SCR contactors are subject to heating due to the high currents that they sometimes have to carry. They are normally air or water cooled but can still over heat if driven too hard or not cooled correctly. Heat is one of SCR’s main failure modes. SCR’s fail in the “ON” shorted mode. This makes caution very important. Over temp monitors on the contactors is recommended by the SCR manufacturers.
Reference: RWMA - Resistance Welding Manual 4th Edition