The invention relates to a thermally controlled steam trap with a casing formed from two disk-like wall portions, with a diaphragm arranged between them and which is tightly welded at its outer edge to the two wall portions and forming with one wall portion a chamber receiving an expansion medium, and with a closure centrally fixed to the diaphragm and cooperating through a bore in the second wall portion with a valve seat. According to the invention the closure 7 is centrally connected to the diaphragm 4 and a counterpart 15 by resistance force welding. In order to obtain a clean weld, the closure 7 is specially designed for resistance forge welding.