Application of a thermogravimetric technique for the determination of low nitrogen solubilities in metals: Using iron as an example

A thermogravimetric technique was explored in order to precisely investigate the low nitrogen solubilities in metals. The thermogravimetry (TG) set-up is presented in detail. By the optimization of the set-up, a good baseline (noise level <2 μg, curve drift: ±1–4 μg/h) was obtained. The measurements of nitrogen solubility in iron are described. The weight signal arising from the nitrogen dissolved in iron was recorded continuously as a function of time under various experimental conditions. The nitrogen solubility can be calculated from the weight change between the beginning of nitrogen introduction and the steady state (equilibrium). There is a good agreement between the present measurements and the data in the literature determined by alternative methods such as chemical analyses of quenched samples, Sieverts’ method as well as thermogravimetric technique. In addition, a so-called isothermal stepping TG process shows that the TG set-up has a sufficient sensitivity to display the effect of the phase transformation of iron from body-centered cubic phase to face-centered cubic phase on the nitrogen solubility.