An evaluation of threshold values and fatigue crack growth (FCG) rates is a fundamental condition for an application of damage tolerance philosophy in numerous machinery components. This philosophy starts to be applied even in such problematic cases like aircraft engine blades. The paper contains results of an evaluation of FCG rates including threshold values in a heat resistant Inconel 718 / René 41 welds used in advanced manufacture processes of jet engine rotor blades. Measurement was performed at load asymmetry R = 0.05, temperature 700 oC, at low load frequency f = 0.3 Hz. Methodological approaches are mentioned. Results of FCG and particularly threshold values are discussed in connection with fractographical analyses. In addition, crack closure phenomenon and closure values are considered and discussed.