Understanding Hydrogen Redistribution During Steel Casting,and Its Effective Extraction by Thermally Induced Up-Hill Diffusion

摘要

The severe loss of ductility caused by hydrogen in high strength steels has been the object of intense research during decades.Because of the high mobility of hydrogen once has entered the metal,even in comparison with other interstitial atoms like carbon and nitrogen,a rigorous description of hydrogen fluxes during manufacturing of steel components is considered to be essential.A physical model of interstitial element diffusion is used to study the fluxes of hydrogen during solidification and cooling of cast alloys.In particular,the present model contemplates diffusion in its most comprehensive description,i.e.,atom diffusion is driven by a reduction of the Gibbs energy of the system.Usually,it is only considered the case in which diffusion occurs to reduce the concentration gradient of an element at constant matrix composition and temperature.However,in a more general view,and as was proven by Darken (1949),diffusion may occur even up-hill the composition gradient as long it still leads to a reduction of Gibbs energy in the system.When diffusion occurs due to a temperature gradient,it has been referred to as thermal diffusion or Ludwig-Soret effect.The model presented incorporates physical description of thermal agitation and atom mobility of interstitial elements,the influence of temperature gradients,solubility and saturation of the interstitial elements as function of temperature and matrix phases,as well as the kinetics of degassing at high temperature.The application of this model shows how hydrogen localizes in some regions of the component to a degree that depends on manufacturing conditions and where it may reach concentrations well beyond the initial average concentration in the metal.The model describes as well how the FCC to BCC phase transformation may drive hydrogen concentration beyond supersaturation,due to the disparate capability of these phases to dissolve hydrogen.On the other hand,this study has also enabled the development of a method for the reduction of hydrogen based on the imposition of severe but controlled temperature gradients to the component during cooling.