A new experimental method for identifying the conditions necessary for diffusion bonding in free cutting steels

Porosity in cast bloom steel may lead to reduced strength or inconsistency of final rolled bar products and components if not properly closed and healed during the hot rolling process. Partial or complete recovery of strength in such porous materials can be achieved by diffusive healing processes at elevated temperatures. Devising an appropriate healing process that does not cause discontinuity in the microstructure, and in the mechanical properties at the bonding sites, whilst preventing distortion of the component during bonding requires an accurate choice of thermo-mechanical processing parameters. Despite work carried out on optimising diffusion bonding in materials such as titanium alloys, aluminium alloys and copper, the diffusion bonding process in free cutting steels has received relatively little attention. To support thermomechanical process optimisation (e.g. rolling) and calibration of theoretical models, a new experimental method was developed to determine combinations of load, temperature and time sufficient for complete diffusion bonding in as-cast Free Cutting Steels (FCS). The extent of diffusive healing and bond strength were examined by tensile testing to failure corroborated by SEM examination of the bond line. This enabled optimal loading conditions for the formation of a complete, strong bond to be identified.