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The effect of cooling rate and Nb addition on the phase transformation, microstructures, and mechanical properties of thermomechanically controlled hot-rolled low-carbon CrNiMnMoB ultrahigh-strength steel were studied. The aspects impeding Nb's benefits on hardenability, phase transformation, and mechanical properties were investigated. The hot rolling process was simulated, and a continuous cooling transformation diagram was constructed for the studied steels. Microstructures were analysed using laser scanning confocal microscopy and field emission scanning electron microscopy, with the latter also used for in-depth investigations of the non-metallic inclusions. Transmission electron microscopy was employed to investigate the precipitate characteristics. The concentrations of Nb in the solution and precipitates were quantified for the martensitic and granular bainitic structures. The effect of Nb in solution on phase transformation and strengthening is diminished, and the hardness is reduced by Nb precipitation at an early processing stage. Decreasing the cooling rate promotes the formation of granular bainitic structures rather than martensite. After being subjected to hot rolling followed by water quenching and air cooling, the microstructures are autotempered martensite and fully granular bainite, respectively. Various non-metallic inclusions, including Al2O3, MnS, and Al2O3–MnS, MnS·BN, Al2O3·BN and MnS·Al2O3·BN were observed. The number density and area fraction of non-metallic inclusions in the Nb-containing steel and the air-cooled samples are higher than those observed in the Nb-free and water-quenched samples. Adding 0.04 wt% of Nb slightly increased the strength, but the toughness properties dropped, mainly due to the higher number density and area fraction of non-metallic inclusions and primary large precipitates.

Ball milling is a fundamental step of powder metallurgy widely employed for composite manufacturing. This work focuses on the influence of ball milling time on the morphological, electrical, and tribological properties of self-lubricating copper-tungsten disulfide (Cu-WS2) composites. The study investigates ball milling times between 1 and 24 h to guarantee different degrees of incorporation of WS2 in the copper matrix. Micro-scratch and wear tests are performed to evaluate the tribological behavior. Optical, scanning electron, and confocal laser scanning microscopy analyze the scratch and wear tracks. The results show the reliability of the production process and a general improvement of the composites' mechanical properties compared to pure copper. The addition of WS2 enhances the tribo-mechanical properties, increasing hardness and wear resistance and decreasing the friction coefficient. Shorter ball milling times result in larger WS2 flakes distributed in the copper matrix, while longer ball milling times result in smaller and more dispersed particles. This homogeneous fine dispersion determines a difference in the composites' electrical conductivity and tribological performance, with shorter ball milling times (i.e., between 2 and 4 h) offering the best trade-off between wear behavior and electrical properties.

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.

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.

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.

The microstructure and mechanical properties of a low-alloy medium carbon steel (Fe-0.5C-0.9Mn-1Cr-0.16V, in wt.%) were investigated after rapid tempering and compared with a conventionally tempered counterpart. The conventional thermal cycle was performed in a laboratory-scale box furnace while rapid heat treatments were carried out using the Gleeble 3800 thermomechanical simulator machine. In the rapid heat treatments, the heating rate was 50 °C/s for austenitizing and 60 °C/s for the tempering process, with a cooling rate of 60 °C/s for both treatments. Austenitization was performed at 900 °C for 3 s and tempering was conducted at 300 °C and 500 °C for 2 s. For conventional routes, the heating rate for both austenitization and tempering was 5 °C/s. Likewise, the austenitization was carried out at 900 °C for 45 min and tempering was carried out at 300 °C and 500 °C for 30 min. The results revealed that rapid tempering resulted in a significantly increased impact toughness compared to conventional tempering, while maintaining a consistent high strength level. The quenched samples showed the highest hardness and tensile strength but obtained the lowest toughness values. The optimum combination of strength and toughness was achieved with the sample rapidly tempered at 300 °C, resulting in a tensile strength of 2050 MPa and impact energy of 14 J for sub-sized CVN samples. These desirable mechanical properties were achieved throughout the tempered martensitic microstructure with a minor fraction of pearlitic strings.

Abstract The widely used stainless steels and their deformed variants are anticorrosive in ambient conditions due to passivation layers composed of chromium oxides. Conventionally, corrosion and erosion of the steels are attributed to the breakdown of such layers but seldomly to the origin that depends on surface heterogeneity at the microscopic level. In this work, the nanometer-scaled chemical heterogeneity at the surface unveiled via spectro-microscopy and chemometric analysis unexpectedly dominates the breakdown and corrosion behavior of the cold-rolled Ce-modified 2507 super-duplex stainless steels (SDSS) over its hot-deformed counterpart. Though relatively uniformly covered by a native Cr2O3 layer revealed by X-ray photoemission electron microscopy, the cold-rolled SDSS behaved poorly in passivity because of locally distributed Fe3+ rich nano-islands over the Fe/Cr oxide layer. This atomic-level knowledge provides a deep understanding of corrosion of stainless steel and is expected to benefit corrosion controls of similar high-alloyed metals.

The novel processing concept of direct quenching and partitioning (DQ&P) has been explored with a medium-carbon (0.4 wt.% C) steel to evaluate and optimize the processing route for excellent property combinations. New compositional design approach was based on physical simulation studies aiming to understand the influence of varying silicon contents (1.5, 0.75 and 0.25 wt.%) and Q&P processing parameters on microstructural development including carbide formation and retained austenite stabilization. Optimized Q&P parameters were selected to design a DQ&P processing route for laboratory hot-rolling trials based on the analyses of physical simulation data. The overall aim of the study was to produce ultrahigh-strength structural steels with yield strength ≥1100 MPa combined with high uniform and total elongation and impact toughness, achieved through designing a low-temperature quenching and partitioning route with effective carbon partitioning. The DQ&P steels gained an excellent combination of mechanical properties comprising of high yield strength of ∼1000–1200 MPa and tensile strength ∼2100–2300 MPa, good elongation (∼11–13%), and moderate impact toughness transition temperature T28J ∼ (−5 to +12 °C). Straining of austenite prior to DQ&P led to an extensive refinement of the final martensitic-austenitic nanostructure. Formation of nanoscale lath-martensite and fine film-like retained austenite structures enabled the observed improvement of mechanical properties. Besides the formation of nano-twinned martensite, inter-lath austenite and transitional carbides were comprehensively characterized. No adverse effect of prolonged partitioning during slow cooling, simulating coiling in actual industrial practice, has been noticed suggesting new possibilities for developing tough, ductile structural steels both for strip/plate products.

A new combination of factors enhancing the stabilization of austenite, including pre-existed austenite among quenched martensite, prior deformation, and partitioning at high temperatures is employed to create a multi-component refined microstructure in a medium Mn steel (Fe–4Mn–0.31C–2Ni–0.5Al–0.2Mo, wt.%). The microstructure evolution and phase fraction during the processing are systematically investigated using various characterization methods. The microstructure of the specimen after 0.4 strain deformation of 73% martensite–27% austenite at 250 °C and subsequent partition-annealing at 600 °C for 20 min was composed of several phases including tempered martensite, fresh martensite, pearlite, 10% of retained austenite (RA) and undissolved cementite. By increasing the annealing temperature, the pearlitic transformation was suppressed, whereas recrystallization of the deformed martensite and carbide dissolution occurred following annealing at 650 °C for 20 min resulting in an ultrafine-grained microstructure composed of equiaxed ferrite, 32% RA along with some fresh martensite during final cooling and few carbide precipitates. The results demonstrate that the combinatorial approach accelerated partitioning of alloying elements from martensite and carbides to largely pre-existing austenite is responsible for the improved austenite stabilization during intercritical annealing of the deformed dual-phase specimens. However, competitive processes are also enhanced so that the RA content is not increased by deformation.

Quenching and partitioning (Q&P) treated steels are traditionally alloyed with silicon (Si), but its precise role on microstructural mechanisms occurring during partitioning is not thoroughly understood. In this study, dilatometric analysis has been combined with detailed microstructural characterization to unravel the competing mechanisms occurring during partitioning either in parallel or in succession. Three 0.4 wt.% carbon steels with varying Si contents were quenched to 150 °C for ~20% untransformed austenite, and partitioned for 10–1000 s in the temperature range 200–300 °C. The steel with low Si content (0.25 wt.%) exhibited substantial bainitic transformation during partitioning at 300 °C and only 4% retained austenite (RA) at room temperature (RT) even after 1000 s hold. In contrast, a high Si fraction (1.5 wt.%) enabled retention of ~18% austenite under similar conditions. While η-carbides precipitated within the martensite laths in the high-Si steel, cementite precipitated in the low-Si variant. Furthermore, carbide precipitation and growth were strongly suppressed by high Si content. Secondary martensite formation occurred from carbon-enriched austenite during final cooling, irrespective of Si-content. Results illustrate that Si retards austenite decomposition at higher partitioning temperatures but does not improve carbon partitioning at lower temperatures.

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.

A typical rare-earth element modified Ce-SAF 2507 super duplex stainless steel was isothermally hot-compressed to reach superior mechanic properties over its pristine and peers. Mechanical, macro- and micro-structural evolutions subjected to hot-modification were studied in detail toward an optimal hot working for the Ce-SAF2507. A dynamic softening phenomenon shows that the increase of hot deformation temperature and decrease of the strain rate were dominated by the dynamic recovery of ferrite at a high strain rate and low deformation temperature. The same phenomenon at a low strain rate but high deformation temperature was ruled by the dynamic recrystallization of austenite. These two processes determined significant phase transformations from austenite to ferrite under higher deformation temperature and strain rate. A hot deformation activation energy Q ∼406 kJ mol−1 was obtained through a unified strain-compensated constitutive equation for Ce-SAF2507. Quantitative grain size refinements further proved the above mechanisms deduced from mechanical and microscopic observations, while structure induced changes of mechanical properties were crosschecked with the microhardness. Insights of microstructure also demonstrated the existences of the Cr2N in both phases, grain boundaries, and α/γ interface. The overall deformation dynamics was explicated based on the structural and quantitative results.

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.

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.

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.

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.

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. Keywords: Low C microalloyed/interstitial-free steels, Flow stress, Deformation induced ferrite transformation, Continuous dynamic recrystallization, Ultrafine ferrite grains, EBSD/TEM analysis

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.

The presence of acicular ferrite (AF) in the heat-affected zone (HAZ) of steels used offshore is generally seen as beneficial for toughness. In this study, the effects of varying fractions of AF (0–49 vol.%) were assessed in the simulated, unaltered and coarse-grained heat-affected zones (CGHAZ) of three experimental steels. Two steels were deoxidized using Ti and one using Al. The characterization was carried out by using electron microscopy, energy-dispersive X-ray spectrometry, electron backscatter diffraction and X-ray diffraction. The fraction of AF varied with the heat input and cooling time applied in the Gleeble thermomechanical simulator. AF was present in one of the Ti-deoxidized steels with all the applied cooling times, and its fraction increased with increasing cooling time. However, in other materials, only a small fraction (13–22%) of AF was present and only when the longest cooling time was applied. The impact toughness of the simulated specimens was evaluated using instrumented Charpy V-notch testing. Contrary to the assumption, the highest impact toughness was obtained in the conventional Al-deoxidized steel with little or no AF in the microstructure, while the variants with the highest fraction of AF had the lowest impact toughness. It was concluded that the coarser microstructural and inclusion features of the steels with AF and also the fraction of AF may not have been great enough to improve the CGHAZ toughness of the steels investigated.

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.

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 scanning electron microscopy (FE-SEM) and electron back scatter diffraction (EBSD) failed to differentiate untempered, high-carbon martensite from retained austenite in the M/RA islands and martensite laths from bainitic matrix, respectively. Transmission electron microscopy (TEM) studies successfully distinguished the RA from high-carbon martensite, as noticed in M/RA islands. The volume fraction of retained austenite estimated by EBSD, XRD and a point counting method on color micrographs of quadruple-step etched samples showed good agreement.

Stress relief treatments were carried out separately with a pneumatic chipping hammer, ultrasonic peening treatment, and heat treatment for metal active-gas welding (MAG) welded joints of 2205 duplex stainless steel. The effects of these methods on the residual stress, microstructure, mechanical properties and corrosion resistance of welded joints were studied. Results show the stress state of the weld and the surrounding area was effectively improved by the pneumatic chipping hammer and ultrasonic peening treatment, and the residual stress field of the surface layer changed from tensile stress to compressive stress. On the contrary, low-temperature stress relieving annealing had no obvious effect on stress distribution. After the pneumatic chipping hammer and ultrasonic peening treatment, the welded joints were machined and hardened. Correspondingly, strength and hardness were improved. However, the heat treatment only led to a slight decrease in strength and hardness due to the static recovery of the welded joint structure. All stress relief methods effectively improved the corrosion resistance of welded joints, with the ultrasonic peening treatment giving the best performance.

Deterioration of the toughness in heat-affected zones (HAZs) due to the thermal cycles caused by welding is a known problem in offshore steels. Acicular ferrite (AF) in the HAZ is generally considered beneficial regarding the toughness. Three experimental steels were studied in order to find optimal conditions for the AF formation in the coarse-grained heat-affected zone (CGHAZ). One of the steels was Al-deoxidized, while the other two were Ti-deoxidized. The main focus was to distinguish whether the deoxidation practice affected the AF formation in the simulated CGHAZ. First, two different peak temperatures and prolonged annealing were used to study the prior austenite grain coarsening. Then, the effect of welding heat input was studied by applying three cooling times from 800 °C to 500 °C in a Gleeble thermomechanical simulator. The materials were characterized using electron microscopy, energy-dispersive X-ray spectrometry, and electron backscatter diffraction. The Mn depletion along the matrix-particle interface was modelled and measured. It was found that AF formed in the simulated CGHAZ of one of the Ti-deoxidized steels and its fraction increased with increasing cooling time. In this steel, the inclusions consisted mainly of small (1–4 μm) TiOx-MnS, and the tendency for prior austenite grain coarsening was the highest.

Low-carbon, low-alloy steels undergo auto-tempering and carbon partitioning to austenite during quenching to martensite. The microstructures of two such steels quenched at two cooling rates have been evaluated using electron microscopy to characterise lath and carbide precipitate morphologies, and the results have been compared with theoretical predictions based on the Thermo-Calc modules DICTRA and TC-Prisma. The modelling tools predicted the carbon depletion rates due to diffusion from the bcc martensite laths into austenite and the precipitation of cementite in the ferrite matrix. The predictions showed a satisfactory agreement with the metallographic results, indicating that the Thermo-Calc based software can aid in the design of new low-carbon martensitic steels.

This paper focuses on understanding the effect of niobium content on the phase transformation behavior and resultant mechanical properties of thermomechanically rolled and direct-quenched low carbon steels containing 0.08 wt.% carbon. Investigated steels contained three different levels of niobium: 0, 0.02 and 0.05 wt.%. The continuous cooling transformation (CCT) diagrams covering cooling rates in the range 3–96 °C/s constructed based on the dilatometer studies showed only a minor effects of Nb on the phase transformation characteristics. In addition, experiments were performed for reheating and soaking the slabs at 1050–1200 °C and the results revealed that for these low-carbon steels, Nb failed to prevent the austenite grain growth during slab reheating. In the case of hot rolling trials, two different finish rolling temperatures of 820 °C and 920 °C were used to obtain different levels of pancaking in the austenite prior to direct quenching. The resultant microstructures were essentially mixtures of autotempered martensite and lower bainite imparting yield strengths in the range 940–1070 MPa. The lower finish rolling temperature enabled better combinations of strength and toughness in all the cases, predominantly due to a higher degree of pancaking in the austenite. The optimum level of Nb in the steel was ascertained to be 0.02 wt.%, which resulted not only in marginally higher strength but also without any significant loss of impact toughness.

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.

The effect of electroslag remelting (ESR) with CaF2-based synthetic slag on the microstructure and mechanical properties of three as-quenched martensitic/martensitic-bainitic ultrahigh-strength steels with tensile strengths in the range of 1250−2000 MPa was investigated. Ingots were produced both without ESR, using induction furnace melting and casting, and with subsequent ESR. The cast ingots were forged at temperatures between 1100 and 950 °C and air cooled. Final microstructures were investigated using laser scanning confocal microscopy, field emission scanning electron microscopy, electron backscatter diffraction, electron probe microanalysis, X-ray diffraction, color etching, and micro-hardness measurements. Mechanical properties were investigated through measurement of hardness, tensile properties and Charpy-V impact toughness. The microstructures of the investigated steels were mainly auto-tempered martensite in addition to small fractions of retained austenite and bainite. Due to the consequences of subtle modifications in chemical composition, ESR had a considerable impact on the final microstructural features: Prior austenite grain, effective martensite grain, and lath sizes were refined by up to 52%, 38%, and 28%, respectively. Moreover, the 95th percentiles in the cumulative size distribution of the precipitates decreased by up to 18%. However, ESR had little, if any, the effect on microsegregation. The variable effects of ESR on mechanical properties and how they depend on the initial steel composition are discussed.

As-quenched low-carbon martensitic steels (

The effect of chromium content in the range of 1 wt.%−4 wt.% on the microstructure and mechanical properties of controlled-rolled and direct-quenched 12 mm thick low-carbon (0.04 wt.%) steel plates containing 0.06 wt.% Nb has been studied. In these microalloyed 700 MPa grade steels, the aim was to achieve a robust bainitic microstructure with a yield strength of 700 MPa combined with good tensile ductility and impact toughness. Continuous cooling transformation diagrams of deformed and non-deformed austenite were recorded to study the effect of Cr and hot deformation on the transformation behavior of the investigated steels. Depending on the cooling rate, the microstructures consist of one or more of the following microstructural constituents: bainitic ferrite, granular bainite, polygonal ferrite, and pearlite. The fraction of bainitic ferrite decreases with decreasing cooling rate, giving an increasing fraction of granular bainite and polygonal ferrite and a reduction in the hardness of the transformation products. Polygonal ferrite formation depends mainly on the Cr content and the cooling rate. In both deformed and non-deformed austenite, increasing the Cr content enhances the hardenability and refines the final microstructure, shifting the ferrite start curve to lower cooling rates. Preceding austenite deformation promotes the formation of polygonal ferrite at lower cooling rates, which leads to a decrease in hardness. In hot-rolled and direct-quenched plates, decreasing the Cr content promotes the formation of polygonal ferrite leading to an increase in the impact toughness and elongation but also a loss of yield strength.

The effect of grain size in the range 72 to 190 μm and carbon content in the range 0.105−0.073 wt.% on the intergranular corrosion of the austenitic stainless steel 301 has been investigated. Grain boundary chromium depletion has been studied directly using energy dispersive X-ray spectroscopy combined with scanning transmission electron microscopy and indirectly using double loop electrochemical potentiokinetic reactivation tests. In addition, chromium depletion has been modelled using the CALPHAD Thermo-Calc software TC-DICTRA. It is shown that the degree of sensitization measured using the double loop electrochemical potentiokinetic reactivation tests can be successfully predicted with the aid of a depletion parameter based on the modelled chromium depletion profiles for heat treatment times covering both the sensitization and de-sensitization or self-healing. Additionally, along with intergranular M23C6 carbides, intragranular M23C6 and Cr2N nitrides that affect the available Cr for grain boundary carbide precipitation were also observed.

In this paper, the effects of different tempering temperatures on a recently developed ultrahigh-strength steel with 0.4 wt.% carbon content were studied. The steel is designed to be used in press-hardening for different wear applications, which require high surface hardness (650 HV/58 HRC). Hot-rolled steel sheet from a hot strip mill was austenitized, water quenched and subjected to 2-h tempering at different temperatures ranging from 150 °C to 400 °C. Mechanical properties, microstructure, dislocation densities, and fracture surfaces of the steels were characterized. Tensile strength greater than 2200 MPa and hardness above 650 HV/58 HRC were measured for the as-quenched variant. Tempering decreased the tensile strength and hardness, but yield strength increased with low-temperature tempering (150 °C and 200 °C). Charpy-V impact toughness improved with low-temperature tempering, but tempered martensite embrittlement at 300 °C and 400 °C decreased the impact toughness at −40 °C. Dislocation densities as estimated using X-ray diffraction showed a linear decrease with increasing tempering temperature. Retained austenite was present in the water quenched and low-temperature tempered samples, but no retained austenite was found in samples subjected to tempering at 300 °C or higher. The substantial changes in the microstructure of the steels caused by the tempering are discussed.

A large number of thermodynamic simulations has been used to design a new Nb-Ti dual stabilized ferritic stainless steel with excellent creep resistance at 1050 °C through an optimal volume fraction of Laves (η) phase stabilized by the alloying elements Nb, Si and Mo. By raising the dissolution temperature of the phase, which also corresponds to the onset of rapid grain growth, the steel will better maintain the mechanical properties at higher service temperature. Laves phase precipitates can also improve creep resistance through precipitation strengthening and grain boundary pinning depending on the dominant creep mechanism. Sag tests at high temperatures for the designed steel showed significantly better results compared to other ferritic stainless steels typically used in high temperature applications at present.

The direct quenching process is an energy- and resource-efficient process for making high-strength structural steels with good toughness, weldability, and bendability. This paper presents the results of an investigation into the effect of molybdenum and niobium on the microstructures and mechanical properties of laboratory rolled and direct-quenched 11 mm thick steel plates containing 0.16 wt.% C. Three of the studied compositions were niobium-free, having molybdenum contents of 0 wt.%, 0.25 wt.%, and 0.5 wt.%. In addition, a composition containing 0.25 wt.% molybdenum and 0.04 wt.% niobium was studied. Prior to direct quenching, finish rolling temperatures (FRTs) of about 800 °C and 900 °C were used to obtain different levels of austenite pancaking. The final direct-quenched microstructures were martensitic and yield strengths varied in the range of 766–1119 MPa. Mo and Nb additions led to a refined martensitic microstructure that resulted in a good combination of strength and toughness. Furthermore, Mo and Nb alloying significantly reduced the amount of strain-induced ferrite in the microstructure at lower FRTs (800 °C). The steel with 0.5 wt.% Mo exhibited a high yield strength of 1119 MPa combined with very low 28 J transition temperature of −95 °C in the as-quenched condition. Improved mechanical properties of Mo and Mo–Nb steels can be attributed to the improved boron protection. Also, the crystallographic texture of the investigated steels showed that Nb and Nb–Mo alloying increased the amount of {112} and {554} texture components. The 0Mo steel also contained the texture components of {110} and {011}, which can be considered to be detrimental for impact toughness properties.

A new experimental steel containing in weight percent 0.3C-2.0Mn-0.5Si-1.0Al-2.2Cr and 0.3C-1.9Mn-1.0Si-1.0Cr was hot rolled in a laboratory rolling mill and directly quenched within the martensite start and finish temperature range. It was then partitioned without reheating during slow furnace cooling to achieve tensile yield strengths over 1100 MPa with good combinations of strength, ductility and impact toughness. Gleeble thermomechanical simulations led to the selection of the partitioning at the temperatures 175 and 225 °C, which produced the desired microstructures of lath martensite with finely divided retained austenite in fractions of 6.5% and 10% respectively. The microstructures were analyzed using light and scanning electron microscopy in combination with electron backscatter diffraction and X-ray diffraction analysis. The mechanical properties were characterized extensively using hardness, tensile and Charpy V impact testing. In tensile testing a transformation induced plasticity mechanism was shown to operate with the less stable, carbon-poorer retained austenite, which transformed to martensite during straining. The auspicious results in respect to microstructures and mechanical properties indicate that there are possibilities for developing tough ductile structural steels through thermomechanical rolling followed by the direct quenching and partitioning route.