Hydrogen determination in 22MnB5 steel grade by use of carrier gas hot extraction technique

摘要

Carrier gas hot extraction (CGHE) technique was used for measurement of hydrogen in press-hardened (and as delivered) condition of 22MnB5 steel primarily coated with a thin layer of Al-Si. The CGHE technique was applied using different temperature programs: isothermal heating, temperature step, and linear heating in solid state. The effusing hydrogen was measured using thermal conductivity device (TCD) and high resolution mass spectroscopy (MS). Single isothermal heating at 400 and 900 degrees C allowed determining absolute value of effusing hydrogen. The linear heating, also known as thermal desorption analysis (TDA), revealed temperature dependent hydrogen effusion peaks. The deconvolution of the TDA spectra by peak fitting allowed the calculation of hydrogen desorption energies for each peak. The results showed good agreement between hydrogen concentrations measured with MS and TCD. In addition, the asreceived ferrite-perlite microstructure showed only hydrogen effusion above 400 degrees C. The subsequent press hardening process leads to hydrogen uptake in the microstructure. In general, the press-hardened 22MnB5 revealed a hydrogen concentration of 0.4 to 0.5 ppm. The biggest concentration was released at isothermal holding at 400 degrees C indicating reversibly trapped hydrogen. TDA results with different heating rates confirmed mostly diffusible and reversible trapped hydrogen due to calculated activation energies in the range from 4 to 20 kJ mol(-1); it was ascertained that nearly 90 % of the hydrogen left the specimens below 400 degrees C. Melt extraction (ME) was performed to measure the total hydrogen amount (including the diffusible and trapped hydrogen) and showed that above 900 degrees C up to 1 ppm hydrogen is trapped.