The Fe-12Mn-1C Hadfield steel is an abrasion-resistant alloy of high technological relevance for mining and heavy machinery. This composition is susceptible to pearlite formation which is detrimental for the material's ductility. Although its spread use, the study of pearlite formation has been preserved to laboratory conditions which cannot be transferred to industrial practices. This manuscript provides updated information about this phenomenon by constructing the time-temperature-transformation diagram of the alloy between 400 and 600 A degrees C. The pearlitic reaction occurs above 450 A degrees C and begins on the grain boundaries. Only 7 min is needed for the transformation to start at 550 A degrees C, and a maximum pearlite fraction of 35% is reached after 150 min at this temperature. Results are compared with the Fe-12Mn-0.8C composition mostly found in literature. The discussion comprises the effects of carbon and manganese content on the pearlitic reaction with the support of thermodynamics calculations.