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An archaeometallurgical characterization of a VI to VII century Lombard steel bar from Ponte di Val Gabbia I site (northern Italy) was performed to reconstruct its manufacturing process and extract processing thermochemical parameters (i.e., temperature and oxygen chemical potential) from the slag inclusion-metal systems. Metallographic observations by optical microscopy and Vickers microhardness measurements indicated that the steel bar was forged from a heterogeneous steel lump, most probably an iron bloom, which was cooled in the air after the forging process. The chemical composition of non-metallic inclusions was determined by scanning electron microscopy coupled with X-ray dispersive spectroscopy. A multivariate statistical analysis of the smelting-derived non-metallic inclusions chemistry allowed to conclude that the steel bar was obtained by the direct or bloomery method. Under the hypothesis of local near-equilibrium conditions for the slag inclusion-metal systems, a thermodynamic-based strategy was adopted to estimate the firing conditions in the bloomery furnace in terms of both temperature and oxygen chemical potential of the reducing gas atmosphere. In particular, the computed temperatures are in the range of 1003 °C to 1171 °C, whereas the values of oxygen chemical potential vary between − 447.47 and − 385.79 kJ/mol.

A metallurgical and technological characterization of a seventh century seax coming from the Lombard necropolis of Campo San Giovanni in Leno (Brescia, northern Italy) was carried out to investigate the smithing technology involved in the blade manufacturing process. At first, a metric-based typological investigation of the seax was performed. X-ray radiography was used to gain a preliminary non-destructive technological assessment of the object. Optical microscopy and Vickers hardness measurements have allowed a metallurgical and mechanical characterization of the metal matrix both at the upper and lower blade portion. The results show that the seax blade was forged using different steel bars and also hypereutectoid steel. In addition, a selective quenching was performed by the ancient blacksmith to enhance the cutting-edge hardness. Scanning electron microscopy coupled with X-ray dispersive spectroscopy was employed to determine the chemistry of a large sample of nonmetallic inclusions entrapped within the metal matrix. A multivariate statistical analysis of the inclusions compositional data was used to carefully examine the composite nature of the artefact and trace the origin of the seax steel.

The diffusion of brake-shoe system has made it necessary to study brake–wheel–rail interactions. This study aims to elucidate this topic with an innovative small-scale test rig, the ‘4-contact machine’. A test procedure is designed in which an ER7 steel wheel is coupled with a R260 steel rail and two different brake-shoe materials (cast iron and sintered material). The results show that the 4-contact machine reproduces the effects of wheel–rail–brake shoe contact and damage similar to those observed on a railway wheel at the end of its life, highlighting marked differences in behaviour between the two brake materials.

The purpose of this work is to gain a better understanding of the complex damage phenomena taking place at the railway wheel/brake block interface due to thermo-mechanical loading. Initially, full-stop braking was studied using Finite Element (FE) simulations to estimate the temperature reached in the wheel rim. Experiments to reproduce wheel damage were conducted with a two-disc machine using test conditions that were based on the results of the FE simulations. Three different wheel steels were tested against the same cast iron shoe material. The evolution of the wheel disc damage was studied at various numbers of cycles under fixed contact pressure and sliding speed. The friction coefficient and the temperature on the wheel disc surface were measured during the tests. At the end of the experiments, the wheel disc was examined and characterized. Cross-sections were observed with an optical microscope and the hardness was measured as a function of the depth to investigate the damage mechanisms that occurred at surface and subsurface. Material transfer from the shoe specimen to the wheel specimen results in the formation of a discontinuous “third body” layer, and that layer plays a key role in the evolution of the wheel disc damage. When the transferred layer of brake material is worn away, detachment of steel from the wheel disc surface occurs, probably promoting the crack nucleation. In addition, wear debris from both disc materials promotes three-body abrasive wear of the wheel disc surface.

Worn railway wheels after service in the desert were investigated by means of failure analysis, hardness measurements, tensile tests, fracture toughness tests and low-cycle fatigue (LCF) tests. The results complete both previous small-scale laboratory experiments and FEM simulations performed by the authors on widely used steels and new steel grades allowing to better understand the main damage mechanisms due to the sand. These new steel grades were developed for forged-rolled solid wheels in alternative to AAR Class B/C/D steel grades and were designed to ensure high resistance to wear and rolling contact fatigue in the presence of sand, debris or gravel on the rails. This paper covers the in-depth mechanical characterization of wheels made of the new steels. Hardness measurements, tensile tests, fracture toughness tests and LCF tests were carried out. The results are compared with AAR Class B/C/D steels showing an excellent combination of fracture toughness, cyclic yield strength and ductility.

A metallurgical and technological characterization of a seventh century bronze buckle coming from the Lombard necropolis of Monte San Zeno in Montichiari (Brescia, northern Italy) was performed to understand the manufacturing techniques. At first, a careful inspection was carried out to highlight the existence of any type of details, which would indicate an intentional design activity or typical casting defects. To avoid destructive measures on the ancient artifact, the buckle was successively characterized only via scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. This investigation was supported by an accurate study on the chemical composition of the matrix. The results suggest that the buckle was produced by sand casting, although the use of this technique in Europe dates back to the fifteenth century. Finally, based on the results obtained, a faithful archaeometallurgical reproduction of the buckle is proposed.Graphic Abstract

Wheel microstructure significantly affects railway wheel performances. Although wheel microstructure traditionally consists of pearlite-ferrite, it is possible to detect some intermediate structures traces, for instance bainite, generated during the thermal treatment process. The difference in mechanical properties between pearlite and bainite causes stress and strain incompatibilities at their interface, and this gives rises to early crack initiation and propagation. This paper aims at investigating the presence of bainitic traces in a ferritic-pearlitic microstructure of a railway wheel through semi-analytical and numerical finite element simulations. The effect of bainitic spot shape, dimension and distribution is studied as well as stress and plastic strain in the proximity of the heterogeneities is analysed. The radius-interaxis ratio parameter, defined as the ratio between the bainitic spots radiuses and the distance between two neighbouring spots centres, is introduced to study the effect of circular-sectioned bainitic spots on the steel plasticity. It is observed that the bainitic traces form regions subjected to overstress and overstrain in the ferritic-pearlitic matrix and that the area of the bainitic spots influences the size of these regions. The bainitic spot distribution is found out to increase further the overstrain in case the radius/interaxis ratio is higher than 0.2, corresponding to about 11% of bainite in the ferritic-pearlitic matrix. Beyond this threshold value, bainite traces can compromise both RCF and wear response.

The effect of material plastic properties in solid-contaminated wheel-rail couplings was studied by simplified finite element models, considering typical couplings of steels following European and ...

An improved railway wheel steel containing higher contents of C, Mn, Si, and V than the traditional ER7 steel was developed by alloy design for shoe-braked high-speed train applications. The effects of the thermal load on the microstructure and mechanical properties of a wheel made from this steel were investigated using a combined experimental and numerical approach. The wheel braking was studied using finite element simulations that account for the thermal loading of the wheel in order to find the temperatures reached in the wheel rim. Hardness measurements, tensile tests, toughness tests, fatigue crack growth tests, and microstructural observations were carried out on samples extracted from real wheels, with and without heat treatments simulating the modification of the microstructure due to the shoe braking. The results on the un-treated samples showed that the improved steel has a better combination of strength and toughness than ER7 steel. The results on the heat-treated samples showed that the improved steel maintains acceptable mechanical properties provided the maximum temperature during braking is below the A3 temperature of the steel (around 790 °C).

Twin-disk tests are an effective method to characterize the material response to rolling sliding contact, to reproduce the damage phenomena of real components at a laboratory scale in controlled working conditions. Usually the monitoring is performed by means of “gross” parameters, such as weight loss, coefficient of friction and Barkhausen noise, and micrographs of the sample sections, at the end of the test. Visual inspection of the sample contact surface at the macro-scale yields further information about the process under analysis. In twin-disk tests, the samples are visually inspected at predetermined steps, typically by acquiring the image of their surface when the samples are stationary. The availability of a system able to capture the images of the samples while they roll during the tests is of interest to better monitor the damage evolution. In this paper, we present the results of the experiments carried out to extract quantitative information from the images captured on railway wheel samples during rolling contact tests: suitable image processing has been designed with the objective of finding meaningful, synthetic indices for the monitoring and the interpretation of the wear process, also in relation to prior knowledge about the process. The experimental work was focused on the definition of the indices, on the analysis of their behavior on different steels, and on their usefulness to predict uneven wear during the tests on the test bench.

The interaction between wear and rolling contact fatigue in wet contact was investigated by means of experiments on a two-disk test bench. Specimens of various railway wheel steels were coupled with specimens of the same rail steel, subjected to rolling and sliding wet contact with varying sliding/rolling ratio. Some tests with a dry rolling–sliding contact phase preceding wet contact were carried out as well (dry–wet tests). The pressurization of the fluid entrapped in the surface cracks was the cause of a rapid and severe damage in the dry–wet tests, due to the nucleation of surface cracks by ratcheting in the dry phase, which subsequently propagate in the wet phase. In the wet tests, the fluid pressurization effect was much mitigated due to the absence of initial surface cracks; the specimens subjected to higher sliding/rolling ratio showed the best performance against rolling contact fatigue, due to the effect of wear in reducing the length of surface cracks. A model for assessing the interaction between wear and rolling contact fatigue was proposed, based on the correction of the classical Paris law for crack propagation by taking into account the effect of wear on crack length reduction. The application of the model to the experimental tests allowed finding a correlation between the model predictions and the occurrence of fatigue failure.

Metallurgical analyses and mechanical tests were performed on two ancient iron nails (dated to the seventeenth and eighteenth centuries) found at a private estate in the village of Limone sul Garda, which is located on Lake Garda in the northern Italian region of Lombardy. The nails were characterized via various metallurgical methods to assess their matrix composition, microstructure, and nonmetallic inclusion type, morphology, and distribution. These investigations elucidated the manufacturing process of the nails. The results indicated that the nails were produced by different forging techniques, and it was hypothesized that the steel utilized was obtained via two different methods (direct and indirect ore reduction), which were still in use at that time.

A recent interest on emergency shoe braking in high speed trains stimulated the research on the damage due to sliding contact between brake blocks and wheel tread. Bi-disc tests, with rolling/slidi...

A historical and metallurgical characterization of a “falchion” sword manufactured in Caino (Brescia, northern Italy) and dating from the early 17th century was performed to understand the manufacture methods of a Renaissance sword. At first, a set of size measurements was carried out to look for the existence of constant and/or recurring macroscopic sizes, which would indicate a standardized production, or of any type of proportionality between different parts of a sword, which would prove an intentional design activity. Light optical microscopy, scanning electron microscopy, energy-dispersive x-ray spectroscopy, quantometer analyses, and Vickers microhardness tests were then employed to analyze the microstructure and obtain the mechanical properties. All the metallurgical work is supported by an accurate study on the chemical composition of both metal-matrix and nonmetallic inclusions, which allowed for rebuilding and evaluating the efficiency of the whole production process.

Weight reduction and material substitution are increasing trends in the automotive industry. High pressure die casting (HPDC) is the conventional casting technology for the high volume production of light alloys, it has recently found wide application in the manufacturing of critical components, such as complex and thin geometry automotive parts. However, the major restriction of this affordable technology is the difficulty to design and realize hollow sections or components with undercuts. An innovative way to further increase the competitiveness of HPDC is to form complex undercut shaped parts through the use of new lost cores that are able endure the high pressures used in HPDC. This paper investigates the use of innovative ceramic lost cores in the production of a passenger car aluminum crossbeam by HPDC. Firstly, process and structural simulations were performed to improve the crossbeam design and check the technology features. The results led to the selection of the process parameters and the production of some prototypes that were finally characterized. These analyses demonstrate the feasibility of the production of hollow components by HPDC using ceramic cores.

An investigation was carried out to study the effects caused by shoe braking on the microstructure and mechanical properties of ER7, CLASS B, CLASS C, and SANDLOS® wheels as a result of the thermal load. Particular attention was given to cases of exposure to medium and high temperatures, namely from 700 °C to 970 °C, followed by air cooling. Hardness measurements, tensile tests, toughness tests, fatigue crack growth tests, and microstructural observations were carried out on samples extracted from new wheels, with and without heat treatments simulating the microstructural modifications caused by shoe braking. The experiments showed that the hardness, yield strength and ultimate tensile strength of the steels all decrease with the heat-treatment temperature up to 750 °C due to globular pearlite formation. However, after the heat treatment at 970 °C, these properties show a trend inversion, as the globular pearlite no longer appears, while, in some cases, traces of bainite and martensite are observed. On the other hand, the fracture toughness, crack propagation threshold, and rate are not significantly altered by the heat treatments, showing the steels to have a good stability under the thermal loads caused by shoe braking.

This work studies the effect of desert sand at the wheel-rail contact interface on two standard railway wheel steels (ER8 and CLASS C) and two upgraded steels (SANDLOS ® S and SANDLOS ® H) specifically designed for sandy environments. Rolling contact tests were carried out by means of a two-disc testing machine with and without sanding at the contact interface. The tests were performed for various durations in order to study the disc damage evolution. The response of the steels to the same contact pressure, rolling speed, sliding/rolling ratio and sand feed rate was evaluated in terms of wear rate, strain hardening and surface and subsurface damage. The ER8 discs performed worst, incorporating sand into the subsurface layer by extremely high plastic flow and were, therefore, subject to material detachment by rolling contact fatigue. The SANDLOS ® H discs performed best, with a wear rate 2.5 times lower than that of ER8 discs, due to the steel having the highest mechanical properties, and thereby restricting the sand embedding mechanism to a thin surface layer. The sanding increased the wear rate of the wheel discs by a factor which varies from 1.4 for SANDLOS ® H discs to 2.2 for ER8 and CLASS C discs. The damage mechanisms were qualitatively studied by finite element simulations, which recognised two different layers for the plastic strain. These layers are related to the dimensional scales of the disc-disc and sand-disc contact areas, respectively, and their influence on the total damage depends on the material cyclic yield stress.

A procedure for damage assessment in wheel–rail contact is still an open task: much work has been done for understanding failure mechanisms such as wear, cyclic plasticity and Rolling Contact Fatigue, but only recently the competition between them has been taken into account. In this study, a numerical procedure for damage assessment of railway wheel steels, considering the complex interaction between various failure phenomena that can occur at the loaded region, is proposed. The procedure is based on various damage mechanism models, integrated into an overall simulation method able to take into account their reciprocal influence. The model was applied for characterising a railway wheel steel (ER8 EN13262), through the elaboration of the results of rolling–sliding experiments, determining the plasticity and the wear model constants that can be used for damage assessment of real wheels. The experiments showed that the main damage phenomenon was surface crack formation due to unidirectional plastic flow (ratcheting); in the presence of water these cracks propagated causing very severe damage. A role of manganese sulphides non-metallic inclusions as preferential site for subsurface crack nucleation was observed. A quantitative evaluation of these phenomena was provided.

Recent research on ancient ferrous artefacts belonging to different historical periods has indicated significant differences in various parameters characterizing the slags entrapped in these artefacts, in cases where they were obtained by using different production methods. Such differences have been observed by comparing “ancient period” artefacts with “subsequent period” artefacts. “Ancient period” products were obtained by direct reduction of iron ore, without carburizing and at temperatures below the melting point of the reduced sponge iron. In the “subsequent period”, the indirect process started to be introduced, with the production, in a first reduction stage, of a liquid cast iron that had to be converted to wrought iron during a second fining operation. The understanding of the characterizing parameters of the slags has in fact progressed to the point where they represent a useful tool not only for inferring the origins of the starting ore but also for distinguishing between direct or indirect production. In the present research work, an accurate study of the entrapped slags has been carried out on an artefact from the Val Gabbia III site, identified in previous studies as a miner’s chisel. This study aims to carry out further metallurgical investigation into the miner’s chisel microstructure and the entrapped slags in order to help ascertain which production method was in use at the Val Gabbia III site; in fact, based on the intrinsic characteristics of the chisel, and the fact that the site where it was found, i.e. layers of the V–VI cent AD in Val Gabbia III site, was characterized by the presence of an almost 3.5 kg cast iron block, previous investigators were led to suppose that it may be a very early site of indirect iron smelting. While the slag characterizing parameters obtained in the present investigation appear to be consistent with published results related to the direct method, the discussion on the relationship between the indirect method production effects on entrapped slag and the experimental findings substantiate, although not definitively, the hypothesis that the production method of the miner’s chisel is indirect.

The aim of this work is to experimentally evaluate the notch ductility of a TWinning Induced Plasticity (TWIP) steel, a Dual Phase (DP) steel and two Quenching and Partitioning (Q&P) steels for automotive components. The notch ductility was investigated by studying the resistance to fracture initiation in tensile specimens with notches of various root radii. Fracture toughness is discussed in terms of the critical J -integral J c . It was found that J c progressively increases with the notch tip radius ρ for all the studied steels, demonstrating a high sensitivity of ductility upon notch severity. The relationship between J c and ρ allowed to calculate the maximum strain at fracture initiation e max , that is proposed as a deformation limit parameter useful to rank the forming capacity of the steels. It can be considered as the maximum strain that can be sustained ahead of the notch tip up to the onset of fracture advancement. Afterwards, a metallographic analysis by means of light optical microscope and scanning electron microscope was carried on etched specimens. The fracture mechanisms were also investigated by means of scanning electron microscope.

The effect of shoe braking on three railway wheel steels was studied by means of bi-disc tests, in order to evaluate the applicability of shoe braking as an emergency stop braking system in high-speed passenger trains. Disc-shaped specimens, extracted from real wheels, were initially paired with discs extracted from cast iron brake blocks, in condition of rolling and sliding such to reach the typical temperature of the surface of a wheel rim during stop braking. Subsequently, the wheel discs were coupled with rail steel discs in rolling and sliding dry contact. Some of them were cut and analysed; other ones were subsequently subjected to rolling and sliding contact with rail specimens in wet condition, with various test duration. Measurements of the coefficient of friction and of the weight loss were carried out during the tests; at the end of the tests, hardness measurements and microstructure observation were made on the disc cross-sections. The main damage phenomena observed in the specimens subjected to wheel-brake contact and dry wheel-rail contact were ratcheting, wear and nucleation of surface cracks. Traces of cast iron, previously transferred from the brake specimens to the wheel ones, were detected. The main damage phenomenon observed in the specimens subjected to wheel-brake, dry wheel-rail and wet wheel-rail contact was rolling contact fatigue, developing by the propagation of surface cracks due to the pressurization of the water entrapped inside the cracks. All these phenomena were found to be similar to those observed in wheel specimens subjected to wheel-rail contact only. The effect of the cast iron traces on surface crack propagation was evaluated by means of finite element simulations and it was found to be limited. Therefore, the application of shoe braking as emergency stop braking system to high speed trains did not show any relevant contraindications.

An experimental investigation was carried out to study and compare the response to cyclic loading of the high-performance railway wheel steels ER8 EN13262 and SUPERLOS®. Rolling contact tests were performed with the same contact pressure, rolling speed and sliding/rolling ratio, varying the lubrication regime to simulate different climatic conditions. The samples, machined out of wheel rims at two depths within the reprofiling layer, were coupled with UIC 900A rail steel samples. The wear rates, friction coefficients and hardness were correlated with the deformation beneath the contact surface. The crack morphology was studied, and the damage mechanisms were identified. The distribution of crack length and depth at the end of the dry tests was analyzed to quantify the damage. The main difference between the steels lies in the response of the external samples to dry contact: SUPERLOS® is subjected to a higher wear and lower friction coefficient than ER8, and this reduces the density of surface cracks that can propagate under wet contact conditions. The analysis of feedback data from in-service wheels confirmed the experimental results.

In recent years, certain foundry processes have made it possible to obtain products with very thin parts, below the 4 mm threshold of the permanent mold casting technology. The safety margins of these castings have been reduced, so the T6 heat treatment conditions adopted for the Al–7Si–Mg alloys need to be investigated to identify the best combination of strength and ductility. Furthermore, the cost and the production time associated with T6 heat treatment have to be optimized. In the present work, an experimental study was carried out to optimize the solution treatment and artificial aging conditions in gravity cast thin bars of B356 aluminum alloy modified with Sr. Two solution temperatures were selected, 530 °C and 550 °C, respectively, with solution time ranging from 2 to 8 h, followed by water quenching and artificial aging at 165 °C with aging time from 2 to 32 h. The results of hardness and tensile tests were correlated with differential scanning calorimetry (DSC) analysis. The best combination of mechanical properties and heat treatment duration was obtained with 2 h solutionizing at 550 °C and 8 h aging at 165 °C. DSC analysis showed that the alloy's mechanical properties reach the maximum value when the β″ phase is completely developed during the artificial aging.

The wear behaviour of two railway wheel steels, ER8 and SUPERLOS®, was studied through pin-on-disc tests, and the results were correlated with those previously obtained with twin-disc tests. The work-hardening of the steels was investigated with Vickers hardness measurements, and the wear mechanisms were studied using scanning electron microscopy. ER8 discs showed higher wear resistance, lower work-hardening ability and less wear damage than SUPERLOS® ones, confirming the results of the twin-disc tests. Therefore, sliding pin-on-disc experiments are recommended as a simple laboratory technique that can be used as a screening method for wheel steel performance prior to more complex and more expensive tests. The damage in both steels was due to the concomitance of oxidative wear, abrasive wear and fatigue wear. Iron oxide formation protects the steels from severe wear, whereas its detachment causes abrasive wear; furthermore, surface fatigue cracks initiate and propagate leading to the detachment of material flakes.

The paper presents a ratchetting model, based on the non-linear kinematic-isotropic hardening law of Leimatre and Chaboche, able to predict the shear strain accumulation during rolling contact loading and including wear as a competitive phenomenon. A procedure was proposed to calibrate the model with the material constants suitable for rolling contact problems, obtaining them from bi-disk contact tests. For this aim and to study the damage evolution at the wheel–rail interface, some twin disk rolling contact tests were carried out on a common wheel–rail material couple under a dry rolling–sliding condition typical of normal service. The tests were stopped at progressive cycles numbers for different couples of specimens, which were then cut and observed with optical and electron microscope in order to analyse the damage evolution in the zone near to the contact interface. A wear-rolling contact fatigue competition was observed with a progressive crack tip advance following a shear band cracking mechanism and crack tail removal due to wear. The numerical model allowed predicting the experimental strain profile along the depth as a function of contact cycles number, also demonstrating through the critical strain approach that in the analysed conditions wear was able to prevent deep crack formation.

Experimental investigations were carried out in order to evaluate the possibility of using warm spray as a repair technique for steel components. A high velocity oxygen fuel system was modified to deposit an AISI 316 stainless steel feedstock powder in solid state. A preliminary study was carried out on the spray conditions to optimise the deposition process. The powder was deposited on a steel sample, and the quality of the coating was evaluated by optical microscope observations: a limited porosity was observed in the repair. Test damage was produced in another steel sample and was repaired by warm spray. Residual stresses and microhardness of the repaired area were measured. It was found that beneficial compressive residual stresses were produced in the repaired area, which also exhibits a good microhardness, slightly higher than that of the substrate. Finally, adhesion tests were carried out, and a strength >46 MPa was found.

A model for assessing wear and rolling contact fatigue (RCF) in rails is proposed, with particular emphasis for their competitive role. RCF was studied taking into account different mechanisms of crack nucleation and propagation in different working conditions, and failure indexes are proposed to predict which damage mechanism prevails. Wear was investigated in terms of wear rate under different working conditions; a method to compare it to crack propagation rate is proposed in order to predict if wear can remove surface cracks due to RCF. The effectiveness of the method was verified by experimental tests.

The age hardening behavior of gravity cast B356 aluminum alloy was investigated by differential scanning calorimetry (DSC), hardness measurements and tensile tests. Three different artificial aging temperatures were selected, namely 155, 165 and 180 °C, with heat treatment time from 40 min to 32 h. DSC analysis results show that cluster formation begins below room temperature (at around −10 °C). Since cluster formation influences the subsequent precipitation of the main strengthening β″ phase, it can be inferred that a delay between solutionizing and artificial aging has a detrimental effect on the mechanical properties of the alloy. It was also confirmed that the hardness and the tensile properties of the alloy reach the maximum values when β″ phase is completely developed during the artificial aging. This happens after 16 h for samples aged at 155 °C, after 6 h for samples aged at 165 °C and after 4 h for samples aged at 180 °C. A subsequent decrease of the mechanical properties, observed only in the sample aged at the highest temperature, with increasing aging time can be associated with the transformation of the coherent β″ phase into the semi-coherent β′ phase. Finally, the activation energy associated with the precipitation of β″ phase was calculated to be 57.2 kJ/mol.

The assessment of damage in rail–wheel cyclic contact requires considering the combined action of different damage mechanisms, such as wear, ratcheting, surface or subsurface crack nucleation and propagation. Models, usually requiring experimental calibration, are available for assessing these phenomena. The best way to calibrate them is based on cyclic contact tests, as these represent the real working conditions more closely. However, some experimental information, such as microstructural changes or crack paths, can be obtained only by destructive methods at the end of the tests, and their evolution cannot be monitored; other parameters, such as the wear rate, can only be determined during time consuming breaks to the tests. In this work, non destructive measurements of vibrations, torque and Barkhausen noise were introduced as indicators of damage evolution in cyclic contact tests on a high performance steel for railway wheels, coupled with a rail steel. In particular, their correlation with surface state, wear rate, subsurface microstructure and presence of cracks was shown.

The possibility of using cold spray as repair technique of martensitic stainless steel components was evaluated through laboratory investigations. An austenitic stainless steel feedstock powder was chosen, instead of soft metals powders like nickel, copper, or aluminum, used for repairing components made in light alloy or cast iron. The present study directly compares the microstructure, the residual stresses, and the micro-hardness of repairs obtained by cold spray and by TIG welding, that is commonly used as repair technique in large steel components. XRD and optical metallographic analysis of the repairs showed that cold spray offers some advantages, inducing compressive residual stresses in the repair and avoiding alterations of the interface between repair and base material. For these reasons, a heat treatment after the cold spray repair is not required to restore the base material properties, whereas a post-weld heat treatment is needed after the welding repair. Cold spray repair also exhibits a higher micro-hardness than the welding repair. In addition, the cavitation erosion resistance of a cold spray coating was investigated through ultrasonic cavitation tests, and the samples worn surfaces were observed by scanning electron microscopy.

In recent years, attempts of engineering the surface of duplex stainless steels were made in order to enhancetheir hardness and tribological properties, without affecting their corrosion resistance. A possibility ofimproving these properties is provided by a family of processes developed by Prof. B.H. Kolster in theNetherlands in the late 1980’s. These processes (usually referred to as Kolsterising® treatments) consist in a lowtemperature surface carburizing, which involves the diffusion of large quantities of carbon atoms (up to 6-7wt.%) into the steel at a diffusion temperature below 450 °C. In the present paper a characterization of thesurface layer of Kolsterised duplex SAF 2205 stainless steel was carried out to study the effects of this treatmenton surface properties. The characterization includes optical metallographic examination, microhardness testsand SEM-EDS investigation on the Kolsterised steel in the as treated condition and after annealing treatments at200, 250, 300, 350 and 400°C for 10 hours, to evaluate the stability of Kolsterised layer’s properties with amoderate increase in temperature. Moreover, complying with ASTM G48-03 Method E Standard, in order toevaluate the effect of the Kolsterising® treatment on steel pitting resistance, the critical pitting temperature wasobtained for Kolsterised duplex SAF 2205 stainless steel compared with the base metal.

The fracture toughness of a 2205 duplex stainless steel (DSS) heavy section forged bar in the transition range (between 0 °C and −60 °C) was investigated. Experimental results were used to estimate the reference temperature T0 and to define a Master Curve according to the ASTM E 1921-11 Standard. Following this Standard, that assumes the Weibull exponent b fixed and equal to 4, the reference temperature obtained using data at T = −60 °C is T0 = −110 °C; using data at T = −40 °C is T0 = −103 °C and using data at T = −20 °C is T0 = −110 °C. As the description of the experimental data distribution does not follow the theoretical Weibull slope of b = 4, the slope obtained by the best fit of the experimental data was also measured and its influence on the assessment of the toughness scatter in the transition range by the weakest link approach ascertained. The recalculated reference temperatures T0 for the data at −60 °C, −40 °C and −20 °C are −114 °C (with b = 6.5), −107 °C (with b = 6), and −113 °C (with b = 4.8) respectively. Experimental data are well fitted by the Master Curve for all the chosen temperatures. Fracture mechanisms were also investigated by means of scanning electron microscope in order to discuss the application of the ASTM E 1921-11 Standard to duplex stainless steels whose microstructure is only partially ferritic and whose fracture transition does not result in a completely cleavage fracture at low temperatures because of the ductile behaviour of the austenite phase.

Rolling contact fatigue tests were carried out on ring specimens made of quenched and tempered SAE 5135 gear steel with three different steel-production processes, through a bi-disc machine under pure rolling condition and water lubrication. Early formation of micro-pits then coalescing into macro-pits was observed on the rolling surface, while the final failure was caused by subsurface originated spalling phenomena. Microscope analysis of specimens section highlighted the complex surface and subsurface crack layout, and permitted to recognise sulphides as preferential sites for cracks initiation. The inclusion content was analysed throughout the extreme value statistics and the maximum expected inclusion in the Hertzian contact zone was introduced in a failure assessment diagram recently proposed, which resulted effective in predicting the specimen failures.

In order to contribute to a better understanding of the mechanical properties of gold alloys this paper investigates the mechanical behaviour of an 18 carat yellow alloy; the strain hardening behaviour during a tensile test was measured in the annealed as well as in different cold-work conditions. In addition, the contribution of different strengthening mechanisms is discussed on the basis of current results and of data reported in literature. In particular, the contributions of grain size refinement and copper solid solution alloying in improving the mechanical strength of gold alloys were quantified.

Rolling contact fatigue (RCF) plays a critical role in railway components, and the characterization of materials used, in terms of RCF life, is still an open task, made complex by the interactions of different phenomena. The contact surface has a direct impact on the pressure exerted and can change during the test, due to wear. The procedure proposed consists in using vibrations of a test bench during RCF?life tests to identify when wear increases and causes a quick flattening of the specimen’s surface, and when this process is complete. The procedure is applied to two case studies regarding wheel and rail steels. In the tests, a wheel steel specimen rotates against a rail steel specimen, while pressed against each other by a constant force. At regular intervals weight loss and surface analysis are performed, while vibrations and torque are monitored continuously. Destructive tests are carried out at the end of each test. Results from non?destructive measurements were used to provide input data to a numerical simulation, used to determine the cyclic plasticity properties of the material. The methodology proposed shows the potential application of vibration measurements for detecting wear rates thus allowing supporting or partially supplanting destructive testing.

This study investigates the modification of the carbide type, size, shape and the associated changes in the toughness properties after prolonged aging at temperatures close to that of maximum hardening during tempering treatment. The aging effects on the microstructure of both steels still consist of a tempered martensite matrix, with an increased amount of precipitates and more marked prior austenitic grain boundaries. For H13 steel, the aging effects on fracture toughness show an appreciable increase in up to 50 h treatment; raising the aging time to 250 h causes considerable embrittlement. For H11 steel, however, the fracture toughness decreases early, from the first stages of aging; this behaviour has been associated with an increase in the carbide size. At increasing aging time, also for H11 steel, the embrittlement of prior austenite grain boundaries prevails in controlling fracture initiation and in decreasing fracture toughness.

KeyLos? 2001 is a new precipitation-hardening steel especially developed for plastic moulds. In this study the precipitation stage of KeyLos? 2001 steel has been investigated and compared to the results obtained with 17-4 PH steel. Precipitation-hardening has been carried out at three different temperatures and the stages of hardening and overageing have been studied in order to clarify the hardening mechanisms. It has been found that hardening and softening mechanism during the precipitation-hardening treatment occur at higher temperatures and in correspondence with more prolonged treatment times than those typical for the best known 17-4 PH steel; hardness is then expected to remain stable also for very extended mould lives. Microstructural investigations by means of Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) have also been carried out and the microstructural parameters responsible for the hardening and overageing have been pointed out.