Papers by Keyword: Martensite

Abstract: Relevance of the work. One of the key challenges currently faced by manufacturers of steel wire rod for welding applications worldwide is achieving maximum plasticity in the metal before delivery to hardware plants. This need arises due to the ongoing technological advancements in hardware plants and the integration of modern equipment capable of producing wire at high drawing speeds with significant degrees of unit deformation. Such processes demand high-quality raw materials (wire rod). Another crucial aspect is ensuring a high-quality surface for the welding wire, free from microcracks and hard inclusions. These defects can negatively impact the welding process by increasing metal spatter, causing electric arc instability, and leading to critical defects in welded joints, such as cavities and cracks. Rational microstructural design is a key principle in producing highly plastic wire rod for the efficient manufacturing of solid-section welding wire from low-carbon alloy steels. The presence of hard components (such as martensite, bainite, or their mixtures) in the wire rod’s structure is undesirable, as they increase the risk of metal failure during intensive cold deformation by drawing. Minimizing these hard phases remains an urgent scientific and practical challenge. The aim of the study is to determine the mechanism of martensite grains formation during slow cooling of low-carbon alloy steel grade CrMoV1Si wire rod, used for producing welding wire, from the austenitization temperature to room temperature. Material and methodology. The material used in this study was steel with the chemical composition (in wt.%) 0.08C–1.30Mn–0.54Si–1.06Cr–0.54Mo–0.24V–Fe_(balance). The samples were subjected to thermal cycles using an automated Gleeble 3800 thermal deformation simulation system. The thermal cycle involved heating the samples to the temperature required for complete austenitization, holding them at this temperature, and then continuously cooling at controlled rates of 0.20, 0.10, and 0.05 °C/s to room temperature. Metallographic analysis was conducted using an optical microscope and a scanning electron microscope. The hardness of individual structural components was measured using a microhardness tester following the standard method. The chemical composition of the phases was determined by energy-dispersive X-ray spectroscopy and Auger electron spectroscopy. Results. The study established an anomalous increase in the volume fraction of austenite shear transformation products in CrMoV1Si steel after continuous slow cooling at rates of 0.20–0.05 °C/s, at the same time, martensite had a relatively high carbon content (up to 1.6 wt.%). The authors attribute this microstructural evolution to dynamic changes in the chemical composition of individual phases during cooling, primarily due to the partitioning of carbon and other alloying elements between the α and γ phases, as confirmed experimentally. Based on the obtained results, a mechanism has been proposed for the formation of high-carbon martensite grains in low-carbon alloy steel wire rod during slow cooling from the austenitization temperature to room temperature.

Abstract: This study investigates the microstructural evolution and mechanical response of 51CrV4 spring steel subjected to flash quenching and tempering using a continuous high-speed induction heating line. The steel, supplied as 8 mm thick sheets with a composition of Fe–0.5C–0.9Mn–1Cr–0.16V (wt.%), was processed through rapid austenitisation at 900 °C (~200 °C/s), followed by water quenching and tempering at 300 °C. Rapid induction heat treatment was utilized to produce a hardened surface layer with refined microstructure and balanced mechanical properties. Optical microscopy revealed a uniform, crack-free martensitic layer extending to approximately ~1.2 mm from the surface, while hardness profiling showed a gradient from 590 ± 20 HV at the surface to 240–330 HV in the core. Electron Backscatter Diffraction (EBSD) analysis confirmed a fully martensitic surface structure with refined prior austenite grains (~3.2 µm), and FESEM imaging indicated minimal carbide coarsening, supporting the effectiveness of short-time tempering. These results demonstrate that flash induction processing can produce a hardened shell with retained core ductility. The consistency between EBSD, FESEM, and hardness data validates the process as an energy-efficient, scalable alternative to conventional furnace-based treatments.

Abstract: The quenched-in microstructures of Ti-15at%Nb-1at%O alloy after solid solution treatment (SST) in β phase for different SST times were investigated. In the as-rolled sample before SST, the α phase formed at the grain boundaries, and coarse martensite laths of α" phase formed in the grains. In the sample after SST for the time from 0.3 to 2.4 ks, a bundle-microstructure containing α" phase laths nucleating in the same crystallographical direction was formed. In the sample with SST for 4.8ks, the α" phase laths did not form in the area at a certain distance away from the grain boundaries, and the β+ω phase formed in that area. The rest of the areas were covered by the acicular laths of α" phase. The sample after SST for 10.8 ks exhibited the acicular laths of α" phase formed uniformly in the grains. The inhomogeneous oxygen distribution would significantly affect the microstructure formation of an oxygen-containing Ti-Nb alloy.

Abstract: The present study is an attempt to provide the solutions to the problem encountered by the components subjected to metal-to-metal wear or galling from grass root level to the advanced stages. Sliding wear or metal to metal wear, galling, can result in seizure due to bonding of the materials and can generate large amounts of damage over small sliding distances. More damage could be expected under unlubricated sliding systems. At lower loads, and particularly at high relative velocities could lead to considerable heating of the metallic surfaces, oxide growth and stripping can also be possibly observed. Under similar conditions cobalt based and nickel based hard facings were considered to be most suited, but due to the high cost of these materials an attempt has been made to achieve adequate hardness and wear resistance at much lower cost, by alloy additions with the help of Shielded Metal Arc Welding (SMAW) process. In this paper the effect of alloying elements on the wear performance of hard-faced components prepared by Shielded Metal Arc Welding (SMAW) process has been undertaken on the low carbon steel substrate by different compositions of iron (Fe) based, hard facing electrodes. The effect of alloying elements especially with varying compositions of chromium and molybdenum on the microstructure, microhardness, and wear resistance of the Fe-based hardfacing alloyed specimens were investigated by means of optical microscopy, and pin on disc wear test. The hardness and wear resistance were improved with the addition of principal alloying elements such as chromium (Cr), molybdenum (Mo) and manganese (Mn) through the consumable electrode during hardfacing by SMAW process. The microhardness of substrate material, i.e., before hardfacing was around 100 HV that latter improved up to 280 HV using first electrode E1, 330 HV using second electrode E2 and 350 HV using third electrode E3. Sliding wear for metal-to-metal wear testing was conducted as per ASTM G99 standards and wear resistance was calculated in terms of the weight loss of the pin after the test run. Wear resistance was found to be improved by 45% approximately with the electrodes E1 and E2 which have chromium content from 2.5% to 4.5 %, whereas an improvement up to 54% was observed with the third electrode E3 corresponding to 6% chromium. The percentage of carbide was found to be more in hardfaced layer in the presence of the molybdenum (Mo). The improvement of hardness and wear resistance of the hardfacing layer is attributed to the solution strengthening of Mo alloying elements. It was further observed that samples that have higher Cr content possessed finer grains with martensitic structure. Role of Mn can also be very important as it removes oxygen and Sulphur from the coatings and improve toughness and overall strength, on the other hand presence of silicon (Si) can attribute to improved yield strength.

Abstract: This study investigates the impact of varied heat treatment parameters on the mechanical and metallurgical characteristics of 9254 steel. Different cylindrical specimens underwent controlled heat treatments targeting three different phases. The interplay of time and temperature was systematically explored to understand their influence on bending strength, bending deflection, hardness, and microstructural evolution. The results revealed that a partially tempered martensitic structure exhibiting an exceptional ultimate strength of 4308 MPa. Achieving this involved a heat treatment, starting at 900°C for 30 minutes, followed by rapid cooling in an oil bath, quenching at 165°C for 5 minutes, annealing at 180°C for 60 minutes, and gradual air-cooling. This treatment regimen produced a specimen with a desirable combination of mechanical properties, showcasing its potential significance in advanced engineering applications.

Abstract: Hydrogen embrittlement (HE) is a well-known issue, especially with ultrahigh-strength steels (UHSS). Various testing methods are utilised to study HE, but they typically require tensile test equipment, or are impractical due to limited stress control with standard geometries. We have developed a novel Tuning-fork test (TFT) to study HE susceptibility of steels with a new specimen geometry, which can be stressed accurately without tensile test equipment. The test method utilises in situ electrochemical hydrogen charging and constant displacement for stressing of the notched specimens by bending. Crack initiation and propagation are controlled with an isolated tensile stress region, and the failure process is monitored with a loadcell. TFT is a simple and fast testing method, which allows ranking of UHSSs, and to investigate, e.g., microstructural effects on susceptibility to HE and H-induced fracture processes. Here in this study, we present the state-of-the-art with the improved more precise second-generation TFT setup, which benefits from a more sensitive loadcell and a more stable fine-tuneable differential screw adjustment. We extend TFT to testing of martensitic steels with nominal hardness from 400 HBW to 600 HBW with the Incremental step loading technique (ISLT). The results show that TFT with ISLT is well applicable for ranking ultrahigh-strength steels based on their susceptibility to HE. Force-time data from ISLT can also be used for the determination of a material-specific threshold stress level, and the last step for the calculation of a crack initiation-time and time-to-fracture. However, the current manual operation of the loading screw can still limit maximum duration of a test.

Abstract: The effects of Mo and V on impact toughness in martensitic steels tempered at low temperatures were investigated using three low-alloy medium-C steels. Previous examination of these alloys had identified differences in impact toughness without a clear cause. In this work, the Base alloy with a reduced Mo addition experienced a significant loss in hardenability leading to the formation of small fractions of bainite during quenching even at relatively high quench rates. The use of different quench media to simulate cooling rates throughout a heavy section demonstrated that the variation in previously reported Charpy V-notch impact absorbed energies was readily explained by some regions cooling fast enough to avoid bainite while others formed some small fraction of upper bainite leading to increased cleavage fracture and decreased impact toughness. Small amounts of bainite transformation were not detected by dilatometry or tensile properties. These results emphasize the importance of effective through-hardening and careful microstructure evaluation in alloys that are meant to maintain good toughness and strength in thicker sections.

Abstract: Rapid induction can be utilized to decrease the time and energy used for heat-treatment of steels. In the present study, a commercial 500 HB grade wear-resistant steel was subjected to rapid induction tempering and compared to conventionally furnace tempered samples. The martensitic ultra-high strength steel was cut to narrow thin sheets, which were tempered at 200, 300, 400, and 500 °C with both methods. The rapid tempering was applied with an in-house built induction line, in which the samples were moved through an induction coil. The velocity of the samples was adjusted to ensure constant temperature control. The applied heating rate was 1000–1100 °C/s resulting in extremely rapid tempering times. The conventionally tempered samples were heated in a pre-heated furnace for 45 min and cooled in still air. The samples were tested for tensile and hardness properties and microstructural characterization was conducted. Results revealed only minor differences between the differently treated steels. Elongation was slightly improved with the induction treatment. Therefore, the induction tempering appeared to result in similar or even slightly better tensile properties and can be considered a promising alternative for tempering processes in future steelmaking.

Abstract: A novel high-frequency induction technology was successfully developed to carry out ultra-flash tempering treatment (UFTT) of high-strength carbon steel (HS-CS) with a heating rate of 1000 °C/s. The as-received HS-CS is a fully martensitic structure with low toughness. UFTT strategy at various temperatures is proposed to produce a tempered martensitic lath structure with promoting carbide precipitates in this structure. Microstructure evolution during UFTT was characterized using secondary electron imaging and electron backscatter diffraction technique in a scanning electron microscope. Micro-indentation hardness tests were measured through the cross-section of the steel to analyze the impact extent of UFTT. The mechanical properties were measured by uniaxial tensile tests. The results revealed that UFTT at various temperatures (550-650 °C) significantly affected the microstructure and the mechanical strength of the steel. A fully tempered martensitic microstructure with various types of carbide precipitates was promoted. Although, the microhardness and tensile strength of flash-tempered steel decreased owing to the breakdown of lath and dislocation structure in the achieved microstructure by UFTT. Hence, it is expected that the promoted microstructure during UFTT in the tested steel will result in a superior strength-toughness synergy. Based on the achieved results, the UFTT technique provides an alternative route for the conventional processing to tailor the microstructure of microalloyed HS-CS, consequently, optimizing the mechanical performance. Meanwhile, Economically, it is a cost-effective route to manufacture advanced high-strength steel.

Abstract: Currently, heavy-haul and passenger rails are joined by a welding process, which can be either flash-butt welding or thermite. The joining process has increased the overall rails strength, but the welding parameters optimization is tricky and must be performed and studied to improve the weld quality. Heavy-haul rails are high carbon steels, containing alloying elements and as such, the weld presents a series of difficulties. On one side, martensite should be avoided during the cooling step, while on the other, the HAZ should be minimized as it is known to be prone to localized wear and rolling contact fatigue. Finite element simulations were performed to map the weld cooling rates and corresponding heat-affected zone (HAZ) width. CCT curves of rail steels were determined using dilatometry for different austenitizing temperatures. Comparing the simulations with the CCT data, processing windows able to prevent martensite formation were determined, usually corresponding to a critical cooling rate of 2 °C/s. The correlation with the simulations showed that the shorter the HAZ length, the greater the chance of martensite formation due to the higher cooling rate. The methodology developed and presented in this paper can be used for simulations considering phase transformations or determining the microstructure formed from different thermal welding cycles, depending on the distance from the heat source during the welding process.