A~traet. The time-dependent deformation and cracking behavior of the quenched and tempered steel 15 NiCuMoNb 5 was investigated in the temperature range 20°C to 400°C. Fracture mechanics tests were carried out under various loading conditions including load-hold times of up to three weeks. The time-dependent crack growth can be described by the crack resistance (JR-) curve. Only above 300°C, the JR-curves obtained from constant-load tests start to deviate substantially from constant-displacement-rate tests. In this temperature range, the C,-parameter, which has been developed for creep crack growth testing, can be used to describe crack growth rates. For practical purposes, it is important to note that hold-time effects reduce the load-bearing capacity of cracked specimens by less than 10 percent for the present material and testing conditions.