A cast iron radiator is stable, durable and dependable, but is there a breakage factor. There are a few different reasons why a radiator section will fail. The number one cause of a broken section is the result of frozen rads. Water, when it freezes, expands significantly. In a closed-loop system, it has nowhere to go and will crack the radiator sections because the ice increases the pressure in the sections. Predominantly, broken radiators are usually caused by a house freezing. Although a cast iron is strong and very heavy, much like porcelain, it’s actually incredibly fragile and can be a very brittle material.
If one was to take an average sized cast iron rad and let it fall over on the floor, the rad would be garbage. There would be cracks down each one or more of the sections, and it would be useless. The fragility of the cast iron radiator can be compared to – of all things – a toilet; a toilet has a structure that appears to be very strong, people sit on them every day and never break them. But if someone knocks their toilet over, the toilet is most likely to shatter. Cast iron radiators react the same way.
In cases where freezing was not the cause of a failed radiator, the only seemingly possible explanation is that at some point in its life the rad had suffered a trauma. It might have been at the factory where it was made, or during its installation, but at some point the rad likely took an impact but the fracture was not substantial enough to leak. The rad will sit there for year, even decades, and perform functionally perhaps without causing incident, but one day, after it’s had enough fatigue, it will just open up and start leaking.
Given that cast iron radiators have been around in various styles since the 1800’s, the majority are still in use today, the likelihood of a radiator cracking is rare.