Thermo-mechanical fatigue approach to predict tooling life in high temperature metal forming processes

In high temperature metal forming processes, strong mechanical and thermal cyclic loads coexist in the formed material and in the tooling. Such a phenomenon has a negative influence in the tool life cycle. The thermal and mechanical ranges lead to the appearance of cracks on the tool surface because of thermo-mechanical fatigue damage (TMF). The objective of this paper is to present a simplified cumulative thermo-mechanical fatigue model which has been developed in order to study the most critical areas in the forming tools according to the predicted number of cycles up to crack appearance considering multi-axial fatigue and oxidation. This model is based on assumptions of a general cumulative TMF model proposed by Neu and Sehitoglu; and critical plane approaches by Smith, Watson and Topper, and Fatemi and Socie. In general, cumulative TMF approaches three agents are considered: multiaxial-fatigue, oxidation and creep. The developed model has been applied to a specific manufacturing process of seamless steel pipes which includes three basic forming steps: backward extrusion, perforation and Pilger rolling mill. This process transforms a blank, or ingot, into a final seamless steel pipe with specific dimensions and characteristics depending on market requirements. These studies have been carried out in order to improve the tool life. To this end, the forming process has been reproduced by means of coupled thermo-mechanical FE simulations using the ABAQUS/Explicit solver. Moreover, a post-processing subroutine has been incorporated which enables to predict the number of cycles until crack appearance (tool life) according to both the general model of Neu and Sehitoglu, and the specific model developed. Good correlations between predicted and actually observed damage areas have been reached in both cases. The advantage of the hereafter proposed TMF model is that, it takes the most convenient features of general TMF models and it is particularized to a reduced input TMF model in order to analyze high temperature metal forming processes.