This paper will present an eye-opening look at some of the historic data sources and correspondence that are the basis of the API RP 941 Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants (a.k.a. The Nelson Curves). The focus of the paper is primarily on carbon steel data but includes interesting information regarding the ‘removal’ of the C-0.5Mo curve. Included are comments highlighting some of the unknown/missing data incorporated into the Nelson Curves.In 2015 a significant change was made to the carbon steel curves in the 8th edition of API 941. It was the first significant change to the curves in the last 25 years and is based primarily on damage of non-post weld heat treated carbon steel that occurred after exposure times of 6 to 14 years. This new non-post weld heat treated carbon steel curve is approximately 50°F below the existing carbon steel line and begins at 50 psia partial pressure H2. Some of the failures that were utilized in the development of this new curve are atypical of traditional HTHA damage. A review of this damage morphology as well as other cases of similar damage of carbon steel and C-0.5Mo are included.