Hold-time fatigue crack growth of Allvac 718Plus

The hold-time fatigue behaviour of Allvac 718Plus was investigated both in the as-heat treated condition and after long-time exposure at temperature close to the maximum operating limit. At temperatures above 450°C the introduction of 90 s hold-time at maximum load significantly increased the fatigue crack growth rate. Both conditions showed purely cycle dependent behavior at 450°C and close to purely time dependent behavior at 700°C. At 600°C intermediate behavior was observed. The long-time exposure had little effect on the cyclic (0.5 Hz) crack growth rates, but the resistance to crack growth with 90 s hold-time decreased. No microstructural effects of long-time exposure (700°C/140 h + 675°C/460 h) could be observed by SEM, but there were indications that the hardness of the material increases somewhat after exposure. (AU) © 2010 by The Materials, Metals, & Materials Society. All rights reserved.