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 pro-duced. Microstructural characterization and mechanical testing were conducted for the steel samples. The weartesting was carried out with high-stress abrasive method, in which the samples were rotated inside a crushed gran-ite bed. A commercial 400 HB grade wear-resistant steel was included in the wear testing as a reference. The exper-imental steel showed very high mechanical properties reaching tensile strength up to 2600 MPa with hardness of750 HV10. Wear testing resulted in only minimal differences between the three variants indicating that the im-proved impact toughness by tempering did not significantly affect the wear resistance. The reference steel hadnearly two times greater mass loss compared to the higher hardness press-hardening steels. Microhardness meas-urements on the worn surface showed drastic increase in hardness for the deformed structure for all samples. Itwas concluded that even the high-hardness martensitic steels exhibit notable wear surface work-hardening. There-fore, hardness was determined to be the most significant factor affecting the wear performance of studied steels.publishedVersionPeer reviewe

The microstructure and technological properties of ultra high strength 1100MPa grade strip steel

The article describes the microstructure and the technological properties of a direct quenched ultrahighstrength strip steel with the minimum specific yield strength of 1100MPa. The microstructure of thislow carbon, Mn-Cr-Mo-Cu-Ni alloyed steel consists mainly of auto-tempered lath martensite. Due to thesophisticated thermo-mechanical controlled processing schedule, the martensite transformation takesplace from a fine and uniform austenite grain structure. State-of-the-art steelmaking and continuous castingoperations guarantee a good inclusion cleanness and low level of segregation. The steel has excellent impactand fracture toughness properties with respect to its ultra-high strength level. The determined transitiontemperature for 28J in Charpy-V test and fracture toughness characteristic temperature, T0, were below-100°C. The weldability tests indicated that the impact toughness of the heat affected zone (HAZ) is excellentand there is no significant softening in the HAZ or in the welded joint in the wide range of t8/5 cooling times.The steel allows crack-free bending with a minimum inside bending radius equal to 3 times material thicknessirrespective of the bending direction. In addition, the steel has a good resistance to atmospheric corrosion

Formation of nanostructured surface layer, the white layer, through solid particles impingement during slurry erosion in a martensitic medium-carbon steel

The extremely altered topmost surface layer, known as the white layer, formed in a medium-carbon low-alloy steel as result of impacts by angular 10–12 mm granite particles during the slurry erosion process is comprehensively investigated. For this purpose, the characteristics, morphology, and formation mechanism of this white layer are described based on the microstructural observations using optical, scanning and transmission electron microscopies as well as nanoindentation hardness measurements and modelling of surface deformation. The white layer exhibits a nanocrystalline structure consisting of ultrafine grains with an average size of 200 nm. It has a nanohardness level of around 10.1 GPa, considerably higher than that of untempered martensitic bulk material (5.7 GPa) achieved by an induction hardening treatment. The results showed that during the high-speed slurry erosion process, solid particle impacts brought forth conditions of high strain, high strain rate, and multi-directional strain paths. This promoted formation of a cell-type structure at first and later, after increasing the number of impacts, development of subgrains following by subgrain rotation and eventually formation of a nanocrystalline structure with ultra-high hardness. The model confirmed that high strain conditions - much higher than required for the onset of plastic deformation - can be achieved on the surface resulting in severe microstructural and property changes during the slurry erosion test.publishedVersionPeer reviewe

Synnynnäiset sukuelinpoikkeavuudet:elämänlaatu aikuisuudessa

Intersukupuolisuuteen lukeutuu monia erilaisia sukupuolikehityksen ja sukuelinkehityksen variaatioita. Kirurgialla on ollut intersukupuolisten lasten hoidossa ensisijainen asema. Lapsen sukupuolen ollessa epäselvä on vauvan sukuelimiä pyritty muokkaamaan kirurgisesti mahdollisimman varhaisessa vaiheessa valittuun sukupuoleen sopiviksi. Aikuisten intersukupuolisten ihmisten elämänlaatua Suomessa ei ole vielä kunnolla tutkittu. Vuosina 1970–1993 on Oulun Yliopistollisessa sairaalassa lastenkirurgialla hoidettu 14 intersukupuolista lasta kirurgisesti. Nämä potilaat identifioitiin ja heidän potilastietonsa kerättiin vuosilta 1970–1993 lasten leikkausosaston leikkaustoimenpiteiden toimenpidekoodeilla. Heistä neljä oli miehiä ja kymmenen naisia. Tutkimus toteutettiin lähettämällä heille kirjeitse kaksi kyselyä. Heistä kuusi vastasi. Heidän elämänlaatua ja hoitokokemuksia kartoitettiin sekä tutkimuksen painopistealueiden selvittämiseksi tehdyllä kyselylomakkeella, että suomalaisella elämänlaatua mittaavalla 15D-kyselyllä. Tutkimusaineisto analysoitiin ilman henkilötunnistetietoja ja tulokset raportoitiin niin, ettei potilaiden tunnistaminen ollut mahdollista. Suurin osa vastanneista kertoi, ettei sukupuolen kokemus ollut muuttunut elämän aikana. Suurin osa vastanneista oli ollut parisuhteessa. Kaikki vastanneet pystyivät saavuttamaan orgasmin, kuitenkin seksin aikaisia kipuja tai fyysisiä ongelmia oli suurimmalla osalla. Lisäksi suurin osa vastanneista oli tyytyväisiä heille tehtyihin sukuelinkirurgisiin operaatioihin. Validoidun 15D-kyselyn mukaan tutkimukseen vastanneet arvioivat elämänlaatunsa verrokkiryhmään nähden heikommaksi

Hot ductility of austenitic and duplex stainless steels under hot rolling conditions

Abstract The effects of restoration and certain elements, nitrogen, sulphur, calcium and Misch metal, on the hot ductility of austenitic, high-alloyed austenitic and duplex stainless steels have been investigated by means of hot rolling, hot tensile, hot bending and stress relaxation tests. The results of these different testing methods indicated that hot rolling experiments using stepped specimens is the most effective way to investigate the relationship between the softening and cracking phenomena under hot rolling conditions. For as-cast, high-alloyed and duplex stainless steels with a low impurity level, the cracking tendency was observed to increase with increasing pass strain and temperature, being minimal for the small strain of 0.1. No cracking occurred in these steels when rolled in the wrought condition. It could be concluded that the cracking problems are only exhibited by the cast structure with the hot ductility of even partially recrystallised steel being perfectly adequate. However, the recrystallisation kinetics of the high-alloyed austenitic stainless steels, determined by stress relaxation and double-pass rolling tests, were found to be so slow that only partial softening can be expected to occur between roughing passes under normal rolling conditions. In the duplex steel, the restoration is fairly fast so that complete softening can occur within typical interpass times in hot rolling, while certain changes in the phase structure take place as well. Sulphur was found to be an extremely harmful element in duplex stainless steel with regard to their hot ductility so that severe cracking can take place with sulphur content above 30 ppm. However, the effect of sulphur can be eliminated by reducing its content and by calcium or Misch metal treatments that significantly increase the number and decrease the average size of the inclusions. It seems that the desulphurisation capacity of an element is the most important property for assessing its usefulness in reducing the detrimental influence of sulphur. The hot ductility of type 316L stainless steel determined by tensile tests was found to be better for nitrogen content of 0.05 wt-% than 0.02%, while in double-hit tensile tests the hot ductility values were identical. The mechanism whereby nitrogen affects hot ductility remains unclear but a retarding effect on static recrystallisation was observed

A study on grain growth using a novel grain size calculation tool

Abstract The growth of prior austenite grains (PAG) of low alloyed martensitic steel is proven to be one of the key attributes contributing to the mechanical properties of ultrahigh-strength steels. The mean linear intercept -method (MLI) is traditionally used to acquire average PAG sizes from light optical microscopy images, which are from experimental test samples. The MLI -method is arduous and time-consuming as well as a highly generalizing method, where you lose information about the grain size distribution. Therefore, a more sophisticated and computerised method is in high demand among metallurgists. A program has been developed that encompasses an importing, digitalizing and calculating tool, which provides grain sizes and their distribution from multiple images. The tool mimics the workflow of manual MLI -method so the user sets the measure lines and marks all the linear intercepts. After this the tool calculates the MLI grain sizes and their 95 % confidence limits. Additionally, the tool provides the size of each intercepted grain and combines them to create a distribution. This information has been used to study the effects of holding temperature and time on grain sizes throughout the test samples in a case where abnormal grain growth at the centreline was expected. In the present study, PAG sizes were studied before and after deformation at ¼ and ½ thicknesses at various temperatures and holding times using the grain size calculation tool. The average MLI grain sizes show very little differences between temperatures and holding times, so information about grain size distribution is needed. Traditional presentation of the grain size distributions also shows too much variation to interpret the data properly. Instead, using the grain size distribution information and grouping grains to small, medium and large instances gives more profound data, especially in cases where grain size variation is significantly large. Distribution data from the test series also showed abnormal grain growth at the centreline of the test sample. The grain size calculation tool is used to quantify the effect of temperature and hold time on abnormal grain growth and its root cause is examined briefly

Flow stress characteristics and design of innovative 3-steps multiphase control thermomechanical processing to produce ultrafine grained bulk steels

Abstract In the present study, at first flow behavior of Nb–Ti microalloyed and interstitial-free (IF) steels was investigated to know the effects of processing parameters on their microstructural evolution. Then, innovative 3-steps multiphase control rolling schedules have been designed to yield submicron size uniform grains structure and successfully achieved ultrafine ferrite+martensite (0.69–0.78 μm) and ferritic structure (0.83–0.88 μm), respectively, in microalloyed and IF steels. The good combination of yield strength and ductility was achieved for the microalloyed (924 MPa, 13.6% elongation) and IF steel (621 MPa, 19.4% elongation) after rolling as per the designed 3-steps multiphase control deformation schedules. Deformation induced ferrite transformation followed by continuous dynamic recrystallization of the dynamically transformed ferrite is found to be the key mechanism for the formation of the ultrafine grained structure. Due to application of high amount of strains specifically within α+γ phase regime, the α-phase subdivided into several subgrains. These α-subgrains are strongly pinned by the γ/α grain boundaries and thereby restrict the dynamic recovery of the ferrite through reknitting and unravelling subgrain boundaries. On the application of further straining, the misorientation angle between these subgrain boundaries increases continuously through accumulation of the dislocations and finally, ultrafine ferrite grain structure is developed through continuous dynamic recrystallization

The effect of microalloying elements on prior austenite grain growth of low-carbon slab material

Abstract The effect of microalloying elements on the prior austenite grain growth of slab materials during slab reheating has been investigated. The investigated materials were laboratory castings with two levels of carbon (0.05 and 0.09 wt.%) and different combinations of microalloying elements, such as V, Nb, Mo and Ti. Experimental results were compared to equilibrium Thermo-Calc® simulations predicting the solubility of different precipitates during slab reheating. Based on slab reheating experiments at temperatures between 1100–1250 °C, Ti is the only alloying element to hinder prior austenite grain growth effectively above 1200 °C. For the steel containing high carbon with Nb, V and Ti microalloying, the average prior austenite grain size was less than 50 μm after the slab reheating to 1250 °C, which can be considered very small. According to Thermo-Calc® calculations, stable Ti(CN) precipitates are formed from the liquid and are in solution until 1500 °C. Until 50–60% of soluble NbC -precipitates, austenite grain size remained relatively small in Nb-alloyed compositions without Ti, but after that grain size increased drastically. For only V-alloyed steels, already at the soaking temperature of 1100 °C hardly any VC precipitates exists based on Thermo-Calc® simulations, and after that grain coarsening can be assumed to be controlled only by the coarsening rate of austenite grains

Effect of ageing on the mechanical properties of cold formed S700 rectangular hollow

Abstract The aim of this work is to study the effect of ageing (250 °C for 1 h) on the mechanical properties of the cold formed S700 rectangular hollow section. The investigated hot rolled steel strip was produced thermomechanical rolling followed by direct quenching. The dimension of the investigated hollow section was 120 x 120 x 10 mm and the corner radii and the other tolerances compliant with EN 10219. Tensile properties and Charpy-V impact toughness were determined for the base material and flat and corner area of the hollow section. The results showed that the tensile strength in the corner was slightly higher in comparison with the flat side, revealing higher cold deformation rate in the corner. Ageing increased the strength level relatively higher than cold forming without losing any elongation properties. The impact energies were at the high level at −40°C and −60°C in cold formed and aged materials. Even at −80°C, the CV results were 118 J/cm². It is also notable that no difference in CV values between the flat and the corner samples were observed. Thus, the results showed that the flat side specimens testing provides sufficient information of mechanical properties of the cold formed rectangular hollow sections and no need demanding corner sample testing when the structural hollow section is produced by using the thermomechanical controlled and direct-quenched base material. Furthermore, results showed that cold formed S700 is excellent for offshore steels, as steels are used even colder conditions as before

Insight to the influence of Ti addition on the strain-induced martensitic transformation in a high (about 7 wt.%) Mn stainless steel

Tiivistelmä The kinetics of strain-induced martensite (SIM) formation in a Ti-bearing 201L stainless steel were evaluated and compared to the existing results of two conventional stainless steel grades; i.e., 201L and 304L AISI. The effects of strain rate and rolling pass reduction on the kinetics of SIM formation during cold rolling were investigated. The Ti-microalloying was found to be intensifying the transformation due to lowering the stacking fault energy. It was seen that decreasing the rolling pass reduction strongly affected the variation of SIM volume fraction. Furthermore, a close correlation between the hardness and strain-induced transformation was found arising from microstructural evolution during the cold rolling process. Three stages in the hardening behavior were detected associated with lath-type martensite formation, transition stage of martensite laths break up and formation of dislocation-cell-type martensite