yeah but i think they're just hi ten steel.
prepare to be educated.
4140 (what they made the pegs out of) alloy steel is allmost exactly the same as 4130 alloy steel (this is what bars are made out of). 4130 is easier for welding and fabrication work, and is more readily sold in tube/sheet. also it's very weldable compared to 4140 which is mainly used in thicker components because it is more readily heat treatable to higher levels and machines easier (which is why he made these pegs out of 4140). chromoly is simply a steel alloy, it has chromium and molybdenum along with the steel.
EDIT: in case i wasn't clear enough,
4140: what pegs are made out of.
reason for this: more machinable and heat treatable (heat treating makes metal stronger and harder)
4130: what bars are made out of.
reason for this: very weldable and readily available in many different sizes of tubing.
i'm comparing the two because 4130 is obviously chromoly. 4140 is just slightly different and sill has chromium and molybdenum added to the steel, making it CHROMOLY.
EDIT AGAIN: just in case you're still confused, 4130/4140 is a code of the American Iron & Steel Institute and says whatever the approximate chemical composition of the steel.
The "41" denotes a low alloy steel containing nominally 1 percent chromium and 0.2 percent molybdenum (thats where it gets the nickname "chromoly"). The "30"/"40" means a carbon content of 0.30/0.40 percent. normal alloy steels, there will be around 0.2-0.5 percent silicon, 0.5-1.0 percent manganese. The remaining 97-98 percent is iron
this IS chromoly. NOT just hi ten steel.