High-stress abrasive wear characteristics of ultra-high strength press-hardening steel

Ultra-high strength steels are widely utilized in many applications operating in harsh abrasive wear conditions. For instance, the machineries used in mining and mineral handling or in agricultural sector require robust, but cost-effective wear-resistant materials. Steels provide excellent combination of mechanical properties and usability. This study encompasses mechanical and wear testing of an experimental medium-carbon press-hardening steel. The as-received material was austenitized at two different temperatures and quenched in water. Additionally, low-temperature tempering was applied for one variant. In total, three variants of the press-hardening steel were produced. Microstructural characterization and mechanical testing were conducted for the steel samples. The wear testing was carried out with high-stress abrasive method, in which the samples were rotated inside a crushed granite bed. A commercial 400 HB grade wear-resistant steel was included in the wear testing as a reference. The experimental steel showed very high mechanical properties reaching tensile strength up to 2600 MPa with hardness of 750 HV10. Wear testing resulted in only minimal differences between the three variants indicating that the improved impact toughness by tempering did not significantly affect the wear resistance. The reference steel had nearly two times greater mass loss compared to the higher hardness press-hardening steels. Microhardness measurements on the worn surface showed drastic increase in hardness for the deformed structure for all samples. It was concluded that even the high-hardness martensitic steels exhibit notable wear surface work-hardening. Therefore, hardness was determined to be the most significant factor affecting the wear performance of studied steels

Effect of prior austenite grain size on the abrasive wear resistance of ultra-high strength martensitic steels

Prior austenite grain size has a marked effect on the hardenability, strength, and impact toughness properties of steels. This study was conducted in order to understand the effect of prior austenite grain size and morphology on the mechanical properties and abrasive wear performance of an ultra-high strength steel. A commercial quenched 500 HB grade wear-resistant steel was selected for the study: the steel was austenitized at two different temperatures and compared to the original, as-received quenched condition. The resulting mean prior austenite grain size was ranging from 14 μm to 34 μm. The decrease in grain size improved the low-temperature impact toughness properties. A high stress abrasive wear testing method with natural granite abrasives was utilized for the evaluation of abrasive wear resistance. The results suggest that decreasing the prior austenite grain size improves the abrasive wear resistance with similar hardness level martensitic steels. In addition, high-resolution electron backscatter diffraction measurements revealed formation of ultra-fine grain structures in the severely deformed regions of the wear surfaces.acceptedVersionPeer reviewe

High-stress abrasive wear performance of medium-carbon direct-quenched and partitioned, carbide-free bainitic, and martensitic steels

Experimental steels, a direct-quenched and partitioned (DQP) steel and a carbide-free bainitic steel (CFB), were tested along with a commercial martensitic 500 HB grade wear resistant steel in high-stress abrasive conditions. The three steels had different microstructures consisting of varying fractions and morphologies of martensite, retained austenite, and bainitic ferrite. The results showed that the CFB steel had a lower mass loss compared to the martensitic 500 HB steel with a similar hardness level. The DQP steel had a higher initial hardness and outperformed the other two steels. Wear surface characterization revealed that the investigated steels had significant work hardening of the wear surface, except with different mechanisms. Transformation induced plasticity (TRIP) increased the hardness of the DQP and CFB steels, while the fully martensitic 500 HB had more white layer formation on the wear surface resulting in increased hardness.publishedVersionPeer reviewe

Effect of microstructural characteristics and mechanical properties on the impact-abrasive and abrasive wear resistance of ultra-high strength steels

Abstract The effect of different microstructural characteristics and mechanical properties on the impact-abrasive and abrasive wear resistance of martensitic and carbide-free bainitic (CFB) steels was investigated. The main objectives were to study the role of retained austenite, tempering, and prior austenite grain size on the wear performance of martensitic steels and to evaluate the wear performance of novel carbide-free bainitic steels. The role of retained austenite in the wear resistance of martensitic steels was investigated by utilizing a direct quenching and partitioning (DQ&P) processing method to produce martensitic steels with varying retained austenite content. The effect of tempering on the wear resistance was studied by applying tempering to a medium-carbon martensitic steel and comparing the tempered samples with direct/water quenched variants. Prior austenite grain size was altered for a martensitic 500 HB grade wear-resistant steel to understand the influence of grain size on the abrasive wear resistance. Impact-abrasive wear testing revealed that no clear benefit could be obtained by having retained austenite in the martensitic matrix. Tempering might have a minor decreasing effect on the wear resistance when compared to untempered martensitic steels of similar hardness levels. Prior austenite grain size and morphology also appeared to influence the abrasive wear resistance of martensitic steels: decreasing grain size and more equiaxed grain structure improved the wear performance. Experimental carbide-free bainitic steels were fabricated via three ausforming routes. The high-stress abrasive wear testing showed that the steels have highly promising wear resistance due to the excellent work-hardening capabilities. Some CFB steels showed better wear resistance than the martensitic reference material despite lower or similar initial hardness levels. Work-hardening was considered a major factor affecting the wear resistance of steels. The hardness of the deformed wear surfaces showed good correlation with wear performance. Characterization of the wear surfaces with the electron backscatter diffraction (EBSD) method provided information about the severely deformed regions showing very fine grain structure. These features of the wear surfaces have a marked effect on the wear resistance of steels.Tiivistelmä Tässä väitöskirjassa on tutkittu martensiittisten ja karbidivapaiden bainiittisten terästen mikrorakenteen ja mekaanisten ominaisuuksien vaikutusta iskuabrasiiviseen sekä abrasiiviseen kulumisenkestoon. Tavoitteena oli ymmärtää jäännösausteniitin, päästön ja perinnäisen austeniitin raekoon vaikutus martensiittisten terästen kulumisenkestoon sekä tutkia karbidivapaiden terästen kulumista. Jäännösausteniitin merkitystä martensiittisten terästen kulumisominaisuuksiin tutkittiin hyödyntämällä suorasammutusteknologiaa (DQ&P), jossa jäähdytys keskeytetään martensiitin faasimuutoslämpötila-alueella jäännösausteniitin stabiloimiseksi. Päästön vaikutusta kulumisominaisuuksiin tutkittiin päästämällä keskihiilinen teräs ja muokkaamalla parametreja niin, että päästöllä saatiin aikaan suorakarkaistuja, vähähiilisempiä teräksiä vastaavat kovuustasot. Raekoon vaikutusta abrasiiviseen kulumisenkestoon tutkittiin lämpökäsittelemällä martensiittinen 500 HB -kovuusluokan teräs. Iskuabrasiivisessa kulumisessa jäännösausteniitin ei kuitenkaan havaittu olevan avuksi martensiittisilla teräksillä. Päästön havaittiin hieman heikentävän kulumisominaisuuksia, kun vertailtiin tuloksia saman kovuusluokan päästämättömiin teräksiin. Raerakenteella ja -koolla havaittiin myös yhteys kulumisenkestoon: hienontamalla raerakennetta ja muokkaamalla siitä mahdollisimman tasa-aksiaalinen saatiin paras kulumisenkesto abrasiivisissa olosuhteissa. Kokeelliset karbidivapaat bainiittiset (CFB) teräkset osoittautuivat erittäin lupaaviksi kulumisominaisuuksiltaan. Hyödyntämällä matalan lämpötilan kuumamuokkausta saatiin aikaiseksi teräksiä, joiden muokkauslujittuminen oli hyvin voimakasta. Osalla CFB-koeteräksistä kovuus jäi martensiittisen verrokkiteräksen tasolle tai jopa alle, mutta niiden massahäviö kulutustesteissä oli siitä huolimatta pienempi. Muokkauslujittumisella todettiin olevan tärkein rooli terästen kulumisominaisuuksia tarkasteltaessa. Kuluneen pinnan kovuudella ja kulumisenkestolla havaittiin olevan vahva korrelaatio. EBSD-menetelmällä tutkittiin terästen muokkautuneen pinnan ominaisuuksia ja havaittiin erittäin hienoa raerakennetta, joka oli syntynyt kulumisen aikana. Tähän muokkautuneeseen kerrokseen vaikuttavat tekijät määräävät pitkälti teräksen kulumisenkeston

Comparison of various high-stress wear conditions and wear performance of martensitic steels

The demanding environments typically encountered by the wear resistant steels create challenges for the materials selection, because the hardness grades of the steels alone do not reveal the true nature of their wear behavior. In this study, five commercial wear resistant steels were tested using three application oriented test methods with five different test variables for abrasion, impact-abrasion, and slurry erosion. All the used test methods produced high-stress conditions that crushed the used mineral abrasive, plastically deformed the sample surfaces, and led to the formation of adiabatic shear bands. When the results produced by the chosen methods were compared, the normalization of the wear losses by the wear area and test time revealed well the differences between the methods. The test methods ranked the steels similarly, but there were clear differences in the wear rates and wear mechanisms between the tests. In addition, the abrasive methods produced surface adiabatic shear bands, while subsurface shear bands were initiated by the more impacting methods. In the studied conditions, the work hardening ability of the steel had a clear influence on its wear resistance, which largely explains the marked differences in the wear rates of the studied commercial 500HB grade steels.acceptedVersionPeer reviewe

Physico-Mechanical Properties of Metal Matrix Self-Lubricating Composites Reinforced with Traditional and Nanometric Particles

Innovative nanostructured materials offer the possibility of enhancing the tribological performance of traditional materials like graphite and molybdenum disulfide (MoS2). In this study, the scratch resistance of two different copper powders, dendritic and spherical, and their composites with traditional MoS2, nanometric MoS2, and graphene nanoplatelets was investigated. Metal powder metallurgy was employed to produce composite materials with 5 wt% and 10 wt% of each solid lubricant. A ball milling step was employed to grind and mix the matrix copper powder with the lubricants. The use of a cold press combined with the sintering in inert atmosphere at 550 °C limited the oxidation of the copper and the degradation of the solid lubricants. The so-produced materials were characterized through a variety of techniques such as micro-indentation hardness, electrical resistivity, contact angle wettability, X-ray diffraction, Raman scattering, and scanning electron microscopy. Moreover, micro-scratch tests were performed on both pure copper and composite materials for comparing the apparent scratch hardness and friction coefficients. The scratches were examined with confocal laser scanning microscopy (CLSM), to identify the evolution of the damage mechanisms during the formation of the groove. The results highlighted the important difference between the dendritic and spherical copper powders and demonstrated a way to improve wear behavior thanks to the use of nanometric powders as solid lubricants

The effects of microstructure on erosive-abrasive wear behavior of carbide free bainitic and boron steels

The wear resistance of carbide free bainitic (CFB) microstructures have shown to be excellent in sliding, sliding-rolling and erosive-abrasive wear. Whereas, boron steels are often an economically favorable alternative used in applications subjected to erosive and abrasive wear. In this study the erosive-abrasive wear resistance of CFB and boron steels with different heat treatments were compared and the effect of microstructure on wear was investigated. An application oriented dry-pot laboratory test method with 8-10 mm granite gravel was used to produce erosive-abrasive wear environment. The tested materials were CFB and boron steels. The CFB steels had hardness values of 500 and 600 HV. The boron steels, both quenched and quenched and tempered, had a hardness of 500 HV. The influence of the microstructures on wear was studied by wear test results as well as by optical and scanning electron microscopy. The phase compositions were determined by XRD. The effect of wear, in addition to weight loss was also characterized by surface profilometry, hardness and hardness profile determinations. The wear resistance of the steels was compared with results achieved in a field test in an industrial mining application. Moreover, the effect of the different microstructures on wear behavior is discussed. The carbide free bainitic steels showed better wear performance than the martensitic boron steels. The boron steels were subjected to microcutting and microploughing, whereas the CFB steels exhibited more shallow impact craters with thin platelets.publishedVersio

Characteristics of carbide-free medium-carbon bainitic steels in high-stress abrasive wear conditions

This study encompasses a comprehensive account of the abrasive wear properties of carbide-free, ultrahigh-strength bainitic steels processed through ausforming at three different temperatures well below the recrystallization stop temperature followed by bainitic transformation at temperatures close to the Ms temperature. Five medium-carbon, high-silicon compositions were designed for the study by suitably varying the alloying levels of carbon, vanadium, niobium, molybdenum, and aluminum. While ausforming at lower temperatures enabled a large number of nucleation sites leading to significant refinement of bainitic laths, the decomposition of austenite at relatively low transformation temperatures was accelerated due to the presence of a high dislocation density, thus enabling completion of bainitic transformation in a reasonable length of time. The steels were characterized in respect of microstructural features and mechanical properties, besides evaluation of wear resistance through a high-stress abrasive wear testing method with natural granite abrasives. The microstructures comprised different fractions of bainitic ferrite and/or granular bainite (56–68%), martensite (0–25%), besides a significant fraction of retained austenite (20–34%) manifesting as pools and also interlath films, depending on the ausforming conditions and subsequent cooling paths. A tensile strength of 1900 MPa level was achieved with hardness exceeding 500 HV for the medium-temperature ausformed steel containing a high carbon content that also showed lowest mass loss in the wear test. The hardness-to-mass loss ratio appeared highly promising with some of the carbide-free bainitic steels on par with or better than the reference martensitic steel. The high work-hardening capability as a consequence of the strain-induced austenite to martensite transformation was considered as the main factor for the superior abrasive wear resistance of the carbide-free bainitic steels.publishedVersionPeer reviewe

Effect of tempering on the impact-abrasive and abrasive wear resistance of ultra-high strength steels

Tempering is an essential part in the fabrication of ultra-high strength steels and it is also widely applied in the processing of wear-resistant steels. In this paper, the effects of different tempering temperatures on the impact-abrasive and abrasive wear properties of martensitic ultra-high strength steels were studied. A novel press-hardening steel with carbon content of 0.4 wt% was received in hot-rolled condition and further austenitized, water-quenched and tempered for 2 h at different temperatures (150–400 °C). Tensile strength values up to 2200MPa and hardness exceeding 650HV were measured. Wear testing was done with impact-abrasive impeller-tumbler and abrasive dry-pot application-oriented test methods simulating mining and mineral handling environments. A laboratory rolled 600HB steel and a commercial 500HB grade wear-resistant steel were included for comparison. The wear surfaces and cross-sections of the samples were thoroughly characterized. Both testing methods produced highly deformed surface layers and strong work-hardening. Wear performance was mainly controlled by the initial hardness of the steels, but differences were found in the highly work-hardened surfaces of the steels.acceptedVersionPeer reviewe

High-stress abrasive wear characteristics of ultra-high strength press-hardening steel

Abstract Ultra-high strength steels are widely utilized in many applications operating in harsh abrasive wear conditions. For instance, the machineries used in mining and mineral handling or in agricultural sector require robust, but cost-effective wear-resistant materials. Steels provide excellent combination of mechanical properties and usability. This study encompasses mechanical and wear testing of an experimental medium-carbon press-hardening steel. The as-received material was austenitized at two different temperatures and quenched in water. Additionally, low-temperature tempering was applied for one variant. In total, three variants of the press-hardening steel were produced. Microstructural characterization and mechanical testing were conducted for the steel samples. The wear testing was carried out with high-stress abrasive method, in which the samples were rotated inside a crushed granite bed. A commercial 400 HB grade wear-resistant steel was included in the wear testing as a reference. The experimental steel showed very high mechanical properties reaching tensile strength up to 2600 MPa with hardness of 750 HV10. Wear testing resulted in only minimal differences between the three variants indicating that the improved impact toughness by tempering did not significantly affect the wear resistance. The reference steel had nearly two times greater mass loss compared to the higher hardness press-hardening steels. Microhardness measurements on the worn surface showed drastic increase in hardness for the deformed structure for all samples. It was concluded that even the high-hardness martensitic steels exhibit notable wear surface work-hardening. Therefore, hardness was determined to be the most significant factor affecting the wear performance of studied steels