Answer
Relaxation refers to the loss of load over time when a spring is held at a constant height and high temperature. This is a form of creep where the elastic strain is converted into plastic strain. For wave springs in turbines, this is critical. If using a standard stainless steel like 302, relaxation begins to occur significantly above $400^{\circ}F$. For temperatures up to $1200^{\circ}F$, Inconel X-750 or Waspaloy is required. The rate of relaxation is governed by the Arrhenius equation, where the loss of load $L_{loss} = A \cdot e^{-\frac{Q}{RT}}$. If a wave spring relaxes, the preload on the turbine seals is lost, resulting in leakage. Designers must over-spec the initial load or use materials with higher creep-rupture strength to compensate for the predicted relaxation over the maintenance interval.
