The SAE/ AISI 1045, a type of medium carbon steel, is used most commonly in various structural and element of machines. Sometime, it failed during the in-service, which assumed to be caused by cracking in material as the effect of casting, manufacturing, or heat treatment processes. The current research was developed to find out the effect of hardening and of tempering processes toward hardness, microstructure and cracking. The objectives of the current research are to obtain the effect of cooling rates toward the hardness and cracking and to define a proper cooling media to get a martensite microstructure without cracking of heat resistant products. Results showed that the chemical composition from the spectrometry test confirmed that the specimens were classified as AISI 1045 or JIS S45C. The hardness values properties increased with increase of temperature, except at 1000 °C. The specimens having the hardness property more than that of ASME II standard were not useable due to its brittle.