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Thermal fatigue of the martensitic hot work tool steel AISI H13 is complicated by structural degradation which in turn is influenced by plastic deformation. These properties are dependent upon plastic deformation, temperature, and time, and thus it is of vital importance to find a description which allows inter- or extrapolation to the thermal load experienced in various practical applications.
Isothermal, low cycle fatigue testing was performed mainly at 500 and 600°C and also to a limited amount at 80, 300, and 750°C. The material was tempered to three different hardness levels (43, 47, and 51 HRC) prior to testing. The main bulk of tests were performed on the 47 HRC variant. At 600°C no difference in life was observed between the different hardness variants. The cyclic stress-strain curves, evaluated at half lives, were also almost identical. Softening is believed to cause this similarity, while the initial differences will only influence the behavior during a small fraction of the life after which all material differences will have vanished. Softening is due to particle coarsening and a decrease of the initially very high dislocation density. By assuming that Ostwald ripening takes place through volume diffusion, that the Orowan mechanism for particle strengthening is valid, and that the dislocation network spacing is directly proportional to the interparticle distance, a fair functional description of the experimental data is obtained. The temperature dependence of the solubility limit for the carbides and the activation energy for self-diffusion are nicely reflected by the data. The model predicts that a low solid solubility and a high volume fraction of the carbides will give rise to a low rate of softening as will a high strain-rate and of course a low temperature. Plastic deformation is found to increase particle coarsening significantly, but the extent is independent of the magnitude of the plastic strain within the limits investigated.
Thermomechanical fatigue testing (simultaneous cycling of both strain and temperature, half a period out of phase with one another) was performed with a maximum temperature of 600°C in the cycle. The minimum temperature, T-min, was varied from 80 to 400°C. In comparison to the isothermal 600°C data, life was reduced when T-min was less than or equal to 200°C, but remained fairly unaffected when a T-min of 400°C was used. As expected, softening increased with increasing minimum temperature.
The same hardness variants as described above were also tested with the 200 to 600°C cycle. No significant differences in life were obtained, although the cyclic stress-strain curves (maximum tensile stress at half life) differed. The Ostergren method for life prediction fit our data reasonably well if the plastic work is normalized with the cube of the shear-modulus.
low cycle fatigue, thermal fatigue, stress relaxation, softening, tool steel
Svenskt Stål, Domnarvet, Borlänge,
Swedish Institute for Metals Research, Stockholm,