts the best this year in the center yes this call the dislocation that all very far away where you inserted the extra half Lane and the structure is is basically the same there is no lattice distortion in the lattice distortion is end of the core of dislocation and it is the motion of this dislocation which allows plastic deformation is nothing but what causes indeed this location motion to be more difficult this and these dislocations are in steals Booker every time you do deformation so for instance you do forging operation at high temperature the Due to the fundamental mechanisms that make the deformation of this foraging possible there are dislocations yesterday at high temperatures or at low temperatures OK so again or you look into you do attend Celtic test yes look into you might
rastructure at very high magnification you will see that what has happened is you've created a lot of dislocations and that that has made the plastic deformation possible so we know where we like to think about this locations it is not alone
in a formal way yes is by this is a formal way of looking at this location but the way it actually happens in in in Paul across lines steel but different different but for the sake of the discussion here we we can just I have this formal approach so if if we wanted to form safe at 1 of the grains in our steel sample we need to somehow slipped this the top part which respected the bottom part and we do this by introducing dislocation of so we would basically Bush this and the crystal planes into the lactose and we create an extra half plane in the process and then this extra have plane moved across the crystal then comes out on this side and when this has happened this crystal has become slightly larger yes currently so what has happened plastic deformation and what is strange strength is how difficult or easy it is to make this dislocation move if it's difficult to move then the crystal is strong steel a strong if it's easy to move Crystal is soft did not it's very important to realize that what is actually moving through the crystal is not
a but an extra half plane that as its word diffuses too because the only thing that really happens is a rearrangement of atomic bombs and this is important a rearrangement of atomic bombs at the Cold War but this dislocation of those this this is as extra half plane I talked about so what gets rearranged that this bond goes to here to connect these 2 atoms and when this has happened to this small rearrangement the dislocation the extra have blamed now looks like it has moved yes whereas it hasn't moved at all right he's Adams of just we've just rearranged the atomic balding at the core of the dislocation but when we do this there is a small shift goes the latter's at which you can compare to the to the way they a snake a move seen as when as make moves it doesn't jump like this no it this inject it it makes a little changed yes in its tail and the kink propagates yes it's this is the same what's happening here this rearrangement of bonds it's like a little kinks that moves through the body of the snake has in mind a kink arrives at his head the snake has moved up a little bit yes so it makes a propagating came the day the dislocation same thing South yes I against you you can see there if dislocations move on these crystal planes used the shape sheer and then you get permanent or plastic deformation save where the power of the year snake that moves so no what happens when you have a when you do
for a material steel what 1 of the things you notice if you take a stress-strain curve is that distressed always increases yes it's so it's as if you know I'm deformed material and for some reason it gets stronger yes it's you would expect well you know if if I only have to get these dislocations to move what 1 does in this state flats yes because they're all the same defects yes but obviously there's something that happens which is called resistance to dislocation motion in this particular case the reason why we haven't increased resistance to disagrees with this because all these dislocations start to interact with each other yes they cut each other and so on then act as an obstacle to each other's emotions and that gives us strengthening which is called strain hardening at each other and so then act as an obstacle to each other's emotions and that gives us strengthening which is called strain hardening but there are other ways in which we can create a the strength the dismisses is the idea of strength why
if you want to prevent the snake from moving you just put a big stones on his head yes he makes a kink in and the king stops at the stove yes and it or you don't need to put it on him you can put just in front of him also In both cases it will be harder for the snake to move yes and it's similar with dislocation strength is obstacles to dislocation motion and therefore ways main ways in which we strengthened the steals but we can strengthened by adding solutes composition dependent the main solutes that's strengthens steels are false arrests silica and manganese that's 1 way in which we can strengthen the atoms can interact with the dislocation motion an act as of barriers to demotion and increase the strength so the this strength is proportional to the content of solutes obviously to certain power and and is usually walk all week in steals we can assume it's 1 for those of you who went into theory it's doesn't necessarily have to be 1 but 1 is the way the exponent we like to use in it in practice engineering practice the other possibility is you increase the dislocation density and that's what we call strain hardening more this locations you have the higher the yield strength of Steel N there is a linear relation between the strength and the square root after dislocation Institute humming the dislocations the have per unit volume the more I have there more obstacles they are for all the dislocations and they get strengthening another with a way to increase the strength it and it's very much used in the steel industry is the grain stocks you reduce the grain size you get strength you don't need to change the composition you don't need to and the defamation you just reduced the grain size and you probably know that the yield strength is increases which read reduction of the grain size proportional to 1 over the square root of the and that's well known whole patch relationship with and finally the before the way we used to increase strength is by adding precipitates it's in fact very similar to this it's just adding particles as such as this rock in front or on top of the snake yes 2 prevented from moving in here we see that it's very much a function of the particle diameter when the particle diameter is small the strengthening increases considerable and when I increased the volume fraction of precipitates so higher volume fraction might also increase the the yield strength or so I will want to have a high density High density of tiny precipitates I will want to have steel which small grain sizes I may want to this form steel to strengthen and I may want to add false 1st silicon or manganese To increase stress so certain of these things are related to Alloy Inc deformation of processing very often a combination of these 3 texts of Gannett will will be talking about the war in detail on on Thursday thank you for your attention
Schaft <Waffe>
Satz <Drucktechnik>
Airbus 300
Institut für Raumfahrtsysteme
Austin Motor Company
Ford Transit
Optisches Bauelement
ISS <Raumfahrt>
Band <Textilien>
Mechanismus <Maschinendynamik>
Band <Textilien>


Formale Metadaten

Titel Modern Steel Products (2015) - lecture 5
Serientitel Modern Steel Products
Teil 5 (2015)
Anzahl der Teile 31
Autor Cooman, Bruno C. de
Lizenz CC-Namensnennung 3.0 Unported:
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DOI 10.5446/18354
Herausgeber University of Cambridge
Erscheinungsjahr 2015
Sprache Englisch

Inhaltliche Metadaten

Fachgebiet Technik
Abstract A series of lectures on steels, given by Professor Bruno de Cooman, Graduate Institute of Ferrous Technology (GIFT), POSTECH, Republic of Korea

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