ABSTRACT The different densification kinetics of metal powders is one of the most important facto... more ABSTRACT The different densification kinetics of metal powders is one of the most important factors negatively affecting their possible co-sintering. This technological limit is further related to the relative fraction of the two powders, their shape and relative mean size. A decisive parameter, with this respect, is the so called particle size ratio defined as the ratio of the average particle size of the two powders. In this work the spark plasma co-sintering behaviour of an 80%vol AISI H13–20%vol AISI M3:2 blend is studied by considering nine different particle size ratios (dH13/dM3:2). The matrix of the relatively softer tool steel is strengthened by high speed steel. The highest values of density, hardness and fracture toughness are related to the smaller size of H13 particles (< 45 μm), the influence of particle size ratio being not so relevant. For bigger H13 size (125, 250 μm), particle size ratios < 1 promote a dispersion of isolated hard M3:2 particles, good densification and final high density. Increasing ratios evidence the formation of hard particles aggregates and a percolative network, respectively, which hinder densification. Above a critical value, however, the particle size ratio promotes densification in view of the lower “excluded volume” by smaller hard particles.
ABSTRACT Mechanical milling is a suitable technique to enhance various properties of copper by th... more ABSTRACT Mechanical milling is a suitable technique to enhance various properties of copper by the severe plastic deformation involved during the process. Contamination from milling media is one of the major problems of mechanical alloying. In this study, the behaviour of copper powder during mechanical milling was investigated in order to minimize iron and chromium contaminations. Hence, three different parameters have been studied to highlight the high influence of ball milling parameters on the final products. The parameters included the type of process control agent (none, toluene or stearic acid), the ball-to-powder weight ratio (33:1 or 10:1) and finally, a change in the milling cycle (interrupted or continuous) in an interactive procedure according to the experimental results. As a result, the best morphology and contamination level combination was observed in powder milled with stearic acid; it was 10:1 for the ball to powder ratio when using a continuous milling cycle. Once the best milling conditions were determined, the resulting samples were exposed to spark plasma sintering (SPS). The sintering parameters were selected based on a previous thermal gravimetrical measurement of the milled powders to avoid residual porosity. The final density of all of the samples is very good, 99%, confirming an effective densification process and sintering activated by severely strain-hardened and nanostructured particles.
In questo lavoro si propone lo studio del trattamento di tempra e rinvenimento di due acciai rapi... more In questo lavoro si propone lo studio del trattamento di tempra e rinvenimento di due acciai rapidi prodotti per colata centrifuga. La microstruttura dei materiali allo stato grezzo di colata è data da un reticolo più o meno interconnesso di carburi primari, in percentuale prossima al 10% in volume e da una matrice costituita da martensite e bainite. Le curve CCT evidenziano per entrambi un naso bainitico al di sotto dei 400°C per velocità di raffreddamento inferiori a 10°Cs -1 . Gli acciai sono stati trattati termicamente secondo il processo impiegato a livello industriale. L'influenza della temperatura di austenitizzazione è stata valutata aumentando tale parametro da 1030 fino a 1200°C. Le relative curve di rinvenimento evidenziano un aumento sia della temperatura che della durezza di picco secondaria, differente nei due materiali considerati, indicando un effetto benefico sulla resistenza a caldo e la necessità di un rinvenimento a temperatura superiore per eliminare completamente l'austenite residua. In prossimità della temperatura più elevata (1200°C) si assiste alla rifusione di eutettici, individuando così in questo valore di temperatura un limite processuale invalicabile. Anche il tempo di austenitizzazione mostra un effetto sulla durezza dopo tempra correlabile alla differente propensione dei due materiali alla risolubilizzazione dei carburi.
Spark Plasma Sintering (SPS) is a newly developed rapid technique for powder sintering. In this s... more Spark Plasma Sintering (SPS) is a newly developed rapid technique for powder sintering. In this study, consolidation by SPS of high speed steel (AISI M2) and hot work tool steel (AISI H13) commercial powders is investigated. The influence of sintering temperature (900 to 1150°C) and time (0 to 30 min) as well as the particle size and distribution is evaluated with respect to final density, hardness and fracture toughness. Properties have been compared with those of samples produced by Hot Isostatic Pressing (HIP).
Deep Cryogenic Treatment (DCT) was applied to two different cold work tool steels, X155CrMoV121 a... more Deep Cryogenic Treatment (DCT) was applied to two different cold work tool steels, X155CrMoV121 and X110CrMoV82, to improve their wear re- sistance. Several heat treatment cycles were investigated , by carrying out DCT both after quenching, after tempering and between quenching and tem- pering. Deep cryogenic treatment always reduces wear rate of the two steels, even if it does not
Evolution of friction and wear of 42CrAlMo7 steels with different nitriding processes was investi... more Evolution of friction and wear of 42CrAlMo7 steels with different nitriding processes was investigated during boundary-lubricated rolling–sliding tests. The wear behaviour of nitrided steel with a thin compound layer (produced by plasma nitriding and by gas nitriding followed by oxidation) was characterised by the early removal of the compound layer, and the wear resistance was thus, given by the underlying
The wear resistance at high temperature of different high speed steels (HSS) for rolls is evaluat... more The wear resistance at high temperature of different high speed steels (HSS) for rolls is evaluated on the basis of a tribological test aimed at reproducing the damage mechanisms occur during hot rolling. The test is positively used to highlight the different wear and friction behaviour of high speed steels and high chromium irons to be used in the early finishing stands of hot strip mill. The microstructure of these materials is given by a tempered martensitic matrix surrounded by an interconnected network of primary and eutectic carbides. In this paper a closer look into the properties of high speed steels is proposed. A wide range of microstructural conditions was obtained by changing the chemical composition and even by tempering the steels at different temperatures, in order to obtain different matrix microhardnesses. The wear behaviour can be explained on the basis of the operating mechanism, given by the combination of abrasion, triboxidation and by adhesion. While abrasion rules the tribological behaviour of HSS, triboxidation becomes more and more decisive in controlling the behaviour of HiCr irons. In these alloys the friction coefficient shows a transition (not observed in HSS), which was ascribed to the transition form a metal-oxide to an oxide-oxide contact, i.e., to the formation of wear protective layers. The tribological behaviour was correlated to the microstructure.
In the present paper the results of secondary phases determination and quantification in ZERON100... more In the present paper the results of secondary phases determination and quantification in ZERON100 duplex steel, heat treated at 850-1000uC for 180-2400 s, are presented. During the isothermal heat treatments, at 850uC, the x phase is the only phase to precipitate at a/c boundaries and triple points, while at 900 and 950uC, x phase is the first phase to precipitate after 180 s, followed by s phase 300 s later. At all the temperatures the total amount of secondary phases is y3% after ,600 s aging.
ABSTRACT The hot tribological contact between a surface engineered hot work tool steel and 6082 A... more ABSTRACT The hot tribological contact between a surface engineered hot work tool steel and 6082 Al alloy was evaluated by means of a block on disc test configuration, under conformal contact. The steel was nitrided, duplex-PVD (CrN, TiAlN, TiCN) and CVD coated (TiC+TiN), following the typical processing conditions used for extrusion dies. Furthermore, different degrees of surface finishing were considered. The contact was characterized in terms of damage mechanism, friction behaviour and tendency towards Al pick up by the die material. The surface engineered steel showed better properties than base material in view of the higher load bearing capacity. This was particularly beneficial for duplex-PVD samples, combining high surface hardness with good stability against chemical dissolution by hot aluminium. The double layered CVD coating also showed excellent properties in view of the high interfacial adhesion, which was a limiting factor for duplex coatings. A lower resistance was highlighted by the compound layer in nitrided samples, due to the higher affinity of Fe to Al. This resulted in a heavy pick up and the formation of big hard particles on the die surface, very detrimental for the surface finishing of the extrudate. In coated samples, the friction diagrams evidenced a delayed transition towards an unstable contact regime, which could be associated to a lower tendency towards the formation of build up layer of aluminium. The positive influence of a better surface finishing was demonstrated, as well.
ABSTRACT The influence of laser hardening on the tribological behaviour of a forged steel for hot... more ABSTRACT The influence of laser hardening on the tribological behaviour of a forged steel for hot profiled rolls is evaluated using a customary test rig which provides the rolling–sliding contact between the roll and a C40 plain carbon steel counterpart. Samples treated with different lasers and processing parameters were considered. Starting from a base value of 300HV the surface hardness was increased up to 800HV and the total case depth ranged from 1.2 to 2mm. The benefits of the laser treatment could be clearly observed at low test temperature, due to the positive influence of a hard martensite microstructure in preventing severe metallic wear by adhesive fracture. The influence of laser hardening was less evident at high temperature, where the tendency of different microstructures towards the formation of wear protective oxide layers could result in a better or worse resistance compared to the base steel, respectively. The capability of the roll material to produce wear particles and their entrapment within the contact interface resulted of fundamental importance for the formation of oxide glazes.
The study of wear mechanisms represents a very important task for the development of roll materia... more The study of wear mechanisms represents a very important task for the development of roll materials, with improved hot tribological properties. In the present paper the wear behaviour of different materials, i.e., high alloy steels (high speed steels (HSS)) and cast irons (high chromium irons (HiCr) and indefinite chill irons (IC)), produced by centrifugal casting, has been comparatively evaluated by means of a rolling sliding hot wear test. A disc specimen of C40 plain carbon steel, induction heated up to 700 • C, is allowed to rotate against a high alloy steel/iron at 200 rpm, corresponding to a slip rate of 28%, for a total period of time comprised between 1 and 3 h. A maximum contact hertzian stress of 300 MPa was imposed. The wear rates have been evaluated by weight measurements of the specimen, before and after each test period. The surface roughness was also determined as representative for the roll profile retention during service. The friction was continuously monitored. The wear mechanism is given by the combination of abrasion and oxidation. High speed steels show remarkable lower wear rates than high chromium and indefinite chill irons, because of their higher hot hardness. The retention of high hardness at elevated temperature is very important to reduce abrasive wear and also to support the protective oxide layer which develops on the surface of the roll material. As a general rule the roughness at the end of test tends to increase with increasing wear rate, as demonstrated by the relatively soft indefinite chill iron. However, the oxidation resistance also influences the final roughness, higher roughness values being observed for poorly oxidized materials. A simplified model explaining this experimental evidence is proposed.
Immersion tests in a molten aluminium alloy of a surface-treated AISI H11 hot work tool steel wer... more Immersion tests in a molten aluminium alloy of a surface-treated AISI H11 hot work tool steel were carried out in order to study the corrosion mechanisms and the effect of different surface treatments on the resistance of the material. The steel has a great reactivity with molten aluminium, which causes a generalised attack on the exposed surface. Plasma nitriding and
Thermal fatigue represents one of the most important stresses encountered in technical applicatio... more Thermal fatigue represents one of the most important stresses encountered in technical applications like aluminum pressure die casting and hot forging. The resulting damage is the well-known heat checking phenomenon, characterized by nucleation of a fine cracks network on die and insert surfaces. Usually, many other detrimental stresses, i.e. wear, erosion᎐corrosion by the molten aluminum and oxidation act during service, thus suggesting the introduction of protective surface treatments. The beneficial effect of nitriding, PVD-coating and, more recently, of their combination in duplex systems, in counteracting these stresses has been proved. On the contrary, the coating influence on thermal fatigue resistance is not yet clearly established. In Ž . Ž this paper the influence of plasma nitriding, multi-layer PVD coating i.e. CrN, ZrN and duplex treatment plasma nitriding and . PVD on the thermal fatigue resistance of AISI H11 hot work tool steel was studied. For this purpose a specific experimental apparatus, based on induction heating and water cooling, has been realized. Particular attention was given to plasma nitriding, Ž . used as pre-treatment in duplex plasma surface engineering. The role of compound layer, diffusion layer and coating s is discussed on the basis of the damage observed during the test. ᮊ
ABSTRACT The different densification kinetics of metal powders is one of the most important facto... more ABSTRACT The different densification kinetics of metal powders is one of the most important factors negatively affecting their possible co-sintering. This technological limit is further related to the relative fraction of the two powders, their shape and relative mean size. A decisive parameter, with this respect, is the so called particle size ratio defined as the ratio of the average particle size of the two powders. In this work the spark plasma co-sintering behaviour of an 80%vol AISI H13–20%vol AISI M3:2 blend is studied by considering nine different particle size ratios (dH13/dM3:2). The matrix of the relatively softer tool steel is strengthened by high speed steel. The highest values of density, hardness and fracture toughness are related to the smaller size of H13 particles (< 45 μm), the influence of particle size ratio being not so relevant. For bigger H13 size (125, 250 μm), particle size ratios < 1 promote a dispersion of isolated hard M3:2 particles, good densification and final high density. Increasing ratios evidence the formation of hard particles aggregates and a percolative network, respectively, which hinder densification. Above a critical value, however, the particle size ratio promotes densification in view of the lower “excluded volume” by smaller hard particles.
ABSTRACT Mechanical milling is a suitable technique to enhance various properties of copper by th... more ABSTRACT Mechanical milling is a suitable technique to enhance various properties of copper by the severe plastic deformation involved during the process. Contamination from milling media is one of the major problems of mechanical alloying. In this study, the behaviour of copper powder during mechanical milling was investigated in order to minimize iron and chromium contaminations. Hence, three different parameters have been studied to highlight the high influence of ball milling parameters on the final products. The parameters included the type of process control agent (none, toluene or stearic acid), the ball-to-powder weight ratio (33:1 or 10:1) and finally, a change in the milling cycle (interrupted or continuous) in an interactive procedure according to the experimental results. As a result, the best morphology and contamination level combination was observed in powder milled with stearic acid; it was 10:1 for the ball to powder ratio when using a continuous milling cycle. Once the best milling conditions were determined, the resulting samples were exposed to spark plasma sintering (SPS). The sintering parameters were selected based on a previous thermal gravimetrical measurement of the milled powders to avoid residual porosity. The final density of all of the samples is very good, 99%, confirming an effective densification process and sintering activated by severely strain-hardened and nanostructured particles.
In questo lavoro si propone lo studio del trattamento di tempra e rinvenimento di due acciai rapi... more In questo lavoro si propone lo studio del trattamento di tempra e rinvenimento di due acciai rapidi prodotti per colata centrifuga. La microstruttura dei materiali allo stato grezzo di colata è data da un reticolo più o meno interconnesso di carburi primari, in percentuale prossima al 10% in volume e da una matrice costituita da martensite e bainite. Le curve CCT evidenziano per entrambi un naso bainitico al di sotto dei 400°C per velocità di raffreddamento inferiori a 10°Cs -1 . Gli acciai sono stati trattati termicamente secondo il processo impiegato a livello industriale. L'influenza della temperatura di austenitizzazione è stata valutata aumentando tale parametro da 1030 fino a 1200°C. Le relative curve di rinvenimento evidenziano un aumento sia della temperatura che della durezza di picco secondaria, differente nei due materiali considerati, indicando un effetto benefico sulla resistenza a caldo e la necessità di un rinvenimento a temperatura superiore per eliminare completamente l'austenite residua. In prossimità della temperatura più elevata (1200°C) si assiste alla rifusione di eutettici, individuando così in questo valore di temperatura un limite processuale invalicabile. Anche il tempo di austenitizzazione mostra un effetto sulla durezza dopo tempra correlabile alla differente propensione dei due materiali alla risolubilizzazione dei carburi.
Spark Plasma Sintering (SPS) is a newly developed rapid technique for powder sintering. In this s... more Spark Plasma Sintering (SPS) is a newly developed rapid technique for powder sintering. In this study, consolidation by SPS of high speed steel (AISI M2) and hot work tool steel (AISI H13) commercial powders is investigated. The influence of sintering temperature (900 to 1150°C) and time (0 to 30 min) as well as the particle size and distribution is evaluated with respect to final density, hardness and fracture toughness. Properties have been compared with those of samples produced by Hot Isostatic Pressing (HIP).
Deep Cryogenic Treatment (DCT) was applied to two different cold work tool steels, X155CrMoV121 a... more Deep Cryogenic Treatment (DCT) was applied to two different cold work tool steels, X155CrMoV121 and X110CrMoV82, to improve their wear re- sistance. Several heat treatment cycles were investigated , by carrying out DCT both after quenching, after tempering and between quenching and tem- pering. Deep cryogenic treatment always reduces wear rate of the two steels, even if it does not
Evolution of friction and wear of 42CrAlMo7 steels with different nitriding processes was investi... more Evolution of friction and wear of 42CrAlMo7 steels with different nitriding processes was investigated during boundary-lubricated rolling–sliding tests. The wear behaviour of nitrided steel with a thin compound layer (produced by plasma nitriding and by gas nitriding followed by oxidation) was characterised by the early removal of the compound layer, and the wear resistance was thus, given by the underlying
The wear resistance at high temperature of different high speed steels (HSS) for rolls is evaluat... more The wear resistance at high temperature of different high speed steels (HSS) for rolls is evaluated on the basis of a tribological test aimed at reproducing the damage mechanisms occur during hot rolling. The test is positively used to highlight the different wear and friction behaviour of high speed steels and high chromium irons to be used in the early finishing stands of hot strip mill. The microstructure of these materials is given by a tempered martensitic matrix surrounded by an interconnected network of primary and eutectic carbides. In this paper a closer look into the properties of high speed steels is proposed. A wide range of microstructural conditions was obtained by changing the chemical composition and even by tempering the steels at different temperatures, in order to obtain different matrix microhardnesses. The wear behaviour can be explained on the basis of the operating mechanism, given by the combination of abrasion, triboxidation and by adhesion. While abrasion rules the tribological behaviour of HSS, triboxidation becomes more and more decisive in controlling the behaviour of HiCr irons. In these alloys the friction coefficient shows a transition (not observed in HSS), which was ascribed to the transition form a metal-oxide to an oxide-oxide contact, i.e., to the formation of wear protective layers. The tribological behaviour was correlated to the microstructure.
In the present paper the results of secondary phases determination and quantification in ZERON100... more In the present paper the results of secondary phases determination and quantification in ZERON100 duplex steel, heat treated at 850-1000uC for 180-2400 s, are presented. During the isothermal heat treatments, at 850uC, the x phase is the only phase to precipitate at a/c boundaries and triple points, while at 900 and 950uC, x phase is the first phase to precipitate after 180 s, followed by s phase 300 s later. At all the temperatures the total amount of secondary phases is y3% after ,600 s aging.
ABSTRACT The hot tribological contact between a surface engineered hot work tool steel and 6082 A... more ABSTRACT The hot tribological contact between a surface engineered hot work tool steel and 6082 Al alloy was evaluated by means of a block on disc test configuration, under conformal contact. The steel was nitrided, duplex-PVD (CrN, TiAlN, TiCN) and CVD coated (TiC+TiN), following the typical processing conditions used for extrusion dies. Furthermore, different degrees of surface finishing were considered. The contact was characterized in terms of damage mechanism, friction behaviour and tendency towards Al pick up by the die material. The surface engineered steel showed better properties than base material in view of the higher load bearing capacity. This was particularly beneficial for duplex-PVD samples, combining high surface hardness with good stability against chemical dissolution by hot aluminium. The double layered CVD coating also showed excellent properties in view of the high interfacial adhesion, which was a limiting factor for duplex coatings. A lower resistance was highlighted by the compound layer in nitrided samples, due to the higher affinity of Fe to Al. This resulted in a heavy pick up and the formation of big hard particles on the die surface, very detrimental for the surface finishing of the extrudate. In coated samples, the friction diagrams evidenced a delayed transition towards an unstable contact regime, which could be associated to a lower tendency towards the formation of build up layer of aluminium. The positive influence of a better surface finishing was demonstrated, as well.
ABSTRACT The influence of laser hardening on the tribological behaviour of a forged steel for hot... more ABSTRACT The influence of laser hardening on the tribological behaviour of a forged steel for hot profiled rolls is evaluated using a customary test rig which provides the rolling–sliding contact between the roll and a C40 plain carbon steel counterpart. Samples treated with different lasers and processing parameters were considered. Starting from a base value of 300HV the surface hardness was increased up to 800HV and the total case depth ranged from 1.2 to 2mm. The benefits of the laser treatment could be clearly observed at low test temperature, due to the positive influence of a hard martensite microstructure in preventing severe metallic wear by adhesive fracture. The influence of laser hardening was less evident at high temperature, where the tendency of different microstructures towards the formation of wear protective oxide layers could result in a better or worse resistance compared to the base steel, respectively. The capability of the roll material to produce wear particles and their entrapment within the contact interface resulted of fundamental importance for the formation of oxide glazes.
The study of wear mechanisms represents a very important task for the development of roll materia... more The study of wear mechanisms represents a very important task for the development of roll materials, with improved hot tribological properties. In the present paper the wear behaviour of different materials, i.e., high alloy steels (high speed steels (HSS)) and cast irons (high chromium irons (HiCr) and indefinite chill irons (IC)), produced by centrifugal casting, has been comparatively evaluated by means of a rolling sliding hot wear test. A disc specimen of C40 plain carbon steel, induction heated up to 700 • C, is allowed to rotate against a high alloy steel/iron at 200 rpm, corresponding to a slip rate of 28%, for a total period of time comprised between 1 and 3 h. A maximum contact hertzian stress of 300 MPa was imposed. The wear rates have been evaluated by weight measurements of the specimen, before and after each test period. The surface roughness was also determined as representative for the roll profile retention during service. The friction was continuously monitored. The wear mechanism is given by the combination of abrasion and oxidation. High speed steels show remarkable lower wear rates than high chromium and indefinite chill irons, because of their higher hot hardness. The retention of high hardness at elevated temperature is very important to reduce abrasive wear and also to support the protective oxide layer which develops on the surface of the roll material. As a general rule the roughness at the end of test tends to increase with increasing wear rate, as demonstrated by the relatively soft indefinite chill iron. However, the oxidation resistance also influences the final roughness, higher roughness values being observed for poorly oxidized materials. A simplified model explaining this experimental evidence is proposed.
Immersion tests in a molten aluminium alloy of a surface-treated AISI H11 hot work tool steel wer... more Immersion tests in a molten aluminium alloy of a surface-treated AISI H11 hot work tool steel were carried out in order to study the corrosion mechanisms and the effect of different surface treatments on the resistance of the material. The steel has a great reactivity with molten aluminium, which causes a generalised attack on the exposed surface. Plasma nitriding and
Thermal fatigue represents one of the most important stresses encountered in technical applicatio... more Thermal fatigue represents one of the most important stresses encountered in technical applications like aluminum pressure die casting and hot forging. The resulting damage is the well-known heat checking phenomenon, characterized by nucleation of a fine cracks network on die and insert surfaces. Usually, many other detrimental stresses, i.e. wear, erosion᎐corrosion by the molten aluminum and oxidation act during service, thus suggesting the introduction of protective surface treatments. The beneficial effect of nitriding, PVD-coating and, more recently, of their combination in duplex systems, in counteracting these stresses has been proved. On the contrary, the coating influence on thermal fatigue resistance is not yet clearly established. In Ž . Ž this paper the influence of plasma nitriding, multi-layer PVD coating i.e. CrN, ZrN and duplex treatment plasma nitriding and . PVD on the thermal fatigue resistance of AISI H11 hot work tool steel was studied. For this purpose a specific experimental apparatus, based on induction heating and water cooling, has been realized. Particular attention was given to plasma nitriding, Ž . used as pre-treatment in duplex plasma surface engineering. The role of compound layer, diffusion layer and coating s is discussed on the basis of the damage observed during the test. ᮊ
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Papers by M. Pellizzari