Steel lined reducers are industrial pipe fittings that combine the chemical stability of PTFE with the mechanical strength of carbon steel. They are mostly employed in piping systems that need to be resistant to wear and corrosion, particularly in situations where pipes need to branch or change direction.
link:https://www.ttptfe.com/steel-lining-ptfe-pipes-and-fittings/steel-lined-ptfe-concentric-reducer.html