F. Iacoviello

Fatigue crack propagation in austeno-ferritic duplex stainless steel 22 Cr 5 Ni

Fatigue crack propagation in Duplex Stainless Steels (DSSs) is strongly affected by microstructure and therefore by the choice of the steel grade or by the heat treatment conditions. In the present work fatigue behaviour of 22 Cr 5 Ni DSS has been investigated both after solution annealing and after different embrittling thermal cycles. Fatigue tests have been done in air considering the influence of both the stress ratio and the loading direction. Roughness analysis has been performed to point out a relationship between fracture morphology and fatigue crack propagation data and it shows that fracture surface roughness influence on fatigue crack advancing depends on loading conditions. A scanning electron microscope (SEM) fracture surface analysis has been performed to investigate the fatigue crack propagation micro mechanisms in 22 Cr 5 Ni duplex stainless steel.

Hydrogen embrittlement in the duplex stainless steel Z2CND2205 hydrogen-charged at 200°C

Abstract Physical and mechanical properties of austeno-ferritic stainless steels depend on the microstructure and phase transformations: many intermetallic phases, carbides and nitrides precipitate at different tempering temperatures. Hydrogen behaviour in steels is affected by the morphology and by the presence of precipitates, both for its diffusional behaviour and for the importance of trapping phenomena. In this paper, the hydrogenation of a 22Cr-5Ni duplex stainless steel has been achieved at 200°C in molten salts bath in potentiostatic conditions, and hydrogen embrittlement has been characterised using low strain rate tensile tests. The influence on hydrogen embrittlement of different intermetallic phases, carbides and nitrides has been considered via tempering heat treatment with tempering temperatures between 200 and 1050°C. The possibility of the recovery of the mechanical properties of charged steel outgassing at room temperature has been considered, investigating also the influence of different intermetallic phases, carbides and nitrides. Moreover, a new thermal technique based on high-temperature outgassing tests has been performed in order to calculate the hydrogen coefficient of diffusion.

Ductile cast irons: microstructure influence on fatigue crack propagation resistance

Microstructure influence on fatigue crack propagation resistance in five different ductile cast irons (DCI) was investigated. Four ferrite/pearlite volume fractions were considered, performing fatigue crack propagation tests according to ASTM E647 standard (R equals to 0.1, 0.5 and 0.75, respectively). Results were compared with an austempered DCI. Damaging micromechanisms were investigated according to the following procedures: - “traditional” Scanning Electron Microscope (SEM) fracture surfaces analysis; - SEM fracture surface analysis with 3D quantitative analysis; - SEM longitudinal crack profile analysis - Light Optical Microscope (LOM) transversal crack profile analysis;

A thermal outgassing method (T.O.M.) to measure the hydrogen diffusion coefficients in austenitic, austeno-ferritic and ferritic-perlitic steels

Abstract A thermal outgassing method (T.O.M.) and a baking method have been applied to measure the hydrogen diffusion coefficient at high temperature (150–600°C) in an austenitic stainless steel (AISI A286), an austeno-ferritic stainless steel (Z2CND2205) and a ferritic–perlitic steel (FM35). The results obtained by the above mentioned methods are in good agreement with those measured by the permeation technique at high temperature. Furthermore, in the austeno-ferritic steel Z2CND2205 the inverse of the total hydrogen diffusion coefficient 1\ D H( α+γ) is equal to f α \ D H α + f γ \ D H γ , where f α and f γ are, respectively, the volume fraction of α and γ phases and D H α and D H γ are the hydrogen diffusion coefficients for each phase.

Statistical behaviour of ΔK threshold values and life prediction analysis in 2091 Al-Li alloy

Abstract Fatigue crack propagation resistance in Al alloys is strongly affected by many parameters such as the loading frequency, mean stress or R load ratio, environment, chemical composition, heat treatment conditions. In the present work a ΔK threshold values statistical analysis has been done performing 25 threshold tests using the “load shedding” technique. This statistical analysis, connected with a former statistical analysis on the Paris–Erdogan relationship parameters (C, m), resulted in a life prediction analysis with the identification of the most conservative combination of ΔKth, C and m values with a defined probability.

Fatigue models for Al alloys

Abstract The Paris and Collipriest models of fatigue crack growth have been applied to experimental data on Al alloys. A pivot point, common to all the tests carried out on the same material, has been shown to exist when the Paris model is adopted. A mobile pivot, which is a function of the load ratio R , is suggested to exist using the Collipriest model on different sets of Al alloy experimental data. Pairs of parameters, A and B , as defined in this paper, can characterize the fatigue crack propagation behaviour of each alloy, irrespective of the load ratio R .

Graphite nodules features identifications and damaging micromechanims in ductile irons

Ductile irons mechanical properties are strongly influenced by the metal matrix microstructure and on the graphite elements morphology. Depending on the chemical composition, the manufacturing process and the heat treatments, these graphite elements can be characterized by different shape, size and distribution. These geometrical features are usually evaluated by the experts visual inspection, and some commercial softwares are also available to assist this activity. In this work, an automatic procedure based on an image segmentation technique is applied: this procedure is validated not only considering spheroidal graphite elements, but also considering other morphologies (e.g. lamellae).

Pearlitic ductile cast iron: damaging micromechanisms at crack tip

Ductile cast irons (DCIs) are characterized by a wide range of mechanical properties, mainly depending on microstructural factors, as matrix microstructure (characterized by phases volume fraction, grains size and grain distribution), graphite nodules (characterized by size, shape, density and distribution) and defects presence (e.g., porosity, inclusions, etc.). Versatility and higher performances at lower cost if compared to steels with analogous performances are the main DCIs advantages. In the last years, the role played by graphite nodules was deeply investigated by means of tensile and fatigue tests, performing scanning electron microscope (SEM) observations of specimens lateral surfaces during the tests (“in situ” tests) and identifying different damaging micromechanisms. In this work, a pearlitic DCIs fatigue resistance is investigated considering both fatigue crack propagation (by means of Compact Type specimens and according to ASTM E399 standard) and overload effects, focusing the interaction between the crack and the investigated DCI microstructure (pearlitic matrix and graphite nodules). On the basis of experimental results, and considering loading conditions and damaging micromechanisms, the applicability of ASTM E399 standard on the characterization of fatigue crack propagation resistance in ferritic DCIs is critically analyzed, mainly focusing the stress intensity factor amplitude role.

Optimal binarization of images by neural networks for morphological analysis of ductile cast iron

This work aims to characterize different objects on a scene by means of some of their morphological properties. The leading application consists in the analysis of ductile cast iron specimen pictures, in order to provide a quantitative evaluation of the graphite nodules shape; to this aim the material specimen pictures are binarized. Such a binarization process can be formulated as an optimal segmentation problem. The search for the optimal solution is solved efficiently by training a neural network on a suitable set of binary templates. A robust procedure is obtained, amenable for parallel or hardware implementation, so that real-time applications can be effectively dealt with. The method was developed as the core of an expert system aimed at the unsupervised analysis of ductile cast iron mechanical properties that are influenced by the microstructure and the peculiar morphology of graphite elements.

Fatigue crack tip damaging micromechanisms in a ferritic-pearlitic ductile cast iron

Due to the peculiar graphite elements shape, obtained by means of a chemical composition control (mainly small addition of elements like Mg, Ca or Ce), Ductile Cast Irons (DCIs) are able to offer the good castability of gray irons with the high mechanical properties of irons (first of all, toughness). This interesting properties combination can be improved both by means of the chemical composition control and by means of different heat treatments(e.g. annealing, normalizing, quenching, austempering etc). In this work, fatigue crack tip damaging micromechanisms in a ferritic-pearlitic DCI were investigated by means of scanning electron microscope observations performed on a lateral surface of Compact Type (CT) specimens during the fatigue crack propagation test (step by step procedure), performed according to the “load shedding procedure”. On the basis of the experimental results, different fatigue damaging micromechanisms were identified, both in the graphite nodules and in the ferritic – pearlitic matrix.