A fitting for liquid or steam piping, comprising: an elastic sealing ring; a locating sleeve placed around said sealing ring; an internal teethed lock ring located on the locating ring outer end face; an inner ring located outside of the internal teethed lock ring, the inner surface of the inner ring matching with the outer surface of the pipe to be coupled, the inner ring further including a tapered surface and external threads on the outer surface of the inner ring; an outer ring with threads on its internal surface for matching the external thread on said inner ring, as well as methods for connecting pipes using the fitting.