Thermal desorption spectroscopy study of the hydrogen trapping ability of W based precipitates in a Q&T matrix

The hydrogen trapping efficiency of different types of W based precipitates is considered in three generic FeCW alloys with increasing carbon content and a stoichiometric amount of W. A martensitic microstructure is prepared and two conditions are compared; i.e. an as-quenched (as-Q) state and a quenched and tempered (Q&T) state in which W based carbides are formed. The tempering time and temperature are modified to change the carbide characteristics. Hence, the hydrogen trapping characteristics are evaluated for the different carbides. Melt extraction is done to determine the hydrogen content, whereas thermal desorption spectroscopy (TDS) is performed to verify whether the tempered induced carbides are able to trap hydrogen efficiently. The trapping capacity is found not only to be size dependent, but also reliant on the morphology and type of the carbides, as investigated by transmission electron microscopy. TDS revealed that the tempered induced W2C in the Q&T at 600 °C for 1 h condition do not trap hydrogen due to their rather large size (>20 nm) and hence incoherent nature. Moreover, the hydrogen content is decreased compared to the as-Q condition due to the reduced dislocation density. The tempering time is reduced to 10 min to evaluate the size effect on the carbide trapping ability. As such, smaller W2C (<20 nm) are induced, which trapped a significant amount of hydrogen, as demonstrated by TDS, with a corresponding activation energy of 42-45 kJ/mol. This compensated the reduction in dislocation density, causing a higher hydrogen level. Furthermore, tempering at 700 °C for 1 h results in an increase of hydrogen compared to the Q&T at 600 °C for 1 h condition. This is confirmed by TDS and TEM to be linked to the trapping ability of W23C6, showing a slightly higher activation energy ranging from 47 to 49 kJ/mol.