Papers by Keyword: AISI H11

Abstract: Tools are exposed to severe working conditions during the hot extrusion process. In particular, dies and mandrels can be subjected to an excessive amount of deformation as a result of the developed high cyclic loads and temperatures. In this scenario, a physical experiment reproducing the thermo-mechanical conditions of a mandrel in a porthole die was performed with the Gleeble machine on the AISI H11 tool steel with the aim to investigate the mechanisms that influence the die deformation. The design of experiment consisted of 4 levels of temperature, 3 levels of stress and 3 types of load, i.e. pure creep, pure fatigue and creep-fatigue. In all the testing conditions, a comparable pattern of the mandrel displacement-time curve was found reproducing the 3 stages of softening typical of the strain evolution in a standard creep test but with a marked primary phase. Thus, with the aim to identify an easy-applicable equation to estimate the die deformation, the time hardening creep law was chosen. Coefficients of the time-hardening law were optimized, for each testing condition, on the basis of experimental data starting from values for similar alloys taken from the literature. Results in terms of mandrel displacement were then compared to experimental data for the creep-fatigue condition at different stress and temperature levels. The values found were validated against additional experimental data performed with different specimen geometries. A good average agreement was found between experimental and numerical results. The developed procedure was then applied to an industrial die.

Abstract: Lifetime of hot-work tool steels is often limited by the development of crack nets as a consequence of thermal or mechanical fatigue loading. In this paper, the isothermal and thermal fatigue behaviour of tool steel AISI H11 (German grade X38CrMoV 5-1) in the temperature range between room temperature and 650°C is investigated. In addition to stress controlled isothermal tension-compression fatigue tests, out-of-phase thermal fatigue tests were carried out using a servohydraulic testing system, keeping the macroscopic total strain of the gauge length constant, while triangular time-temperature cycles were applied. The temperature range was chosen in such a way that numbers of cycles to fracture between 1,000 and 10,000 were achieved. The development of stress amplitudes, plastic strain amplitudes as well as of mean stresses was analysed during the tests. Results of isothermal and thermal fatigue tests are compared and discussed.