Massimo De Sanctis

Hydrogen Embrittlement of Automotive Advanced High-Strength Steels

Advanced high-strength steels (AHSS) have a better combination between strength and ductility than conventional HSS, and higher crash resistances are obtained in concomitance with weight reduction of car structural components. These steels have been developed in the last few decades, and their use is rapidly increasing. Notwithstanding, some of their important features have to be still understood and studied in order to completely characterize their service behavior. In particular, the high mechanical resistance of AHSS makes hydrogen-related problems a great concern for this steel grade. This article investigates the hydrogen embrittlement (HE) of four AHSS steels. The behavior of one transformation induced plasticity (TRIP), two martensitic with different strength levels, and one hot-stamping steels has been studied using slow strain rate tensile (SSRT) tests on electrochemically hydrogenated notched samples. The embrittlement susceptibility of these AHSS steels has been correlated mainly to their strength level and to their microstructural features. Finally, the hydrogen critical concentrations for HE, established by SSRT tests, have been compared to hydrogen contents absorbed during the painting process of a body in white (BIW) structure, experimentally determined during a real cycle in an industrial plant.

Microstructural Features Affecting Tempering Behavior of 16Cr-5Ni Supermartensitic Steel

In industrial production processes, the respect of hardness and UTS maximum values of 16Cr5Ni steel is of utmost importance and a careful control of chemical composition and thermo-mechanical treatments is a common practice. Nevertheless, some scatter of properties is often observed with consequent rejection of final components. To better understand the role played by different factors, two heats of 16Cr-5Ni supermartensitic stainless steels with very close chemical compositions but different thermal behavior during tempering have been studied by means of TEM observations, X-ray diffraction measurements, dilatometry, and thermo-mechanical simulations. It has been found that Ms–Mf temperature range can extend below the room temperature and the relative amount of retained austenite in as-quenched conditions plays a significant role in determining the thermal behavior. When present, the γ-phase increases the amount of reversed austenite formed during tempering and accelerates the process kinetics of martensite recovery. Moreover, increasing amounts of retained austenite after quenching lower the critical temperature for austenite destabilization and influence the optimum temperature–time combination to be adopted for controlling final mechanical properties. In the studied cases, the very close chemical composition of the heats was not a sufficient condition to guarantee the same as-quenched structure in terms of retained austenite amount. This was proven to be related to solute segregation effects during solidification of original heats.

Microstructural Evolution during Tempering of 16Cr-5Ni Stainless Steel: Effects on Final Mechanical Properties

In this work, the structural behaviour during tempering of two different heats of 16Cr-5Ni supermartensitic stainless steel has been studied by means of dilatometry, transmission electron microscopy and X-ray diffraction. A thermomechanical simulator (Gleeble 3800) has been also used to characterize the effects on final mechanical properties of different tempering temperatures in the range 600 °C to 700 °C and the influence of sub-zero cooling on industrial double tempering treatments. It has been found that the pre-existence of retained austenite in as-quenched conditions can induce significant differences in the microstructural evolution during tempering and on the final mechanical properties of industrial components, thus inducing problems in controlling final maximum hardness allowable by normative requirements.

A Dental Prosthesis from the Early Modern Age in Tuscany (Italy)

BACKGROUND During archaeological excavation, carried out in the S. Francesco Monastery at Lucca (Tuscany, Italy), a golden dental appliance was discovered. The prosthesis was found, together with commingled human remains, in the collective tomb of the aristocratic family of the Guinigi, a powerful family who governed Lucca from 1392 until 1429. The exact archaeological dating of the prosthesis was not possible, but some elements suggest a dating to the beginning of the 17th century. PURPOSE Aim of the paper is to study and describe the dental appliance trough a multidisciplinary approach. MATERIALS AND METHODS Macroscopical and micro-CT examinations were performed to investigate the technics used for the realization of the dental prosthesis. SEM analysis was performed to study alloy composition of the metallic fixing lamina and microstructure of the deposits on the dental surface. RESULTS The dental prosthesis consists in five mandibular teeth: three central incisors and two lateral canines linked together by a golden band inserted into the dental roots to replace the anterior arch of the jaw. Micro-CT scan revealed the presence of two small golden pins inserted into each tooth crossing the root and fixing the teeth to the internal gold band. SEM examination of the lamina indicated a homogeneous composition, with average contents of 73 wt% gold, 15.6 wt% Ag, and 11.4 wt% Cu. Apposition of dental calculus on the teeth indicated that the prosthesis had been worn for a long period. CONCLUSIONS This dental prosthesis provides a unique finding of technologically advanced dentistry in this period. In fact, during the Early Modern Age, some authors described gold band technology for the replacement of missing teeth; nevertheless, no direct evidences of these devices have been brought to light up so far.