Borides in austenitic experimental alloy and its influence on the properties : Development of TTT-diagram and evaluation of borides influence on mechanical and corrosive properties

Sammanfattning

The boride formation and its influence on mechanical and corrosive properties was investigated in a nickel rich austenitic experimental alloy with 19 ppm boron. Samples retrieved from the tube walls were heat treated isothermally to develop a time-temperature-transition (TTT)-diagram. Samples were analysed using scanning electron microscope (SEM) and energy dispersive electron spectroscopy (EDS). The process window established for boride precipitation was 850–1090 °C in the investigated time span 1–8 minutes. The average fraction and size of borides in the temperature range 900–1050 °C and for durations of 1–15 minutes were quantified. The fraction of borides was generally seen to increase with ageing-time and temperature within the quantified region. The size of the borides was suggested to increase with ageing-time and temperature, while the number of borides observed were decreased. Solution annealing boride containing material at 1200 °C for 3 minutes suggests a dissolution of the borides. Additionally, tests were performed in continuous cooling condition. It was observed that the borides can nucleate during cooling in the interval 0.5–1.75 °C/s, with increasing amounts towards slower cooling rates. Using electron probe microanalysis (EPMA) it was established that the borides likely were of the type M2B, rich in chromium. It was also established that heat treating material at 700 °C for 5 hours yielded material containing borides without any sigma phase. Mechanical and corrosive tests were performed on the boride induced material and material in extruded condition. The mechanical properties were evaluated using hardness, tensile and impact toughness tests. The corrosive properties were assessed through ASTM G150 pitting corrosion testing. No significant difference was observed in mechanical properties. A slight increase in pitting corrosion resistance was observed. The increase could be related to boron segregating to lattice imperfections, lowering the strain energy and reducing the number of sites susceptible to pitting corrosion.