Reduction of fouling in thermal processing of olefinic feedstocks

A process to reduce fouling in a petrochemical thermal process for treating a feed stream comprising naphtha, pyrolysis oils or a mixture thereof said feed stream having a combined olefinic content from 10 to 50 weight % and the balance inert hydrocarbons at a temperature from 100° C. to 300° C. which comprises decreasing the amount of carbon steel in the apparatus contacting said feed stream and increasing the amount of stainless steel having an iron content from 60 to 75 weight %, a chrome content from 15 to 21 weight %, a nickel content from 1 to 15 weight %, a manganese content from 0 to 3 weight %, a molybdenum content from 0 to 3 weight % of and the balance comprising one or more elements selected from the group consisting of Si, Co, Cu, and other trace elements provided no single such element is present in an amount of greater than 1 weight %.