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Hydrogen permeation technique is a widely used testing method for the determination of hydrogen diffusion coefficient (D), which is an important parameter considering hydrogen embrittlement. A palladium (Pd) or nickel (Ni) coating is often utilised on the hydrogen detection side of the test specimens. Here, we investigate the effect of Pd and Ni coatings on hydrogen diffusion in a martensitic 500 HBW hardness low-alloy steel in the thickness range of 0.5–0.8 mm using a refined successive transient method and compare against an uncoated reference specimen. Both coatings yield similar average D values (6–6.6 × 10⁻⁷ cm²/s), but the best repeatability is achieved with Pd coating. With Ni coating, D values decrease with the increasing specimen thickness, which is partly caused by a slower hydrogen diffusion in Ni, and therefore a concentration gradient at the specimen-coating interface. The uncoated specimen has a poor transient fit, and significantly lower D (2.1 × 10⁻⁷ cm²/s) due to surface oxidation. With both coatings, the steepness of the last decay transient was highly affected by specimen thickness, and therefore the density of reversible hydrogen traps is only comparable for similar thicknesses.

Metal matrix self-lubricating composites exhibit outstanding performance in various environments, reaching the required properties by modifying the reinforcement–matrix ratio and the production method. The present research investigated the effects on tribological performance and electrical properties of different pressure loads, maintaining pressing time, and sintering temperatures during the production of copper–10 wt% tungsten disulfide (Cu–WS₂) composite via powder metallurgy. Moreover, additional thermo-mechanical treatments were evaluated, namely second pressing and second sintering steps. The density and the hardness of the produced composites were measured, as well as the electrical resistivity, considering sliding electrical contacts as possible employment. The outputs of the wear tests were considered together with the analysis of the wear track via scanning electron microscopy and confocal laser scanning microscopy to understand wear mechanisms. Different production routes were compared in terms of electrical resistivity, wear coefficient, and specific wear rate, calculated by the confocal laser scanning microscopy, and friction coefficient, measured during the wear test. The main results highlighted that the increase in sintering temperature was detrimental to the hardness and tribological properties; higher load and additional pressing step determined a general improvement in the tested properties.

The precipitation of nanosized carbides in the hot-deformed low-alloy Ti-Mo-V-Nb and Ti-Mo-V- steels during the isothermal dwell at three different temperatures (590 °C, 630 °C, and 680 °C) was investigated. Scanning transmission electron microscopy (STEM) revealed that the formation of regularly and irregularly distributed interphase precipitates (IP) and their orientation relationship with the polygonal ferrite matrix depended strongly on steel chemistry, dwelling time, and temperature. Results also showed that precipitation occurred not only at the austenite/ferrite phase boundary during the phase transformation, but nanosized precipitates also formed within the supersaturated polygonal ferrite grains after the phase transformation. Interpretation of the results suggested that finely dispersed interphase precipitated V(Mo)C carbides, formed rows in which the spacing and the size of the precipitates grew slightly during the isothermal dwell time in the Ti-Mo-V-Nb steel at the temperatures of 680 °C and 630 °C, whereas spherical V(Mo)C carbides formed within the supersaturated ferrite matrix at all temperatures. In the case of the Ti-Mo-V steel, the formation of the interphase precipitated Ti(Mo,V)C carbides occurred only at the highest dwell temperature of 680 °C. No IPs was detected at the other dwell temperatures in this steel, but an irregular carbide distribution consisting mostly of spherical Ti(Mo,V)C carbides was formed within the supersaturated polygonal ferrite matrix.

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.

Both 0.3C (high) Si-Al and 0.3C medium-Mn steels are considered ideal recipes for developing the third-generation advanced high strength steels using the innovative quenching and partitioning (QP) and austenite reversion transformation (ART) treatments, respectively. This study reports the effects of QP and ART processing on the evolved microstructural characteristics, focusing on phase transformation behavior and resultant mechanical properties, augmented by the presence of finely divided retained austenite (RA). While QP processing trials were carried out with (high) Si-Al steels, both QP and ART treatments were utilized to optimize the processing of medium-Mn steels to achieve high strength-ductility combinations. The processes were later scaled up for laboratory rolling simulations, involving thermomechanical rolling followed by direct quenching and partitioning (TMR-DQP) in the case of (high) Si-Al steels, and TMR-QP and TMR-ART treatments for medium-Mn steels. The prime objective was to gain in-depth knowledge about the effects of processing on evolved microstructures, operating structural mechanisms and corresponding mechanical properties. Physical simulation experiments were conducted on a Gleeble simulator, followed by scaling up of the experiments on a laboratory rolling mill. Detailed microstructural characterization, including the differences in fractions, morphology, C-content, and stability of RA were performed by X-ray diffraction (XRD) and electron probe microanalysis (EPMA), besides using various materials characterization and electron microscopy techniques. Mechanical properties were evaluated in respect of hardness, tensile and impact toughness properties, and structure-property correlations established. The result showed that fine division of RA between martensitic laths, besides the refinement of martensitic packets and laths, notably improved the low-temperature impact toughness. Various combinations of high tensile strengths with good, Tiivistelmä 0.3C (korkea) pii- ja alumiiniseostetut sekä 0.3C keskimangaaniteräkset ovat houkuttelevia kandidaatteja uudeksi kolmannen sukupolven autoteollisuuden muovattavaksi suurlujuusteräkseksi. Tässä väitöskirjassa on tutkittu näiden teräksien mikrorakenteiden ominaisuuksia, austeniitti-martensiitti- ja martensiiti-austeniittifaasimuutosilmiöitä sekä näiden vaikutusta mekaanisiin ominaisuuksiin. Keskeytettyä sammutusta yhdistettynä matalan lämpötilan hehkutus (QP) prosessiin sekä austeniitin reversiomuutos (ART) -prosessiin on käytetty hyvien lujuus-sitkeysominaisuuksien saavuttamiseksi. Tavoite oli lisätä tietoa siitä, kuinka mikrorakenteen eri piirteet vaikuttavat termomekaanisesti valssatun suorasammutetun QP (TMR-DQP) ja termomekaanisesti valssatun ART (TMR-ART) teräksen ominaisuuksiin. Termomekaanista Gleeble-simulaattoria ja laboratoriokuumavalssainta on käytetty mikrorakenteen ominaisuuksien ja jäännösausteniitin hajaantumisen tutkimisessa. Lisäksi käytettiin erilaisia elektronimikroskooppeja ja analysaattoreita. Mekaanisten ominaisuuksien määrittämiseen käytettiin kovuusmittareita, veto- ja iskusitkeyskokeita. Tulokset osoittivat, että iskusitkeyteen vaikuttaa jäännösauteniitin määrän lisäksi myös rakenteen sälekoko. QP- ja ART-käsittelyillä voidaan saavuttaa erilaisia suuria vetolujuus-/venymäyhdistelmiä riippuen jäännösausteniitin määrästä, koosta, muodosta, jakaantumasta ja jäännösausteniitin hiilipitoisuudesta sekä martensiitin ja mahdollisen bainiitin morfologiasta. On haasteellista ottaa DQP-käsittely käytäntöön, koska keskeytetyn sammutuksen lämpötila vaikuttaa teräksen ominaisuuksiin ja sen hallinta tarkasti teollisessa mittakaavassa on haasteellista. ART-käsittelyssä haasteena on kapean prosessi-ikkunan (lämpötila ja hehkutusaika) hallinta ART-hehkutuslämpötiloissa.

A combination of physical simulation and laboratory rolling experiments, including thermomechanical rolling and low-temperature ausforming, was conducted for designing a suitable processing route to enable phase transformation from austenite to ultrafine bainite in a medium-carbon steel. Following low-temperature ausforming at 500–550 °C, two different cooling and holding paths were tried in the study: (1) water cooling close to martensite start temperature (300 °C), followed by isothermal holding (route A), and (2) air cooling to 350 °C followed by isothermal holding (route B). For reference, a third sample was directly water-cooled to 300 °C after hot rolling without ausforming treatment, followed by isothermal holding (route C). Field emission scanning electron microscopy and electron backscatter diffraction, as well as x-ray diffraction, were employed for microstructural analysis and correlations with the mechanical properties evaluated in respect of hardness and tensile properties. The low-temperature ausforming and subsequent cooling schedules resulted in the decomposition of austenite into ultrafine bainite and some martensite, while stabilizing a fraction of finely divided, carbon-enriched interlath austenite. Results suggested the development of a novel, multiphase bainite-martensite-austenite microstructure, achieved via low-temperature ausforming and subsequent air-cooling (route B), was beneficial in respect of mechanical properties. Most of the bainitic plates were in the range of 50–200 nm with the occasional presence of coalesced plates as wide as 2000 nm. Despite the differences in the microstructure, the mechanical behavior of non-ausformed samples was not significantly different. The technique paves way for developing medium-carbon nanostructured/ultrafine bainitic steels with high mechanical properties achieved via innovative modification of processing routes including low-temperature ausforming.

Suppressing hydrogen embrittlement in martensitic steels is a longstanding challenge. Here, we studied the effects of prior austenite grain (PAG) shape and size with a 0.25C steel utilising novel in situ H-charging, constant-displacement Tuning-fork testing (TFT) and H-permeation tests. Anisotropic elongated PAG structure has enhanced HE resistance transverse to the rolling direction (RD) with slower crack propagation rate (CPR) and quasi-cleavage fracture. Larger elongated grains are prone to intergranular fracture when crack propagates in RD. Reaustenitised equiaxed PAGs fail with intergranular cracking, which accelerates max CPR up to threefold compared to quasi-cleavage. All the microstructures have similar H-diffusion ∼5 × 10−7 cm2/s and density of reversible H-traps NT ∼ 3 × 1016, irrespective of PAG surface area, indicating that PAG boundaries are not effective diffusion paths. Deformed PAG boundaries mitigate susceptibility to intergranular cracking.

Non-metallic inclusions and their effects on the toughness and ductility of ultra-high-strength steels (UHSS) and high strength offshore steels have been investigated in this study. In the case of UHSS, the scope covered the martensitic base material, while in the case of offshore steels, the coarse-grained heat-affected zone (CGHAZ), consisting primarily of various types of bainite, caused by the thermal cycles during the welding were mainly studied. The focus was on the degrading effect of inclusions on the toughness and ductility, but the beneficial effects of certain inclusion types through inducing the acicular ferrite (AF) formation in the CGHAZ of offshore steels were also considered. In UHSS, it was found that small differences in impurity levels (IL), i.e. sum of S, N and O contents, had a significant impact on the inclusions. The number density of coarse inclusions increased with increasing IL from 0.7 mm-2 to 1.2 mm-2. In the steel with a lower IL, the coarse inclusions were mainly calcium aluminates with a calcium sulphide cell around them, while in the steel with a higher IL, a majority of the coarse inclusions consisted of titanium nitrides (TiN) and elongated manganese sulphides (MnS). The MnSs were found to deteriorate the transverse ductility relative to the rolling direction, while it was suggested that TiNs impaired the impact toughness at –40 °C. In the offshore steels, CGHAZs were simulated using various cooling rates in order to produce different microstructures and to be able to compare the combined effect of inclusions and microstructures on toughness properties. Low heat-input welding and a lack of coarse TiNs were found to be important for excellent toughness in the CGHAZ of 500 MPa offshore steels. For high heat-input welding, it might be necessary to lower the carbon content of the steel in order to achieve good toughness in the CGHAZ. Increasing prior austenite grain size, decreasing cooling rate, and the presence of small, Tiivistelmä Tässä väitöskirjassa on tutkittu epämetallisia sulkeumia ja niiden vaikutusta ultralujien terästen sekä lujien offshore-terästen sitkeysominaisuuksiin. Ultralujien terästen osalta tarkasteltavana oli perusaineen sitkeys, kun taas offshore-terästen osalta keskityttiin hitsauksesta johtuvan karkearakeisen lämpömuutosvyöhykkeen (CGHAZ) sitkeyteen. Pääpaino oli sulkeumien haitallisessa vaikutuksessa sitkeyden kannalta, mutta lisäksi käsiteltiin myös asikulaarisen ferriitin (AF) muodostumista ja sen mahdollisesti sitkeyttä parantavaa vaikutusta offshore-terästen CGHAZ:ssa tietyn tyyppisten sulkeumien avulla. Ultralujissa teräksissä pienen eron epäpuhtauspitoisuuksissa havaittiin vaikuttavan merkittävästi sulkeumarakenteeseen. Isojen sulkeumien määrä nousi epäpuhtauspitoisuuden noustessa. Pienemmällä epäpuhtauspitoisuudella isot sulkeumat olivat enimmäkseen pyöreitä kalsiumaluminaatteja, kun taas suuremmalla epäpuhtauspitoisuudella isot sulkeumat olivat titaaninitridejä (TiN) sekä valssauksessa venyneitä mangaanisulfideja (MnS). MnS:n todettiin heikentävän teräksen sitkeyttä valssaussuuntaan nähden poikittaisessa suunnassa, kun taas TiN:n havaittiin heikentävän iskusitkeyttä –40 °C:ssa. Offshore-terästen CGHAZ:t simuloitiin käyttäen useita jäähtymisnopeuksia. Näin saatiin tuotettua erilaisia mikrorakenteita, jolloin voitiin vertailla mikrorakenteen ja sulkeumien yhteisvaikutusta sitkeysominaisuuksiin. 500 MPa offshore-teräksissä matalan lämmöntuonnin hitsausmenetelmillä voidaan saavuttaa erinomainen CGHAZ:n isku- ja murtumissitkeys, kun teräksessä ei ole isoja titaaninitridejä. Käytettäessä korkean lämmöntuonnin hitsausmenetelmiä riittävän hyvän sitkeyden saavuttamiseen vaadittaneen teräksen hiilipitoisuuden laskemista. Riittävän karkean perinnäisen austeniitin raekoon sekä pienien MnO-TiOx(+MnS) sulkeumien havaittiin edesauttavan AF:n muodostumista 420 MPa offshore-terästen CGHAZ:ssa. Lisäksi AF:n osuus mikrorakenteessa kasvoi jäähtymisnopeuden laskiess

New steel alloying concepts are designed in order to produce a fully ferritic, low-alloy steel with high (1 GPa) ultimate tensile strength (TS). A simulated hot-deformation process of the Ti–Mo–V–Nb and Ti–Mo–V steels is designed for that purpose, and the strengthening mechanisms of the steels are evaluated after the isothermal dwell at three different temperatures (590, 630, and 680 °C). The TS and the yield strength (YS) of the test alloys are estimated via hardness measurements. Results show that the estimated TS of over 1000 MPa and YS of over 900 MPa can be achieved in both steels, although the contribution of different strengthening mechanisms to the YS varies between the steels. The effect of the dislocation strengthening can especially compensate the reduced effect of the precipitation strengthening at all tested coiling temperatures (CTs). Based on the results, a CT range of 590–630 °C with the 1800 s dwell time seems to be a potential process window for the studied steels after the present thermomechanically controlled processing (TMCP) route.

In the present study, the hydrogen embrittlement (HE) susceptibility of an additively manufactured (AM) 316L stainless steel (SS) was investigated. The materials were fabricated in the form of a lattice auxetic structure with three different strut thicknesses, 0.6, 1, and 1.4 mm, by the laser powder bed fusion technique at a volumetric energy of 70 J·mm⁻³. The effect of H charging on the strength and ductility of the lattice structures was evaluated by conducting tensile testing of the H-charged specimens at a slow strain rate of 4 × 10⁻⁵ s⁻¹. Hydrogen was introduced to the specimens via electrochemical charging in an NaOH aqueous solution for 24 h at 80 °C before the tensile testing. The microstructure evolution of the H-charged materials was studied using the electron backscattered diffraction (EBSD) technique. The study revealed that the auxetic structures of the AM 316L-SS exhibited a slight reduction in mechanical properties after H charging. The tensile strength was slightly decreased regardless of the thickness. However, the ductility was significantly reduced with increasing thickness. For instance, the strength and uniform elongation of the auxetic structure of the 0.6 mm thick strut were 340 MPa and 17.4% before H charging, and 320 MPa and 16.7% after H charging, respectively. The corresponding values of the counterpart’s 1.4 mm thick strut were 550 MPa and 29% before H charging, and 523 MPa and 23.9% after H charging, respectively. The fractography of the fracture surfaces showed the impact of H charging, as cleavage fracture was a striking feature in H-charged materials. Furthermore, the mechanical twins were enhanced during tensile straining of the H-charged high-thickness material.

The influence of martensite on the transformation rate of bainite as well as enhancement of various properties is well established, yet the combined influence of both the primary martensite and low temperature ausforming below the martensite start temperature (MS) on the characteristics of microstructural evolution comprising ultrafine/nanostructured bainite, martensite and retained austenite (RA) in medium C steels has not been unambiguously clarified. A series of physical simulation experiments involving isothermal holding at various temperatures below and above the MS was conducted with or without prior ausforming (∼0.5 strain, 0.5 s⁻¹ strain rate) in order to assess the influence of ausforming on subsequent transformation behaviour. The results suggest that when ausforming was applied above MS, it not only accelerated the bainite transformation kinetics, but also initiated strain-induced bainite/martensite formation. At temperatures below MS, prior ausforming did hinder the onset of bainite transformation, but the transformation rates were faster suggesting the strain-induced acceleration in kinetics. Due to a remarkable refinement of the bainitic structure achieved by prior ausforming in samples held below MS, the Vickers hardness improved appreciably unlike in the case of samples held above MS. Ausforming promoted significant refinement and uniformity of the bainitic plates, and also the orientations of the plates became more randomized. The dislocation density of RA decreased with the reduced content of secondary (fresh) martensite that formed during final cooling to room temperature.

Rapid tempering is a remarkable sustainable and energy-efficient way to modify properties of as-quenched martensite, particularly its toughness and ductility. In this work, tensile and fracture toughness properties, and microstructural evolution of a medium-carbon (0.4 wt%), low-alloy steel under different rapid tempering circumstances are discussed. The hot-rolled and direct-quenched specimens were subjected to rapid tempering treatments (heating rate of about 90 °C/s) to the different tempering temperatures (320–720 °C). As a reference material, one sample was subjected to conventional tempering at 420 °C for one hour. The results indicated that the mechanical properties and microstructural features are influenced considerably by tempering temperature rather than the holding time. Despite the short tempering duration, the strength of tempered martensite dropped (from a max. tensile strength of ∼ 2100 MPa to a min. of ∼ 1100 MPa) while ductility increased with increasing tempering temperature (from a tensile elongation of 4% to ∼ 15%). However, the rapidly tempered sample at 420 °C experienced a loss of fracture toughness as a result of coarse stick-like cementite precipitation. Microstructural observations revealed that, even with a short time frame, tempering temperature plays a significant role in the resulting cementite morphology. At lower tempering temperatures, the cementite precipitates have a stick-like structure, while at higher temperatures, they become more globular/spheroid-like. This change in cementite morphology led to an enhancement in fracture toughness. Overall, the results indicated that, by a proper design of the rapid tempering process, an excellent combination of tensile properties and fracture toughness can be obtained compared to conventional tempering while significantly saving time and energy.

Nanostructured bainite is recognized as a novel structure engineered through innovative processing thus enabling tensile strength as high as 2000 MPa in medium carbon steels. Low martensite start (MS) temperature of a typical high-carbon steel enables bainitic transformation at relatively lower temperatures, thus facilitating the formation of fine bainite platelets at the nanometre level. In this study, the target was to understand the physical metallurgy principles governing the operating structural mechanisms leading to the desired ultrafine/nanostructured bainitic microstructures and mechanical properties in medium carbon steels. The results suggest that ausforming applied above MS not only accelerated the bainite transformation kinetics but also initiated a strain-induced transformation of austenite into bainite/martensite. Ausforming below MS, however, hindered the onset of bainite transformation but subsequent transformation rates were faster, suggesting strain-induced acceleration of transformation kinetics. The influence of strain-induced phase transformation on flow stress was obvious at slow strain rates 0.01–0.5 s-1, particularly at temperatures below 400 °C. Ausforming led to significant refinement and uniformity of the bainitic plates with a concomitant increase in Vickers hardness. Low-temperature ausforming and subsequent isothermal holding and cooling schedules resulted in the decomposition of austenite into bainite of different morphologies. This was alongside the formation of small fractions of high-carbon martensite during cooling, together with the retention of desired fractions of finely divided, carbon-enriched austenite of various morphologies. A novel, multiphase bainite-martensite-austenite microstructure, achieved via low-temperature ausforming and subsequent air-cooling, was beneficial in terms of mechanical properties. The bainitic plates could be refined as fine as 20–50 nm, though with the presence of coalesced plates as wide a, Tiivistelmä Nanobainiitti tunnetaan matalan lämpötilan isotermisen pidon tuloksena saatavana uudenlaisena mikrorakenteena, joka mahdollistaa jopa 2000 MPa murtolujuuden keskihiilisissä teräksissä. Korkea hiilipitoisuus laskee martensiitin muodostumislämpötilaa, mikä mahdollistaa matalan bainiitin muodostumislämpötilan (MS), joka puolestaan johtaa hienojen, jopa nanometriluokan bainiittisäleiden muodostumiseen. Tämän tutkimuksen tavoitteena on ymmärtää fysikaalisen metallurgian mekanismeja, jotka johtavat toivottuun nanobainiittiseen mikrorakenteeseen ja mekaanisiin ominaisuuksiin. Tulokset osoittavat, että vaikka teräksen ausforming-käsittely MS-lämpötilan alapuolella viivästytti bainiitin muodostumisen alkamista, se kuitenkin kiihdytti bainiitin muodostumisnopeutta, ja sai myös aikaan venymän aiheuttaman faasimuutoksen austeniitista bainiitiksi tai martensiitiksi. Venymän aiheuttama faasimuutos voitiin nähdä muodonmuutoslujuuden huomattavana nousuna erityisesti hitailla muokkausnopeuksilla 0.01–0.5 s-1 matalissa, alle 400 °C lämpötiloissa. Ausformingin nähtiin pienentävän ja yhdenmukaistavan bainiittisäleitä, sekä nostavan terästen Vickers-kovuutta. Matalan lämpötilan ausforming, senjälkeinen isoterminen pito ja jäähdytys huoneenlämpötilaan saivat austeniitin hajaantumaan erimuotoisiin bainiitteihin ja martensiittiin, joskin pieni osa austeniitista säilyi rakenteessa jäännösausteniittina. Uudenlainen, matalan lämpötilan ausformingilla ja sopivilla pidolla ja jäähdytyksellä saavutettu bainiitti-martensiitti-austeniitti-rakenne oli mekaanisten ominaisuuksiensa puolesta edullinen mikrorakenne, jossa hienoimmat bainiittisäleet olivat jopa vain 20–50 nanometrin levyisiä. Karkeimmat, yhteenkasvaneet säleet olivat kuitenkin jopa 2000 nanometrin levyisiä. Tämä tutkimus antaa suuntaa erinomaiset mekaaniset ominaisuudet mahdollistavien nanobainiittisten keskihiilisten terästen kehittämiselle innovatiivisten prosessointiratkaisujen kautta.

A novel processing route is proposed to create a heterogeneous, multiphase structure in a medium Mn steel by incorporating partial quenching above the ambient, warm deformation, and partial recrystallization at high partitioning temperatures. The processing schedule was implemented in a Gleeble thermomechanical simulator and microstructures were examined by electron microscopy and X-ray diffraction. The hardness of the structures was measured as the preliminary mechanical property. Quenching of the reaustenitized sample to 120 °C provided a microstructure consisting of 73% martensite and balance (27%) untransformed austenite. Subsequent warm deformation at 500 °C enabled partially recrystallized ferrite and retained austenite during subsequent partitioning at 650 °C. The final microstructure consisted of a heterogeneous mixture of several phases and morphologies including lath-tempered martensite, partially recrystallized ferrite, lath and equiaxed austenite, and carbides. The volume fraction of retained austenite was 29% with a grain size of 200–300 nm and an estimated average stacking fault energy of 45 mJ/m2. The study indicates that desired novel microstructures can be imparted in these steels through suitable process design, whereby various hardening mechanisms, such as transformation-induced plasticity, bimodal grain size, phase boundary, strain partitioning, and precipitation hardening can be activated, resulting presumably in enhanced mechanical properties.

In the present study, very high cycle fatigue (VHCF) characteristics and properties of a direct quenched and partitioned (DQ&P) medium carbon (0.4 wt.% C) steel having ultrahigh-strength (UTS > 2 GPa) was investigated using an ultrasonic-fatigue testing technique (∼19 kHz frequency) up to ∼10¹⁰ cycles at the load ratio (R) of −1 (fully reversed tension-compression). The occurrence of crack initiation in the VHCF regime was observed at interior non-metallic inclusions. The microstructural changes following VHCF failure underneath the fracture surfaces near the crack initiation sites were further investigated through transmission electron microscopy. Formation of a clear nanograined subsurface layer adjacent to the crack initiating site at the interior inclusion was noticeably evident. The variation of average grain size (60 − 140 nm) and thickness (300 − 1200 nm) of nanograined layers at different number of cycles to failure were measured in detail. Apparently, localized plastic deformation caused the fragmentation of martensitic laths and hence, the formation of aforesaid ultrafine-grained layers in the microstructures.

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.

Prior austenite grain (PAG) structure is an important factor influencing hydrogen embrittlement (HE) susceptibility of ultrahigh-strength steels. In this study, the effect of PAG shape and size on HE behaviour is investigated using a novel tuning-fork testing method and hydrogen thermal desorption spectroscopy (TDS). Different PAG structures were acquired via re-austenitization (860°C = A860, 960°C = A960) and rapid quenching of an as-received 500 HBW direct-quenched (DQ) steel, which has an auto-tempered lath-martensitic microstructure and elongated PAG morphology. Fractography reveals different crack propagation mechanisms depending on the PAG shape. With the elongated PAG structure, hydrogen-induced crack propagation transverse to elongated PAGs was transgranular quasi-cleavage. Propagation was partially intergranular with the equiaxed PAG structures, regardless of the PAG size, leading to equally faster fracture. The TDS results show that there are no significant differences between the total hydrogen contents, but re-austenitized A860 and A960 steels contain a higher fraction of weakly trapped hydrogen. This indicates that the PAG boundaries are not the dominant hydrogen traps, and the different crack propagation mechanisms are rather linked to the geometrical shape of the grain structure.

Third generation advanced high-strength steels, e.g., quenched and partitioned steels, are forthcoming structural materials, which consist of a martensitic matrix and a substantial proportion of stabilized residual austenite for improved deformability. A novel less energy-intensive processing route of direct-quenching and partitioning advances this concept by facilitating carbon partitioning to untransformed austenite directly from the quench-stop temperature. However, a major challenge also with these steels is how to maintain structural integrity in the welded end-products after additional heat-input reaching above a temperature where given microstructure is still stable. Heat-input limiting beam welding processes are a solution to this by minimizing degradation of the heat-affected zone (HAZ) and producing even-strength welded joints for S1100 and above. In this study, we report toughness properties of an electron-beam (EB) welded 0.2C-1.5Mn-0.5Si-0.8Al-1.1Cr-0.8Ni (wt.%) direct-quenched and partitioned steel (DQ&P) having a yield strength of ∼1100 MPa, and a direct-quenched (DQ) was used as a reference. Low-temperature post-weld heat treatment (PWHT) was considered, too. Weld seam, coarse-grained HAZ, and the base materials were tested for impact toughness. Both the DQ and DQ&P base materials have excellent impact toughness transition temperatures T28J below -100°C. The weld seam has very good low-temperature toughness already at this stage of optimisation with T28J of -66°C, which shows robustness of the chosen alloy. Increased residual austenite content increased upper shelf toughness but not T28J. Furthermore, both the DQ and DQP HAZs have T28J below -70°C, pointing to the weld seam as the weakest link. PWHT reduced low-temperature impact toughness in all the cases with T28J being above -40°C, clearly demanding reassessment of its feasibility.

Hydrogen embrittlement (HE) is a well-recognized issue with ultrahigh-strength steels (UHSS) and an intensively studied subject to minimize the risk of sudden, catastrophic failures in structural applications. A novel tuning-fork test (TFT) is developed in this thesis to study HE of UHSS. The testing method utilizes constant displacement for stressing of the tuning-fork specimens by bending, combined with electrochemical hydrogen charging. With an isolated tensile stress region, crack initiation is controlled and can be monitored with different clamping arrangements. The main objectives were to create a simple and fast testing method, which allows ranking of UHSS, and to investigate the effects of prior austenite grain (PAG) structure. Traditional hydrogen permeation tests with a set-up built within this thesis complement the results. The first part of the TFT implementation is composed of an evaluation of the HE susceptibility of 300–600 HBW martensitic steels with relative threshold stress levels. Increasing the strength level leads to lower threshold stress levels and increased susceptibility to HE, especially for steels with over 400 HBW hardness. With the addition of a loadcell to the TFT clamping system, a more precise investigation was conducted for 500 HBW steels with various PAG structures. The effect of PAG morphology (elongated/equiaxed) on HE susceptibility was studied with steels that had different alloying compositions and with steels that had different PAG morphologies but the same alloying. An elongated PAG structure leads to enhanced resistance against HE in comparison to equiaxed PAG structures. Crack propagation rates were slower with more elongated microstructure, which is linked to transgranular quasi-cleavage crack propagation and slower hydrogen diffusion. Equiaxed microstructure exhibits partly intergranular cracking with faster crack propagation rate and hydrogen diffusion. Crack propagation was affected by the microstructure alignm, Tiivistelmä Ultralujien terästen vetyhauraus on tunnettu ilmiö, jonka tutkiminen on tärkeää koska se voi pahimmillaan johtaa äkkinäisiin, rakenteiden katastrofaalisiin vaurioihin. Tässä työssä kehitetään uutta testausmenetelmää nimeltään äänirautakoe (Tuning-Fork Test, TFT) ultralujien terästen vetyhaurauden tutkimiseksi. Äänirautakokeessa yhdistetään vakiosiirtymän kautta määritetty elastinen jännitystila ja sähkökemiallinen vetylataus. Erityyppisten geometrioiden ja puristimien avulla äänirautanäytteeseen vaikuttava jännitystila saatiin rajattua tarkasti alueelle, jossa särön ydintyminen tapahtuu, jolloin särönkasvua voidaan kontrolloida ja seurata. Väitöskirjan päätavoitteena oli kehittää nopea ja yksinkertainen testausmenetelmä, joka mahdollistaa ultralujien terästen vertailun ja mikrorakenteellisten tekijöiden vaikutuksen tutkimisen vedyn aiheuttamassa murtumisessa. Täydentävää tutkimusta tehtiin perinteisellä vedyn permeaatiokokeella. Ensimmäisessä vaiheessa 300–600 HBW kovuusluokan martensiittisten terästen taipumusta vetyhaurauteen arvioitiin määrittämällä terästen suhteellinen kynnysjännitys. Tulokset osoittavat kovuuden/lujuuden kasvun lisäävän teräksen alttiutta vetyhaurauteen, joka korostuu teräksen kovuuden ollessa yli 400 HBW. Voima-anturin lisääminen testauslaitteistoon paransi mittaustarkkuutta, ja mahdollisti perinnäisen austeniitin raerakenteen vaikutuksen tutkimisen. Vertailussa käytettiin 500 HBW teräksiä, jotka erosivat perinnäisen austeniitin muodon (venynyt/tasa-aksiaalinen) ja koon suhteen ja osa seostukseltaan. Venynyt perinnäisen austeniitin raerakenne paransi vetyhaurauden kestoa. Särön etenemisnopeus oli hitaampi venyneessä mikrorakenteessa, joka johtui rakeiden läpi etenevästä murtumasta sekä hitaammasta vedyn diffuusiosta. Tasa-aksiaalisessa rakenteessa vedyn diffuusio oli nopeampaa ja särön etenemisnopeus kasvoi ja murtuma tapahtui osittain raerajamurtumana. Mikrorakenteellinen anisotropia vaikutti särön etenemiseen venyneissä rakeissa

InterDendritic Solidification (IDS) is a thermodynamic–kinetic software combined with a microstructure tool developed to simulate the nonequilibrium solidification (non-EQS) of steels. Herein, its main calculation module, solidification (SOL), is introduced, and some essential results of that module, such as the formation of ferrite and austenite in different types of steels during their solidification, and the formation and dissolution of precipitates during subsequent cooling and heating processes, respectively, following solidification, are presented. The non-EQS is compared with equilibrium and poor-kinetics solidification to demonstrate the effect of kinetics on the results using finite solute diffusion and microstructure data. The poor-kinetics solidification is comparable with the modified Scheil simulation ignoring the solid-state diffusion of slowly moving metallic elements. A particular emphasis is made on demonstrating how to use a postprocessing treatment to control the residual ferrite amounts in stainless steels and the extent of precipitation in particular steel. In this context, the phenomena occurring behind the results are discussed. Finally, to validate the simulations of the SOL module, its calculations are compared with numerous solidification measurements, such as the liquidus and solidus temperatures of different steels and the residual ferrite amounts in stainless steels.

The residual compressive stresses and dimensional changes related to the lattice strains of retained austenite (RA) phase in a high-Si, medium-carbon steel (Fe-0.53C-1.67Si-0.72Mn-0.12Cr) are investigated for samples austenitized and quenched for isothermal bainitic transformation (Q&B) in the range 5 s to 1 h at 350 °C. Also, samples are directly quenched in water (DWQ) from the austenitization temperature for comparison with Q&B samples. Field emission scanning electron microscopy (FE-SEM) combined with electron backscatter diffraction (EBSD) analyses, and X-ray diffraction are used to investigate the microstructural evolution, phase distribution, and lattice parameters of RA phase. While the Q&B samples showed formation of bainite and high-carbon fresh martensite in conjunction with stabilization of various fractions of RA, the DWQ samples displayed nearly complete martensitic microstructure. For short holding durations (≪200 s), there was limited formation of bainite and the inadequate carbon partitioning to the adjacent untransformed austenite areas resulted in significant martensite formation and the associated c/a ratio of martensite resulted in high compressive residual stresses within the RA phase. While, at long isothermal holding times (≫ 200 s), there was a significant formation of bainite. The DWQ samples displayed maximum lattice strain in a small fraction of untransformed RA phase.

The static recrystallization characteristics and microstructure evolution in hot-deformed austenite were evaluated for a newly developed low-carbon CrNiMnB ultrahigh-strength steel with and without molybdenum addition. The time for 50% static recrystallization (t50%) over a wide range of strains and hot-deformation temperatures were obtained using the stress-relaxation technique on Gleeble thermomechanical simulator. Moreover, effect of deformation parameters on the size distribution and average size of prior austenite grains are investigated. A novel semi-automatic stress relaxation test reading tool with a graphical user interface was created and used successfully for the current study. The obtained results of strain‘s power and the apparent activation energy are within the range stated in literature for C-Mn and microalloyed steels. Addition of molybdenum increase the power of strain and the apparent activation energy from − 1.9 to − 2.6 and 206 to 212 kJ/mol, respectively. The retardation effect of molybdenum addition was shown by a new regression equation devised for calculating t50%. The developed equations show a good agreement with the experimental data and can be used in the designing of roughing during thermomechanical processing. The deformation parameters i.e., temperature, strain and holding time have a significant effect on the size distribution and average size of prior austenite grains.

The overwhelming impact of complex-phase microstructures to mechanical response in multiphase steels requires accurate constitutive properties of the individual phases. However, precise prediction of individual phase properties to their mechanical response is critical and sophisticated, and commonly requires multiscale characterizations and numerous approximations. In this work, by employing full phase information and semi-empirical analytical models, we accurately predict the yield strength of deformed variants of Ce-modified SAF2507 super duplex stainless steel (SDSS). High energy synchrotron X-ray diffraction (HE-SXRD) reveals the phase fractions of major phases along with secondary phases of Cr₂N and eutectic CexFey. Average lattice strain/crystallite size of the austenite and ferrite/martensite phases from the measured volume is estimated through the Williamson-Hall method. A unique composite strengthening type analytical model is used to estimate yield strength by taking individual strengthening contributions from all phases, their grain sizes, stored dislocation densities, solid solution, and precipitates. Close agreement between reconstructed and experimental yield strength is observed for several cold and cryogenic rolled SDSS. A combination of HE-SXRD and analytical model offers a time-effective virtual design pathway to engineer high-strength steel.

The influence of load ratio on the high and very high cycle fatigue (VHCF) strength of Ck45M steel processed by thermomechanical rolling integrated direct quenching was investigated. Ultrasonic fatigue tests were performed under uniaxial and torsional loading at load ratios of R = −1, 0.05, 0.3, and 0.5 with smooth specimens and specimens containing artificially introduced defects. Up to 2 × 10⁵ cycles, failure originated from surface aluminate inclusions and pits under both loading conditions. The prevailing fracture mechanisms in the VHCF regime were interior crack initiation under uniaxial loading and surface shear crack initiation under torsional loading. The mean-stress sensitivity and the fatigue strength were evaluated using fracture mechanics approaches. Equal fatigue limits for uniaxial and torsional loading were determined considering the size of crack initiating defects and the appropriate threshold condition for Mode-I crack growth. Furthermore, the mean-stress sensitivity is independent of loading condition and can be expressed by \( {\sigma}_{\mathrm{w}}(R)={\left.{\sigma}_{\mathrm{w}}\right|}_{R=-1}\cdotp {\left(\frac{1-R}{2}\right)}^{0.63} \) and \( {\tau}_{\mathrm{w}}(R)={\left.{\tau}_{\mathrm{w}}\right|}_{R=-1}\cdotp {\left(\frac{1-R}{2}\right)}^{0.63} \).

Over the last ten years, global demand for high-strength steels has grown significantly. Stricter environmental criteria and more demanding applications have accelerated the development of high-strength steels and improved the properties of these steels. The chemical composition, as well as the microalloying, of direct quenched steels, together with the manufacturing process, substantially affect the properties of the resulting steel. The use of various more special alloying elements has become more common, especially in steels made for the most demanding applications. Used correctly, these alloying elements can clearly improve the mechanical properties of steel. The aim of the dissertation was to study the effects of certain alloying elements on the microstructure and mechanical properties of direct quenched ultra-high-strength steels, and to optimize their use for maximum benefit. The compositions of the steels in the study were based on existing commercial steel grades and therefore, the achieved results can be used in the development of new steel grades. The results of the dissertation showed that phase transformations in steel can be influenced by microalloying, and especially the use of molybdenum can strongly promote the formation of lower temperature phases such as bainite and martensite. With molybdenum alloying, together with the correct process parameters, it is possible to obtain excellent mechanical properties in direct quenched steel. The results also showed that molybdenum increased tempering resistance without actual precipitation strengthening. The greatest advantage of niobium alloying with direct quenched steels was seen to be in slowing down the recrystallization process, which led to the formation of a finer martensite structure, increasing the strength as well as improving the toughness. The effect of niobium was also significant in tempering resistance, where the strong precipitation strengthening clearly increased the yield strength, Tiivistelmä Viimeisen kymmenen vuoden aikana lujien terästen kysyntä on kasvanut maailmalla merkittävästi. Tiukentuneet ympäristökriteerit sekä yhä vaativammat käyttökohteet ovat nopeuttaneet lujien terästen kehitystyötä sekä parantaneet näiden terästen ominaisuuksia. Suorasammutettujen terästen kemiallinen koostumus sekä käytettävät mikroseosaineet yhdessä valmistuspraktiikan kanssa vaikuttavat olennaisesti syntyvän teräksen ominaisuuksiin. Erilaisten seosaineiden käyttö on lisääntynyt varsinkin vaativimpien käyttökohteiden teräksissä. Näillä seosaineilla voidaan oikein käytettynä parantaa selvästi teräksen mekaanisia ominaisuuksia. Väitöstyön tavoitteena oli tutkia tiettyjen seosaineiden vaikutusta ultralujien terästen mikrorakenteeseen ja mekaanisiin ominaisuuksiin, sekä optimoida niiden käyttö maksimaalisen hyödyn aikaansaamiseksi. Tutkittavien terästen koostumukset pohjautuivat jo olemassa oleviin kaupallisiin teräksiin, joten saatuja tuloksia on mahdollista hyödyntää uusien teräslajien kehityksessä. Väitöstyön tulokset osoittivat, että mikroseostuksessa voidaan vaikuttaa teräksen faasimuutoksiin ja etenkin molybdeenin käytöllä voidaan voimakkaasti edesauttaa alempien lämpötilojen faasien, kuten bainiitin ja martensiitin muodostumista. Molybdeeniseostuksella, yhdessä oikeiden prosessiparametrien kanssa, on mahdollista saada erinomaiset mekaaniset ominaisuudet suorasammutetulle teräkselle. Tulosten perusteella nähtiin myös, että molybdeeni lisäsi päästönkestävyyttä ilman suoranaista erkautuslujittamista. Niobiseostuksen suurin etu suorasammutetuilla teräksillä nähtiin olevan rekristallisaatioprosessin hidastamisessa, mikä johti hienomman martensiittirakenteen syntyyn nostaen lujuutta sekä parantaen sitkeyttä. Niobin vaikutus oli merkittävä myös päästönkestävyydessä, missä voimakas erkautuslujittaminen nosti myötölujuutta selvästi verrattaessa ilman niobia olleisiin teräksiin. Boorivapaa niobiseostettu teräs omasi erittäin hyvät iskusitkeysominaisuudet johtuen te

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.

This work deals with the kinetic aspects of bainite formation during isothermal holding above and below the martensite start (Mₛ~275 °C) temperature using a low-alloy, high-silicon DIN 1.5025 steel in a range suitable for achieving ultrafine/nanostructured bainite. Dilatation measurements were conducted to study transformation behaviour and kinetics, while the microstructural features were examined using laser scanning confocal microscopy and electron backscatter diffraction (EBSD) techniques combined with hardness measurements. The results showed that for isothermal holding above the Mₛ temperature, the maximum bainitic transformation rate decreased with the decrease in isothermal holding temperature between 450 and 300 °C. On the other hand, for isothermal holding below the Mₛ temperature at 250 and 200 °C, the maximum rate of transformation was achieved corresponding to region I due to the partitioning of carbon and also possibly because of the ledged growth of isothermal martensite soon after the start of isothermal holding. In addition, a second peak was obvious at about 100 and 500 s, respectively, during holding at 250 and 200 °C due to the occurrence of bainitic transformation, marking the beginning of region II.

Ferritic–austenitic duplex stainless steels are known to offer favorable combinations of good mechanical properties and corrosion resistance to be used for structural purposes. Lean duplex grades have already been introduced for consideration to replace standard 18–8 austenitic stainless steels in various industrial applications. Ferrite and austenite represent different deformation behaviors and contribute to the resulting fracture toughness characteristics. Micromechanical crystal plasticity based assessment of this cleavage fracture behavior is the subject area of current work. The objective is to bridge cleavage fracture models to full field crystal plasticity imaging based modeling of microstructures of ferritic–austenitic duplex stainless steels. The goal is to introduce means to computationally assess the effects of different multi-phase microstructural morphologies to cleavage fracture toughness and develop both the respective constitutive and cleavage fracture modeling capabilities. Such means can be used as an aid to develop better cleavage resistant, while in the current context lean, steel grades. Three different steels are investigated with differing austenite phase morphologies, and their behavior with respect to fracture mechanical response evaluated by micromechanical modeling. The effect of austenite fraction and morphology in terms of improving fracture toughness, a critical parameter concerning these steel grades and improvement of their cleavage fracture properties, is investigated. Deleterious features such as large ferrite grain size and microstructural property mismatching are identified and their implications to fracture toughness and development of applicable modeling capabilities for ferritic–austenitic duplex steels discussed. Simple design task of varying the austenite phase fraction is performed using synthetic microstructural modeling and the results evaluated with respect to their influence on the fracture toughness ductile-to

In this study, fresh attempts have been made to identify and estimate the phase constituents of a high-silicon, medium carbon multiphase steel (DIN 1.5025 grade) subjected to austenitization at 900 °C for 5 min, followed by quenching and low-temperature bainitizing (Q&B) at 350 °C for 200 s. Several techniques were employed using different chemical etching reagents either individually (single-step) or in combination of two or more etchants in succession (multiple-step) for conducting color metallography. The results showed that the complex multiphase microstructures comprising a fine mixture of bainite, martensite and retained austenite phase constituents were selectivity stained/tinted with good contrasting resolution, as observed via conventional light optical microscopy observations. While the carbon-enriched martensite-retained austenite (M/RA) islands were revealed as cream-colored areas by using a double-step etching technique comprising etching with 10% ammonium persulfate followed by etching with Marble’s reagent, the dark gray-colored bainite packets were easily distinguishable from the brown-colored martensite regions. However, the high-carbon martensite and retained austenite in M/RA islands could be differentiated only after resorting to a triple-step etching technique comprising etching in succession with 2% nital, 10% ammonium persulfate solution and then warm Marble’s reagent at 30 °C. This revealed orange-colored martensite in contrast to cream-colored retained austenite in M/RA constituents, besides the presence of brown-colored martensite laths in the dark gray-colored bainitic matrix. A quadruple-step technique involving successive etching with 2% nital, 10% ammonium persulfate solution, Marble’s reagent and finally Klemm’s Ι reagent at 40 °C revealed even better contrast in comparison to the triple-step etching technique, particularly in distinguishing the RA from martensite. Observations using advanced techniques like field emission sca

Supersonic fine particles bombarding (SFPB) technology opens a new territory for engineering materials towards improved performances. Owing to its merits and emerging applications, 300M steel (tensile strength ≥1800 MPa) was treated with SFPB to create surface gradient nanostructures. The time dependent SFPB process was implemented on various 300M steel surface to investigate the microstructural evolution and mechanical property. 300M steel surface grains were sufficiently refined down to nanometer scale under high energy SFPB. In the subsurface layer, acicular martensite was found to be bent and broken, resulting in the high-density dislocation. At the early stage of SFPB, the impact affected area of 300M steel surface was deepened with increasing SFPB time, and the grains were constantly refined, which further lead to higher strength and improved hardness. However, after longer treatments of more than 90 s, bombardment energy accumulated at 300M steel surface resulted in grain growths and deteriorations of hardness. In particular, the newly formed microcracks substantially reduced the tensile strength. After SFPB treatment, the dimple size of the 300M steel surface fracture decreased significantly, and a large area of cleavage plane appeared, showing typical characteristics of ductile-brittle mixed fracture.

Phase transformation in low alloyed Fe-C steels during quenching and partitioning (Q&P), though authentically attributed to carbon diffusion, is scarcely quantified experimentally in terms of microstructural evolution and lack of fundamental backing at a quantum mechanics level. Herein, we report on a combined in-situ high energy synchrotron X-ray diffraction and density functional theory (DFT) study to unveil the physical mechanism of phase transformation in a Q&P processed advanced Fe-C steel. Beside a small fraction of bainite, a low air-quenching rate of ∼6 °C/s in the Ms to quenching temperature range leads to carbon enrichment into the untransformed austenite, which simultaneously turns up with the later stage of martensitic transformation at the existing austenite/martensite interfaces. The resulting transformations are ascertained by DFT results and attributed to a second energy barrier in ferrite/martensite facilitation to carbon diffusion to austenite at elevated temperatures, along with the well-known carbon solubility difference/equilibrium in austenite and martensite. The development of cubic martensite in the carbon steel is theoretically elucidated and attributed to more probable hopping sites and diffusion paths of carbon in the ferrite/martensite than in the austenite.

The interaction of different alloying elements has a significant impact on the mechanical and microstructural properties of steel products due to the thermodynamic and kinetic effect. This article presents a statistically developed and validated model for austenite decomposition during cooling, based on a set of experimental continuous cooling transformation diagrams available in literature. In the model, two-way interactions of the alloying elements are included as add-on terms, and the procedure for the analysis ensures there is no overfitting. The model can be used to predict phase transformation temperatures and critical cooling rates for the formation of polygonal ferrite, bainite or martensite for the production of steel.

Density functional theory based computational study has been conducted in order to investigate the effect of substitution of Cr and Co components by Si on the structure, mechanical, electronic, and magnetic properties of the high entropy alloy CrCoNiFe. It is found that the presence of a moderate concentration of Si substitutes (up to 12.5%) does not significantly reduce the structural and mechanical stability of CrCoNiFe while it may modify its electronic and magnetic properties. Based on that, Si is proposed as a cheap and functional material for partial substitution of Cr or Co in CrCoNiFe.

Quality assurance is one of the largest challenges to the widespread adoption of metal additive manufacturing technology. Deposition pattern can significantly impact the temperature distribution during manufacturing process and thus the overall quality and residual stress formation of the manufactured components. In order to explore the intricate relationship, three different deposition patterns in DED-Arc additive manufacturing, the meander pattern, the spiral pattern and the newly developed S pattern, were experimentally and numerically scrutinized in terms of the resulting temperature distribution, grain size, porosity as well as residual stress formation. The study reveals that the variation of the deposition paths results in characteristic temperature fields and gradients with distinct local peak temperatures that determine the deposition quality, and simultaneously offers great degrees of freedom for optimal pattern design. Comparing the results with different deposition patterns, the S pattern leads to a more homogeneous temperature distribution, showing beneficial effects on the microstructure, porosity and residual stress formation in the deposited Al-4046 material, and is thus regarded as a promising alternative for improving additively manufactured parts quality.

Herein the effects of molybdenum and niobium on the microstructures and mechanical properties of laboratory‐rolled and direct‐quenched and direct‐quenched and tempered steels are revealed. The microstructures are martensitic with yield strength of 766–1119 MPa in direct‐quenched condition and 632–1011 MPa in direct‐quenched and tempered condition. Mo and Nb additions lead to a fine martensitic microstructure that imparts a good combination of strength and toughness. Steel with 0.5 wt% molybdenum has a high yield strength of 1119 MPa combined with low 28 J transition temperature of −95 °C in direct‐quenched condition. Molybdenum and niobium increase the strength significantly during tempering due to enhanced solute drag and precipitation hardening. Addition of 0.25 wt% molybdenum increases yield strength from 632 to 813 MPa after tempering. However, the combination of niobium and molybdenum results in even greater increase in yield strength during tempering compared to the nonalloyed version, producing almost 400 MPa increase in yield strength. Transmission electron microscopy reveals that only niobium forms stable precipitates during tempering, indicating that molybdenum largely remains in solution. X‐ray diffraction analysis elucidates that molybdenum and molybdenum–niobium alloying prevents annihilation of dislocations, leading to the presence of high densities of dislocations after tempering.

The present paper aims to compare the microstructural and mechanical properties of CK45 carbon steel plates, joined by friction stir (FSW) and tungsten inert gas (TIG) welding methods. Besides visual inspection, the welded joints and the base material were subsequently evaluated in respect of optical microstructures, hardness and tensile properties. Sound joints could be accomplished using both the FSW and TIG welding methods through proper selection of process parameters and the filler metal. The influence of a water-cooling system on the FSW and various filler metals on the quality of TIG welding were further assessed. Both the FS welded sample as well as TIG welded samples with two different filler metals ER70S-6 and ER80S-B2 exhibited brittle behavior that could be mitigated through optimized water cooling and use of R60 filler metal. A drastic reduction of brittle martensite phase constituent in the microstructure corroborated significant improvements in mechanical properties of the welded zones for both the FSW sample as well as TIG welded samples with R60 filler metal.

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, 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

New continuous cooling transformation (CCT) equations have been optimized to calculate the start temperatures and critical cooling rates of phase formations during austenite decomposition in low-alloyed steels. Experimental CCT data from the literature were used for applying the recently developed method of calculating the grain boundary soluble compositions of the steels for optimization. These compositions, which are influenced by solute microsegregation and precipitation depending on the heating/cooling/holding process, are expected to control the start of the austenite decomposition, if initiated at the grain boundaries. The current optimization was carried out rigorously for an extended set of steels than used previously, besides including three new solute elements, Al, Cu and B, in the CCT-equations. The validity of the equations was, therefore, boosted not only due to the inclusion of new elements, but also due to the addition of more low-alloyed steels in the optimization. The final optimization was made with a mini-tab tool, which discarded statistically insignificant parameters from the equations and made them prudently safer to use. Using a thermodynamic-kinetic software, IDS, the new equations were further validated using new experimental CCT data measured in this study. The agreement is good both for the phase transformation start temperatures as well as the final phase fractions. In addition, IDS simulations were carried out to construct the CCT diagrams and the final phase fraction diagrams for 17 steels and two cast irons, in order to outline the influence of solute elements on the calculations and their relationship with literature recommendations.