Subject: Re: ordinance in the Eastern Front
From: jschaefe@genesee.freenet.org (John W. Schaefer)
Date: Jun 02 1996
Newsgroups: soc.history.war.world-war-ii

In article <Ds8BFG.92E.4.relay.ga.unc@ecsvax.uncecs.edu>, 
rberezni@maildrop.srv.ualberta.ca says...

>If memory serves me right, the T-34 was the only tank to have sloped
>armour, which was very tough and tended to take a lot of punishment before
>buckling. The  sloped nature of the armour tended to cause shells to
>bounce off (to some extent). 

	Buckling failures only occur when tanks are hit by large-caliber 
artillery shells, sizable bombs, or are exposed to similar conditions. The 
common failure mode when impacted by a conventional AP shell, one of the 
solid-slug shells or a shaped-charge device was small-area penetration, or 
sometimes non-penetration but high-velocity spalling of metal inside the 
tank. 

	The engineering reason for providing a slight slope on turret 
profiles is not so much to increase the effective trigonometric thickness or 
encourage a slightly higher percentage of off-angle shots to glance off 
without transferring much energy. Rather it is to keep the effective target 
cross-section of the uppermost extent of the tank, which is the part that is 
unavoidably exposed to enemy fire under defensive conditions, as small as 
possible, while maximizing the turret bearing diameter for maximum strength 
and stability, and maximizing the room at the base of the turret for such 
essentials as the gun breech and loader, recoil clearance and crew bodies.

	More modern turrets that provide major amounts of slope, of course, 
are intended to achieve all performance advantages, not just some.

	Of course, modern turret design thinking came to some countries' AFV 
designers rather slowly. Thus we were stuck with the Sherman, with the 
welded-plate turret versions being especially bad.

	Frontal armor slope, on the other hand, has always been done 
primarily to increase the effective thickness, and to increase the tendancy 
for incoming shells to glance off without transferring energy effectively. It 
happens to be consistent with conventional tank design, with the front track 
wheel mechanisms and the driver's legs requiring more extension than the 
driver station and upper body, so that the armor slope provides only that 
enclosed volume that is actually needed. 

>As far as I know, no other tank in WWII had sloped armour.
>I have two questions: (1) Was there another tank that had sloped armour,
>and (2) Did the Panther have sloped armour or not?

	Many WWII tanks had sloped frontal armor, including the Panther and 
Konigstiger, and even the Shermans. The Panther and Konigstiger had slight 
turret profile slopes for the technical reasons cited above, not particularly 
for thickness effects. The late US M24s are one example of the beginnings of 
more modern turret thinking.

                                     John Schaefer

Subject: Re: WWII tanks
From: "John W. Schaefer" <jschaefe@genesee.freenet.org>
Date: Jun 11 1996
Newsgroups: soc.history.war.world-war-ii

In article <DsK043.5t7.4.relay.ga.unc@ecsvax.uncecs.edu>,
adrianw@cassius.ee.usyd.EDU.AU says...

>>>The problem with American WWII AFV engineering was that for some
>>> reason we had no effective feedback to lead to an improved design. We had 
>>>a huge amount of tool-and-die capability, and so many plants building 
>>>Shermans and related AFVs that a new family could have been introduced 
>>>on-the-fly without an unacceptable drought of supply. (...)

>>Actually, part of the problem with American WWII AFV engineering was that 
>>the U.S. did not have the metallurgical expertise or facitilies to produce 
>>large, casted turrets...unlike the British, Germans, and Soviets (...)

>A company in Granite City (sorry, I've forgotten their name) was happily
>casting the entire frame, with integral cylinders, in special alloy steel
>for very large steam locomotives and had been doing so for at least ten 
>years prior to the war starting. I'm sure a tank turret would not have been 
>beyond them.

	My comment about lack of engineering feedback (first quote above) got
lost in this discussion of metallurgical capability, some of which I
think is wrong.

	Certainly the US had the expertise and facilities to produce
cast AFV hulls and turrets. As another poster detailed, most of the
many M4s we produced had cast turrets, and a majority had cast
hulls. Castings, though, have certain inherent disadvantages. No
matter how metallurgically sophisticated a nation is, the physics
dictates that grain orientation cannot be controlled, and grain growth
is mostly uncontrollable, in large castings.  Thus even the most
elegant tool steel alloys are not particularly strong as raw castings,
without work hardening to make a fine grain structure. Casting's main
advantages, once the tooling is built, are production rate and lower
(not higher!) technological requirements. A casting
facility--especially one that works with simple alloys--can be not
much more than a big building, an overhead crane and a furnace.

	Rolled plate, on the other hand, may not be perfect, but it can offer
fairly good and consistent grain size, work hardening, and especially
grain orientation.  Welding of fabricated plate sections, if done
properly, does not compromise these advantages. The gains from plate's
metallurgical advantages more than offset the small trigonometric
differences in effective thickness from small angles of incidence of
shell impact against angled sections of cast hulls and
turrets. Sophisticated heat treating can be combined with rolling to
make very strong steel. But good rolled plate requires a very, very
expensive and complicated mill. There are not many in the world. I
believe the reason the Russians did not make tanks of plate is that
they simply did not have the mill capability.

	The ideal tank structure is a forging. Forged steels can be
roughly twice as strong as cast, pound for pound, and much more so
around curves, unlike fabricated plate. If you want me to admire a
nation's military metallurgical capability, show me a tank with a
forged hull. Not very easy to do. To the best of my knowledge, no one
forged tank structures in WWII.


                                       John Schaefer

Subject: Re: Can not do everything: (improved Sherman)
From: jschaefe@genesee.freenet.org (John W. Schaefer)
Date: Jul 07 1996
Newsgroups: soc.history.war.world-war-ii

In article <Du1GpL.5M3.4.relay.ga.unc@ecsvax.uncecs.edu>, 
nightjar@pavilion.co.uk says...

>In article <DtxAKM.J4o.4.relay.ga.unc@ecsvax.uncecs.edu>, Richard Becker 
<rbecker@virginia.edu> says:

>>Were there any Allied AFV's in WWII with sufficient frontal armor to 
>>stop an 88 AP at 500m?

>According to some wargaming data sheets I have, the PaK 43 
>88mm anti-tank gun could penetrate 136mm of armour and 
>the KwK 43 88mm tank gun, 139mm. The Flak 36 / KwK 36
>88s could, however, only penetrate 95mm. These are figures
>at 1000 yards, so at 500 yards penetration would be greater.

	I always take information provided with individual-weapons-level 
wargames with a very large grain of salt.

	Numbers like these are so over-simplified as to be meaningless, 
except for rough qualitative rankings of performance. No one can run 
ballistics tests on AP projectiles and determine a 2% distinction (136mm 
vs. 139 mm) in penetration performance, because the individual variations in 
any number of parameters--barrel wear, temperature, and amount of fouling; 
air temperature and humidity; propellant conditions; several projectile 
variables, etc., etc.--were greater than 2% in practical tanks, particularly 
for vehicles that had been in combat for a while. That's why WWII gunners 
fired ranging rounds--to determine how to adjust for the performance of the 
moment.

	Probably some wargame designer did calculations based on theoretical 
muzzle velocity and projectile weight, and scaled from known data for some 
particularly-well-studied weapon. That introduces several more variables, 
because different projectile designs have very different ballistics.

	The biggest variable of all in penetration, though, is one that was 
touched on in another thread here recently, namely armor metallurgical 
conditions. 

	Ideal armor is extremely tough and fairly hard at its outer surface, 
with two goals: (1) reflecting enough energy back into the arriving 
projectile to cause it to shatter or deform, thereby diffusing its kinetic 
energy (if square-on to the outer face) or deflect and carry off most of its 
energy (if at an angle to the outer face), and (2) maintaining as close to a 
perfectly stiff outer surface on the armor as possible during the extremely 
dynamic energy flow following impact, so that the incoming energy is spread 
over as large an area of the outer face of the armor as possible, to maximize 
the volume of metal behind the impact area into which the energy shock wave 
is transmitted. 

	Ideal armor is quite ductile for a significant depth from its inner 
face, so that a shock wave arriving from the outer face is dissipated to the 
greatest possible extent in deformation, so as to avoid spalling or 
fragmentation. Even if the inner face reaches its melting point during 
deformation, a minimum amount of molten metal will be ejected inward, and 
hopefully at low velocity. If the melting point is reached during spalling, 
on the other hand, relatively massive pieces of molten metal can be projected 
at high velocity, which is very undesirable. Even for minor impacts that do 
not cause inner-face melting, spalling is very likely to cause damage. If 
complete fragmentation occurs, catastrophic damage can be caused by the 
fragments and by passage of some or all of the arriving projectile through 
the armor.

	The reason that simple penetration figures are meaningless is that 
the best plate armor, or composite armor with plate or forgings attached to a 
cast or forged structure, can deliver a much greater degree of this kind of 
ideal performance. Simple, crude WWII castings, on the other hand, were 
homogeneous all the way through at best, and were uncontrolled and variable 
in counterproductive ways in other cases. Very good armor may deliver three 
or four times the performance, inch for inch, of the best possible 
homogeneous casting. I'm sure that many mid-war Russian castings, for 
instance, were not good quality. Some of the mid-war welded-plate German 
tanks may have been fairly good--I don't have any specific tank-history 
knowledge, but the metallurgical sophistication and rolling mills existed, at 
least for a while.

                                      John Schaefer

Newsgroups: sci.military.moderated
From: "John W. Schaefer" <jschaefe@genesee.freenet.org>
Subject: Re: T34 engineering
Date: Sat, 14 Sep 1996 00:52:10 GMT

Chris Steadman <chris@steadman.demon.co.uk> wrote:

>The British who evaluated the T34 found the armour to be of a reasonably high
>quality (even with roughness of castings) with an armour hardness measurement
>of about 350-400 brinell. This put it on par with some of the better German 
>Armour.

        But while face hardness is relevant, it is by no means the only 
parameter of importance.

        Face hardness can be accomplished fairly easily with a process such
as oven surface carburizing. While high face hardness will help the armor
defeat glancing impacts from soft-steel slugs, it also will tend to face-
crack on heavy impact. Under this dynamic condition, the strength and 
ductility of the metal behind the point of impact will determine whether the 
shock wave progressing through the metal thickness will break metal away from 
the inner face. 

        The flexural strength and notch sensitivity of the metal are perhaps
the most important characteristics that distinguish sophisticated fine-grained
differentially cold-rolled high-alloy plate, with not only a hardened face
but maximum toughness of the plate core and maximum ductility of the inner face
(to resist spalling), from homogeneous hot-rolled plate or varying-
characteristic cast sections of carbon steel with no heat treatment beyond 
surface carburizing. Sophisticated plate depends on the flexural strength of 
the plate core to contribute its local stiffness to the spreading of the
incoming shock wave over a larger area, so that the inner face will experience
a lower peak force. Notch sensitivity, of course, is relevant to whether the
inner face of the armor stays in one piece under dynamic conditions.

                                         John Schaefer

Newsgroups: sci.military.moderated
From: "John W. Schaefer" <jschaefe@genesee.freenet.org>
Subject: Re: APFSDS vice HESH rounds
Date: Wed, 18 Sep 1996 18:23:53 GMT

Andrew.wicken@stonebow.otago.ac.nz (Andrew Wicken) wrote:

>(...) Churchill (I think) tank commander
>in WW2. One incident he recalled was when his tank was hit by an armour
>piercing round. Big clang, small entrance hole, crewman has untimely
>but very quick death, exit hole.

        This illustrates the upside of abysmally manufactured hardened-
through armor.

        Good armor has a hard outer face to reflect as much energy as
possible and spread the remainder over as large an armor area as possible; 
a high-strength tough interior to resist deformation and convert kinetic 
energy to heat; and a ductile inner face to resist spalling due to the 
propogated shock wave and local deformation. When an AP round hits it, 
the armor undergoes tremendous local heating. If the AP round overwhelms 
the armor and penetrates, a sizable amount of armor metal is typically 
ejected into the tank interior ahead of the penetrating round. That 
metal is at least very hot and sometimes molten. It can have a much 
higher velocity inside the tank than the remnant velocity of the 
penetrating round. This spray of liquid steel generally ignites
whatever is flammable. Also, having lost much of its kinetic energy
penetrating the first armor surface, the projectile is likely to bounce 
off the inner face of the far armor surface and richochet around the 
interior.

        Bad armor is hard through. When an AP round hits it, a few large
chunks of armor pop inward to make a hole a bit bigger than the diameter 
of the projectile. These chunks may initially be moving fairly quickly, 
but having broken out at relatively low force levels, they are probably 
barely warm. Thus their damage contribution is minimal. The projectile 
itself, having lost little velocity, continues onward. If it richochets 
off the gun or a wall, it may lose all of its energy bouncing around tens 
or hundreds of times inside, but if it hits the second wall cleanly, it 
may break out another clean hole and exit.

        For a given incoming round, the odds are very much higher that
bad armor will result in a penetration. The reason that the results of a 
good-armor penetration are so much more violent is that the armor 
died its own violent death trying to keep the projectile away from the
crew. You're much, much more likely to end up dead behind bad armor than
behind good armor. 

        If you have to ride around in a badly armored tank, you want to 
be as lucky as that tank commander, and not as unlucky as his deceased 
crew member. If they had been in a well-armored tank, they would have 
been more likely to have gotten away with nothing more than sore ears.

                                        John Schaefer

Subject: Re: T34 engineering
From: "John W. Schaefer" <jschaefe@genesee.freenet.org>
Date: Sep 27 1996
Newsgroups: sci.military.moderated

From "John W. Schaefer" <jschaefe@genesee.freenet.org>

In article <Dxp6Ey.Kn4@ranger.daytonoh.ncr.com>, "John W. Schaefer"
 <jschaefe@genesee.freenet.org> wrote about tank armor:

>>(...) while face hardness is relevant, it is by no means the only 
>>parameter of importance.

>>Face hardness can be accomplished fairly easily (...)

>>The flexural strength and notch sensitivity of the metal are perhaps
>>the most important characteristics that distinguish sophisticated fine-grained
>>differentially cold-rolled high-alloy plate, with not only a hardened face
>>but maximum toughness of the plate core and maximum ductility of the inner
face
>>(to resist spalling), from homogeneous hot-rolled plate or varying-
>>characteristic cast sections of carbon steel with no heat treatment beyond 
>>surface carburizing.

Chris Steadman <chris@steadman.demon.co.uk> responded:

>So is any of this relevant to the T -34? Were any of these techniques
>applied to Soviet armoured plate?

        I cannot cite sources here; if you want a referenced discussion, you
might want to move your question to soc.history.war.wwii (post to 
ww2-sub@acpub.duke.edu), where technical discussions of AFV characteristics
are old hat.

        It is my understanding, though, from previous discussions and years-
ago engineering study, that during WWII, only the Germans utilized decent-
quality rolled alloy plate for tanks. Both the Russians and the Brits were 
short on both first-class mill capability and alloy steel capability, 
relative to the widely disparate numbers of AFVs they built. We in the 
U.S.A. had no absolute resource limits, but chose not to build even a better 
version of the tank design we had, because of the armor-utilization 
doctrinal mess in the U.S. Army at the time. We even moved in the opposite 
direction, choosing the theoretically preferable course of design
modifications for faster production and thereby more tanks in the field at
any given time, rather than improved protection and thereby increased average 
crew survival and skill. We did this by going to an efficiently built cast
body which served as a unitized structural frame. Unfortunately, the cast 
body was designed full-thickness to provide a degree of protection not much 
better than the early inferior welded bodies. An alternative would have been
to design the cast frame just thick enough for its structural job, and make 
up the remainder of the allowable weight with applique forged shapes or rolled 
plate, at least frontally. This would have provided some real shell-rejection 
capability on top of the efficiently produced ductile cast body. However, it 
would have required a bit more labor and resouces, and we had plenty of cheap 
steel and cheap GIs.

                                     John Schaefer

Subject: Griddling that armor
From: Robert Livingston
Date: 8/20/98 7:00:01 PM

If by griddling you mean marking by filing, the answer is that the harder
types of armor will resist files and the softer types will not.  US WWII
armor was of the softer type, about 250 BHN, while most other nations
used harder steel.  Files are usually case hardened high carbon steel,
and should cut armor up to 375 BHN or so.  Russian tank armor was at
400-450 BHN during the later stages of the war; the 1941 and '42 KV was
around 250.  German armor started the war very hard, then lost hardness
as thickness and production quantities increased.  The Germans used
face-hardened armor at first, with file-resisting hardness, then dropped
the face hardening and relied on the core hardness of 250-300 BHN,
similar to US tank armor.  Late-war German armor on the front of a
Jagdpanther was measured at about 200 BHN, as was Hetzer side armor.  The
Elefants were measured in the low 200's after capture by the Russians, as
early as 1943.  These are the softest examples of German armor I can
recall.  I would expect easy filing on them, and maybe easy griddling,
too.

Generally, hard armor is expected to break up attacking projectiles,
which it can do when it is thicker than the diameter of the projectile.
Soft armor is best at absorbing projectile impact through slower
deceleration.  The switch from the earlier face-hardened or
hard-all-the-way-through steel came about when the major combatants
introduced penetrating caps on their ammo, which protected against
shatter when hitting hard surfaces.  These caps were so effective that
the FH armor resisted less well than softer homogeneous armor.

Armor under 375 BHN is called Machineable, which means that it can be cut
with normal machine-shop cutting tools.  The harder it gets, the more
often you have to sharpen the tools, until you get to a hardness which
resists cutting completely.  Tungsten carbide has been used to cut the
harder steels without excessive resharpening.  By the same token, TC was
(is) used for armor-penetrating projectiles; during WWII there was
constant tension in Germany between those who thought it should be
reserved for the machining of steel and those who thought it should be
used on the battlefield for the penetration of armor.

Subject: FH/ Homo; Cast/Rolled
From: Robert Livingston
Date: 8/21/98 3:27:45 PM

Face Hardened armor is best at defeating uncapped AP when it overmatches
the projectile, that is, the diameter of the round is less than the
thickness of the armor. Caps on APC and APCBC defeat FH by encouraging
crack formation in the hard brittle surface. The nose of the round is
supported by the cap during the impact stage of penetration.  The cap
blows out of the way for the rest of the trip through, with penetration
either by deepening cracks and ejection of material (plugging), or by
"ductile push-aside".  FH tank armor generally had 80-95% of its depth at
machinable homogeneous levels.  It was, in fact, made out of RHA. You can
see why it was more expensive as it took time, materials, and other
effort.  After the additional heat treating, the plates tended to curl,
and so were flattened cold in presses.  This cold-working introduced
locked up stresses which could be relieved catastrophically under
ballistic impact.

Homogeneous armor was "the best" by the end of WWII, when 3-6+" thick.
Even so, the Germans had considerable industrial plant dedicated to
production of FH plate, some made by the novel method of induction
hardening.  People ask why the Pz III and IV remained in production too
long, to which we should add that much of their plate was expensive to
produce and difficult to assemble.  Against uncapped Russian small bore,
capless AP and APBC (ballistic windshield only) it probably worked well.
These weapons were more likely to hit the Panzers than the
76-85-100-122s, due to the quantities of 45s and 14.5s on the field.

Rolled armor is ballistically superior to cast armor due to the
compaction and consolidation of grain structure which occurs during
rolling.  Rolled armor is made directly from cast ingots, so you an see
that cast armor could be cheaper, as it dispenses with a huge and costly
step in fabrication.  Mold making cost offsets this, but in mass
production allows savings on long term pattern use.  The USA pioneered
cast armor during WWII, taking the lead from the French with their S-35s
and all.  We had developed big casting techniques for for our locomotives
a good decade earlier.

Cast hulls and turrets can easily be curved, which results in less
exterior surface area for the same volume enclosed (the igloo principle).
Cast hull Shermans were good at taking glancing hits on the curved sides.
The armor was soft and ductile, and photos don't usually show cracks in
punctured Shermans.  Cast armor was subject to poorly controlled
thickness, resulting in oddities such as 44mm M4A1 hull sides as measured
by the Germans 38mm nominal thicknes), and 2" rather than 3" inner gun
shields found on an early/mid M4A3.  Crystalline grain structure up
through 11/43 limited ballistic resistance of cast and rolled US plate.

Curved surfaces distribute stress better than sharp-cornered welded
boxes, so curved mantlets acted a bit thicker than their weak granular
structure would lead us to expect. That is, they were kinda the same as
if they were RHA.

The relation of optimum hardness to thickness has been covered in
previous threads, and it can be put briefly that higher hardness was best
at defeating undermatching projectiles.

Another important factor is the "scale effect", which causes armor to
grow more brittle as it increases in thickness, when the scale of the
attack is stepped up proportionately.  The reason is that the necessary
rapid temperature drop (in quenching) is difficult to obtain deep inside
multi-inch thick plates.  Certain alloys such as chromium deepen the
internal hardening, but Cr supplies in Germany quickly became limited.
Due to the difficulties in making thick plate, optimum BHN drops as the
thickness increases, as softer plates are more forgiving of heat treating
errors.

Nothing much below 210 BHN was used with good results, I think.  The US
Army rejected an M4A3E2 Jumbo turret at 212 BHN (thickness was OK at 6").
A captured Ferdinand in Russia was measured at 212-223 BHN on its 86,
110, and 200mm plates (Brit intell, 16 Feb '44).  Spielberger tells us
that the plates for the Ferdinands were taken from Naval stocks, which
could mean it was made to different specs.  German 85-200mm specs at the
end of the war called for 220-266 BHN.  55-80 was 250-290, and 35-50mm
was 300-350 BHN.  Much armor in that range was face hardened, with a
450-600 BHN face.  The German specs point out the general relation
between optimum hardness related to plate thickness with respect to
attack by late war KE weapons capable of having a chance of defeating the
armor.  The USA developed similar specs by the end.

Austempered steel was used on the SdKfz 234 8-wheel armored cars we all
think were so great.  It was mediocre armor, confined to the plates of
14.5mm and thinner, but it was cheap and quick to make, with a simple
heat treating procedure, resulting in crackable thin homogeneous
low-alloy armor.

Light armor was variable in resistance and tended to be brittle and to
fly into large fragments when overmatched.  During the Spanish Civil War
I've read that a US volunteer tanker observed that the Russian tanks
(T-26's?) had thin ductile armor which allowed German shells to pass
right through, and provided no crew member was hit the shells could
penetrate without injury.  He said the Krupp tanks (Pz Is?) shattered
like glass, with dramatic casualties. Although the Russians went to
high-hardness armor, probably because they could forego tempering, the
Germans relied on their Ruhr Valley craftsmanship to avoid the pitfalls.
Emergency conditions led to acceptance of substandard lots of ammo and
armor, however, as the craftsmanship and raw materials dwindled.

At the beginning of the war the Germans tested French and British armor
as found on captured tanks at Dieppe and in N. France, finding it
comparable to German armor.  By the end of the war the US had tested
German projectiles and found them significantly better than ours, against
our own and British plates.  As their penetration data for their own guns
showed, the Germans were able to make VERY resistant plate through the
end of the war for their own test programs.  The quality of AFV
production armor suffered, though.  The loss of nickel and molybdenum
supplies was critical, and they could only compensate successfully on a
proportion of the plate delivered, due to the finicky and troublesome
interrupted-quench system, wherein plates were hoisted in and out and
back into huge quench pools, with timing to the nearest second.  I
believe the Panther glacis often was defeated at the mill, with a 10-20%
reduction in effective thickness due to incorrect quenching and
tempering.  A metallurgical report on a Panther glacis showed the
presence of bainite, a crystalline form of steel, in an interior layer
(like plywood).

Difficulties with armor production led to hull front, sides, and rear of
the Nashorn/Hummel and possibly Wespe to be plain mild steel, according
to Guderian's memoirs.  As for the comment that some later Japanese tanks
had non-armor, well, maybe so.  I've seem absolutely nothing about the
quality of Japaneses AFV armor.  Their Naval armor was good enough.

Sources include the BIOS report GERMAN TANK ARMOUR, and ARMOR HANDBOOK
(1952) "The Development and Manufacture of the Types of Cast Armor
Employed by the US Army during WWII", Briggs et al, Ordnance Corps, 1942.
EFFECTS of IMPACT & EXPLOSION, and THE PENETRATION OF ARMOUR PLATE.  The
BIOS report came from the Bovington tank museum library (you might want
to Search Messages to get a better explanation by me about these reports
& how to get 'em). It is based on captured German records, site visits,
and interviews with industrial bosses.  The last two are official
summaries of US and British armor and penetration studies during WWII,
which I got from the NTIS (phone numbers are listed at www.ntis.gov).  If
you can get ahold of a live operator you can get her (him?) to go look it
up in their old card file. Last time I looked the computers hadn't caught
up with the past.  Currently I am looking into the Federal Depository of
Records library system to see what I can find.  The above was written on
one cup of coffee while my wife is away.-- Robert

X-Source: The Tankers' Forum
Subject: US Cast vs Rolled Armor
From: Robert Livingston
Date: 1/10/99 5:59:50 PM

Cast armor resists less well than rolled of the same hardness and
thickness. US tests of production quality armor in 1942 and '43 showed
this clearly, in which 2" thick test pieces of cast armor showed 10-20%
inferiority compared to 2" rolled plates, when hit by 75mm projectiles.

Rolled armor can be raised to higher hardness levels than cast without
losing ductility (and therefore ballistic resistance). This was also
demonstrated in tests.  US cast armor was of the following BHN:

<32mm:  302-325 BHN
32-64mm:  235-269
76mm: 235-260
102-152mm: 220-250
>152mm: 200

In contrast, the following BHNs apply to US rolled armor:

25mm: 310-350 BHN
38mm: 280-320 
51mm: 260-290
64mm: 240-275
76-127mm: 240-260
>127mm: 220

Not that, on average, hardness of cast was less than rolled. The US
accepted the lower ballistic quality of cast armor in making the M4A1
Sherman, relying on a little extra thickness and the rounded corners (as
Troy rightly points out) to make up for the essential weakness of the
armor material. The net effect was that the later versions of M4A1 were
less well protected than the later versions of rolled-hull Shermans. One
unit which had both (the 743rd Bn) kept their cast hull Shermans out of
combat, a lesson apparently learned the hard way.

Rolled armor is esentially cast armor which has been further worked and
shaped, which aligns the grain structure which increases ballistic
strength. Rolled steel armor is made by first pouring molten metal into
molds and allowing it to cool and solidify into ingots. These big
barrel-shaped pieces which come out of the molds are then pounded with
hammers (BIG hammers) to form billets, which are then rolled at the
rolling mill to become slabs, which can be sized in thickness to fit the
tank design. They are flame cut to the profile of the desired tank part,
then heat treated, then welded into the tank. Cast steel armor components
bypass all that working, and are made simply by pouring molten metal
directly into tank component shaped molds. They are removed from the
molds, rough spots, risers and gate marks ground off, and heat treated.
Then they are built into the tank.

The differential in hardness between outer surface and inner surface
found on some WWII cast armor is more a result of poor heat treatment or
insufficient alloy content than any intentional effect intended to
increase ballistic resistance. Homogeneous armor works best when it is
the same hardness throughout, as changes in hardness form stress
concentration boundaries which destroy ballistic resistance. This too has
been shown empirically. Face hardening is another matter, but was not
used by the US on tanks after the early models of the M3 light, due to
expense, difficulty in working it, and relatively poor resistance to
German APCBC in comparison to homogeneous armor.

X-Source: The Tankers' Forum
From: Robert Livingston (rlivingston01@snet.net)
Date: 10:17:44 PM

Some of you (well, one of you, Andrew) have asked if I might list some of
the research papers I've quoted on the last Tankers' Forum, which is now
lost.  These papers were written during WWII and represent the state of
the art as it was, in the area of armor penetration by kinetic energy
projectiles.  Comments, questions, and contributions are welcome.


PENETRATION OF ARMOUR PLATE by US ORDNANCE BOARD, ABERDEEN PROVING
GROUND, March 1950. NTIS call number PB91-127506, currently available.
This report is actually the British National Physics Laboratory report as
published by the Ordnance Board Press in England, not APG.  It covers
penetration formulae, the effects of plate hardness on ballistic
resistance as determined by actual tests, the phemonena of shot shatter,
and a discussion of tungsten core dynamics as they were then understood.
There is a huge bibliography of WWII research papers included which is a
good start on doing your own research.

GERMAN TANK ARMOR by the British Intelligence Objectives Sub-committee
(BIOS) 1946. This is held in the Tank Museum Library at Bovington,
England, and is a giant-size 260 page exploration of captured German info
on the specifying, making and testing of German rolled and cast armor
throughout the war, including face hardened and odd stuff like induction
hardened and austempered steel.  Some is based on interviews with German
engineers.  Some captured Krupp gun performance data is included.  For
several (!) dollars you can buy photocopies from Bovington.

EFFECTS OF IMPACT AND EXPLOSION, by the Office of Scientific Research and
Development, et al, 1946. Available from the NTIS under call number AD
221 586. This fatboy (511 pages) is filled with articles on terminal
ballistics, bombs, explosions, armor, and numerous semi-legible charts.

The NTIS has a report which is apparently an index of all the reports
from the NDRC (National Defense Research Committee) which can be obtained
under the call number AD-221610.  This would be WWII era technology
pertaining to weapon development of all kinds. I have not seen this
report.