exploding cartridge heater
Cartridge heaters are typically intended to be buried in a deep hole in a piece of metal. You run them at full power in open air- this is a typical result. 

The watts per square cm or inch is too high for them to operate in free air. You might be able to operate the heater at a fraction of rated power. 

You might do better with an IR or calrod type of heater- like the elements inside a home electric oven.  Image from this site.