Search Results (108)

This study investigates the mechanical, tribological, and electrochemical behaviour of a novel precipitation-hardenable martensitic alloy produced by selective laser melting (SLM). The alloy was specifically engineered with an optimised composition, free from cobalt and molybdenum, and featuring reduced nickel content (7 wt.%) and 8 wt.% chromium. It has been developed as a cost-effective and sustainable alternative to conventional maraging steels, while maintaining high mechanical strength and a refined microstructure tailored to the steep thermal gradients inherent to the SLM process. Several ageing heat treatments were assessed to evaluate their influence on microstructure, hardness, tensile strength, retained austenite content, dislocation density, as well as wear behaviour (pin-on-disc test) and corrosion resistance (polarisation curves in 3.5%NaCl). The results indicate that ageing at 540 °C for 2 h offers an optimal combination of hardness (550–560 HV), tensile strength (~1700 MPa), microstructural stability, and wear resistance, with a 90% improvement compared to the as-built condition. In contrast, ageing at 600 °C for 1 h enhances ductility and corrosion resistance (Rp = 462.2 kΩ; Ecorr = –111.8 mV), at the expense of a higher fraction of reverted austenite (~34%) and reduced hardness (450 HV). This study demonstrates that the mechanical, surface, and electrochemical performance of this novel SLM-produced alloy can be effectively tailored through controlled thermal treatments, offering promising opportunities for demanding applications requiring a customised balance of strength, durability, and corrosion behaviour. Full article

The direct energy deposition (DED) metal additive manufacturing process enables rapid deposition and repair, providing an efficient approach to producing durable tool steel components. Here, 5 wt% Cr cold-work tool steel (Caldie) was developed by reducing carbon and chromium to suppress coarse carbide formation and by increasing molybdenum and vanadium to enhance dimensional stability. In this study, Caldie tool steel was fabricated via DED for the first time, and the effects of post-heat treatment on its hierarchical microstructure and mechanical properties were investigated and compared with those of wrought (reference) material. The as-built sample exhibited a mixed microstructure comprising lath martensite, retained austenite, polygonal ferrite, and carbide networks, which transformed into full martensite with fine carbides after heat treatment (DED-HT). The tensile strength of the DED Caldie material increased from 1340 MPa to 1949 MPa after heat treatment, demonstrating superior strength compared to other heat-treated, DED-processed high-carbon tool steels. Compared to DED-HT, the wrought material exhibited finer martensite, a more uniform Bain group distribution, and finer carbides, resulting in higher strength. This study provides insights into the effects of heat treatment on the hierarchical microstructure and mechanical behavior of Caldie tool steel manufactured by DED. Full article

This study examined the impact of tempering temperature on the microstructure and properties of vanadium (V)-microalloyed medium-carbon bainitic steel. A series of heat treatments were performed on the steel, and the microstructural evolution and mechanical properties were systematically investigated through X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and mechanical testing systems (MTS). The findings revealed that tempering temperature has a significant influence on microstructural changes. Specifically, at 350–450 °C, retained austenite begins to decompose and carbides start to precipitate. At 550–600 °C, bainitic ferrite laths undergo coarsening. Regarding mechanical properties, both tensile strength and yield strength initially increase with tempering temperature before decreasing as the temperature continues to rise. The diffusion and redistribution of carbon atoms during tempering enhance the elongation of all tempered samples compared to their untempered counterparts. Optimal comprehensive mechanical properties are achieved at 450 °C, where precipitation strengthening from vanadium, enhanced stability of retained austenite, and synergistic strengthening effects of decomposition products are most pronounced. This research provides a theoretical foundation for optimizing the heat treatment process of such steels and offers insights into the synergistic effects of V-microalloying and tempering. Full article

In modern industries, gears function as pivotal transmission elements whose operational performance is directly dependent on the microstructural characteristics of gear steels. While high-temperature carburizing (950–1050 °C) substantially improves process efficiency through accelerated carbon diffusion, it inevitably promotes austenite grain coarsening. This study investigates the effect of Nb microalloying on grain stability in SAE8620H gear steel during high-temperature carburizing. Experimental steels with varying Nb contents were prepared via vacuum induction suspension melting, followed by hot rolling, solution treatment, and pseudo-carburizing. Thermodynamic calculations, optical microscopy, transmission electron microscopy, and energy-dispersive spectroscopy were employed to analyze the mechanisms. Thermodynamic results revealed that higher Nb content retains more Nb(C, N) phases at elevated temperatures, effectively suppressing grain coarsening. Without preheating, increased Nb content refined grains but exhibited limited inhibition at high temperatures. Preheating (1330 °C × 10 min + water quenching) promoted uniform and fine Nb(C, N) precipitates, significantly enhancing grain refinement. When Nb content exceeded 0.053 wt.%, grain coarsening was fully inhibited under 1050 °C × 2 h carburizing. This study establishes the optimal Nb content range, elucidates the micro-mechanisms, and proposes a preheating process to improve high-temperature carburizing performance in gear steels. Full article

In this study, friction stir welding was first applied to the 1.4 mm thick TRIP590 steel sheets at a constant transverse speed of 100 mm/min and different rotation speeds from 200 to 500 rpm. Then, the obtained joints received deep cryogenic treatment in liquid nitrogen for 24 and 48 h, respectively. It was revealed that the content of retained austenite in the stir zone of the welded joints decreased from 3.3% to 0.2% when the rotation speed increased from 200 rpm to 500 rpm. The stability of retained austenite increased due to grain refinement and work hardening at low rotation speeds. After deep cryogenic treatment of the welded joints, the retained austenite in the stir zone partially transformed into martensite, which led to the precipitation of nano-sized carbide in the ferrite matrix and the release of local stress. As a result, both the strength and plasticity of the stir zone after 48 h of deep cryogenic treatment increased from 798 MPa, 15% to 927 MPa, 17% for the 200 rpm joint, and from 914 MPa, 14% to 1086 MPa, 16% for the 300 rpm joint during the tensile tests. Full article

To improve the contact fatigue life of gears, we studied the effect of adding a certain proportion of the Al element to a 20MnCr5 steel FZG spur gear under different heat treatment processes, characterizing the retained austenite and residual compressive stress on the tooth surface. The stability of the microstructure grain size on the gear surface under different heat treatment processes was studied, and the surface microstructure, phase structure, and composition of the gear were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The changes in the retained austenite content and grain size on the gear surface at a microscale of 2–100 μm were investigated. In addition, this study revealed the effect of adding the Al element and the optimization of the carburizing and quenching process on the residual compressive stress on the gear surface at a depth range of 200–280 μm. The effect of higher residual compressive stress and fewer non-metallic inclusions on the gear surface on the stress intensity factor of fatigue crack propagation was considered, along with the effect of deeper hardened layers on the improvement in wear resistance. The experiments in this study significantly improved the contact fatigue life of 20MnCr5 steel gears. Full article

In this study, microstructure evolution during prior austenite decomposition and reverse phase transformation processes was revealed in a high-strength medium-Mn steel. Furthermore, the relationship between deformed prior austenite characteristics and deformation behavior was studied. The results indicated that the recovery and recrystallization of the deformed prior austenite were significantly inhibited during hot rolling in the non-recrystallized zone, the grain size was obviously refined along the normal direction (ND), and that the strain hardening of prior austenite via hot deformation could increase the resistance of shear transformation, resulting in the preservation of high-density lattice defects in the quenched martensite matrix. Before the nucleation of intercritical austenite, the dislocation and grain boundary can provide fast diffusion paths for C and Mn, and the enrichment of C and Mn before intercritical austenite formation can reduce the critical temperature of ferrite/austenite transformation. The nucleated sites and driving force for intercritical austenite were strongly increased by rolling in the non-recrystallization region. The resistance of crack propagation was found to be enhanced by the sustained transformation-induced plasticity (TRIP) effect (via retained austenite with different stability) and for the laminated microstructure, the optimum properties were obtained as being a combination of yield strength of 748 MPa, tensile strength of 952 MPa, and total elongation of 26.2%. Full article

Cyclic heat treatment is an effective approach for enhancing the mechanical properties of 18Ni(C250) maraging steel, and the selection of cyclic heat treatment temperature is a key factor. In this study, a cyclic heat treatment process with a two-step solution treatment is employed to investigate the influence of cyclic heat treatment temperature, specifically the first solution treatment temperature (920 °C, 950 °C, and 980 °C), on the microstructure and mechanical properties of 18Ni(C250) maraging steel. The results indicate that with an increase in the cyclic heat treatment temperature, the average grain size of the 18Ni(C250) maraging steel decreases initially and then increases. When the cyclic heat treatment temperature reaches 950 °C, the grain size is at its minimum, exhibiting optimal grain uniformity. Additionally, the increase in cyclic heat treatment temperature results in a reduction in the size of martensitic lath with the same orientation inside the grains, along with an increase in the relative quantity of low-angle grain boundaries. Furthermore, the volume fraction and size of retained austenite show a monotonous increase with the rise in the temperature of the cyclic heat treatment, and the rate of increase becomes notably larger when the temperature is raised from 950 °C to 980 °C. Based on the observed microstructural changes, the variation in the mechanical properties of the 18Ni(C250) maraging steel was analyzed. Specifically, as the cyclic heat treatment temperature increases, the tensile strength of the 18Ni(C250) maraging steel initially increases and then stabilizes, while the elongation and fracture toughness exhibit a monotonic increase. Full article

The effects of austenitizing and austempering temperatures on the bainite transformation kinetics and the microstructural and mechanical properties of a medium-carbon high-silicon ultra-fine bainitic steel were investigated via dilatometric measurements, microstructural characterization and mechanical tests. It is demonstrated that the optimum austenitizing temperature exists for 0.3 wt.%C ultra-fine bainitic steel. Although the finer austenite grain at 950 °C provides more bainite nuclei site and form finer bainitic ferrite plates, the lower dislocation density in plates and the higher volume fraction of the retained austenite reduces the strength and impact toughness of ultra-fine steel. When the austenitizing temperature exceeds 1000 °C, the true thickness of bainitic ferrite plates and the volume fraction of blocky retained austenite in the bainite microstructure increase significantly with the increases in austenitizing temperature, which do harm to the plasticity and impact toughness. The effect of austempering temperature on the transformation behavior and microstructural morphology of ultra-fine bainite is greater than that of austenitizing temperature. The prior martensite, formed when the austempering temperature below Ms, can refine the bainitic ferrite plates and improve the strength and impact toughness. However, the presence of prior martensite divides the untransformed austenite and inhibits the growth of bainite sheaves, thus prolonging the finishing time of bainite transformation. In addition, prior martensite also strengthens the stability of untransformed austenite though carbon partition and enhances the volume fraction of blocky retained austenite, which reduces the plasticity of ultra-fine bainitic steel. According to the experimental results, the optimum austempering process for 0.3 wt. %C ultra-fine bainitic steel is through austenitization at 1000 °C and austempering at 340 °C. Full article

The damage properties in the shear-punched surface layer, such as the strain-hardening increment, strain-induced martensite fraction, and initiated micro-crack/void characteristics at the shear and break sections, were experimentally evaluated to relate to the stretch-flangeability in three types of low-carbon high-strength TRIP-aided steel with different matrix structures. In addition, the surface layer damage properties were related to the mean normal stress developed on shear-punching and microstructural properties. The shear-punched surface damage of these steels was experimentally confirmed to be produced under the mean normal stress of negative to 0 MPa. TRIP-aided bainitic ferrite (TBF) steel had the smallest surface layer damage, featuring a significantly suppressed micro-crack/void initiation. This was due to the fine bainitic ferrite lath matrix structure, a low strength ratio of the second phase to the matrix structure, and the high mechanical stability of the retained austenite. On the other hand, the surface layer damage of TRIP-aided annealed martensite (TAM) steel was suppressed next to TBF steel and was smaller than that of TRIP-aided polygonal ferrite (TPF) steel. The surface layer damage was also characterized by a large plastic strain, a large amount of strain-induced martensite transformation, and a relatively suppressed micro-crack/void formation, which resulted from an annealed martensite matrix and a large quantity of retained austenite. The excellent stretch-flangeability of TBF steel might be caused by the suppressed micro-crack/void formation and high crack propagation/void connection resistance. The next high stretch-flangeability of TAM steel was associated with a small-sized micro-crack/void initiation and high crack growth/void connection resistance. Full article

Impact fracture behavior at low temperatures was investigated in medium manganese steel with bcc-fcc duplex microstructures. The impact energy was above 150 J (−80~20 °C) and the fractography showed dimples for inter-critical annealing at 630 °C (QHA) because of the high retained austenite stability and low martensite dislocation density. The impact energy was from 180 J (20 °C) to 60 J (−80 °C) and the fractography was intergranular for inter-critical annealing at 610 °C (QLA) because of the low stability of RA and carbides precipitated at the prior austenite grain boundaries. The impact energy was below 60 J (−80~20 °C) and the fractography showed cleavage for direct quenching (DQ) because of the high dislocation density of martensite. Full article

The variations of the microstructure and mechanical properties of medium-Mn steel after vanadium (V) microalloying with different contents were investigated. After a one-step intercritical annealing (IA) at 730 °C, the steel containing 0.04 wt.% of V exhibited excellent comprehensive properties. The steel maintained an ultimate tensile strength (UTS) of 1000 MPa while also exhibiting a total elongation (TEL) of 37% and a product of strength and plasticity (PSE) of 37.7 GPa%. V-microalloying improved the yield strength (YS) and UTS of the experimental steel by refining ferrite grains and precipitation strengthening, however, it deteriorated its plasticity, which is difficult to compensate for through grain refinement and due to the TRIP effect of retained austenite (RA). The largest amount of RA and the appropriate stability also make a significant contribution to the outstanding UTS of the steel containing 0.04 wt.% of V through the TRIP effect. However, the further increase of V content led to decreased RA content and stability, weakening the TRIP effect and resulting in a weaker strength ductility balance. Full article

Conventional heat treatment is not capable of converting a sufficient amount of retained austenite into martensite in high-carbon or high-carbon and high-alloyed iron alloys. Cryogenic treatment induces the following alterations in the microstructures: (i) a considerable reduction in the retained austenite amount, (ii) formation of refined martensite coupled with an increased number of lattice defects, such as dislocations and twins, (iii) changes in the precipitation kinetics of nano-sized transient carbides during tempering, and (iv) an increase in the number of small globular carbides. These microstructural alterations are reflected in mechanical property improvements and better dimensional stability. A common consequence of cryogenic treatment is a significant increase in the wear resistance of steels. The current review deals with all of the mentioned microstructural changes as well as the variations in strength, toughness, wear performance, and corrosion resistance for a variety of iron alloys, such as carburising steels, hot work tool steels, bearing and eutectoid steels, and high-carbon and high-alloyed ledeburitic cold work tool steels. Full article

To analyze various types of cold formability in TRIP-aided polygonal ferrite (TPF), annealed martensite (TAM), and bainitic ferrite (TBF) steels, the effects of the mean normal stress on the strain-hardening, strain-induced martensite transformation, and void-formation behaviors were investigated. The strain-hardening behavior was influenced by positive mean normal stress and was hardly influenced by zero and negative mean normal stresses in all steels. Positive mean normal stress promoted the strain-induced martensitic transformation behavior, especially in TBF steel due to the high mechanical stability of the retained austenite. The void-formation behavior was also promoted by positive mean normal stress, especially in TPF steel. These behaviors were also related to the microstructural properties, such as the matrix structure, retained austenite characteristics, and second phase. Full article

This article presents the results of a study on reducing deformations resulting from high-temperature vacuum carburizing and post-carburizing heat treatment. The idea was to increase the strength of steel at elevated temperatures by pre-carburizing at heat-up to the process temperature (SC—stage carburizing). It has been shown that the use of carburizing in stages from a lower temperature to the target temperature, compared to traditional vacuum carburizing at a constant temperature (Constant-Temperature Carburizing—CTC), has a significant impact on the chemical and phase composition of the technological layer, surface after the process and, consequently, on its mechanical properties. It was shown that the retained austenite content after stage carburizing was reduced by approximately 45%, as was the thickness of the gear teeth measured at the pitch diameter. Additionally, uniform stress distribution was demonstrated for the SC process. Carbon saturation of austenite increases the yield strength, and therefore the dimensional stability of steel heat-treated at elevated temperatures also improves, which effectively permits high-temperature treatment of critical steel parts such as, for example, gear wheels, for which high dimensional accuracy is required. Full article