A. Solina

Influence of titanium and carbon contents on the hydrogen trapping of microalloyed steels

The presence of traps for hydrogen atoms influences the diffusivity and solubility of hydrogen itself in steels. In the present work, permeation measurements were carried out on hot-rolled microalloyed steels with different C and Ti contents in order to evaluate the number of irreversible and reversible traps. The number per unit volume of irreversible traps was correlated to calculated volume fraction of Ti(C,N) precipitates. These results, combined with microstructural investigations by transmission electron microscopy (TEM), showed that the largest number of irreversible traps was associated with steels having the largest volume fraction of fine and coherent Ti(C,N) precipitates. The reversible traps were associated with free Ti atoms, dislocations, and ferritic grain boundaries. Theoretical calculations confirmed the hydrogen binding energy of Ti free atoms (−27.1 kJ/mol).

Air bake-out to reduce hydrogen outgassing from stainless steel

Hydrogen outgassing is the most significant factor limiting the attainment of outgassing rates below 10−12 mbar l s−1 cm−2 in stainless steel vacuum systems. This limit turns out to be crucial in very large vacuum systems, like the VIRGO vacuum tubes (2 tubes 1.2 m diam, 3000 m length). Heating the raw material at 400 °C in air was suggested as a money saving alternative to the classical vacuum heating at 950 °C. We report the results of hydrogen content analysis performed on stainless steel samples submitted to different treatments, and also the measurement performed on a prototype tube (1.2-m-diam, 48-m-long). We concluded that air bake-out drives out most of the hydrogen absorbed in the bulk stainless steel, while the presence of an oxide layer does not reduce the hydrogen outgassing.

Origin and development of residual stresses induced by laser surface-hardening treatments

The variations of residual stresses and microstructures obtained by surface hardening with a 15-kw, continuous-wave CO2 laser on samples of AISI 1045 (C43) and AISI 4140 (40CD4) have been studied.The ranges for laser operating parameters were 10 to 38 W/mm2 s for the power and 0.024 to 1.12 seconds for the beammetal interaction time. From residual stress measurements, it is found that the stresses in the more superficial layers may vary, with changing laser operating conditions, from compressive stress values (>200 MPa) to strong tensile stress values (up to approximately +800 MPa). The maximum temperatures reached during laser treatment, the depth of heating, and the boundary conditions imposed influence the residual stress state. It seems possible to limit the onset of tensile stress states at the surface or even to obtain a compressive stress state by heating the material for relatively long periods of time (0.5 to 1.5 seconds) and by avoiding high surface temperatures. These operating procedures, however, do not provide a particularly finegrained martensitic structure. It has, therefore, not proved possible to optimize treatment from both a microstructural and residual stress point of view simultaneously by means of single laser treatment.Some preliminary tests involving suitably programmed repeated treatments of samples seem to indicate that improved combinations of the depth of hardening, the characteristics of the martensite, and the state of the residual stress can be obtained. To account for the experimental results, hypotheses have been advanced to explain the stress evolution during laser heating cycles.

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.

Residual stresses induced by localized laser hardening treatments on steels and cast iron

Residual stresses originating from laser hardening of localized areas on disk-shaped samples of steels AISI 1045 (C45), AISI 4140 (40CD4) and nodular iron (UNI G90-55-5) were studied.When the measured residual stresses are compared with the stress results obtained in a previous work, in which the whole surface of analogous samples was treated, the influence of the constraint exerted by the material surrounding the treated area became evident: the residual stresses values measured in the present work are significantly higher for the same operative parameters.The role played by the maximum temperature reached on the surface during heating was made evident: the resulting surface stresses tend to approach higher tensile values with increasing surface temperatures. From the data obtained it is therefore evident that the control of this variable is essential to avoid the development of unfavourable surface stress states. Furthermore, in the case of cast iron, the maximun temperature reached influences the retained austenite content which, at high levels, modifies significantly the resulting stress state.Repeated thermal treatments were also carried out. By this means a compromise is achieved between the characteristics of the martensite obtained and the resulting surface stresses.

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).

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.

Elastic and anelastic properties of Marval 18 steel

Marval 18 is a precipitation hardened steel with particularly high hardness and low creep, presently used for constructing parts of the interferometers for the detection of gravitational waves in the experiments VIRGO and LIGO. The elastic moduli have been measured in samples subjected to the same treatments as the parts of the interferometer VIRGO. In addition, the anelastic spectra of samples subjected to different thermal treatments have been measured between 50 and 350 K. It is found that, in the absence of plastic deformation, the elastic energy loss coefficient under flexural vibrations around 1 kHz can vary by more than one order of magnitude depending on the thermal treatments, and is dominated by the thermoelastic effect. The main reason for such strong variations is supposed to be the dependence of the thermal conductivity on the average sizes and distances between the precipitate particles.

Model of hydrogen behaviour in enamelling grade steels

A model has been developed of the behaviour of hydrogen in enamelling-grade steels in relation to the delayed defect of blow-off of enamelled surface (fishscaling). The model is based on current theories concerning reversible and irreversible trapping of hydrogen in metallic materials. It leads to the establishment of a “free hydrogen” parameterCL which can be used to assess the susceptibility of a steel to fishscaling following the usual enamelling processess. The model can also be used to study the effect of both thermomechanical steelmaking cycles and enamelling processes on resistance to the defect.