I am not a welder. Seems to depend upon the nature of the 'cast iron', and, the requirement for pre-heating is probably impractical for bridge components in-situ. https://www.twi-global.com/technical-knowledge/job-knowledge/weldability-of-materials-cast-irons-025
I know that cast iron engine blocks can be repaired..perhaps this is a different kettle of fish.
@wildwest started
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I am not a welder.
Seems to depend upon the nature of the 'cast iron',
and,
the requirement for pre-heating is probably impractical for bridge components in-situ.
https://www.twi-global.com/technical-knowledge/job-knowledge/weldability-of-materials-cast-irons-025