Hello there!
At the time of this recording SpaceX has
two fully built Starship prototype tanks
ready for testing.
However, they are currently building a
third one and while SN5 and SN6 are made
out of 301 stainless steel,
the SN8 will be made out of 304L
stainless steel.
This is a significant switch and Elon
or SpaceX did not share much about the
reasons behind it,
which gives us a great opportunity to
brush up some basic mechanical
engineering knowledge
and come up with some good guesses as to
why they think that 304L stainless steel
is better for Starship prototypes.
This is going to be a deep dive so get
comfortable.
In fall of 2018 Elon announced switching
from carbon fiber to stainless steel,
even after successfully building and
testing 12 meter diameter humongous
carbon fiber tank.
The motivation behind this change has
been in depth covered by others,
so if you are interested in that I
recommend you video from Everyday
Astronaut on this topic.
This video is about the differences
between 301 and 304L stainless steels
and what differences does the 304L
stainless steel bring for Starship.
Both 301 and 304 stainless steel are
so-called austenitic stainless steels,
which means that they have similar
crystalline structure, are non-magnetic,
and contain high level of chromium
and nickel and low level of carbon.
The chromium and nickel provide good
corrosion resistance and because of that,
they're among the most commonly used
steels in the world.
The 301 is used for springs and coils.
The 304 contains even higher level of
chromium and less carbon,
which makes it more corrosion resistant,
and that is why it is used for kitchen
equipment and pots.
Look at this kitchen, you could build
three starships out of that.
The fact is that the high resistance to
chemical corrosion made a 304 stainless
steel
the most commonly used stainless steel
in the world. And finally,
the letter L in 304L stainless steel
denotes even lower level of carbon.
While the regular 304 can have up to
0.08 percent of carbon,
the 304L can have up to 0.03.
Finally, let's get to the first possible
reason... did SpaceX switch to 304L
stainless steel because the 301 was not
strong enough?
Or was it too brittle? Well to answer
this we need a refresher in basic
mechanical engineering terminology and
tools.
The behavior of a material under stress
is usually described by so-called stress-strain
curve. Let's say that we are
investigating basic properties of a
metal.
We make a sample from this metal in the
shape of a rod and then place this
sample onto a machine capable of
slowly stretching it. As we start
stretching the rod by an increasing
amount, 
we monitor how much force the machine
has to use.
We also divide the force by the
cross-sectional area of the sample
in order to make it independent of the
sample thickness.
As the metal starts stretching we see a
linear relationship between the increase
in length and the pressure that is
needed to stretch it.
This is called elastic deformation
because if we let go,
the sample returns back into its
original length and form.
However as we continue this process, at
some point the behavior changes.
Suddenly the relationship is not linear
anymore and we entered into the region
of plastic deformation,
which means that the sample is now
unable to fully return into its original
form.
The point of transition between elastic
and plastic deformation
is called the "yield point" and the
required pressure associated with it is
called "yield strength".
At some further point the pressure
reaches maximum after which the material
starts to neck
and eventually breaks. The value of
maximum is called
"ultimate tensile strength" and the strain
value at break is called elongation.
The engineers usually design so that the
stress on all parts remains safely under
the yield strength.
Depending on an application, different
material properties are required.
An ideal steel would be strong and
ductile, which would mean that it takes a
lot of force to stretch and it could
stretch a lot before breaking.
However in practice, there is a trade-off
between those properties.
The strongest steel will be brittle -
think of ceramics for example.
It takes a lot of force to stretch but
once it stretches just by a little it
breaks,
because it's not ductile. Most alloys
including steel can be made stronger
or more ductile with a specific after
treatment. Steel can be made harder
but, of course more brittle, by a process
of work hardening, 
For example cold rolling. Ductility is
increased by so-called annealing, which
is a process of heating up the steel
by several hundreds of degrees for a
prolonged time, which allows the internal
structure to reshape itself into the
original unstressed form.
Now we can really go back to the
original question - is 304L
steel stronger than 301? I will make a
reasonable assumption here that a
Starship will experience the highest
structural loads during the launch
because that iss when it is full of fuel and
therefore the heaviest
That means that we are most interested
in comparison between those steels at
cryogenic temperatures
corresponding to liquid oxygen and
methane. This plot shows the value of
yield strength based on the material's
temperature.
Remember that the yield strength is the
pressure value at which the material
becomes permanently deformed.
The higher it is, the more force it takes
to stretch it. As mentioned before,
a single steel alloy comes in many
flavors based on their after treatment.
For this plot I drew both extreme
versions of the respective alloys - 
the strongest, fully hardened, and the
most ductile made by so-called annealing.
And by the way if you are interested I
will link the original data sources
below in the description.
The conclusion from this data is simple:
the 301 stainless steel has higher yield
strength at any temperature,
so the material strength at cryogenic
temperatures cannot be the reason for
switching to 304L.
But I hear you ask: What about ductility?
This is a similar plot to the previous
one, except the yield strength axis has
been replaced with elongation
which denotes how much a sample can
stretch before breaking.
A value means high ductility. If you look
at the room temperature region
on the right of the plot, the 301 steel
is more ductile. That must be why it is
used for springs
and coils. However, for the cryogenic
temperatures around the two vertical
lines,
the 301 becomes more brittle, but the
304L
remains much more ductile for both full
hard and annealed versions.
Is that enough of a reason for switching
the steel type?
We actually do have one hint from Elon
about why they switched to 304L.
In his tweet he said that it has higher
toughness at cryogenic temperatures.
Well if toughness is the reason, we have
to dive into its engineering definition!
Thanks Elon. Remember this original
stress-strain plot that i drew before?
well, the toughness is defined as the
amount of energy a material can take
before breaking.
The value of toughness is determined by
the area under the curve until the
failure point.
It is a combination of both strength and
ductility and to have a high toughness
value
it has to be good at both. There is an
ingeniously simple method for measuring
toughness of real samples - 
It is called Charpy impact test. This
test uses a prepared material sample,
usually with a notch machined in the middle.
The notch is there to simulate a surface
imperfection and give the material an
opportunity to develop a fracture.
The other part of the system is a
pendulum which is designed to break the
sample,
First, the pendulum is released from a
fixed height
and the point where it stops on the
other side is recorded.
Based on the pendulum's mass and
achieved height, the potential energy at
that point can be computed.
Then the sample is placed into the
holder and the pendulum is released
again.
When the sample gets hit by it, it breaks.
But the process of breaking the sample
slowed the pendulum down,
because some of its energy was used to
break it.
now the pendulum will not reach as high
as before and the difference between the
potential energies of the free swing
and this swing equals to how much energy
it took to break the sample,
which is exactly how we defined the
toughness before.
Let's now look at some toughness
measurements from both types of steel
,but before we do let's make this test
even more realistic.
Starships are made out of many sheets of
steel welded together so the weakest
point will always be the weld.
So having the samples on pendulum made
out of two pieces welded together
will provide us information about its
weldability.
Here are the toughness data for the two
types of welded steels
at cryogenic temperatures. Unfortunately,
I could not find the data for direct
comparison between 301 and 304L,
but I found two separate comparisons
between 301 and 304
and between 304 and 304L so that should
do nicely.
While the 301 welds are slightly
stronger at higher temperatures
at cryogenic temperatures the strength
of the 304 welds
is 4 times stronger! Now that's a good
argument for building starship from 304!
There is a partial solution to this
weakening of welds at cold temperatures
and that is treating the finished welds
by annealing, which consists of heating
the welded part to roughly 1000
degrees Celsius
for half an hour and then cooling it
down.
This process clearly improves the
toughness of both 301 and 304 steel,
but annealed 304 is still significantly
tougher.
I think that at this point in time in
starship construction,
annealing the welds is impossible to do.
Maybe when the welds are made
robotically?
So far we have shown the advantage of
304 steel over 301
in toughness of welds at cryogenic
temperatures, but what about 304L?
At minus 196 degrees centigrade,
which is close to the temperature of
liquid oxygen, the 304L
provides additional 25 percent of toughness over
the regular
304 steel. There is one more advantage of
304L steel
over both 301 and 304 and it is related
to its low carbon content.
When heated to high temperatures, for
instance by welding,
the chrome and carbon react to form
areas that are more suspectible to both
cracks and corrosion. Finally, I think we
figured it out.
The motivation behind the switch from
301 to 304L stainless steel seems to be
all about welding,
even though 304L itself is weaker, its
welds at cryogenic temperatures are much
stronger and much less prone to
corrosion.
This switch should allow SpaceX to keep
the welds of starship as they are when
finished,
without any complicated, expensive or
even impossible after treatment.
I hope you enjoyed this detective story
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