Gilles Dour

Image analysis of microscopic crack patterns applied to thermal fatigue heat-checking of high temperature tool steels

Surface cracking or heat-checking is investigated at a microscopic scale on a hot work tool steel (X38CrMoV5) tested under thermal fatigue. Thermal fatigue tests are periodically interrupted to observe the surface of the specimens by scanning electron microscopy (SEM). A non destructive and semi-automatic method is developed to assess and evaluate the two-dimensional crack pattern initiated on the oxide scale layer formed on the specimen surface. The crack pattern is characterized by image analysis in terms of density, morphological and topological features. This technique allows to determine the number of cycles to initiate the microscopic heat-checking and to follow its evolution.

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.