First of all, we need to know some things about armour. Making armour during this age was complicated, required skilled labour and lots of know-how. It was not any steel mill that could make armour plate. Various alloys were used - chrome was commonly mixed into the steel to create armour.

You wanted armour to be very hard, but also spongy or soft - armour that became too hard often became brittle, which meand that a non-penetrative hit could cause a bulge that would cause splinters to fly from the inside of the armour plate, wopunding or killing the crew inside, or destroying vital systems, igniting ammunition.

The quality of armour and the necessary know-how to make it made a lot of difference. It is known that late war German armour was worse than early and mid-war, since it was harder to get the rare materials for the alloys and their supply of skilled Labour was slowly drained by the need of Manpower at the fronts. Likewise, the Soviet armour quality was decent early war, declined mid-war and improved again late war as they had free access to all the rare materials they needed through lend-lease.

Making large armoured plates is hard - basically, the larger the armour plate, the harder it is to make it well and of even quality without faults and invisible cracks. You need more skilled Labour and know-how the larger the armour plate.

That said, there were three main methods of forging armour for tanks in the 30s and 40s. Each had their own advantages and disadvantages.

Rivetting or bolting.

This method was the most common in the 30s and early war. Notable examples are Soviet early T-26s, Italian M13/40s, British Crusaders and German Pz 38(t)s.

Basically you take armoured plate and use rivets or bolts to make them stick together. The advantages of this method are obvious - bolting or rivetting is fairly straight forward and do not require that much skill - it is hard work, but not very difficult. Note for example how many women were employed as rivetters in the US industry - it was possible to make someone who had never worked in industry before a decent rivetter rather quickly (see Rosie the Rivetter for example). Rivetting is also a very strong sonnection, which means you can take many small plates and rivet them together into a strong whole. The big advantage here is that small armour plates are much easier to produce of even quality than large ones.

It is also rather easy to repair or replace plate that has been damaged or penetrated and can at times even be done by field repair shops (as long as the hit damaging or penetrating the plate has not caused a fire or destroyed vital systems in the vehicle).

Vehicles made of rivetted armour is thus cheap to produce (both the armour plate and the assembly), requires little skilled Labour and know-how and is both a quick and tried method that makes tanks that can have their armour repaired in the field (at least somtimes).

The disadvantages are that rivets and bolts weigh a lot. This was not that much of an issue for smaller vehicles, but could become for larger vehicles. The main disadvantage was that bolts and rivets had a tendency to come loose when the vehicle was hit - bolts and rivets could fly around like shrapnel inside a vehicle even at a non-penetrative hit, killing or injuring crew and damaging or destroying vital systems inside the vehicle.

Casting

This method was most common before and early war. Notable examples are the French Suoma S35, the British Matilda II and the early versions of the US M4 Sherman.

Casting an armoured vehicle offered many advantages - cast armour was often soft (ie non-brittle) and still strong and hard. It was easy to make rounded forms that would help make solid shot armour-piercing shells glance or bounce off the armour (these kind of shells were common early war).

The disadvantages were that it was expensive and slow, and required very skilled labour and much know-how to cast quality armour. It was also impossible to repair cast armour once it had been severly damaged or penetrated.

Welding

Welding became the standard way to produce armoured vehicles as the war progressed. Notable examples are the T-34, the later versions of the M4 Sherman, the Pz IV and many others.

The advantages of welding is that it offers a light-weight and rather quick way to connect armour - the tolerance and margin for error is also higher - you can apply a coarser weld to compensate for them (which is notable with many of the mid-war T-34s). You also do not need any tungsten (a rare material iused for metal working in ww2 before diamonds became more common) tipped tools to drill holes and grind off cast residue.

The disadvantage is that it does not connect the pieces as strongly as rivetting, which means that larger pieces are needed (which are harded to produce). It also requires more skill and know-how than rivetting (but less than casting). Welding armour is pretty hard and was pioneered by the Swedish company Landsverk for their tanks and armoured cars (used by Hungary, Ireland, the Netherlands and Sweden) in the 1920s.

Welding was thus a bit of the best of both worlds and less of the disadvantages of both of them.

As far as I know, Italian factories didn't choose either method. Tank hulls were mostly riveted simply because of practical reasons - this method was easier, given the limited equipment available, and also considerably faster; factories, because of the country's limited degree of industrialisation, were struggling to meet the Army's demands and had to cut corners in order to deliver more equipment. Of course, such armours were often way too thin not to mention prone to cracking, as the men of the Ariete division found out by themselves...

Is this true? Did they actually cast the hull? That sounds completely stupid, and I say that as someone who has done a lot of analysis on steel vessels.