Welding Filler Materials and Pre / Post Heat

We are currently setting up a WPS for A36 to A668. Preheat and Postheat would be dictated by the material thickness. A668 is a forged form of AISI 4140 which would make it at the edge of "Low Alloy" steel regardless of class (F,K,etc.). Depending on the code/spec used, most 70XX-H4 (ie; E7018, E70S-6, etc.) fillers should work due to the A36 part of the weldment. Most of the failures we have seen are in the HAZ on the A668 part at or near the weld toe. Higher preheats would be recomended. You may want to ref. the Table of preheats in AWS D1.1.