Pascal Lamesle

Isothermal Oxidation Behaviour of a Hot-Work Tool Steel

Isothermal oxidation behaviour of a hot-work tool steel (X38CrMoV5) was investigated at 600degreesC and 700degreesC in dry and wet air. Growth kinetics were determined by using TGA and oxide scales were characterised by means of SEM (EDS, X-ray mapping) and XRD examinations. Moreover, as the microstructural properties of the studied hot-work steel strongly depend on the carbides precipitates formed during its heat treatment, these carbides were extracted from the X38CrMoV5 matrix and their oxidation behaviour in dry and wet air was also studied. Oxidation behaviour of X38CrMoV5 is very sensitive to the presence of water vapour : a large increase of the scale growth kinetics was observed as soon as the water vapour partial pressure exceeds a value of 9 mbar. Microstructural characterisations showed that scales grown in wet air are porous and sometimes cracked and deformed. They are composed of an external iron-rich oxides scale (hematite alpha-Fe2O3), an internal oxide scale enriched in Cr (spinel oxides (Fe,Cr)(3)O-4) and a narrow zone of internal oxidation. Whereas no significant influence of the water vapour partial pressure has been observed on the oxide scale microstructure (composition, morphology), texture of the superficial hematite scales becomes more pronounced when p(H2O) increases from 9 to 310 mbar. Preferential orientation of alpha-Fe2O3 scales is also favoured by increasing thickness of oxides. On the other hand, the oxidation behaviour of carbide precipitates is rather complex and strongly affected by the presence of water vapour in air.

Oxidation and Corrosion Effects on Thermal Fatigue Behaviour of Hot Work Tool Steel X38CrMoV5 (AISI H11)

Effects of atmosphere and specimen geometry on thermal fatigue (TF) crack initiation and propagation in a low Si content hot work tool steel X38CrMoV5-47HRC were investigated. The TF specimen’s geometry enhances the uni-axial TF loading conditions. A high frequency induction heating (3 to 4 MHz) is used. A new TF rig, working under air and/or inert atmosphere with reduced PO2 has been set up. The reduction of PO2 results in localized oxidation sites. Whatever geometry and atmosphere conditions, TF cracks initiate exclusively in the oxide layers. Damage mechanisms are environment dependant. Under laboratory air, parallel macroscopic cracks initiate perpendicular to the hoop stress. Under argon and nitrogen, SEM surface observations show that initiated cracks coalesce by zigzagging along crystallographic paths between non-oxidized zones. In-depth crack propagation mechanism is mainly trans-granular. TF crack initiation life under air and in presence of Fe-Al intermetalllics is decreased in comparison to inert atmosphere.

Experimental conditions and environment effects on thermal fatigue damage accumulation and life of die-casting steel X38CrMoV5 (AISI H11)

Thermal Fatigue (TF) is a life-limiting factor in die-casting dies. The effect of the maximum temperature of thermal cycle, Tmax, and the heating (or heat flux density) of thermal fatigue (X38CrMoV5 AISI H11) at 47 HRC is investigated. Two microscopic and macroscopic heat-checking cells are identified. The microscopic heat checking is limited to the double-layer oxide scales formed on the external surface of TF specimens. An engineering Paris-type law using Kmax, can describe TF surface crack propagation of all tests examined. TF life based on σmax is used to rationalise TF micro- and macroscopic cracking.