Localized heat treatment to improve the formability of steel pipes for hydraulic applications: process design and mechanical characterization

In the present paper, authors have demonstrated how a localized induction heat treatment can be advantageously applied, controlled, and mechanically characterized on a specific part-i.e., on steel hose fittings for hydraulic applications. More specifically, the study shows how this specific type of heat treatment facilitates significant localization effects on mechanical properties, and how such a treatment could act as a powerful tool for material optimization in diverse applications. The instrumented micro-indentation test was adopted as the investigation method for mechanical characterization and, due to the reduced amount of material required for the test, has the double advantage of retrieving potential spatial gradients of the mechanical properties without causing permanent damage to the analyzed parts. A measurement of both Vickers hardness and plastic work are required in order to make the indentation necessary to quantify the strength and ductility capability of the parts' materials. In addition, a customized tensile test, based on a strains measurement obtained through an optical full-field method-i.e., digital image correlation (DIC)-was developed with the aim of identifying and quantifying the correlation between the material properties attainable through a conventional tensile test and those measured by the instrumented micro-indentation test. Finally, it was demonstrated that the proposed customized tensile test, due to the localized heat treatment, is capable of retrieving potential spatial gradients of the material properties.