Hastelloy Weld Metal Dilution
The degree of mixing between the nickel-base filler material and the unalloyed or low-alloy steel substrate. Controlled dilution is necessary to prevent cracking in the transition zone.
When welding Hastelloy onto structural steel, you inevitably mix the nickel-based weld metal with the base material. You have to control this dilution, otherwise you'll get hard, brittle microstructures in the transition zone that are prone to cracking. The part won't hold up as promised. You mainly control this through heat input and welding technique. Too much dilution is bad, but too little can also be problematic because the bond won't carry cleanly. In practice, this means: you aim for a controlled penetration and make sure the weld bead is clean and uniform. If you're unsure, do a test grind and check the microstructure before welding the whole component.