Abstract: A heat exchanger and especially a blast preheater (or cowper) for blast furnaces with refractory lining, insulating layer and steel jacket is described, the insulating layer of which is designed so that a steel jacket temperature of from 20.degree. to 60.degree. C. can be set and that said steel jacket is made of chromeless structural steel having a molybdenum content in the range from 0.01 to 2% by weight. The blast preheater (or cowper) has a prolonged service life and a low tendency to nitrate tension crack corrosion of the jacket sheet, for which reason it is very suitable for use in a process for the temperature control of gas and blast for the blast furnace. The process is suitable for blast preheaters (or cowpers) with external or internal combustion chamber or checker chamber and downstream collector ducts.