Generation of a Multi-Layered Diffusion Coating on an Interstitial-Free Steel

Abstract

Boronizing and titanium diffusion were applied to an interstitial-free steel substrate separately and one after another. Boronizing was carried out in a liquid environment by applying an electric current, but titanium diffusion was performed in a closed chamber containing solid chemicals. The generated single-layered and multi-layered diffusion coatings were characterised by utilising an optical microscope, a scanning electron microscope equipped with an energy dispersive spectrometer, and a Vickers microhardness tester. Phase analysis was carried out by an x rays diffractometer. Some chemical reactions were supposed to occur regarding titanium diffusion process and the standard formation enthalpies of these reactions were calculated by using a database. Microstructural investigations revealed that titanium diffusion was able to be applied after boronizing, causing the formation of a titanium based diffusion layer over the previously generated boride layer. On the other hand, when a titanium diffused substrate was boronized, the titanium based diffusion layer acted as a diffusion barrier to the introduced boron atoms, preventing the formation of a boride layer. However, if boronizing duration and the current density applied during boronizing were increased to a certain degree, boron atoms could surpass this barrier in small quantities and form small discontinuous regions. It was observed that the hardness of a phase which was formed within the titanium based diffusion layer was more than 4000 HV in some regions when titanium diffusion was applied after boronizing. The standard formation enthalpies of the six assumed chemical reactions were negative, indicating that these can occur at 1000 °C at which titanium diffusion was applied. Moreover, most of the products of these reactions were detected in x rays diffraction analyses.