Thermo-mechanical fatigue life assessment of hot forging die steel

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.