A process for the thermochemical treatment of thin-walled structural elements made of steel comprising enriching an edge zone of a thin-walled structural element with nitrogen and carbon at 590.degree. to 700.degree. C. to harden the edge zone whereby an intermediate layer (3) is formed in the diffusion zone (2), said intermediate layer comprising 1 to 3% by weight of nitrogen and at most 0.8% by weight of carbon and cooling the said thin-walled structural element under an inert atmosphere at a rate of 50.degree. to 100.degree. C. per minute to room temperature to form a nitrogen austenite and/or braunite phase between the connection layer (1) and the basic structure (4) with increased fatigue strength.