How to connect black pipe without threads?

To connect black pipe without threads, you can use a coupling or a fitting designed for welding or brazing. First, clean the surfaces of the pipes to remove any rust or debris, then align them and apply heat to fuse them together if using welding or brazing techniques. Alternatively, you can use a mechanical connection method, like a compression fitting, designed for black pipes. Always ensure the connection method is suitable for the application and complies with local codes.