Pulsed Steels
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Transcript: English(auto-generated)
00:00
Okay, the next presenter is Wangshan Xin from Imperial College London who will be talking
00:35
about the applications of his work of electro-pulsing in steel formation.
00:50
Thank you very much. I'm interested in treating steel in extremely short time. So how short is that? If you blink your eye once, I should have done my experiment 1,000 times.
01:07
That means if you blink your eye, blink your eye take 0.1 second and my experiment should take less than 100 microseconds. Okay, so I'll firstly show some of the result and then we'll discuss the mechanism
01:23
about this. The way I'm using is electric current pulse, okay, because it's very short time, it's a pulse and I use electric current passed through the steel. So I have quite a few authors here and I especially should acknowledge those people
01:45
who are also interested in this very special processing method, this BOSET, the project. And also those people who helped me a lot. So let me first show you some results.
02:09
So this is a piece of steel, it's a commercial pearlitic steel containing 0.8 percent of
02:21
carbon. It's a commercial one, you can see it everywhere. So the sample was more than 10 centimeters long and the diameter is about 2 millimeters. So the mass structure, if you look at the mass structure, you must go. So it should be like this because our cold draw is still wired with 60 percent of reduction
02:46
at room temperature, cold draw date. So without any adhesion afterwards. So if you look at the mass structure, it's something like this. This is a cement-tight place, the ferrite is etched away, okay, so this is the one.
03:02
And then I put this steel into my processing facility at room temperature with one pulse, so totally it's less than 100 microseconds. At that, during the time, you don't see anything happen. You didn't hear a voice, so bong or something.
03:22
You didn't see the steel wire actually become powder or whatever. It's a steel wire and you didn't see the color of the steel wire change into some others, like a red color or so. And afterwards, I put my finger on the steel wire and didn't feel, I can't hold my finger for the wire.
03:42
So that's a kind of phenomenon. However, if you see that steel wire at SEM, you see there's a cement-tight plates, a region was this, now changed into this, okay? Be careful of the scale, here is a 500 nanometer.
04:02
So in this 10 centimeter steel wire, I choose 12 different points, exam, and everywhere got exactly same mass structure. So those cement-tight plates change into a particle with average size of 30 nanometers.
04:21
So when we exam the mechanical property, we found this one, the hardness of this one was about 400. And this one was a little bit higher than this. You know, this is a coat of gold without annealing. And this one has already removed all the internal stress, and
04:40
it is still much harder than this. Another surprising thing, what we found is electrical conductivity of this state is improved 100%. It's almost a double, the electrical conductivity of this steel is almost a double of this.
05:01
So that is one situation for pearlitic steel wire. And then I have another one. This is another steel, okay? We call it trip steel. However, at the moment, it's a hot road steel and
05:21
without any retained austenite, although we call it trip. So if you look at it here, we have a lot of ferrite particles here combined with this complicated mass structure. If you look at this, you do not see any anisotropy here. Max structurally, you don't see that.
05:42
And again, at room temperature, electrical pulse this steel with 3,000 time passes through 3,000 pulses through this piece of steel at room temperature at a frequency of 1 hertz,
06:01
which means each second passes through 1 pulse. And the pulse width at this moment is 20 microseconds. Okay? So after 3,000 time, if you put that into optical mass structure, you will see lots of lines, black lines along the electric current direction.
06:23
This is an image of SEM. You see this original isotropic property has completely lost. All of these phase change into these along the electric current direction. So if you use a higher resolution SEM, you will see even inside those lines,
06:45
the cementite and ferrite lines are all along electric current direction. So now we are interested to develop a steel with all the, with this kind of mass structure, it's everywhere, not only just this.
07:04
So this is again at room temperature for this steel. So now let me show you another example. So this is a 316L stainless steel. We know this problem with this steel is if you do an annealing,
07:25
and that secondary phase will fall. So if you anneal that at a temperature around 1,000 degree, you will see a lot of those white particles, those are secondary phase particles. So this 316L stainless steel contains very high chromium,
07:46
moly, and nickel. So if you form these secondary particles, so these secondary particles are very rich in chromium and molybdenum. So in that case, the area surrounding these will have a poor chromium and
08:03
a poor moly. So if these things happen at that, those poor chromium and a poor moly area will have poor corrosion resistance. So we try not to have these secondary phase particles.
08:22
However, it's very, very difficult to do. So what we are doing is we put two samples in the furnace, exactly the same samples. But one of them linked with electropalsy, another without. So when we did our heat treatment, and then analyzed two samples,
08:44
one of them is this much structure, another is this much structure. So the secondary phase is not formed. The electropalsy has suspended the formation of secondary phase. That's one thing. You see much less secondary phase particles at this side.
09:02
Again, if we analyze the composition inside here, we find even though we form some secondary phase particles inside the chromium and the molybdenum difference between the particle and the matrix are much, much smaller. So by this one, the electropalsy has reduced
09:22
the segregation of this chromium, nickel, and moly in stainless steel. So that's another thing we have done. So now, let me tell you another result, which we just made very recently, but this has already published.
09:40
Okay, this is, we use this technology at liquid steel. So it's in liquid state. The sample here is a 1.5 centimeter diameter ingots, okay? We mount that without electropalsy at a quarter.
10:02
What we see is inside this steel, we have lot of manganese-sulfur inclusions, okay? So those inclusions are almost round shape and distributed everywhere. If you look exam your cast, your cast is everywhere.
10:23
However, if we do the same thing, but connect with electropalsy for 20 minutes, pulse is 20 microsecond, each pulse is 20 microsecond, and use a frequency of one hertz. And then, what we found seems that very differently is that inside
10:43
the ingots, internal ingots, is inclusion free. All the inclusions are gone from inside. However, at a boundary, top side and some of the bottom, we have lots of inclusions. All the inclusions are moved from the central part to the outside part.
11:03
So the ideal way is we put slag on top of that and collect all the inclusions so that we can make super clean steels. So this is one thing. So we have expel inclusions from inside the liquid steel to the surface.
11:23
Another thing what we noticed is, at this case, this is almost a struck particle, and look at it here, all the magnets of sulfur inclusions form this very strange morphology. It's almost everywhere. This is something very strange, and we don't really know what's that.
11:43
Has electropath affected the interfacial energy? Otherwise, why don't these go to a round shape? Round shape will have much smaller surface area. And the lower free energy.
12:01
So that's some of the experiments we have found, use of the facility. Now, let's come into the detail of the technology and analyze what's happening and what can be done further, okay? So firstly, when I talk about passing through high-density
12:22
electric current to steel, many people immediately think there's a heat there. Okay, so now, let's calculate what kind of heat has generated by this electric current. I come very easily to go through this. I know people don't like the equation, but this equation is very special,
12:42
because they are so easy, and we know them from secondary school. So if we calculate the total energy added by this electro-passing. So in my facility, I have two different options. One is I can fix the voltage.
13:02
Another is I can fix the current, the electric current. So suppose I fix the electric current, and this is the current, this is the electrical resistance of the sample, and this is the pulse duration. So use this equation, so I can calculate the total
13:24
energy of this electric current pulse, okay? Now, I suppose all of this energy has changed into heat, and heat of the sample, okay? So in that case, I have, this is the total mass of the sample.
13:43
This is the specific heat of the sample, and this is temperature rising. So suppose all of the electric current didn't do anything else, just the heat of the sample. And then this equal to this. From here, I can immediately get the temperature rising due to one pulse is this, just this divided by mc, okay?
14:06
So here again, this is the resistance of the sample. I can calculate resistance of sample equals to the density multiplied by the length of sample divided by the cross sectional area of the sample.
14:22
Okay, I have this r, and this is a mass equals the density of the sample, steel sample, multiplied the volume of sample. So it's a length, it's a cross section. So if I substitute r to here, m to here, and I can get the temperature rising due to one electro passing should be this,
14:44
because current divided by cross sectional area is the current and density. So you actually have this, so you know it's a very, very simple mathematics. And I have assumed all the electric current energy has been converted
15:00
to the heat to heat up the sample. And then let's apply this one to a calculation. I substitute all the parameters of the 316L stainless steel capacity, heat capacity, this, density, we know 7.8.
15:20
And electrical resistivity is this, okay? Now what I need is, each pulse is 20 microsecond. And this is current density. Look, this is astonishing high. Actually, what I used to generate those results are not that high.
15:43
This is 100 times high than what I used. So it's a ridiculous high. So, because I want to do a maximum calculation. So I use 100 times high that what I actually use. If you substitute all of that into this, so each pulse can actually
16:00
increase the temperature of the sample by 0.217 degree, okay? It's nothing. And remember, for all the experiment I used, the pulse frequency is 1 hertz. So if I put that in, that means the heat
16:21
time per rate is 0.217k per second. You know, steel processing, this is nothing, and it's almost 100 times high than what I've done. This is average current. But some people may argue, okay, there's a percolation here.
16:41
Somewhere I got a high density in the other place. But I already have used 100 times high than the average. So that's one thing. So the electro-passing effect is not a heat effect, okay? So that's one thing. Another, so another thing is, some people say,
17:04
you can't do this in a laboratory because your sample is small. But you may not be able to scale up to industry, because industry need to have a very, very big steel. So can you do that? Let's calculate the energy consumption in a pulse.
17:25
So the total work in a single electro-passing is this, okay? It's the same. So required electric power, power is a work divided by time. So here I got a pass of frequency, I normally use one hertz.
17:42
So if we substitute all the parameters, we use the B4 in. So each, so get all those parameters in. So each pulse actually consumed zero point, the power need to generate this electro-passing process is 0.304 watts.
18:04
That's again, nothing. So I can tell you, in my laboratory, the facility I use to generate the electro-passing is 80 watts. And the electric current was generated at electrical potential of 20 watts.
18:21
So for that kind of power, it's nothing because you have a home, we can easily find a bulb, the power is 100 watts. And my facility is only 80, so it's very easy to scale up. So by those, we know we can apply this, but what is the mechanism?
18:45
Okay, so there's one thing, let's just, it's complicated. I don't believe we have solved this. But one thing is let's analyze this from some dynamic way. So if we have some max structure change, whether phase transition or
19:01
just the structure relaxation or whatever. So the total free energy, and here is free energy change, is a chemical free energy change plus surface energy change plus electric current associated free energy change, okay? Those two we all know, but what about this term?
19:23
So this one, fortunately we know this one can be calculated by this equation. So this equation is free energy associated with the electro-passing equals this is the magnetic permeability at pi.
19:40
This is the current density distribution after the max structure evolution. This is the current distribution before the max structure evolution. And then you have this term, which make this calculation very complicated, very, very non-linear.
20:03
So what we have done is we have actually developed a software. We are able to calculate the current distribution if you have any kind of phase transition or any kind of max structure change, not only phase transition.
20:20
For example, if I have an inclusion, at the moment the inclusion is here, but the next moment the inclusion move up to here. So if you look at that case, it's a chemical free energy difference, it's the same, you don't have any change because there's no phase transition. This one is the same, but this one is different. Because if you have an inclusion moving from this part to the surface,
20:44
and then you have a completely different electric current distribution. So if you have this completely different electric current distribution, you will have a completely different free energy change. And then, because that one, so some of them move will be favored
21:01
by this electro-passing, but some of them will not. Okay, so that's our case about inclusion removing. So we tested this against the, so this is a show I've analyzed for the current distribution. And we are able to calculate all the free energy change.
21:22
This is something like our pearlite, you know, layered structure. But that one, all the layers actually the change separate. This is the more disintegrated part. And we are able to calculate the current distribution.
21:44
And again, we can get a total free energy. So we will be able to know in this electro-passing, you actually will let your grades to separate into different, several things, or you actually will have a causing. So, this one, when we calculate,
22:03
use that equation to calculate the max structure refinement. Because sometimes we use that. So we, and this is our theoretical value, and this is our experimental value, not to do it by ourself. It's a group in America, yeah, professor has core in this group.
22:27
So, we see it's agreed very well. And we further predicted, and to fabricate nanostructure material, those are required current density.
22:41
Those are all theoretical work. And again, we found if we use electro-passing, we are able to increase the fatigue life of the material. So we calculated that if we, for different material, if we use electric current density above a certain value, and we are able to hear the crack.
23:04
So we have done this before, and we were able to improve the fatigue life of low-carbon steel for 34 times. Okay, so those are just my advantage in this steel process is the conclusions.
23:23
Thank you very much. Okay, that's a very novel topic there indeed. And does anyone have any questions? Straight away, yes?
23:42
See if we can get both our microphones over to this row. Just use that one.
24:03
Thank you very much for your great presentation. So you show us the microstructural change due to the electric current. So the last part of this presentation, you said just thermodynamic approaches.
24:21
But to change the microstructure, you, we must consider the kinetics. So what do you think on that? Yeah. We did some work, and also in literature you can find some work. Passing electric current can lower the connected barrier of some transformation.
24:41
But in another group of transformation, you can, electric current can actually increase the kinetic barrier. So there are some works, and we also did some. However, kinetic understanding of this mechanism is very poor at the moment. There are a lot of things we don't understand.
25:02
So your parsing time is very short, I think, right? So point, one second, we're loading one second. Yeah, if you use one pulse, it's certainly less than 100 microsecond. Yeah, pulse time is very short. There are some solid results people found.
25:23
The electric parsing can increase the diffusivity many times. That's what people agreed. Another is a change of kinetic barrier. Some of them reduce a lot, or even completely remove the kinetic barrier. Another case, you can actually increase the kinetic barrier.
25:43
But for the rest of things, it's very, very basic, the early stage. Thank you very much. Okay, Joe? Thank you. You talked about putting this into industry as a limit on the sample size, and you said that the power doesn't limit it.
26:01
Is there a limit imposed by the magnetic skin effect, that if you actually want to put a sharp rise and fall of current into something, the current only tends to flow through the surface of the thing you're applying to? So the skin effect is related to the current frequency. So you can calculate that accurately.
26:20
If you use a direct current, and then you won't have a skin effect at all. Right, but you're pulsing it, though. Is that not equivalent to a single wave of an AC pulse? You need to calculate it, because for example, a lot of automotive screws, what we are interested in is a thin plate or steel wires.
26:41
For those of you who are a few millimeter thicknesses, you can ignore that. If you calculate by one hertz. If you increase the frequency. It's not the one hertz, though, is it? Is it not the 20 microsecond period that actually determines the skin effect? It's a hertz. It's the one hertz is the limiting one.
27:00
One hertz. If you use a very high hertz, a very high frequency, for example, 300 hertz, and you see the skin effect. So in that time, there are some of the experiment reported before. People use that to modify the surface property of the material. You can actually generate amorphous at a surface layout of the material.
27:22
Oh, you deliberately do it. Yeah, extremely high frequency. Interesting idea. Yeah. Thank you. Okay. Yes, Longchen, thank you for your interesting presentation. I have a very basic question, like you did, very elementary calculation in the beginning. So if you are going to melt steel with an electrical furnace,
27:43
normally you spend something around 400 kilowatt hours for tons. So let's say four kilowatt hour, four kilogram. So do you have any number for something on mass of iron, of course, or anything about related moles about your...
28:00
So how much energy really spent for unit of mass in your process, basically? Comparing it to the melting energy, roughly speaking, the entropy of melting, we know which is a good number for any metal. So total amount of energy we spent is extremely small.
28:23
It's not comparable with your mounting material, heat type of material, heater. So extremely small. What we did, the calculation. So this can be, this can be illustrated by our facility. So my facility, the power is only 80 watts.
28:41
So what can you do, use 80 watts facility. If you have a bulb, it's 100 watts. You will not be able to heat up steel by 100 watts above. So the energy consumed here is extremely small. If you want to lift up your temperature, you have to use other ways to increase the temperature, not by this.
29:03
This is just to induce those amongst the structure, but not heat it. Not lift up the temperature. Okay, thank you. Thank you. Okay, I think we've just got time for maybe two more questions. So I'll come down to Pedro at the front. He's been waiting a while, and then we'll go to you at the back. A very interesting talk. Two short questions. One is the parameter for
29:23
liquid processing is the same as the solid state processing. The second question is the inclusions, many kinds of inclusions. Some are in extra contactive, some are not in extra contactive.
29:41
Was any results on different kinds of inclusions in steels? We only did these inclusions first of all. Magnesium sulfur, because the history is like this. Although I showed you all the experiments firstly, but the history is that we got a theory firstly.
30:01
So we got the equation, the calculation, and then use that to apply for funding. So I got a funding and I hired a post doc, and that's that experiment. That's the kind of story. So when we did the calculation, we found is this removing inclusion by this method is completely different from the existing method of removing inclusion.
30:24
Existing methods use other physical properties like density or size of inclusion or those kind of things. But this is according to the electrical property difference. So no metallic inclusion normally is not conductive. And your liquid steel is a reasonably good conductivity.
30:45
So, and then when we start the calculation, we can have, we can choose up with the more important is alumina inclusion or whatever. But those mechanism sulfur have relatively high conductivity.
31:01
Than alumina or silicon. So we tried the most difficult one, and the first try was successful. So we are, but we are still do other inclusions and remove them and try to optimize the technology. Thank you. Okay, one last question, Patrick. Okay.
31:21
You mentioned the refinement of the pearlitic structure associated to a harness increase, but you didn't mention which one was that? You, you, one of your first slides was the refinement, the fragmentation of the pearlitic structure. And you mentioned there was a harness improvement. Yeah. Couldn't hear which one was that.
31:41
The other thing is that for that case, I made a quick mental calculation based on 0.2 joules per square meter of inter-passional energy. And they come, came to this staggering amount of 10 to the 4 kilo, 10 to the 4 watts to, to fragment that microstructure. So, am I doing something wrong in the calculation, and
32:02
what is the harness increase? Only that one, only this one, we got a hardness increase. Because what we find is that because you form the nanostructure. What is the increase? Marginally, marginally increase. This is about a 430, this about a 455.
32:21
But important thing is, this one, you got all the stress here, the thermal stress here. This one, you have completely removed the stress. So, this one, you are not able to draw it anymore. But that one, you can draw it to double its stress. So that's the case. So if the internal mirror spacing is 100 nanometers,
32:42
you'll have about 10 to the 4, 10 to the 2 meters of interfacing in a cube of 1 cubic centimeter. And that gives you for those pulses, 10 to the 4 watts. What is wrong in my calculation? 10 to the 4. 10 to the 4. It could be percolation, so you have to talk about that
33:05
all the time for a moment. The interfaces are broken or created. So this is an interesting idea. So when we talked about the percolation, you talked about it, correct? You talked about it, you talked about it.
33:22
300 millimeters. I agree, but remember that the energy is given only to the interfaces. Yeah, yes, you're right. So we did a calculation, when we first done this, we did a calculation. If you just use a region or the surface energy, transform the original surface energy to the particle,
33:40
what we found is this size that we got is a smaller than your theoretical size. So we should have a big one, but we actually got a smaller one. A problem is that we have put in those extra energy by electro-pulsing. Okay, this might run and run and run. You can talk about this after, so I'm afraid we have to move on.
34:00
So thank you very much.
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