← Glossary

Invariant reaction

A thermodynamic state in a phase diagram where three phases coexist in equilibrium with zero degrees of freedom. Temperature and phase compositions are strictly fixed at this point.

An invariant reaction means that melt, solid phase, and gas are in equilibrium simultaneously. There are no degrees of freedom – temperature and composition are fixed until one phase disappears. In welding, you notice this when you hit an alloy that lies on such a point. Then you have to control the heat input carefully, otherwise the weld pool will tip over and you won't get a proper structure. Keep the melt calm and make sure you don't sloppily go past that point – then it'll be fine.

MetallurgieSchweißtechnik

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