Nickel-Iron Invar Effect
Anomalous thermal expansion behavior of nickel-iron alloys with approximately 36 percent nickel within a defined temperature interval.
Invar is an iron-nickel steel with a high nickel content. The trick is that it hardly expands when heated. That makes it ideal for precision components where dimensional stability over temperature changes is crucial. Welding is tricky: the heat from welding can destroy the effect in the heat-affected zone. Then the material behaves normally again, and you get distortion or stresses. So you have to keep the heat input low and produce a clean, low-stress weld. Best to use a matching filler that matches the base material in composition. And remember: after welding, the weld is not necessarily Invar-true anymore. If utmost precision is required, you may need to post-machine or stress-relieve the part afterwards.