Superinvar

Superinvar has very low thermal expansion, which stays that way after welding if you work cleanly. The problem is that the heat input during welding can change the structure, so this great property quickly goes down the drain. You have to keep the heat low and let the weld cool down well after welding, ideally in a controlled way. If you put too much energy in or weld too slowly, you'll get cracks or the dimensional accuracy won't be right afterwards.