Figure 4 illustrates the maximum service temperatures for various manifold materials. A typical stress level at room temperature for a room to 800C cycle is 35 ksi. This test procedure permits determination of the flow stress during both the manifolds heating and cooling cycles, which can now be used in simulation studies and manifold design considerations. The amount of bulging in fatigue test specimens is an indication of the amount of yielding and creep. The higher the moly content, the less the amount of bulging.
Fatigue failures initiate at nodule sites and voids.
High temperature creep and yielding produces internal voids. Other failure origins can be areas at DI cell boundaries. These can transform to pearlite on thermal cycling above the critical temperature. Thus, high critical temperatures promoted by Si and Mo contents have a significant beneficial effect.