Assessment on the creep-fatigue performances of tool steels

Extrusion dies are typically produced by hot work steels with a great diffusion of AISI H-11 (1.2343) and AISI H-13 (1.2344) steels. Nevertheless different steels for hot working application are available on the market and several new steel grades are proposed every year for this type of application. A main concern for extruder and die maker that are interested into testing new steel grades, is how to evaluate the performances of the new material. If the material is directly applied for manufacturing a die, even if the die is an identical duplication of a previously tested, no reliable data are usually obtained as consequence of process variance. On the other side, standard testing (like tensile testing), even when performed at several high temperatures, offer only a partial view of the mechanical behavior of an extrusion die. Indeed dies are not affected by static loading being fatigue and creep the most damaging mechanisms of an extrusion die properly designed. In the present paper an innovative testing method, (previously proposed by the authors in [1] for investigating separately fatigue, creep and creep-ftigue interaction), was carried out on a TQ1 (X36CrMoV5-2) hot-work tool steel in cooperation with Constellium R&D center. A creep-fatigue loading type was applied to the specimens, i.e. a cyclic load with a dwell-time, in order to properly reproduce the conditions acting on extrusion dies. The specimens were tested at four different loading conditions (from 26 to 45 kN), wirth two different dwell times (160-300 s) and also with different surface conditions (tempered/tempered plus nitriding). Tests were performed on a 100kN MTS fatigue machine equipped with a heating furnace and the temperature was kept constant at 520°C. Final results were presented in terms of fatigue diagram curve of the TQ1 steel and compared to the performances of the H11 tool steel tested in a previous research by the same authors.