Out of Phase Thermo Mechanical Fatigue (OP TMF) 100–950°C tests with 5 min hold time, some including intermediate ageing for 4000h at 950°C after 25 cycles, have been performed on IN738LC at Siemens Industrial Turbomachinery (SIT) AB. In the standard tests, the variation in stress range during the tests was neutral. When tests with intermediate ageing were added to the analysis, the combination of test data strongly suggest that the neutral response above was because of a fairly even competition between softening due to degradation (particle coarsening etc.) and work hardening. We have concluded that the work hardening occuring for a lab hold time of 5 min. will be insignificant under more realistic, i.e. component-near, hold times. Based on this, we have devised a method for reduction of lab TMF test results, with their too short hold times, to handle actual, i.e. significantly longer, component hold times.