A process for the decoking of a hydrocarbon steam cracking furnace having a firebox, radiant coils, a transfer line exchanger, and an oil quench connection wherein liquid quench oil is injected to directly cool the steam-cracked effluent. Decoking feed comprising steam and air is supplied to the furnace under conditions sufficient to at least partially combust coke accumulated on the interior of the radiant coils, the transfer line exchanger, and the quench connection. Quench steam is supplied and injected into the decoking process effluent in an amount sufficient to cool the decoking process effluent below the metallurgical temperature limit of downstream piping. Also, a pyrolysis furnace for the production of ethylene is also provided.