A reaction tube system of a steam reformer, more generally referred to as tubular cracking furnace, for the indirect heating of cracking feedstock, such as the catalytic cracking of hydrocarbons, includes a plurality of reaction tubes and headers, the reaction tubes penetrating through the bottom of the steam reformer and being extended to the associated header by low-alloy tube sections attached to the tubes by circumferential welds. The reaction tubes are surrounded in the area of the weld(s) by a coaxial protective tube having one open end and defining a free annular space around the reaction tube. The protective tube prevents deterioration by corrosion of the weld(s) joining the tube sections of high-alloy steel with low-alloy extension sections with unalloyed steel extension section.
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