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Face Hardened Armour On Tanks.


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#1 Inhapi

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Posted 22 February 2020 - 0953 AM

I know the different types and evolution of armour and its production for warships quite well.

 

I am completely in the dark on the subject of face hardening or even the different types of steel armour (composision, heat treatment, mechanical treatment etc...) in tanks. 

 

What was face hardened armour in tanks ? Surely not cemented. Was it a matter of diffential cooling speeds between a surface and the full plate thickness ? What were the methods used to produce this ?

 

Is there a comprehensive source on this out somewhere ? (I'm interested in WWII and before armour types/production.)

 

thanks

 

Inhapi


Edited by Inhapi, 22 February 2020 - 1014 AM.

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#2 Rick

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Posted 22 February 2020 - 1621 PM

Here are some notes I downloaded awhile back.

 

If I recall correctly, as a rule-of-thumb face-hardened armor performed very well against uncapped projectiles which were of lesser or slightly greater diameter to the armor's thickness. (ex. the Pz IV's 50mm FH armor vs Soviet 45mm and 76mm guns or Shermans with M72 ammo) However, if the enemy were using capped ammunition or very large-caliber shells (Shermans with M61 ammo or Soviet 85, 100, 122, 152mm guns), you'd be better off with regular RHA, as the hard surface would tend to crack. Of course, face-hardening only affected the first 5-20% of the armor plate, so you're not losing a massive amount of strength...
 And yeah, there was a developer answer a while back that stated that cast armor had 95% of the effectiveness of RHA. Not sure if that's still the case but yes, cast armor should have a lower effectiveness. Studies vary on the difference, with some stating 3-8% reduction in effective thickness, while some US tests indicating 10-20% reduction.
Cast armor has poorer "compaction and consolidation of structure" due to being poured into form rather than rolled from ingots. The thing is, the thicker the armor plate you're trying to cast, the worse it gets, the more likely it is to have trapped air bubbles, crystalline structures, and uneven hardness caused by uneven cooling. Granted, very thick RHA could have problems with uneven cooling too, but they went through complex interrupted-quench systems to alleviate this.
 Casting is also somewhat imprecise, and results in some variation in true armor thicknesses. (sometimes as bad as 44mm Sherman side armor being 38mm thick and 3" gun mantlets closer to 2") Not that that's something that is reasonable or fair to include in game, it's probably just as likely that there were some cast-hulled Shermans with 3.5" thick 3" plates as well.
I'm more curious whether there will be any simulation of homogeneous armor hardness (independent from face-hardening). German and American RHA (with the exception of some very late-war German armor after supply losses and sabotage) had a hardness of around 200-300 Birnell. Soviet armor was around 400-450 Birnell, making armor more brittle, and more likely to spall. Anecdotally, British (at least early-war) and Czech armor had these problems as well. (Matildas being knocked out in the desert at extreme range, not by penetration, but by catastrophic spalling)
I remember my grandfather, whom was a steel worker back in WW2 and the Korean War era, said they used to get the best ratings for their large steel castings from government inspectors. Their "secret" was how they'd mix the steel, then pour the steel at the right temperature all the while they would vibrate the mold platform... it was very people knowledge oriented and it was as much an art as it was a procedure. There was a hand full of people that were very good at it and they were the ones that supervised the making of any armor grade castings.
Rolled Homogeneous Armor, is made from a single red hot piece, which is usually squeezed between a number of rollers, and until it is the desired shape. Doing this has pros and cons, ; Pro: easy to mass-produce, and cheap. Con: softer than other types of metals.  Commonly referred to as, "Hot rolled", it has a lower melting point than other metals, on the opposite end of steel, you'd have "Cold Rolled", much harder, heavier, and denser than the RHA. Most tanks in the US made factories used RHA, some of the tanks were then had areas covered with plates of Cast. The logic behind that is different types of metals have, like wooden planks, a 'grain' to them, and the two different types layering the other would provide better protection. But as with many Theories, that one was...wrong. So, since they were often used together, neither one is better than the other. Properties of Cast; it has ductility, better at absorbing an impact: it will dent a lot before it opens a hole, and better at resisting heat, but not necessarily harder than RHA.
 Such as they are, my Credentials: Certified Welder, Blacksmith, and Amateur Metallurgist.  I encourage you to do your own research and learn for yourself these things in the world of Metals.  
We have chilled face, cemented, and even direct hardened face. All 3 have their different uses and some FHA far out perform RHA up to a certain angle. Depending on how the FHA is designed, it can shattered caps and destroy rounds before they penetrate, the issue comes with repeat impacts. If the FHA isn't properly treated, repeated impacts cause cracks to form in the transition layer between the hardened front and the soft rear. This causes a weak point to form and can cause a catastrophic failure in the armor upon impact from subsequent rounds.
 Properly treated FHA at low obliques of no more than a 35 degree angle from vertical surpass RHA by a large margin. The issue is once that angle gets increased the FHA face allows a capped round to literally grip the armor and better normalize. So instead of the round grazing off, it can cause a partial penetration or a full one due to the armor. This was actually proven in combat on the French Battleship Dunkerque, where a 15 inch round from HMS Hood would have deflected off the turret roof had it not been FHA, and instead the shell normalized and tumbled sideways allowing half of the projectile to make it into the right half of the turret killing the crew. It failed to detonate but still did it's job.
 So FHA and RHA have different uses and different practices. I could go into much further detail later on with actual chemical analysis and formula's on why they are so different if people would like.
 The main reason why RHA and cast armor was preferred over FHA had everything to do with "Cost". It's cheaper and faster to produce over FHA. Also, you can directly weld on RHA and Cast where as you cannot weld FHA or it ruins the face properties. This is why you see rivets and bolts used on FHA over welding.
 n article <[email protected]>, [email protected] 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.
Certainly the US had the expertise and facilities to produce cast AFV hulls and turrets. As another poster detailed, most of themany 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 physicsdictates 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 soaround curves, unlike fabricated plate. If you want me to admire a nation's military metallurgical capability, show me a tank with aforged hull. Not very easy to do. To the best of my knowledge, no one forged tank structures in WWII.  John Schaefer
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. mposite 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.
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 designmodifications 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 castbody 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 beento 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
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 orhard-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.  Theharder 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.
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 underballistic 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 controlledthickness, 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 granularstructure 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 gunsshowed, 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
Having worked at General Dynamics for 16 years (building the M1) as a Welding Engineer, I would like to share a few comments.The basic answer to your question in respect to cast vs. welded construction is basically "available technology". First, let me say that in TODAY"S world, welded construction is FAR superior to casting when it comes to main battle tanks. Leopard, Abrams, etc. are welded.
The issue in WWII was welding technology. Having an electrode available that was suitable for ballistic welding applications did exist, but was very difficult to obtain logistically for Britain. The ability to cut armor plate at mass production rate, and a core of certified welders to execute mass production was another hurdle. 
The Germans pioneered most of the advanced welding (armor) technology during the war, and the vast difference in allied and axis vehicle production numbers reflects the advantage of cast armor over welded. The disadvantage (castings) of course, was ballistic performance. 
Now days, weld metal is actually superior in its chemistry than armor plate. Full penetration weldments are required for optimum performance. Weldments are X ray'd, and ballistically validated at Aberdeen Proving Ground. The introduction of MIG welding with specially formulated wire, shielding gas, etc., have made welded construction superior in the construction of Submarines, Tanks etc. Complex shapes (race ring, road arms) are still cast steel in the Abrams, so cast metal is still a vital factor in armor vehicle technology. During wartime, the US poured out the Sherman in the vast numbers necessary to overwhelm the enemy, and cast construction was the cheapest and the fastest way to get the tanks on the battlefield.

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#3 Mobius

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Posted 22 February 2020 - 1814 PM

What Rick said.

There is usually an active thread going on over at the axis history form on some kind of armor quality question.

https://forum.axishi...0ec2da616a1a20e


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#4 bojan

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Posted 22 February 2020 - 1815 PM

...

 And yeah, there was a developer answer a while back that stated that cast armor had 95% of the effectiveness of RHA. Not sure if that's still the case but yes, cast armor should have a lower effectiveness. Studies vary on the difference, with some stating 3-8% reduction in effective thickness, while some US tests indicating 10-20% reduction....
 

US noted larger difference because their cast armor was softer than already (relatively soft) RHA. Efficiency of cast is also dependent on the sloping and type of projectile hitting it.

 

 

I'm more curious whether there will be any simulation of homogeneous armor hardness (independent from face-hardening). German and American RHA (with the exception of some very late-war German armor after supply losses and sabotage) had a hardness of around 200-300 Birnell. Soviet armor was around 400-450 Birnell, making armor more brittle, and more likely to spall. Anecdotally, British (at least early-war) and Czech armor had these problems as well. (Matildas being knocked out in the desert at extreme range, not by penetration, but by catastrophic spalling)
 

 

German armor was harder than that, about 330-380 on Pz-IVh front plate and it was not just late war German armor that had cracking problem, catastrophic cracks were noted already when Soviets tested Pz III in 1940. And not only high hardness armor had that problem, Ferdinand armor cracked a lot and it was sub 300 BHN

Soviet armor hardness varies a lot, KV-1 was 270-290, T-34 was 350-450 etc, IS was about 320 etc.

 

 


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#5 Mobius

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Posted 22 February 2020 - 2000 PM

 

...

 And yeah, there was a developer answer a while back that stated that cast armor had 95% of the effectiveness of RHA. Not sure if that's still the case but yes, cast armor should have a lower effectiveness. Studies vary on the difference, with some stating 3-8% reduction in effective thickness, while some US tests indicating 10-20% reduction....
 

US noted larger difference because their cast armor was softer than already (relatively soft) RHA. Efficiency of cast is also dependent on the sloping and type of projectile hitting it.

 

 

I'm more curious whether there will be any simulation of homogeneous armor hardness (independent from face-hardening). German and American RHA (with the exception of some very late-war German armor after supply losses and sabotage) had a hardness of around 200-300 Birnell. Soviet armor was around 400-450 Birnell, making armor more brittle, and more likely to spall. Anecdotally, British (at least early-war) and Czech armor had these problems as well. (Matildas being knocked out in the desert at extreme range, not by penetration, but by catastrophic spalling)

T-34 was 350-450 etc, 

There's a theory that in the Yugo test the T-34/85 was a post war Czech made model as it had 350 BHN UHF.  US measured captured Russian T-34s UHF as 429-437 BHN.


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#6 bojan

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Posted 23 February 2020 - 0730 AM

There's a theory that in the Yugo test the T-34/85 was a post war Czech made model as it had 350 BHN UHF.  US measured captured Russian T-34s UHF as 429-437 BHN.

 

There were no Czech made T-34s in Yugoslavia. Unlike many other things this can be said with 100% certainty.


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