The affected area by heat was studied in ASTM A36 carbon steels, with thicknesses of 12 mm, analyzing the effect of preheating on their properties. The welding process used was SMAW, then tensile, hardness, metallography and numerical calculations were carried out, for which a temperature range of 100 ° C was established; 120 ° C; 145 ° C; 160 ° C, concluding that the optimum preheating temperature is 145 ° C, with which the following results of the tensile test (515.82 Mpa) and hardness (232 HV) were obtained; according to recommendations of the percentage of carbon equivalent and hardness ratios, the material will not present fissures when it will get cold. On the other hand, the results of stress (424.15-500.36 MPa) and hardness (209-250 HV) due to preheating effect show that both are inversely proportional. In addition, the effect of preheating decreases the cooling rate in order to avoid brittleness of the material and the maximum martensite formation, in addition, the width of the heat-affected area increases as the heat input increases. Then it was determined that the influence of preheating on the microstructure is essential to reduce the amount of martensite through a slow cooling and do not cause hardening levels in the affected area caused by heat; avoiding so risks of cold cracking. Finally, it was possible to understand the microstructural variations (tensile and hardness), of the affected area by heat, in order to avoid the appearance of fissures in the weld seam.