Thermal fatigue is an imporant issue for hot-working tool steels and is usually the main reason for tool failures. High working temperatures reduce its mechanical properties. In such softened form it cannot withstand the strain caused by large temperature gradients and working loads, which leads to deformations, intense wear and cracking.
In this work we used a new test rig for testing of thermal fatigue resistance. The main focus was assessment of thermal fatigue resistance for three different prepared surfaces. The test enabled testing at equal testing conditions and thus providing comparable results. We tested nitrided and two DUPLEX treated surfaces, i.e. with two different PVD coatings (TiAlSiN and (Ti,Al)N). A total of 2000 cycles of heating to a temperature of 650 °C and air cooling were performed on a system Gleeble 1500 D apparatus. A failure has occurred at the beginning of water-cooled cycles after a few 10 cycles.
Cross-section revealed that DUPLEX treated surfaces had a higher concentration of short thermal cracks, while nitrided surfaces had fewer, longer cracks. Also, large portions of (Ti,Al)N had peeled off because of weak adhesion caused by interruption during coating application.
|
