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Dies for hot forging operations are subjected to coupled mechanical and thermal cycles which deeply influence their thermo-mechanical fatigue life. Crack initiation and propagation on die surface are induced both by thermal gradients acting in the layer near the contact surface with the billet and by the superimposed stresses due to the mechanical cycles. At present, die life cannot be estimated either by experimental tools, or by simulation software. Therefore, a programme of research has been started in this field. A new laboratory test has been developed that is able to reproduce in specimens the thermo-mechanical conditions derived from industrial cases. A description of the test equipment and the relevant procedure is summarized in the first part of the work. Then, the paper focuses on the experimental investigation of a typical hot forging die steel. Design of Experiments (DoE) techniques were used in designing and analysing the experimental programme. A thermo-mechanical fatigue life assessment model, based on the experimental data and using response surface methodology (RSM), is proposed. Effects on life, due to variation of some forging parameters, are evaluated

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A B S T R A C T In hot forging operations, the die surfaces and the nearest surface layers of the die undergo mechanical and thermal cycles which significantly influence their service life. The real thermal and mechanical cycles have been previously investigated in forging plants by measurements and numerical simulation, and a reasonable variation window of process parameters has been determined. A new simulative test applied to AISI H11 hot working die steel has been used to generate failure data in conditions similar to those of the forging dies, but under a more controlled and economical method. Fracture surfaces of specimens for different tests observed by scanning electron microscopy (SEM) indicate that both thermo-mechanical fatigue (TMF) and creep phenomena can be considered to be main damage mechanisms and their contribution to the failure differs as testing conditions vary. As a result of the experiments, the failure is affected by both thermo-mechanical cycle and resting time at high temperature. Therefore, the authors developed a new lifetime prediction model obtained by combining the damage evolution laws, at each cycle, for pure creep and pure TMF. This combination was based on the linear accumulation rule. The damage evolution law for pure creep is obtained by modifying Rabotnov's law in order to suit the case of actual hot forging cycles, where temperature and stress vary widely. The damage evolution law for pure TMF is based on a generalization of the Wöhler-Miner law. This law is modified in order to take into account the presence of thermal cycle and thermal gradient. Comparison between the experimental cycles to failure and the predicted ones was performed using tests excluded in the determination of the coefficients. The conclusion was that the accuracy of prediction appears to be quite good and that the linear accumulation and interaction of TMF and creep is confirmed.Keywords creep damage; creep and thermo-mechanical fatigue interaction; forging die; life prediction; thermo-mechanical fatigue damage. N O M E N C L A T U R EA 0 , r = creep damage coefficients for the material b = mean stress coefficient D C = creep damage at time t D F = fatigue damage k = coefficient relevant to the damage rate M 0 = coefficient that depends on the cumulative damage effects N C = predicted number of cycles to fracture in pure creep N F = predicted number of cycles to fracture in pure thermo-mechanical fatigue N RE = experimental number of cycles to failure N RP = predicted number of cycles to failure (creep + TMF) R = stress ratio

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