In-situ investigations of the physicochemical mechanisms of surface activation of stainless steels during heat treatment applying brazing-process-like conditions in reducing process gases

The deoxidation of work piece surfaces in a furnace brazing process using reducing process gases is the precondition for its wettability with braze metal and determines the success and the quality of the resulting brazed joints. While the necessary thermodynamic conditions e.g. for the reduction of native oxidized stainless steel surfaces with hydrogen or monosilane are known, the kinetics of such reactions is not investigated up to now on the atomic scale. However, the latter is essential for a general understanding of the process and is the precondition for further developments in brazing technology. In this context, the use of monosilane doped nitrogen as cost efficient and resource saving alternative to hydrogen, which is state of the art in furnace brazing, is of mayor scientific and technologic interest.
Scope of this project is the investigation of the physicochemical mechanism of surface deoxidation, when brazing stainless steels in a conveyor belt furnace using hydrogen and monosilane containing process gases. The experiments planned are expected to provide detailed information of the chemical reactions and surface conditions during brazing, which are essential for the advancement of fluxless brazing processes with regard to lower process temperatures, robust processes and demanding stainless steel specifications.
Starting point of the project are thermodynamic calculations of possible reactions, for which analytical transport models of oxide layer formation are specified and adjusted for the actual problem. These theoretical considerations are tested by in situ analysis of surface reactions - also time resolved - covering typical process conditions in a conveyor belt furnace, in order to get kinetic information about changes in the surface region of stainless steels with respect to crystal structure, atomic coordination (bond distances, coordination numbers), chemical bonding and atomic diffusion. For this purpose TR-XRD (Time Resolved X-ray Diffraction) and time resolved EXAFS/XANES (Extended X-ray Absorption Fine Structure/ X-ray Absorption Near Edge Structure) measurements using synchrotron radiation are performed. The simulation of realistic furnace conditions during this measurements are carried out in a high temperature cell for X-ray experiments, which is manufactured specially for the requirements to be simulated.
The mentioned X-ray measurements are performed at the DELTA synchrotron light source in Dortmund and at the Deutsche Elektronen-Synchrotron (DESY, Hamburg)  On the basis of the performed measurements and complementary brazing experiments in a conveyor belt furnace with ex-situ analysis of the heat treated specimen a physical model will be developed, which takes into account all physicochemical aspects of surface changes observed in the simulation of the brazing processes.