Pascale Bruckel (V.2)

Abstract

Oxidation behaviour of the X38CrMoV5 tools steel at 600-700°C and in presence of water vapour

Pascale Bruckel - 3 december 2003.

Tools steels for hot forming processes such as the X38CrMoV5 are usually subject to severe thermo-mechanical cycles. Because of the presence of lubricants, water vapour …, the surrounding atmosphere might be chemically aggressive. Several sources of damages (thermal, mechanical and chemical) can thus be involved in the rupture of hot forming tools during their service.

The aim of this study was to get a better understanding of the effect of the water vapour presence in the oxidising air on the behaviour at 600-700°C of X38CrMoV5. Thermogravimetry (TGA), X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) as well as scanning electron microscopy (SEM) have been used to characterise the oxidation products after cooling down of the samples.

The oxidation resistance of X38CrMoV5 depends on the oxidation temperature and the exposure time as well as on the water vapour partial pressure. Finally, the oxidation kinetics faster than in dry air, the oxides scales microstructure, the oxidised surfaces morphology, and also the texture of the outer hematite layer are affected by the presence of water vapour in the oxidising air.

A transient period can be seen on some oxidation curves during the first 15-20 hours exposure in wet air. The duration of the transient regime decreases when the water vapour partial pressure is increased. A transition to faster kinetics is then observed being related to the growth of duplex oxides scales : their development consists first in the formation of oxides nodules whose further growth (laterally and through thickness) allows the formation of a continuous outer oxides layer.

Observations of in situ oxidation have also been performed in an environmental SEM in order to monitor the oxides scales growth on the steel surface exposed to dry or wet air. Based on the morphological evolutions of the surfaces oxidised in the presence of water vapour, a phenomenological model has permitted to relate the TGA curves irregularities to a multi-layered- growth process of the oxides scales. This process might be the reason of the oxides scales porosity.

Usual models for oxides scales growth do not explain the behaviour of the X38CrMoV5 tools steel satisfactorily. Further propositions for modelling should take into account both the presence of pores and the multi-layer formation during the oxides scales growth.

Keywords:
oxidation, dry air, wet air, water vapour, kinetics, morphology, iron oxides, tools steels, in situ growth, multi-layered-growth.

Last modified: 11/23/2005 03:37 AM