Renzo Valentini

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.

The maraging-steel blades of the Virgo super attenuator

The blades are crucial components of the Virgo super attenuators. The material used for their construction is maraging steel, a low-carbon-content alloy with high ultimate tensile strength and low creep under stress. Youngs modulus, the shear modulus, the Poisson ratio and the corresponding elastic energy-loss coefficients have been measured. The measurements have been performed on specimens subjected to the same thermal treatments as those of elements for the Virgo interferometer realized with maraging steel. In addition, anelastic properties of the material subjected to different thermal treatments have been measured. It has been found that, for a maraging-steel structure (one free of plastic deformation), which undergoes an excitation with flexural vibrations, the elastic energy-loss coefficient can vary over a wide range as a function of the thermal treatment of the material and it is dominated by the thermo-elastic effect. The main reason for such a great alteration is supposed to be the dependence of the thermal conductivity on the average sizes of the precipitate particles and their relative separations.

Experimental studies on hydrogen diffusion and trapping in martensitic and austenitic stainless steels for fusion application

Abstract In the development of materials for the first wall and blanket, the presence of hydrogen, its permeation and the possible degradation of mechanical properties represent a key issue. This article presents experimental methods and results concerning measurement of the diffusion coefficient, of the critical concentration for hydrogen-induced cracking and determination of the amount and binding energy of traps for several steels of interest for nuclear fusion, among them Manet. The diffusion coefficient measurements were conducted electrochemically or by high temperature thermal analysis and data were extended, on the basis of the traps theory, to a temperature range of considerable importance for nuclear fusion applications. With respect to the materials studied, analytical correlations are provided which describe the diffusion and trapping of hydrogen. The tests on the critical concentration of hydrogen are also presented and discussed. In the case of Manet these tests highlighted strong dependence of this parameter on the microstructure.

A way to reduce environmental impact of ladle furnace slag

Abstract In the last decades the European steel industry has made continuous efforts to reduce residues and byproducts and to increase recycling in order to reduce its environmental impact. While some steelmaking slags have been widely characterised and, to a certain extent reused, ladle furnace (LF) slag is used in different applications because of its specific properties. The main purpose of the case study presented in this paper concerns the reduction of potential LF slag environmental impacts, because of its intrinsic physicochemical properties. During the handling and cooling of LF slag, it disintegrates into a powder due to instability of the dicalcium silicate, causing an increase in dust emissions to the environment. The approach presented in this paper aims to reduce this phenomenon in order to achieve a more sustainable solution in term of reduction of powder dispersion in the environment, of costs saving and of nuisance reduction in the surroundings areas.

Effect of hydrogen on the ductility reduction of F82H martensitic steel after different heat treatments

The influence of heat treatment on hydrogen embrittlement (HE) susceptibility of F82H martensitic steel, a candidate material for the first wall and blanket of the DEMO reactor, was investigated by means of low strain rate tests conducted at room temperature on notched cylindrical specimens, pre-charged with hydrogen. Three types of thermal treatment were compared, which produced quite different material hardness and strengths. As a general rule, F82H steel exhibited a strong susceptibility to HE in all conditions, with transitions between ductile and brittle behaviour ranging from 0.5 to 1.0 wppm H. The comparison of results produced by the different heat treatments confirmed for F82H also the tendency toward an increase of the susceptibility with material hardness. Moreover, the examined steel appeared to be more sensible to HE than another martensitic steel (MANET II), which was also proposed as a candidate material for fusion reactor first wall application.

Effect of hydrogen on tensile properties of martenistic steels for fusion application

Abstract The present work is aimed at giving a contribution to the characterisation of hydrogen embrittlement (HE) resistance of two martensitic steels, i.e. the modified F 82 H and the MANET. The study is based on tensile low strain rate tests, conducted on notched cylindrical specimens which were previously charged with hydrogen. In the case of modified F 82 H steel, two different heat treatments were considered, i.e. the as-received (AR) and the aged condition. The results of the tests indicated that the analysed steels show a noticeable reduction of the area reduction coefficient (Z%), even for rather low global hydrogen content. As an example, 1–2 wppm are sufficient to lower the Z% of mod. F 82 H to 14–18%, from the 35–40% value typical of the virgin material. In the paper, these results are discussed with the aid of microstructural investigations and SEM analysis which allowed to characterise the microstructural properties of the materials.

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.

Reversible and irreversible hydrogen trapping in metals: new computer-based code THYDA

Abstract Use of metallic materials for fusion reactor components and their performance in the presence of hydrogen isotopes require a development of a physical transport model that permits to explain the dynamics of hydrogen absorption and release. Trapping and release processes will strongly affect hydrogen isotopes inventories. The non-linear nature of hydrogen diffusion equations requires a numerical analysis. In this work a new computer code named THYDA was developed. In relation to thermal cycles, THYDA permits to calculate the concentrations of trapped and movable hydrogen atoms in metallic samples as well as hydrogen evolution rates during thermal desorption at elevated temperature with variable heating rates. In these calculations it is possible to take into account both the internal and external sources of hydrogen isotopes (i.e. generation in (n, p) nuclear reactions and by D-T ion implantation).

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.

Model of Hydrogen Behaviour in enamelling grade steels: Part II – Application

A check has been made of a new model of hydrogen behaviour in enamelling-grade steels in relation to the fishscale surface defect. The quantityCL of free hydrogen which remains in the enamelled product is determined as a function of its solubility and diffusivity in the steel. The seriousness of the defect or the resistance to the defect, resulting from various metallurgical treatment conditions, can be correlated with the hydrogen parameters. It is shown that the parameterCL can be adopted in quality control as a new criterion for fishscaling prediction and in planning thermomechanical cycles for enamelling-grade steels in order to prevent the defect.