In this research , medium carbon steel type (AISI 1045 ) sheets with 8 mm thickness were welded by Gas Metal Arc Welding Process (GMAW), this study included the application of numerical analyses by using control volume method (CVM) to obtain the effect of preheating process with different temperatures (75 ˚C , 125 ˚C and 225 ˚C ) on temperature distribution of the welded joint in addition to determination of cooling rates which were achieved for 3-D heat transfer in the weldment. Results showed the analyses contributed effectively to predict the temperatures distribution for the welded joint and get mathematical models for the cooling rates at welding variables ( welding speed was equal 2.5 mm / sec , and welding current was equal 180 Amp ) with different preheating temperatures.