In production it is necessary to achieve conditions that lead to the minimum decarburization of a steel product’s surfaces. In this study, the hypo-eutectoid carbon steel C45 was annealed in air in the temperature range T a = 600–1100 °C. The annealing times were between t a = ½ h and t a = 2 h. Different decarburizations occurred in different microstructures: ferrite–pearlite (T a = 600°C and 700 °C, T a < AC1, no visible decarburization); ferrite–austenite (T a = 760 °C, AC1 < Ta < AC3, visible decarburization); austenite at the beginning, ferrite–austenite after the incubation period (Ta = 850 °C, AC3 < T a < 912 °C, visible decarburization); and austenite (T a = 950 °C and 1100 °C, T a > 912 °C, visible decarburization and overheating of steel). The edges were more prone to decarburization and to overheating. Stress relieving, normalizing and annealing before quenching of the steel C45 can be carried out in air.