Nickel-Iron Thermal Expansion Coefficient

Invar alloys expand only minimally when heated – that's why they're used for precision parts. When welding, though, it's a double-edged sword: because the material barely moves, stress builds up quickly. If you cool it down too sharply, the structure will crack. So let the part cool slowly after welding, ideally in air and not on cold metal or in a draft. And remember: the heat input itself should be even, then you'll have less trouble with distortion and cracks.