Kind: captions
Language: en
let's torch it previously on veritasium
we learned how Hans gold Schmid
discovered that a metal oxide like rust
can react with aluminum powder to
produce pure liquid
metal reactions of this type are known
as thermite
reactions
wa
yeah that is crazy
wow initially thite found limited
applications repairing heavy machinery
in remote locations its much more common
use the one where it's widely employed
today took a bit more
convincing there is one sound that
everyone Associates with
railroads this is the sound of train
Wheels passing over the join between one
rail and the
next in the early days of railroads
steel mills produced rails that were
around 12 M long then out in the field
these were bolted together using fish
plates that would create these little
gaps between the pieces of
rail and cause this kind of like
signature sound tat tat right and
everybody who thinks railroad thinks
this t t t
t even though we don't hear that
anymore but we still imagine it because
it was so signature for the sound to
ride on the train
the track would Flex causing the train
to rock and roll and it would also
increase the wear on the wheels and
create vibrations in the tracks so they
required frequent maintenance so you
might expect that after thermite was
invented railroads would have been
excited to weld their rails together
with liquid
metal but that was not the
case they would never have the idea to
weld them if he'd come to a railroad
manager and told him I'm Dr Hans
goldshmid I have a process by which I
can weld your thumbs together to these
railroad managers that would have
sounded more useful than what he
actually proposed which was welding
their
rail see the gaps between rails were
considered essential because railroads
are exposed to the elements all year
round so they shrink when they get cold
in Winter and expand In The Heat Of
Summer an increase of 40° C would cause
a 12 M length of rail to grow by 6 mm
without gaps in the rail people expected
kilomet of Railway to expand and buckle
leading to the worst possible
outcome train
derailments so if someone pitched we're
just going to connect all the rails
together that would be a terrible idea
that would be a terrible idea the longer
the rail the bigger the Gap the bigger
the problem and now somebody says hey I
can make an infinite rail right that
just causes an infinite
problem the first ones that were
actually willing to discuss this were
the tram
rails tram rails they're in cities
they're embedded into the streets tram
rail is much much smaller so you know
the forces are much less so the tramells
were the first ones that were interested
in how can we kind of get rid of this
tat tat because it's kind of annoying
you know in the middle of the street
that was basically the first attempts
and I think in essence and then later on
in other
cities for this video I got the
opportunity to weld two rails together
using thermite and I'm going to take you
through the eight steps in the
process first I'll show you how I did it
man this is difficult and then we'll
compare that with how the pros do it all
experiments were performed under the
supervision of professionals with proper
safety precautions okay we are out here
in the German Countryside and tonight
they're going to be welding this rail
they're going to do like 150 welds
tonight when you're welding rail for
real you want to do it at the neutral
temperature of the rail that is a
predefined temperature at which the rail
should have no stress in it and is the
temperature by which that piece of track
that you're welding in is supposed to
have zero tension when it gets colder
than this there's going to be negative
tension and when it gets warmer there's
positive tension and since we were
filming in Germany in the summer they
weld at night after the rail has cooled
off initially when the rails are laid
out they're placed so they're touching
each other and the first step in welding
rail is therefore to create a gap into
which We'll pour the liquid thermite
steel so you have to cut about 2 and 1/2
CM off the end of one rail this can be
done with a circular saw let's do this
cutting a rail out in the field they cut
their Gap with a torch
[Music]
good take a little break oh that is
really
[Applause]
exhausting the next step is to line up
the rail so the weld will make a
seamless connection both rails have to
be an slightly up toward the Gap this is
because when the liquid thermite steel
is poured in it will contract as it
cools which creates an inward pulling
Force but that Force won't be the same
everywhere since there's more metal at
the top there's more Force pulling the
rail together there so that's why the
rails have to be angled up so that after
the liquid steel contracts the rails
will be perfectly
flat now you take one of the wetes and
put it a little bit under the rail like
that there um or like this sideways
sideways would be a little bit better
yep y That's fine okay same goes for the
other side we will check there's no Gap
at the moment we need 2.4 to 3.6 here
hit
it but now this rail is way higher than
this one and we want to have both on the
same high means now we have to hit this
bedge we do that so long until we have
correct h
okay
[Music]
soop
up in addition to the vertical alignment
the rails also have to be horizontally
aligned if we take a look here here's a
large gap between the straight edge and
the rails there's a gap there and they
need to we need more of a peak this side
yeah okay and it's a frustrating process
because trying to align them
horizontally throws out the vertical
alignment and vice versa I'm lucky I had
help or I would have been here for
hours when we were finally done that we
double checked the alignment at the head
and foot of the rail and we made a
terrifying Discovery our rail is Twisted
even though both rails were aligned at
the top one rail was slightly Twisted
relative to the other which you can see
by it not lining up at the foot we have
a maximum tolerance that is allowed here
of 2 mm yep here we have more than 2 mm
so we have to untwist the rail this of
course throughout the vertical and
horizontal alignment so we had to do
that all over
again are we getting closer little bit
more but out in the field the guys align
the rail so quickly and easily it really
is an
art now we can adjust the clamping your
wise this will hold the mold shoes and
preheating torch in place the preheating
torch will sit in here yeah and we will
keep the molds in place with that arms
here okay the next step we will align
the burner it's really important to
align the torch properly so that both
sides of the rail get heated equally I
mean I think it looks pretty good but
maybe a little bit that way so maybe it
could go just a touch there's one mold
and the pl for you next we'll put the
molds inside the mold shoes attach them
to the clamps line them up and align
both sides it's essential to make the
seal watertight because liquid steel is
as liquid as water but almost eight
times more dense so if it's not sealed
properly the steel would just pour out
it looks pretty good I hope so I didn't
expect us to get it this good on our
first uh first go after uh just the
trouble alining the
rails the lining the rails is one of the
hardest part at start it's Madness
so this is sand like beach sand no it
has larger amount of clay inside it
holds in place and seals I will start
sealing this side you can start sealing
this side I start sealing with hand and
I will compact it with the stamping
de and to make sure that none of the
liquid metal can spill out we pack in
additional sand all over on the sides so
there's absolutely no
gaps then it's time for one of the most
crucial steps to preheat the rail put it
in tighten the screw and then opening
the VA we also prepared a demo behind
glass so we can see into the welding
process including
preheating preheating is essential for a
few reasons first it gets rid of any
moisture and other volatiles in the mold
which would create bubbles when the
molten steel is poured in second it
makes both Rail and
really hot which is crucial because the
rate of heat transfer depends on the
difference in temperature between two
materials if we poured liquid Steel in
between two room temperature rails well
the liquid steel would cool rapidly and
that's a
problem we will heat up two pieces of
rail and we will cool down one really
fast in
water and the other one we will cool
down in sand so
slowly just to show how the hardness
increase depending on the cool down
time and now you try to break
it that's hard to bend
[Music]
actually yeah it's hard to bend
that's the one that was in the water
yeah and we cool it down it really
[Music]
fast and that is how to break it and now
you see how the structure of the steel
has changed it breaks very easily that's
because when steel cools rapidly carbon
atoms get trapped in the lattice
creating this needle likee micro
structure and that forms a kind of Steel
known as Martin site Mar site is really
hard but also brittle since the lattice
can no longer absorb energy without
fracturing now we can take a look on the
clock y we have 1.5 minutes until we
have to remove it again okay the next
step is to ignite the thermite and it's
important to do this right after we turn
off the preheating 5
4 3 2 1
so I put the plug in the mold and then
the thermite bucket on top and then I
ignited
it oh my goodness
no in making these videos the team and I
often travel around the world talking to
scientists and filming at research
facilities I mean guess where I am right
now we filmed part one of this video in
Germany Casper's been in the Netherlands
learning about straw Bas and Peter has
just come back from trying and failing
to break glass in New York it is a great
job perk but there is one really
annoying problem which is that it's hard
to stay connected with the rest of the
team while we're out on location we
either have to pay ridiculous roaming
charges or find a local SIM card or just
search around for public Wi-Fi and as
you can imagine this isn't something we
really want to be dealing with while
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to thermite
[Music]
welding oh my goodness
woo
wow so the thermite reaction is taking
place inside we're forming molten steel
and also molten aluminum
[Music]
oxide all these demos are are incredibly
difficult to shoot because they're going
to be many many orders of magnitude
brighter than the normal scene so just
getting exposure is a huge challenge
something this reminds me of is that the
power radiated by something thermally is
proportional to temperature to the power
of four so slight increases in
temperature result in huge increases in
power
[Music]
in the previous video we showed what
happened in The Crucible through glass
so you can see the liquid steel and the
liquid aluminum oxide forming and
separating out and then the liquid steel
dropping out the bottom well here we get
to see that liquid steel falling into
the mold
so the liquid steel comes in first and
it goes all the way to the bottom next
the slag starts to come out but since
aluminum oxide has a lower density than
steel it's just going to float on top
and go out the sides into the slag pans
so that makes sure that we don't get any
slag mixed in with the liquid
steel that is so awesome yeah
that is just
spectacular whoa ah that is
great and now we
wait we need the steel to cool long
enough so that it's solid but not too
long so that the excess steel and slag
is difficult to remove you have to get
the time right what does he think you
guys tell me it's not liquid anymore I
believe you now 2 minutes passed so
let's wait 30 seconds more after that
time has elapsed then it's time to
essentially clean up this weld so we can
remove the slag pans and the mold
shoes and we break off the top of this
mold these are really single use molds
they get destroyed in this
process and now you put it on side put
this to the by the SL pants that's fine
then we used a weld Shear that can
create up to 20 tons of force to Shear
off the excess steel from the top of the
[Applause]
Weld and after that's done we hammered
off the remaining excess
parts that's fun actually I like that
step The Next Step is to grind down the
weld so it's exactly the same level as
the rail obviously you don't want any
bumps when the train is going over it
but also you don't want any indents so
this grinding step is really important
so we put on the grinding machine and
grind until maximum one mm of materials
left and we try to only grind on top of
the W yeah
[Music]
it's normally conducted in two phases
first there's a rough grind and then a
finer grind is applied
later and we only grinded the top
surface yep we did not grind the running
flank then you have to lift down the
grinding device and hold everything so
it's even heavier than the part we've
done
before that's rough grinding right there
I'm sweating
it's like a serious
workout the guys in the field were
obviously much better at grinding down
the rail than I was so they would move
on leaving a still glowing hot section
of the rail behind
[Music]
them there probably will be millions of
people watching this video what is the
likelihood that they have ridden over
one of your thermite welds they will
100% 100% anywhere in the world anywhere
in the world of course there is
countries where we are not the market
leader or something but um finally they
will ride above our
wells we'll just cut it out and then
take it over to the mety
lab that is
heavy oh that's my weld my first
thermite weld man that is hot
this is what you have done
yesterday we can now put it in in the
acid what does the acid do to the
surface the acid attacks the
steel but because the orientation of the
crystals inside the steel is not the
same the acid attacks it not in the same
way
everywhere and because this is different
you can see different structures you can
see some fine structures in the rail in
the heat affected Zone h and in let's
say the fusion Zone mhm can I touch it
yeah yeah you can't really feel it some
micro Copic basis you cannot feel it
it's interesting you can see it but not
yeah and it's a very fine line I mean
you can see the different structures of
the crystal SE in the weld you can see
three different zones the normal rail is
on either side with a horizontal grain
due to how it was rolled in the steel
mill in the middle is the thermite steel
you could still see a bit of horizontal
grain but here it doesn't come from the
steel being rolled since this just
solidified from liquid instead it formed
from the solidification front moving
from the sides to the center and when
the crystals from both sides meet it
creates tiny microscopic pores resulting
in this darker line in the
center now the boundary of the thermite
steel isn't sharp and that's because as
the liquid steel pours in it actually
melts some of the rail this helps it
form a strong
bond a good weld would be enough with
just what Atom layer uh fused but we are
using the process on a construction side
and on the construction side not
everything is so good handled like in a
laboratory the Gap is not 100% always
the same so we need something robust
with functions on construction sites in
Germany regulations require melting of
about 3 mm off each rail the finals zone
is in between the normal Rail and the
thermite steel this steel didn't melt
but it received enough heat that the
crystal structure changed resulting in a
smaller grain structure this is known as
the heat affected Zone if you measure
the hardness across the weld this zone
is actually the weakest both in terms of
hardness and yield the amount of force
it takes to permanently deform the steel
so if a weld breaks it's almost always
here that's a line you could see on that
macro eding thermit y That's the fusion
line and here we have now rail seel heat
affected so the structure is changed by
heating up and then
recrystallization of the
steel so then I guess the question is
how strong is rail that's been welded
well at Gold Schmid they actually take
sections of rail that they have welded
and they test them until failure every
200 basically we create one test Weld
and we analyze it in terms of chemistry
we analyze it in terms of hardness and
we analyze the bending this is the
bending press they put this in right and
then the force comes from top and then
goes down and it breaks it and we
measure the speed and also the
force I'd say it looks like it's flexing
a bit so we're loading up that weld
bending it until it snaps
see that curve go we want to see how
much force it takes to snap and exactly
where that brake
occurs 140
tons it's getting really deflected now
yes yeah 150
tons oo that was cool I was just
expecting a little crack like that was
catastrophic failure just over 150 tons
so we think the crack propagated from
the bottom upwards cuz it's being pushed
down like that so this is under a lot of
tension yes so it breaks under tension
down here rips up this way and so it
doesn't break in the middle of the weld
you're saying this Zone beside it which
is a previous rail material that got
heated up in the weld is actually weaker
[Music]
yes every year around 2 million welds
are performed using thermite and around
half use gold Schmid's thermite now each
weld creates about 2 and2 cm of rail so
in total that's 50 kilm of railroad
created every year out of liquid
steel you have a very demanding process
and you have to make sure that there's
certain properties with respect to
vending with respect to tension with
respect to
hardness that's a process that needs to
be kind of replicated every every every
time a million times a
year so with all this continuously
welded rail around the world why doesn't
it Buckle in summer when it heats
up well that's because there's another
way to change the length of the rail
okay imagine a piece of rail if we use a
powerful machine to push on it from both
sides it will shorten slightly and if on
the other hand we pull on the rail it
will lengthen so you can use mechanical
stress to change the length of the rail
the fractional change in length is
called strain and if you plot the
relationship between stress and strain
you find it has a linear relationship at
least with elastic deformation as long
as the stresses applied are not too
large the rail will spring back to its
original length when the stress is
removed and this is key because as long
as we do that we have two ways to change
the length thermal and mechanical and we
can use one to compensate for the other
when the rail thermally wants to expand
we can use mechanical forces to compress
it this is what the sleepers and the
ballast are
for the sleepers pin down the rail and
the ballast locks the sleepers in
place so as the temperature increases
the rail doesn't lengthen instead the
stress in the rail the compressive
stress increases and the rail just
expands to the top and sides which are
unconstrained it's also interesting to
note that railroads typically use as
high of a neutral temperature as they
can because a rail that shrinks too much
in Winter and cracks under tension is
less problematic than one that expands
too much in summer and therefore buckles
it's also easier to detect a cracked
rail using conductivity measurements
than one that has buckled so thermal
expansion and contraction are
counteracted by mechanical stress that
is the reason why we don't need
Expansion Joints in
Railways it allows trains to go faster
with fewer bumps and vibrations and much
less mainten
Grady from practical engineering
actually made a whole video about this
and it's excellent I'd recommend it so
far we've seen thermite under normal
conditions
wow conditions that are expertly tuned
to protect thermite from its
environment but in the next thermite
episode we'll find out what happens when
this is not the case and we'll learn how
to work with thermite under these
conditions so trust me you don't want to
miss part three
my hands are shaking right make sure
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