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Alexis Oudin (V.2)
Abstract
Thermo-mechanical fatigue of hot work tool steels
Alexis Oudin - 19 october 2001.
Thermal fatigue damage is a life limiting factor of hot metal forming tools. A Thermo-Mechanical Fatigue (TMF) facility was developed to assess such damage mechanisms on the 55NiCrMoV7 and X38CrMoV5 hot work tool steels. A full compressive out-of-phase TMF cycle with different minimal and maximal temperatures were examined.
The cyclic behaviour of both steels is characterised by two successive regimes of accommodation and softening. Accommodation is a gradual adaptation of the microstructure to TMF loading while softening corresponds to a cyclic decrease of the steel strength. The maximal temperature controls the cyclic behaviour. For the X38CrMoV5 steel, the temperature 550°C constitutes a boundary above which thermally activated phenomena control the cyclic behaviour. The 55NiCrMoV7 steel showed more rapid accommodation and global higher softening than the X38CrMoV5 steel.
Preferential oxidation and oxidation-fatigue cracking seem to be the dominant crack initiation mechanisms. A parabolic law describes the oxide scale thickness growth higher for the 55NiCrMoV7 steel. For the X38CrMoV5 steel, an oxide scale through thickness cracking law was proposed. The TMF crack growth rate was deduced from striations accounting. A strain intensity factor range and a Paris law type were used to correlate the crack growth rate. The oxidation, tensile going strain and cyclic plasticity seem to control the crack growth. For the oxidation and the crack growth laws, the minimal temperature effect is neglected.
The TMF lives are reduced with maximal temperature rise. A simple phenomenological TMF life prediction model was proposed using the Manson-Coffin relationship mechanical strain range based. A detailed partitioning of the fatigue life was proposed. For the X38CrMoV5 steel, the oxide scale through thickness cracking and the crack growth laws were used to estimate some fractions of the TMF life. The X38CrMoV5 steel presents a global better TMF strength than the 55NiCrMoV7 steel.
Keywords:
thermo-mechanical fatigue, high temperature low cycle fatigue, thermal
fatigue, martensitic steels, hot work tool steels, cyclic softening,
oxidation, fatigue crack initiation, fatigue crack growth, fatigue life
prediction.
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