Mechanical properties of steel 1: introduction


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Mechanical properties of steel 1: introduction
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Cooman, Bruno C. de
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The first in a series of lectures given by Professor Bruno de Cooman of the Graduate Institute of Ferrous Technology, POSTECH, South Korea. This particular lecture introduces the general concepts which will be explored in detail in subsequent lectures.
The Graduate Institute of Ferrous Technology (GIFT)
Mechanic Computer animation Mechanical watch Ship class Spring (device) Steel Thorns, spines, and prickles Ship class Steel
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Vehicle Machine Ship Computer animation Transmission (mechanics) Spring (device) Schiffbau Engine Powertrain Steel Hose coupling Material Schiffbau Kurbelwelle
Coating Coating Finishing (textiles) Printing press Printing press Ammunition Reisezugwagen Welding Sheet metal Computer animation Automobile BMW 501 Automobile Steel Steel Material Sheet metal
Electric resistance welding Wire Key (engineering) Chemical substance Reisezugwagen Buckelschweißen Computer animation Automobile Cartridge (firearms) Tank Spring (device) Automobile Spare part Spring (device) Steel Musical ensemble Engine Wire Steel Kurbelwelle Kurbelwelle
Blast furnace Nut (hardware) Hüttenindustrie Vehicle Scrap Roots-type supercharger Mechanic Computer animation Mechanical watch Cartridge (firearms) Alcohol proof Typesetting Spare part Internal combustion engine Steel Steel Material International Space Station
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Kopfstütze Truck Train Machine Multiple birth Screwdriver Forging Sizing Forging Computer animation Cartridge (firearms) Typesetting Steel Material
Schlicker Electric resistance welding Stagecoach Camel (cigarette) Chemical substance Buckelschweißen Computer animation Cartridge (firearms) Typesetting Remotely operated underwater vehicle Book design Engine Material Steel Material Movement (clockwork)
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Mechanismus <Maschinendynamik> Mechanic Computer animation Mechanical watch Automobile Ship class Ford Explorer Spare part Tool Remotely operated underwater vehicle Steel Engine Tool Ship of the line Steel Material
so far welcome to the chorus GFT 6 6 9 which is about the mechanical property steel in the 1st session and shall be talking about about the subject of the courses subject matter in an introductory fashion this course is offered in the spring semester during the fall semester of 2 thousand 13 the textbook for the courses currently not available will be published in 2014 it be called direction of the count to the mechanical properties of steals currently in preparation with Professor it friendly at the Colorado School of Mines In the meanwhile as textbook is not available we will have you know you got on all and make sites available on but through the years the classes we meet twice a week 9 30 to 10 45 on Tuesdays and Thursdays and the the course grading is based on a weekly quizzes that power will be held on Thursday morning before the start of every class there's no no mid-term on final exam just east of the Weekly quizzes the subject
matter that will cover that in the course of this semester are basically subjects that are probably a most of you know the fundamentals of the big difference between this course and on the courses you've taken in the past is that the entire no the cost is focused on steel and only state in contrast to the lectures you may have had as the an undergraduate where the mechanical metallurgy or introductions to the mechanics of materials but was illustrated with a wide variety of suitable examples we need reverse here we take steel as there are starting point and we tried to see and understand what happens in steals when we focusing on the mechanical properties of state and so on so today that we would be talking introduction and India later lectures will start 1st of all to repeat or read the and go through some of the fundamentals are you may not temporary wells will talk about elasticity and plasticity on the way you probably seen it in the introduction to the mechanics of materials then will go will also have to repeat a few things related to crystallography and crystal defects there we will spent some time talking about dislocations and their properties again only as in relation to steals dislocations in steals and and crystal defects as such as Pichler vacancies we will talk about elements of crystal plasticity and then will go into the the subject matter Of the costs will be talking of solid solution strengthening strain hardening grain size strengthening precipitation strength stress-strain curve compilation yes it is possible to calculate stress-strain courage of steals even complex steals it is possible to be relatively easy actually to calculate stress-strain curve of the spilled using what you know about and the strengthening mechanism due to the composition of the migrants will also talk about localization of deformation of a strain localization of this very important in steals will talk about creep fatigue and will close with a fracture well right so let's
start I don't have to tell you this but you students at the FTC you know that still is I'm very widely used in France instructional applications well going anywhere from rebars too roof tiles In packaging applications of manufacture of furniture Suma
appliances it's widely used examples a all around us in the air conditioning systems in the kitchen equipment microwave washing machines sector
but automotive big users of steel base materials for out of body of course but also for the powertrain components of the Power and and we have the use of steel and engineering applications such as the oil industry shipbuilding where the ship or a foreign offshore construction is almost entirely built of steel so he's a couple of examples here of his offshore constructions it's or let us
1st start by a member of the simple concept here so when a car is made the steel and that is used to has gone through an incredible number of transformation processes actually in the steel it sells started as St sometime in the past as as a slap in of a long slap of cereal here that you see coming out of a continuous caster undergoes rolling treatments undergoes various the remote regions may undergo surface treatment and eventually leaves the steel plant as this coiled material and here is the process that this steel ghosts of material has still has specific properties specific composition Pacific Mike rastructure that is the result of these but it is the
process now of course the sheet of steel is not a car yet so this material goes through press forming low temperature the thermal treatments welding etc coating processes and of foreign suspects hop in the car industry manufactures car body which is then fertile assembled into a passenger car and that is the final product is a huge amount of processing between the
slap in the passenger car same holds for other products they you have large steel I goes through the various forging forming processes surface treatments thermal treatment to eventually become a crankshaft that will be part of a motor wire steel will go through various amounts of treatments chemical thermal 2 for instance and of it as a useful high-strength fall for instance but so we create properties also through the transformation of these problems that that emissions still into and once
we have a problem at Suffolk product aware the the steel based element is part of the we have to consider it in Service property for instance in the case of passenger cars but move it is important that the car body provides passenger so but in a high-speed in collision test the idea this Steelers must have high-speed strength and so that the car body has high-speed crash resistance in this case here heavy spring steel spring used for the real cars the Spring must have long term resistance against torsion this so I must have guaranteed portion fatigue life resistance resistance against fracture in these conditions in the case of heat the 2 we must have guarantees for high-temperature creep resistance and so does our performance requires so make any throughout the the the making and shaping of steals throughout the manufacturing of the a a product and throughout the use of this product the mechanical properties of the steel played a key role Frank so
however these properties achieved while they're achieved the composition of the material of the steel by the mechanical and thermal power processing that this material goes through end of the Mike rastructure that is generated as a consequence of these 2 element composition and a mechanical and thermal processes and this gives us a mechanical properties that such and also specific there in surface mechanical problem with roots in the case of the world automotive structural parts made out of steel high strain rate defamation property
so let's start with composition 1st of all people who don't have much experience with dealing with steals or with ferrous alloys but sometimes today can have the idea that steals are but contains a very large number of elements in fact so many bad it doesn't seem reasonable that doesn't seem to be a simple explanation for the complexity of the composition and and that and looks as if the steals contains almost all the elements of the periodic table if I may be allowed to exaggerated that is not take some so if we look at the periodic table of elements and we selected elements of that are commonly found still spoke the need we get about 20 elements that Avery ,comma ended at these elements are not there by chance as there as a consequence of the manufacturing of the steel and their consequences that also but the manufacturing of the production of this irony in this field and that a consequence of the island of testing the let's have a look at some of the elements in that sense the 1st of all you have elements such as manganese silicon loomed carbon that are always present in most fields to 0 small degree of very large government larger degree and that is because they're part of the bill or from which we extract the fire during and manganese is that such an element silicon aluminum get reduced when we used carbon to reduce iron ore for instance in the blast furnace and because we use carbon to reduce iron oxides in the war but the metallic Byron Nelson using the formation of the old but we always have some carbon in our steel calcium is the key element that is added during the metallurgy steelmaker steel metallurgy to control the economic calendar conclusions and will come back to that in a moment and so we always find that element to small degree it back in the states because we I always use scrap To make steel or whether it's to make electric arc the use of steel producers produce by using an electric arc furnace or that we make steel by the so-called blast furnace the U.S. Route we always used scrapped because of an electric arc furnace 100 per cent of the charge is almost none of some of the charges it's so the scrap the Suarez metal goes and in the case of the BU effort we use scrap for instance to control the temperature in the Buick so that when we do this they're always contaminants yes that come from strapped with cycling element just as it did in copper Foster sulfur hydrogen etc. And we call these elements tramp elements and we try to keep their content as little as possible then we have elements such as cruel Mollie Nichols boron and and other at the elements that we used to control the transformation in state transformation I'm referring to the gamma alpha transformation which is used to harden steel by for instance letting the steel goods will be needed court markets in the region in order to allow this to happen without having to use to hide cooling rates but we add these elements to steals very often and that so you'll find these elements in steals that where there required that would transformations are
actively being control and then we have special allocation which are very much depending on the patient's so for instance if we want to have a very low alloy levels very low carbon levels that have strong material Riverwalk very often micro alloyed these deals with fighting in Canadian you'll be so that these elements Carbide's or nitrite of Pablo nitrites In the microscope to and give us precipitation strengthening grain refinement In a case of tool steel we need extremely hard Carbide's to be formed in the microscopic and will have thanks Molly chrome Carbide's into Mike restriction will have had these elements to high levels precipitation hardening conferences be achieved by adding color part to the steel and there are for instance certain steals we will add elements as sold for war led led being phased out of Gulf can also be used as machine in addition to ease the machining of parts you will add these elements that in all the steel grades you will want to avoid and then add another element that is in that category as phosphorus that this can be used his strength and to do so give a solid solution strengthening of although in very many applications such as witnesses lined by its prosperous and inserted himself sulfur or are very much avoided in this part of the composition of the state and finally we also have as you know we don't only have carbon steel which we also have higher more highly alloyed steel such as stainless steels and there we have typically following editions of Kroll Mollie Nichols and nitrogen a way in which chrome Molly that nitrogen additions are specifically to guarantee the corrosion resistance of this and nickel is added to control the the type of crystal structure I will have or Florida give Nichols was also that the microscope you need to have a sizable amount of such sequestration of the so that's for the composition of many elements that we can use as a tool box to get but some sexual the effects in Para state then we can go through following through processing through thermal cycles we can get different to microscope and so we have some examples here on the left single faced the rights of simple procedures small grains are right next to a juicy perlite very different microscope where we get on well seem tides iron carbide alternating their heads with in this later patter you can get the 9th and last bottom side if this deal is made to transform at lower temperatures and you can pass steals and very many steels are in that category with a multi fixed such as for instance in the example that show the jewel face stiff so in addition to the east and simple Mike macroscopic but Michael structure yes we also have in the migrants truck trailers things that that are the result of the iron making sealing 1 of the more problematic aspects to the steel Microsoft are the norm metallic include most of the cases we'd like to keep them with the concentration of the nonmetallic inclusions as low as possible but there is always the chance that some of them will be remain they can be the melt inclusions making the calcium sulfide includes the manganese sulfide inclusions that can be calcium aluminum inclusions but depending on what inclusions you have that you will have conclusions with different properties some of them will be able to deform plastic the others will not be able to perform plastic and in general and their impact on the mechanical properties are negative but this is something we will be coming back to talk about toward the end of the course when we talk about fracture because stuck with inclusions second-phase particles nonmetallic inclusions nonmetallic precipitated on play a key role in the In fracture the L we don't we not only have nonmetallic inclusions all the time oxides and sulfides which are created during the and the steelmaking process we also have precipitates and precipitates art in general but much smaller particles In the MicroStrategy that are introduced into my instructor in a controlled manner and I show
here are 2 examples and in on on the on the screen the look much larger than the inclusions and dished show due to reverse is actually true these particles are much smaller than the nonmetallic who himself on let me see things nitrite inclusion and on top of this so museums sulfide has precipitated a and on the right you see Titaniums nitrite inclusion which enabled him carbide has precipitate and was and 1 of the things we tried to do I would precipitates its control mechanical properties in a in a positive way In particular because these precipitated because of their size their density and the distribution allow us to control the great signs of our skills and they also allow for precipitation strength these precipitates have much more positive role there is a lot of little positive impact on mechanical properties that the nonmetallic inclusions and so will be talking about that was also when we talk about the future and when we are I'm working on the chapter about precipitation pardon me but these precipitates can be the it's like nitrites and Carbide's of the picture so they can also be the result of aging treatments with the words in this case we have the small we see carbines In the stainless steel which are not formed during the manufacturing this year but as a result of temperature and temporary and also these particles have been interesting effect on the mechanical problem but it's a complex issue and the reason is the following Is that so the carbon in this particular steel can be In solution or can the present as a car now when the carbon in solution here we get an effect called it's a solid solution strength because when we precipitate the carbide out as a Clark at the carbon out as a Carbide we reduce the solid solutions however at the same time if these precipitates a small enough and distributed in the right manner we get precipitation hardening so I In this case the present the solid solution hardening reduction is partly compensated by an increase in precipitation hardening and say you can see we have a complex situation and part of the the cost will be devoted to trying to understand and determinant calculate How much loss of strength you get and the rules transportation of carbide and how much increase of strength get from the formation of the precipitous it'll turn out this way that there so you know in advance that the solid solution strengthening bikes harbor in variety is a very very strong so that the result of this hampering treatment of all right and the and the resulting Carbide formation results in the overall reduction instructor despite the formation of the scoreboard so whenever you we talk about mechanical properties and uh strengthening effect is all it's also important to me the realize at the time scale at which do the discussion group of a certain mechanisms for a certain theory is about so we talk about the macro scale when we consider for instance the slab we presented corals or a bar of material that's macros that's meter scale or larger when we have test samples yes are they mentions a very different than our dimensions are of the order of centimeters 10 centimeters 20 centimeters foot on the island when we go into the mike rastructure for instance in an optical microscope that we have observed the grain size of a steel and we are in which currently being hauled them as a scale in the 10 micron levels to 1 micron levels 100 to 1 micron level the inclusion so we were talking about their of the Order of Micron's unless the precipitates who were talking about wherein the the nanometer level and at times of and solutions we're in the summer nanometer level so we have tools to study the mechanical properties that all these levels bought we have to be we always have to be aware about the scale at which the discussion occurred the
properties that steel offers us all 4 of us are very wide and we can have steals the but with strengths levels from 100 to 200 years commercial steels and steals that have strength levels of 2 3 thousand mega Pascal and more these deals are all commercially available this and some barriers in a wide variety of applications they're not although they will most of the steals contained 90 to 95 per cent of Irish yes there will be very different in microscope church and composition the and so in order to work when we have very soft steals we have the specific MicroStrategy hope that it will have very hard and strong steals we have of very specific like restrictions and composition but achieving extremely high strengths in their in instills is actually not that much not that difficult where you and the reach at this stage wire restraints there are of the level of about Formica Pascal pursued before they get Pascal at the end the the challenge with steals is to make the mechanical properties property is appropriate for the application not so much whether or not it is possible to achieve extremes of former ability or or plasticity or extremes of struck it's you have the right combination for an application wrecked so let's
have a look at these scales Newfoundland skills to see what is of what is important saying we you know you just tested they a round steel bar broken so what's happened inside the material look with your optical microscope using a microscope which is very often the complex multi-phase Michael structure you don't really see much more than general parameters of this might affect the worst the grain size the volume fraction of this yellow phase here which happens to be Martin signed and so if you want to actually see what causes the of the plastic deformation have to take this sample and put it in a PML-N which is seen are dislocations and so on would you actually see in your microphone are not the dislocations themselves but the interaction between your electron beams and the strains the lattice trains introduced around the this location this location cost him and if you look able to look at the atomic level at this these dislocations what you will find is that they consist of a and that means that they are although very specific lattice defects which is key to plastic deformation in many crystallize materials and in steel dislocations are the and dislocation formation dislocation ability and dislocation multiplication will be key processes then took them to describe the blast city bustier strength also posted but 2 let
me be very clear it doesn't matter whether you are producing for instance the heavy forgings and high temperature of in this open process of forging a machine here inside the Microsoft truck driver yours steel yes can therefore because you have dislocations in the microscopic In the MicroStrategy you create dislocations you can multiply this location these dislocations can move around and make plastic deformation possible so in this industrial process at high temperature but also when you are doing simple tensile test it is the dislocations in the Microsoft product that will give you the list that will be responsible for strengthening the properties of this principle will explain why you have strength when you have strengthening and the and and why you have blessed us it was clear and why
is that well when you take it off the so-called cancel tests but will steal new strain at new measure to strengthen you measure that stress that's necessary to to obtain this strange new find what's called a stretch during which is new most of you are familiar with this structuring Rink?vi?s's stress-strain curve for stealing typical yield .period it's not very good looking as good as of interest certainly is is this increase in the In the stress that's necessary to achieve defamation and I'm well what is happening here is that the plastic deformation i.e. the strange that I achieved is due to the formation the creation of an emotional of the dislocations in the little crystals that make out like steel and it is the motion of these dislocations that allow elastic information to occur and strange is actually nothing else than resistance to the motion of these dislocations in a nutshell that's what the course this chorus here that the 6 6 life is all about his father what happens to this location when they are trying to move in the lattice under the influence of stresses applied stresses on the encounter obstacles to their motive in the lattice
10 such let's think a little bit Back on what we know From the introduction in the chemical melody and and look at our stress-strain curve so we have a strange guilt .period then we have a hardening stage in at a particular point which we call the ultimate tensile strength we see that the samples developed a diffuse neck and neck and then in certain cases we get we get a local nite well very well defined a defamation then before the fracture book for these parameters that we can measure the yield strength tensile strength uniform illumination that stipulation that the old and and fracture stress on total elimination of art very much used in engineering practice there are however not material constants but these the properties would be very different if we had gone the test I think all the conditions yes so In all say few words about these material problems properties that you see in catalogs of steel makers or other material producers strengths etc. They are not material comes they are depending on the following they are
depending on for instance the temperature change the temperature at which you do the test you change that properties that them that you make this an example here for a sporadic stainless steel at nothing very special in terms of like rastructure but even that still has the stress-strain which changes a lot when you change in temperature so we're all familiar from our introductions in materials a properties as undergraduates at that time and you know when you reduce the temperature the 1 of the strangles up and when you increase the temperature the strangles down so let's have a look at whether this rule is all buried in this case well but the black line Disney stress-strain curve at room temperature as and when might decrease the temperature to minus 30 degrees C distressed drinkers moved upward again a material that strong yet so that's OK that's what we expect and if I increase the temperature to 200 degrees C the material becomes soft that's nice too however you can see that something very funny happens to distressed at home the stress-strain curve looks very sickly he has gone is the smoothness of the line and we see all these would have called separations and so on I will something has happened to the material we don't know what it is yes but it's certainly not something has happened to the material which which I cannot explain just on the basis of material becoming stronger when you reduce the temperature and and softer when you increase in temperature let's continue to look at these strange strange behavior but if I look at them now not a forensic steel but in Boston that Dixie which hasn't FCC structure and let's look at often it take iron manganese carved wood steel and let's do the same thing change the temperature all the 1st thing we noticed is that this steel but that reaches a UTS at room temperatures it's the black lines of over a thousand may have passed let's go back to the far-right I
just had the UTS 500 so In this case the steel has the strength unity years it's double over that of the Why is that well but there are many reasons but that to do the prime reason is because I'm looking at the different crystal structure sporadic steel has a particular crystal structure it's BCC in also netic Steel has a particular crystal structure its Boston and the dislocations in these 2 structure of vastly different as a consequence I get very different properties even at room temperature what happens when I go from minus 70 to plus 300 degrees C CIC indeed what I expect that the the strength of the material generally decreases so that's what I expect from my undergraduate knowledge however again but in certain in a certain temperature range here saying I at around 200 100 200 received the distress strained the occur suddenly develops these very ,comma this very complex pattern of celebrations has which are not obvious where they're coming from and we'll be talking about them because they're due to what we call localization of strength right so In case you didn't believe me that Austinite and fair I have vastly different mechanical properties let's look at the very Nice example where the 2 materials are tested Austinite and the far-right tested at 1200 degrees think again and as a special experiment we did where we had also denied most cosmetic alloy and affair callow almost the same composition yes so that you could have these 2 phases at 1200 deg C and and both would be stable at that temperature the into a test where you just have to crystal structures and you can see here that there are the shape of the curve don't worry too much about shape of the curve shape of the because this is partly due to the fact that we have tested on a very high temperature 1200 decreasing but you can see that the Fahri is much much the softer then D-Austin who in the stress-strain curves that are obtained that were obtained by portion tests were intent let's come back to this obvious I I think we were talking about that materials get the softer when we see and spending and leftist try to check isn't always the case do we really understand what's happening and don't go ashore yuan example of steeled that itself very common steel it's very common alloy itself it's basically the nitrogen alloy and very low nitrogen levels and we're talking about a few hundred 100 ppm of nitrogen and it's the ultra-low carbon steel with it doesn't have much strength week and some outlets we do this experiment where we we on the go from minus speeding plus 200 years and that and let's measure the strength while looking well let's look at room temperature room temperature curve is as black lines and now let's look at 200 degrees C. and of course the material should be softer but it and you can see this huge increase in stride 200 deg C and you can see it here also and in addition you get these very very heavily separated so materials do not have to become softer as you increase the temperature there maybe processes that will cause the material to get stronger as you increase the temperature and this is 1 of the things will try to understand that in the course of the these factors but also illustrates the fact that steals but there being a representing the very very wide range of ferrous alloys also it can display but very very wide range and very rich spectrum of mechanical properties which we in the course of his lectures we will expand the Explorer yeah the
same thing with outside z the following properties the union if I ask you to put to you the question what happens when you get to being involved in a collision in the car collision the and properties that are usually made available to you about a material a by a manufacturer room temperature properties measured at relatively low strain rates typically 10 to the minus 3 10 to the minus 2 per 2nd knows or strain rates that we used to do consultants typical consultancy vary widely used in the industry materials industry and not only in the steel industry polymers and other non-ferrous metals including so what happens when you get into a coalition yes William material come stronger or weaker than it turns out that this words this steals that are designed for this particular the situation against the crash absorbing the structural steels in your car the stress-strain curves at high strain rates in palace that the material strengthens even more at the high street and there is no reduction in classes of the black line here shows a static test basically meaning into the minus the 2nd and then the red green and blue line so distressed hide the string and you see if anything the stress-strain curves look even better so it is your car has crush members structural crush members as part of the car bodies which are carbon manganese silicon trips deals you can be confident that In this particular of unfortunate events of the collision that this the properties of distilled will will actually be better than the ones that you have in static situation so very important in service property the reason why this have is not obvious you need to know what is in my perspective of this deal the 2 to understand why why it behaves this way but that will be part of the cost to Explorer this together so to wrap up my
introductory lecture that so did lecture will be focusing on the understanding that the and fundamentals of the various strengthening mechanism and steel and and make sure that we translate this hearing into engineering information right but will in interviews theories that we can actually use in practice relatively simple calculation which will allow us to gain insight and calculate quantitative things such as stress-free so I will give you the tools to calculate specific mechanical properties of steels and will show you how to use them by means of many worked examples in the course of the lecture thank you for your attention and look forward


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