Materials
How hot and fast: amp and speed tips for 4130 chromoly
Quick answer
Similar to mild steel; control heat input to protect the HAZ — then set travel speed by watching the pool, not the clock: a consistent-width bead with proper wetting at the toes is the target on 4130 chromoly.
Starting numbers
Take the standard thickness-based amperage for steel and apply the material adjustment above. On 4130 chromoly, test coupons are not optional — set two or three parameter sets on scrap and pick the bead that wets in without undermining the edges.