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RAMONA SOLA

DIPENDENTE ALTRA UNIVERSITA presso: Dipartimento di Ingegneria "Enzo Ferrari"


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Pubblicazioni

2018 - Influence of a post-tempering cryogenic treatment on the toughness of the AISI M2 steel [Abstract in Atti di Convegno]
Sola, R.; Veronesi, P.; Giovanardi, R.; Merlin, M.; Garagnani, G. L.; Soffritti, C.; Morri, A.; Parigi, G.
abstract

A deep cryogenic treatment, carried out after quenching, can improve several mechanical properties of many metallic materials, but there are few reports on the effect of post-tempering deep cryogenic treatments on steels. The main aim of the present research work is the study of the effects of a post-tempering cryogenic treatment on microstructural and mechanical properties of the AISI M2 tool steel. In particular, the results of impact strength, fracture toughness, scratch test and microhardness show that the microstructural changes, induced by the post-tempering cryogenic treatment, promote a relief of residual stresses and a little improvement of mechanical properties, except for the hardness. Microstructural analysis performed by means of Optical Microscopy (OM) and Scanning Electron Microscopy (SEM), equipped with an Energy Dispersive X-ray Spectroscopy (EDS) microprobe, confirmed the precipitation of small and homogeneously dispersed secondary carbides. The induced precipitation of fine and homogeneously dispersed sub-micrometric carbides does not alter hardness but reduces residual stresses and increase fracture toughness. Finally, scratch test proved to be an alternative simple technique to determine fracture toughness of cryogenically treated steels.


2018 - Nitriding of stainless steels: Combined plasma-gaseous process [Articolo su rivista]
Giovanardi, R.; Veronesi, P.; Sola, R.; Borg, L.; Parigi, G.
abstract

In this work a combined process for the nitriding of stainless steels was developed, applying a first plasma nitriding treatment, which acts as an "activator" of the surface, then followed by a gaseous nitriding step, to increase the diffusion of the nitrogen and therefore the penetration of the treatment. The substrates for the process are two different stainless steels, AISI 304 and AISI 422, subjected to the single treatment of plasma nitriding (NP) and to the combined treatment of plasma nitriding followed by gaseous nitriding (NPNZ). The samples obtained were characterized (and compared with the two non-treated materials, NT) by cross-section micro-hardness profiles, metallographic analysis (SEM), wear tests (pin-on-disk tribometer) and accelerated corrosion tests (polarizations in acidic environment, H2SO4, and neutral saline, NaCl). The results obtained confirm that the combined treatment (NPNZ) is able to significantly increase the hardening depths, without excessively compromising the corrosion resistance in chlorides of the tested steels.


2017 - A Novel Duplex Treatment of C20 Steel Combining Low-Pressure Carburizing and Laser Quenching [Articolo su rivista]
Sola, R.; Veronesi, P.; Giovanardi, R.; Parigi, G.
abstract

The aim of this experimental study is to improve hardness layer thickness and wear resistance through laser quenching. Laser quenching treatment is used as a part of duplex treatments to improve the thickness and hardness of surface layers of steels. This duplex treatment allow to reach a very high hardening depth, improving wear resistance and load bearing capacity. In present study the surface properties of a low carbon steel treated by low-pressure carburizing prior to a laser quenching process are investigated. Results demonstrate that laser quenching is an effective means of improving wear resistance of low-pressure carburized layer and the improvement is significantly affected by increase of hardened layer thickness.


2017 - A novel method for fracture toughness evaluation of tool steels with post-tempering cryogenic treatment [Articolo su rivista]
Sola, Ramona; Giovanardi, Roberto; Parigi, Giovanni; Veronesi, Paolo
abstract

Cryogenic treatments are usually carried out immediately after quenching, but their use can be extended to post tempering in order to improve their fracture toughness. This research paper focuses on the influence of post-tempering cryogenic treatment on the microstructure and mechanical properties of tempered AISI M2, AISI D2, and X105CrCoMo18 steels. The aforementioned steels have been analysed after tempering and tempering + cryogenic treatment with scanning electron microscopy, X-ray diffraction for residual stress measurements, and micro-and nano-indentation to determine Young’s modulus and plasticity factor measurement. Besides the improvement of toughness, a further aim of the present work is the investigation of the pertinence of a novel technique for characterizing the fracture toughness via scratch experiments on cryogenically-treated steels. Results show that the application of post-tempering cryogenic treatment on AISI M2, AISI D2, and X105CrCoMo18 steels induce precipitation of fine and homogeneously dispersed sub-micrometric carbides which do not alter hardness and Young’s modulus values, but reduce residual stresses and increase fracture toughness. Finally, scratch test proved to be an alternative simple technique to determine the fracture toughness of cryogenically treated steels.


2017 - Effetto del trattamento preliminare sulle proprietà dell'acciaio AISI M2 sottoposto a trattamento criogenico [Articolo su rivista]
Sola, R.; Veronesi, P.; Giovanardi, R.; Forti, A.; Parigi, G.
abstract

Cryogenic treatment is widely used to enhance mechanical and physical properties of tool steels, hot work steels and high carbon steels. The application of cryogenic treatment on cutting tools improves wear resistance, hardness, dimensional stability, cutting tool durability, tool life and it reduces tool consumption, leading to a general production cost reduction. These benefits are achieved by deep cryogenic treatment because it decreases retained austenite content and it promotes the precipitation of fine carbides uniformly dispersed in martensite matrix. Retained austenite is a soft and unstable phase that reduces steel hardness and could be converted into martensite in working conditions and under stress, forming brittle (not tempered) martensite, with an increase of volume of 4%, inducing local stresses. Cryogenic treatment, by transforming retained austenite to martensite, improves dimensional stability. In addition to the transformation of retained austenite to martensite, secondary and fine carbides are formed in the structure, increasing mechanical properties, toughness and wear resistance. Cryogenic treatment generate an high internal stress state due to thermal stresses and transformation of martensite into austenite. Furthermore, thermal stresses increase the number of structural defects and the martensite, carbon-supersaturated, becomes unstable. Carbon atoms move towards the new structural defects created, martensite is decomposed and carbide precipitation takes place during the warming up phase to room temperature, producing a reduction of residual stress and resulting in an homogeneously dispersed network of tiny carbides. the greatest improvement in properties is obtained by carrying out the deep cryogenic treatment between quenching and tempering. However, in case of tool steels, an improvement can be obtained even performing cryogenic treatment at the end of the usual heat treatment cycle, i.e. to treat the finished tools. This last solution is more flexible than the previous one and can extend the use of the treatment to many practical applications. Cryogenic treatment barely changes the tensile mechanical properties and hardness of tool steel and hot work steel. But it is worth noting that cryogenic treatment notably improved fracture toughness of such steels because a fine, homogeneously dispersed carbide precipitation and a tougher martensite matrix are formed (with lower carbon content). In this framework, toughness measurement is an important tool to assess the effectiveness of the cryogenic treatment on such steels, but standard methods require careful sample preparation and dedicated equipment, while a simpler technique could be easily adopted as a quality control tool, as an alternative to ASTM E399 e BS 5447 standard method. Akono et al. in [14,15] proposed an alternative novel technique to measure the fracture toughness by scratch testing. Hence, aim of the current study is to investigate the effect of post-tempering cryogenic treatment on the microstructure and mechanical properties of AISI M2 steel. The samples were treated with two different cycles before cryogenic treatment: 1. quenching, cooling at-80°C, three tempering; 2. quenching, three tempering. All the samples were cryogenically cooled for 4h, 12h and 24 h to investigate also the influence of the time at-196°C. The results of microstructure, wear tests, X-ray diffraction, instrumented microhardness and scratch tests shows that posttempering cryogenic treatment promotes the precipitation of fine and homogeneously dispersed carbides, improves wear resistance and fracture toughness, without decreasing hardness. Also the heat treatment cycle before cryogenic cooling influences the wear and mechanical properties of the steel.


2017 - Laser hardening of steel sintered parts [Relazione in Atti di Convegno]
Veronesi, P.; Sola, R.; Colombini, E.; Giovanardi, R.; Parigi, G.
abstract

The possibility of applying rapid and localized laser hardening to near-net shape parts, like the ones deriving from powder metallurgy (P/M) is investigated, demonstrating that even low alloyed steels (Fe + 2% Cu + 0,7% C) can be successfully heat treated with minimal or no dimensional variations. Laser hardening conditions have been selected on the basis of the results of the previous research, carried out by means of an Nd-YAG high power system [1]. To avoid some carbon loss, observed on previous activities, the samples have been protected by neutral atmosphere. The microstructural features of the laser hardened steels have been analyzed by optical microscopy, whereas the surface micro-geometry has been characterized by scanning electron microscope. Hardened depth (HD), hardened width (HW) and hardened area (HA) have been measured as well. As expected, the micro-hardness profiles present a sharp drop at low distance from the hardened surface. The typical splitting between hardened zone and heat-Affected zone (HAZ), well known from laser hardened fully dense steels, has been observed also on low-Alloy sintered steels. The use of a protective atmosphere has been helpful to control surface decarburization and to prevent oxidation. The research confirm that Laser transformation Hardening (LTH) is a suitable hardening process of P/M components, through the action of a scanning laser beam. The short heating time and the modest volume fraction structurally modified can contribute to avoid part distortion, in comparison with other hardening methods.


2016 - Laser hardening of low-alloyed PM steels [Abstract in Atti di Convegno]
Bocchini, G. F.; Colombini, E.; Mannini, A.; Parigi, G.; Poli, G.; Sola, R.; Veronesi, P.
abstract

The good results obtained by laser-hardening of high-strength P/M steels drove to extend the research to low-alloy material (Fe-Cu-C; 6,6 g/cm3 density). Process conditions have been selected on the basis of previous results. To avoid carbon loss, observed on previous tests, the samples have been also protect-ed, by neutral atmosphere, during any critical step. The microstructural features of the laser hardened Cu-C P/M steels have been analyzed by LOM, whereas the surface micro-geometry has been characterized by SEM. Hardened depth (HD), hardened width (HW) and hardened area (HA) have been measured. As expected, the micro-hardness profiles present a sharp drop at low distance from the hardened surface. The typical splitting between hardened zone and heat-affected zone (HAZ), well known from laser hard-ened fully dense steels, has been observed also on low-alloy sintered steels. The addition of a protective atmosphere has been helpful to control decarburization of thin surface layers or surface oxidation. The re-search confirm that Laser transformation Hardening (LTH) is a suitable process to get hard, wear resistant surface, or defined spots on P/M components. The short heating time and the modest volume fraction structurally modified can contribute to avoid part distortion, in comparison with induction hardening. The possibility of very selective and precise hardening treatment has been confirmed. The results of the re-search open new possibilities to widen the use of high-property P/M parts, based on low-alloy materials, for wear-resistant and high precision applications.


2016 - Tempra laser di acciai sinterizzati basso-legati [Articolo su rivista]
Bocchini, G. F.; Colombini, Elena; Mannini, A.; Parigi, G.; Poli, Giorgio; Sola, Ramona; Veronesi, Paolo
abstract

An experimental investigation with 4 kW diodes laser system was carried out to study the effects of laser hardening process parameters on properties of P/M steels, based on prealloyed and diffusion-bonded powders. Eight different process parameters have been applied to optimize the process afterwards they have been set to two different compositions. The independent variables investigated have been: surface temperature, T, and laser travel speed (mm/min). The microstructural features of the laser hardened P/M steels were analyzed using LOM and the surface morphology has been characterized by SEM. Hardened depth both via hardness test and via optical measures were evaluated as well and used as responses for the ANOVA of the experimental data gathered. This work reveals the possibility to apply a selective and precise hardening treatment, like Laser Transformation hardening (LHT), to high-property P/M steel parts. Thanks to ANOVA analysis the correlation between energy density (ED) and optical dimension of treated zone has been identified. By increasing temperature and speed the density energy ED decreases, indeed at high temperature and speed the heated zone is larger. At the same speed, a temperature increase leads to a surface hardness decrease. A similar value of hardness depth (about 0.8 mm) has been detected by means of hardness profiles. The result has been strengthened by scratch test. The parabolic zone presents a very fine martensitic structure on the top of laser trail, and bainite microstructure on transition zone, as literature related. The transition zone between the laser affected zone and the base metal exhibits a micro hardness gradient. It is also possible to notice some Cu diffusion from outer to inner zone. By comparison among LHT specimens and sintering-hardened and stress-relieved one, (characterized by similar chemical compositions) penetration depth and hardness values appear similar. Transformation Hardening (LTH) is a suitable process for producing hard surface, on defined spots of P/M components, through the action of a scanning laser beam. The short heating time enables to decrease part distortion and surface oxidation, in comparison with sinter hardened. The possibility of very selective and precise hardening treatment seems to open new possibilities to widen the use of high-property P/M parts, based on advanced design, for demanding applications.


2015 - Effect of deep cryogenic treatment on the properties of AISI M2 steel [Relazione in Atti di Convegno]
Sola, Ramona; Poli, Giorgio; Defanti, Silvio; Veronesi, Paolo; Parigi, Giovanni
abstract

Deep cryogenic treatment is a special kind of bulk hardening heat treatment performed on a big variety of tool and carburized steels, to improve mechanical properties and wear resistance. The mai reason for this is the complete transformation from austenite into martensite plus the formation of submicrometric carbides dispersed in the tempered martensitic structure. The greatest improvement in properties is obtained by carrying out the deep cryogenic treatment between quenching and tempering. However, a significant improvement can be obtained even by treating the tools at the end of the usual heat treatment cycle, i.e. the finished tools. This last solution is more flexible than the other one and can extend the use of the treatment to many practical applications. In order to check the potential of deep cryogenic treatment on the performance of the finished products, an investigation was carried out on the AISI M2 high speed steel quenched, tempered and deep cryogenically treated. The findings shows that the cryogenic treatment promotes the precipitation of submicrometric carbides, that increment wear resistance and nano-hardness and decrease residual stresses.


2015 - Laser hardening of homogeneous and inhomogeneous p/m steels [Articolo su rivista]
Colombini, E.; Bocchini, G. F.; Parigi, G.; Sola, R.; Veronesi, P.; Poli, G.
abstract


2015 - Tempra laser di acciai sinterizzati omogenei o disomogenei [Articolo su rivista]
Colombini, Elena; Bocchini, G. F.; Parigi, G.; Sola, Ramona; Veronesi, Paolo; Poli, Giorgio
abstract

An experimental investigation with 4 kW diodes laser system was carried out to study the effects of laser hardening process parameters on properties of P/M steels, based on prealloyed and diffusion-bonded powders. Eight different process parameters have been applied to optimize the process afterwards they have been set to two different compositions. The independent variables investigated have been: surface temperature, T, and laser travel speed (mm/min). The microstructural features of the laser hardened P/M steels were analyzed using LOM and the surface morphology has been characterized by SEM. Hardened depth both via hardness test and via optical measures were evaluated as well and used as responses for the ANOVA of the experimental data gathered. This work reveals the possibility to apply a selective and precise hardening treatment, like Laser Transformation hardening (LHT), to high-property P/M steel parts. Thanks to ANOVA analysis the correlation between energy; density (ED) and optical dimension of treated zone has been identified. By increasing temperature and speed the density energy ED decreases, indeed at high temperature and speed the heated zone is larger. At the same speed, a temperature increase leads to a surface hardness decrease. A similar value of hardness depth (about 0.8 mm) has been detected by means of hardness profiles. The result has been strengthened by scratch test. The parabolic zone presents a very fine martensitic structure on the top of laser trail, and bainite microstructure on transition zone, as literature related. The transition zone between the laser affected zone and the base metal exhibits a micro hardness gradient. It is also possible to notice some Cu diffusion from outer to inner zone. By comparison among LHT specimens and sintering-hardened and stress-relieved one, (characterized by similar chemical compositions) penetration depth and hardness values appear similar. Transformation Hardening (LTH) is a suitable process for producing hard surface, on defined spots of P/M components, through the action of a scanning laser beam. The short heating time enables to decrease part distortion and surface oxidation, in comparison with sinter hardened. The possibility of very selective and precise hardening treatment seems to open new possibilities to widen the use of high-property P/M parts, based on advanced design, for demanding applications.


2014 - Effects of Surface Morphology on the Wear and Corrosion Resistance of Post-Treated Nitrided and Nitrocarburized 42CrMo4 Steel [Articolo su rivista]
Sola, Ramona; Poli, Giorgio; Veronesi, Paolo; Giovanardi, Roberto
abstract

The surface of alloyed carbon steel was subjected to thermochemical modification by nitrocarburizing and nitriding with or without postoxidation in order to improve its mechanical properties, corrosion, and wear resistance. Treated samples were characterized by testing their basic properties (compound layer thickness, nitriding, nitrocarburizing depth, and surface hardness) according to standards. Detailed estimation of the modified metal surface was performed by additional testing: X-ray diffraction, microstructure, surface roughness and topography, and wear and corrosion resistance. The surface layer obtained after nitrocarburizing treatment consists mainly of ε-Fe2-3(N,C) and γ'-Fe4(N,C); similarly, the nitrided surface is formed by ε-Fe2-3N and γ'-Fe4N iron nitrides. The surface layer after postoxidation contains additionally Fe3O4. The results obtained show that nitrocarburization, nitridation, and postoxidation result in better mechanical, wear, and corrosion resistance of 42CrMo4 steel, and postoxidized sample properties are influenced by surface morphology.


2014 - Influence of process parameters and alloying type on properties of laser quenched PM-steels [Relazione in Atti di Convegno]
Colombini, Elena; Bocchini, G. F.; Parigi, G.; Sola, Ramona; Veronesi, Paolo; Poli, Giorgio
abstract

Different alloyed PM steels have been laser quenched in industrial equipment laser diodes (4 kW, controlled by material surface temperature). The aim of this work is to investigate their responses to different process condition and different alloying metals, i.e. Cu, Ni, Mo, Cr and C. Furthermore the microstructure of hardened layer, heat affected zone (HAZ) and bulk zone Pre-alloyed, diffusion bonded and hybrid raw materials have been used. Design of Experiments has been the approach for evaluating the effect of treatment parameters (i.e temperature, spot size and speed) and to develop predictive models, correlating such parameters to hardening depth and scratch hardness number. Results demonstrated which valuable properties could be achieved, even through relatively low alloying. The promising results are encouraging since they allow to forecast a possible positive combination of high local hardness and wear resistance of high precision PM part.


2014 - Laser Quenching of Ionic Nitrided Steel: Effect of Process Parameters on Microstructure and Optimization [Articolo su rivista]
Colombini, Elena; Sola, Ramona; Parigi, GIAN BATTISTA; Veronesi, Paolo; Poli, Giorgio
abstract

The surface properties of 40CrMnMo7 steel have been increased by ionic nitriding thermo chemical treatment followed by laser quenching. Nitriding treatment increases the wear resistance and decreases the wear coefficient, guaranteeing an uniform, though shallow, hardening depth. Combining nitriding with laser quenching has been shown to increase of the hardening depth, while retaining the advantages obtained by the first treatment. In this work the effect of laser quenching parameters on microstructure and mechanical properties of 40CrMnMo7 steel have been investigated and predictive models developed suitable for further optimization of the process. DoE has been exploited to reduce the number of experiments and for evaluating by statistic methods the optimized process parameters.


2014 - Laser hardening of homogeneous or not-homogeneous PM steels [Relazione in Atti di Convegno]
Bocchini, G. F.; Colombini, Elena; Parigi, G; Sola, Ramona; Veronesi, Paolo; Poli, Giorgio
abstract

An experimental investigation with 4 kW diodes laser system was carried out to study the effects of laser hardening process parameters on properties of P/M steels, based on prealloyed, admixed or diffusion-bonded powders. The surface temperature of treated zone has been measured by pyrometer (to avoid local melting or surface damaging) and the travel speed on scanning the surface of test samples has been an investigated variable. A special attention has been given to specimen clamping, to avoid any misalignment with the beam movement. The independent variables investigated have been: surface temperature, T, travel speed (mm/min). The microstructural features of the laser hardened P/M steels were analyzed using LOM and the surface morphology has been characterized by SEM. Hardened depth (HD), hardened width (HW) and overall cross-sectional hardened area (HA) were measured as well and used as responses for the ANOVA of the experimental data gathered. The microhardness profiles present a sharp drop at low distance from the hardened surface. The typical splitting between hardened zone and heat-affected zone (HAZ), well known from laser hardened fully dense steels, holds also for P/M (porous) steels. The research showed that Laser Transformation Hardening (LTH) is a suitable process for producing hard, wear resistant surface, on defined spots of P/M components, through the action of a scanning laser beam. The short heating time enables to decrease part distortion and surface oxidation, in comparison with induction hardening. The possibility of very selective and precise hardening treatment seems to open new possibilities to widen the use of high-property P/M parts, based on advanced design, for demanding applications.


2013 - Effect of quenching method on the wear and corrosion resistance of stainless steel AISI 420 (TYPE 30Kh13) [Articolo su rivista]
Sola, Ramona; Giovanardi, Roberto; Veronesi, Paolo; Poli, Giorgio
abstract

The effect of different kinds of quenching, i.e., laser, vacuum, and induction ones, on the mechanical properties and wear and corrosion resistances of stainless steel AISI 420 is studied. It is shown that all the three kinds of heat treatment raise considerably the wear resistance of the steel due to growth in the hardness. Laser and vacuum quenching also increases the corrosion resistance. After induction quenching the resistance to corrosion is lower than in untreated steel.


2012 - Corrosion resistance improvement of nitrocarburised and post-oxidised steels by oil impregnation [Articolo su rivista]
Giovanardi, Roberto; Poli, Giorgio; Sola, Ramona; Veronesi, Paolo
abstract

Nitrocarburising is a well known process developed to improve superficial hardness and wear resistance of steels; a subsequent post-oxidation step is often useful to enhance the corrosion resistance. In this work, an additional step was evaluated: nitrocarburised and post-oxidised parts were impregnated in lubricant oil in order to improve their wear and corrosion resistance. The effectiveness of this new treatment, in terms of corrosion resistance, was assessed for two steels, 20MnCr5 and 42CrMo4, using two different oils. The results obtained from corrosion tests show that the impregnation treatment increases the corrosion resistance, in both acidic and chlorurated media.


2012 - Effect of surface finishing and post-oxidation atmosphere on the properties of 41CrAlMo7 nitrided steel | [Effetto dell'atmosfera di post-ossidazione e della finitura superficiale sull'acciaio 41CrAlMo7 nitrurato] [Articolo su rivista]
Sola, Ramona; Poli, Giorgio; Veronesi, Paolo; Giovanardi, Roberto; S., Mamei; A., Zanotti
abstract

Notoriamente la nitrurazione è un trattamento termochimico di indurimento superficiale sviluppato allo scopo di migliorare le resistenze a usura e a fatica di leghe ferrose. I componenti così trattati soffrono solitamente di una carenza nella resistenza a corrosione. Un obiettivo tecnologicamente importante è lo sviluppo di nuovi trattamenti post-nitrurazione per ottenere simultaneamente buone proprietà estetiche, meccaniche, tribologiche e di resistenza a corrosione. Mediante una analisi sistematica basata sull’uso di numerose tecniche sperimentali in questo lavoro di ricerca sono state approfondite le conoscenze sullo stato superficiale indotto dalla nitrurazione dalla successiva post-ossidazione eseguita in tre diverse atmosfere (protossido di azoto, anidride carbonica e aria) sull’acciaio 41CrAlMo7 fornito in quattro differenti finiture superficiali (lappata, rettificata, tornita e pallinata) al fine di quantificarne gli effetti sui parametri tribologici e sulla resistenza a corrosione.


2011 - Improvement of wear and corrosion resistance of ferrous alloys by post-nitrocarburizing treatments [Articolo su rivista]
Sola, Ramona; Giovanardi, Roberto; Veronesi, Paolo; Poli, Giorgio
abstract

Gas nitrocarburizing and post-oxidation treatments were performed on 42CrMo4 and 20MnCr5 steels, G30 and GS600 cast irons, to improve wear and corrosion resistance. In both materials the thickness of the compounds layer, mainly composed by epsilon- and gamma’- phase, resulted about 15-20 micron and the measured diffusion layer was about 150 micron thick. A subsequent oxidizing step, followed by impregnation with a two different lubricating oils was performed in order to further enhance corrosion resistance. Wear resistance against alumina was measured using a ball-on-disk tribometer and the corrosion characteristics of the samples were studied using salt spray test in accordance with ASTM B117. Experimental results show that the nitrocarburizing treatment improves significantly the wear resistance of the steels but not the wear resistance of the studied cast irons. The steels present almost the same tribological behaviour, ascribable to the formation of similar compounds, while cast irons present different wear resistance due to their chemical composition and the graphite morphology. The application of nitrocarburizing alone does not significantly improve corrosion resistance and it may even promote localized corrosion. The subsequent post-oxidation step leads to a slight decrease of the corrosion rate, because it partially seals the porous nitrocarburized layer. The final oil impregnation step resulted much more effective in further decreasing the corrosion rate and this final treatment in some cases improves also the wear resistance.


2011 - Influence of different quenching technique on wear and corrosion resistance of AISI 420 stainless steel [Relazione in Atti di Convegno]
Sola, Ramona; Giovanardi, Roberto; Veronesi, Paolo; Poli, Giorgio; Parigi, G.
abstract

Food industry requires materials with high corrosion resistance, chemical stability, pore-free surfaces , impact resistance and thermal stress resistance. In many structural applications, food industry requires steels not only with the ability to be easily cleaned and healthy but also with high mechanical properties. In this work high performance AISI420 stainless steels were developed, improving bulk mechanical proprieties and surface characteristics such wear and corrosion resistance, the latter by different quenching techniques: laser quenching, induction quenching and vacuum quenching. Metallurgical characterization, tribological characterization, corrosion tests (polarization tests in H2SO4, Na2SO4 and NaCl solutions) have been used on treated and untreated AISI 420 steel. Results demonstrated that as expected all the three quenching techniques, due to the increase of surface hardness, highly improve wear resistance. Laser quenched and vacuum quenched samples present the best wear resistance, accompanied by a good corrosion resistance in all the solutions and conditions tested, while the induction quenching decreases the corrosion resistance even with respect to untreated material.


2011 - Microwave ignited combustion synthesis of metal and intermetallic matrix composites [Relazione in Atti di Convegno]
Rosa, Roberto; Sola, Ramona; Colombini, Elena; Veronesi, Paolo; Leonelli, Cristina
abstract

Aim of this work is to present the results concerning the application of microwave assisted combustion synthesis (MACS) approach in the production of different kind of cermets and intermetallic matrix composite (IMC). Combustion synthesis allows taking advantages from the heat generated from exothermic reactions, which can derive from the synthesis of the intermetallic matrix, of the reinforcement, or both. Microwaves were used to ignite such combustion reactions exploiting their heating selectivity. The use of a single mode applicator allowed also investigating any "specific" microwave effects ascribable to separate electric and magnetic fields.


2010 - Comparison - By nanoindentation - Among PM steels obtained from diffusion-bonded powders (nominally equivalent) [Relazione in Atti di Convegno]
Bocchini, G. F.; Poli, Giorgio; Sola, Ramona; Veronesi, Paolo
abstract

In principle, powdered raw materials, produced by a given process and having equal chemical com-position, are supposed to be equivalent. In previous research works, the differences among response to sintering behaviour have been investigated on P/M steels based on four diffusionbonded powders (Fe + Ni + Cu + Mo), with atomized iron base, at the same alloy contents. Two levels of carbon (0.3 and 0.6 wt%) and two sintering conditions have been investigated, in industrial manufacturing conditions. However the comparison of dimensional changes, carbon content, and hardness reveals only modest differences. The comparison of microhardness distribution, fractal analysis, pore features, microstructure patterns show some clear differences, sometimes not univocal. The previously achieved results show that the claimed equivalence is not adequately and completely confirmed by closer and more complete comparative evaluation of experimental data. Only accurate analyses of microhardness and microstructure distributions seem powerful tools to rate, with modest sensitivity, the real similarity of the so claimed equivalent ironbase powders. Clearer differences have been shown by dilatometric analysis. To verify these observed differences, nanoindentation tests have been carried out. Each material has been characterized by nanoindentation measurement, for any process condition. The measured values, plotted on normal distribution graphs, only partially confirm the differences revealed by dilatometry. Some different ranking of diffusion level results from nanoindentation. This recently developed method, suitable to characterize very small volumes of material, appears suitable to find even minor differences among materials that could appear equivalent if evaluated by simpler and easier test methods.


2010 - Wear and corrosion resistance modification of nitrided and nitrocarburized steeels | [Modifica della resistenza a usura e a corrosione di acciai nitrurati e nitrocarburati] [Articolo su rivista]
Sola, Ramona; Poli, Giorgio; Giovanardi, Roberto; Veronesi, Paolo; Calzolari, C.; Zanotti, A.
abstract

Nitriding and nitrocarburizing induce surface chemical modification that are influenced by alloying element present in steels [1]. Nitriding treatment forms this layers sequence: a. the more external one, named white layer, formed by e-Fe2-3N e y'-Fe4N nitrides, b. the underlying diffusion layer in which nitrides of alloying element precipitates at grain boundary. The simultaneous presence of active carbon and nitrogen during nitrocarburizing treatment forms an external compound layer, composed by e-Fe2-3(N,C) and Y'-Fe4(N,C) carbonitrides laying on diffusion layer. The use of thermochemical treatments to enhance fatigue and wear resistance performances of highly stressed mechanical components is well known and established [2-5]. However it's technologically interesting post-nitriding and post-nitrocarburizing treatment development to guarantee good aesthetic and mechanical properties and high wear and corrosion resistance. Post-oxidation treatment, already studied in years [6-12], causes: a. nitrides and carbonitrides partial decomposition; b. Fe3O 4 growth on the surface; c. partial surface microporosity closure. Remaining open porosity, filled with lubricant oil (impregnation), could further increase wear and corrosion resistance. Aim of the work is the characterization and optimisation of post-treatments on nitrided and nitrocarburized 41CrAlMo7 and X37CrMoV5-1 steels, in order to improve corrosion resistance and to reduce wear during dry sliding against counterparts. In this study, two kinds of post-treatments have been performed after gaseous nitriding and nitrocarburizing: a) oxidation, b) oxidation and oil impregnation using two different oil in water emulsions (10wt% and 30wt% oil). The morphological features of the post-treated surface have been characterized using scanning electron microscopy and surface roughness measurements, in order to evaluate the surface morphology influence on wear and corrosion resistance. In can be concluded that, in the applied experimental conditions: 1. in both steels, nitrocarburizing a compound layer 11-16 m thick forms, constant in whole material, while nitriding a white layer, 9-10 (xm thick; 2. in both steels, the nitrided diffusion layer is thicker than nitrocarburized one and therefore nitriding allows higher total and effective hardening depth; 3. after the treatments of Tab. II, 41CrAlMo7 shows higher diffusion layer, while X37CrMoV5-1 achieved higher surface hardness than other steel. For this reason X37CrMoV5-1 presents lower wear rate; 4. nitrocarburizing and nitriding originate surfaces with very different morphology: nitrided surface is smooth, regular with a low porosity, while after nitrocarburizing the surface presents higher roughness parameters and abundant open microporosity; 5. oxidation forms Fe 3O4 on both nitrided and nitrocarburized samples. The oxide surface morphology is influenced by the substrate chemical composition and mainly by the surface morphology of the treated substrate: on the nitrocarburized sample the oxide is smooth, non porous and well adhered to the substrate; on nitrided substrate the oxide presents higher roughness value and a high porosity content. As a consequence, oxidation promotes an improvement of wear and corrosion resistance when performed on nitrocarburized substrate, but it has a detrimental effect on corrosion and wear resistance when it is performed after nitriding; 6. nitriding and nitrocarburizing promote an significant improvement of wear resistance, and nitrided steels exhibits the highest wear resistance. All the nitriding post-treatments decrease wear resistance; 7. nitriding and nitrocarburizing promote a significant corrosion resistance increment, oxidation has positive effects only after nitriding, because of post-oxidized surface morphology; 8. oil impregnation effect on wear resistance is moderate, but it is decisive on corrosion resistance, because the oil acts as an insulating.


2008 - Development of post-oxidation treatments to improve wear and corrosion resistance [Articolo su rivista]
Sola, Ramona; Giovanardi, Roberto; Veronesi, Paolo; Poli, Giorgio; S., Masini; A., Zanotti
abstract

Gas nitrocarburizing and post-oxidation treatments were performed on 42CrMo4 steel and GS600 cast iron, to improve wear and corrosion resistance. In both materials the compounds layer after nitrocarburizing, mainly composed by ε-Fe2-3(N,C) and γ′- Fe4N phase. First the oxidizing step, a subsequent impregnation with two different lubricating oils was performed in order to further enhance corrosion resistance. Wear resistance was measured using a ball-on-disk tribometer with Al 2O3 and 100Cr6 counterpart and the corrosion resistance of the samples was studied using salt spray test in accordance with ASTM B117 and electrochemical cell. Experimental results show that the nitrocarburizing treatment improves significantly the wear resistance of the steel but not the wear resistance of the studied cast irons. The application of nitrocarburizing alone does not significantly improve corrosion resistance and it may even promote localized corrosion. The subsequent postoxidation step leads to a slight decrease of the corrosion rate, because it partially seals the porous nitrocarburized layer. The final oil impregnation step resulted much more effective in further decreasing the corrosion rate by one order of magnitude.


2008 - Electroless Ni coatings for the improvement of wear resistance of bearings for lightweight rotary gear pumps [Articolo su rivista]
Veronesi, Paolo; Sola, Ramona; Poli, Giorgio
abstract

The possibility of improving the performances of current aluminiumalloys in the bearings region of lightweight external gear pumps has beeninvestigated. Two different electroless Ni-based coatings, one containing highpercentages of P (10–14%) and the other modified with the addition of 25 wt%PTFE micrometric spheres, deposited on 2011 Al alloy, were characterisedby scratch test and ball-on-disc tests. Comparison with an antifrictionSn-containing Al-alloy was performed. The PTFE-containing coating presentsthe lowest wear rate and friction against steel. However, the presence of PTFEspheres and the hardness mismatch with the substrate tends to favour tensilecracking of the coating.