Sorry to interrupt five pages of hero worship but somebody should point out to our author that there is no such thing as 5100 steel. The "00" in 5100 would indicate 0.00% carbon, and a steel with 0% carbon is not a steel.
Let's assume he was speaking generically of the 51xx class and the steel in question is something like a 5140, that would indicate a 0.40% carbon steel with 1% chromium. In other words, it's like a 4140 minus the molybdenum.
Look it up folks, the 51xx series is less alloyed than the 41xx series, and has lower hardness and strength across the board. Most of that quoted portion of the first post is pure bunk.