Search Results (365)

The heat treatment residual stress of 8Cr4Mo4V steel bearings seriously affects the contact fatigue life. The micro stress concentration at the carbide interface leads to the initiation of micro cracks. Therefore, in this paper, the systematic analysis of heat treatment residual stress of 8Cr4Mo4V steel is conducted. FEA was used to analyze the residual stress of type I after heat treatment process. Based on numerical simulation and EBSD results, CPFEM was carried out to study the distribution of type II residual stress. Using high-resolution characterization results, GPA was performed to study type III residual stress caused by crystal defects. The FEA results indicate that thermal strain and phase transformation strain dominate the macroscopic stress change before and after martensitic transformation. During the first tempering process, the phase transformation leads to the release of quenching residual stress. The large stress concentration at the carbide interface is revealed by CPFEM. High-resolution characterization of coherent interface between carbide and matrix reveals that the micro residual strain at this interface is small. Through a systematic analysis of the residual stress of 8Cr4Mo4V steel, a basis is provided for modifying the macroscopic and microscopic residual stress of heat treatment to improve the bearing performance. Full article

In the design of L-DED (laser-directed energy deposition) cladding processes, the chemical composition of the metallic powders is typically assumed to match that of the intended coating. However, during the deposition of the first layer, dilution with the substrate alters the weld metal composition, deviating from the nominal powder chemistry. Although the application of multiple layers can gradually reduce this dilution effect, it introduces additional complexity and processing time. This study proposes an alternative strategy to counteract substrate dilution from the very first deposited layer, eliminating the need for multilayer coatings. Specifically, to achieve a corrosion-resistant monolayer of AISI 316L stainless steel on a high-strength, quenched-and-tempered AISI 4140 steel substrate, a dilution-compensating alloy powder is added to the standard AISI 316L feedstock. Single-layer coatings, both with and without compensation, were evaluated in terms of chemical composition, microstructure, and corrosion resistance. The results show that unmodified coatings suffered a chromium depletion of approximately 2 wt.%, leading to a reduced pitting potential of Ep = 725 ± 6 mV in synthetic seawater. In contrast, the use of the compensation alloy preserved chromium content and significantly improved corrosion resistance, achieving a pitting potential of Ep = 890 ± 9 mV. Full article

To optimize heat treatment of gears for high-end equipment and enhance their fatigue resistance, this paper studied the effects of Al, Mn and Cr content on surface microstructure, i.e., martensite, retained austenite, grain size, hardened layer depth and residual stress under different carburizing temperatures and low tempering of 20MnCr5 steel FZG gear. With numerical simulation combined with experimental verification, this paper establishes a simulation model for the carburizing process of 20MnCr5 steel FZG gear, analyzing the microstructure and retained austenite volume of the gear surface, after carburizing and quenching, by a scanning electronic microscope (SEM) and X-ray diffraction (XRD). In addition, the paper reveals the influence of the optimized heat treatment on the residual stress of the gear regulated with Al, Mn and Cr content in the meshing wear range of 200~280 µm. This study provides a guiding model theory and experimental verification for regulating proportions of alloying elements and optimizing the heat treatment process of low-carbon-alloy steel. Full article

A total of 0.3%C-Cr-Mo-V steel, a high-strength alloy steel widely used in rocket motor housings, suspension systems in high-performance vehicles, etc., is noted due to its high strength-to-weight ratio. However, its high carbon equivalent (CE > 1%) makes it challenging to weld, as it is prone to brittle martensitic formation, which increases the risk of cracking and embrittlement. The present paper focuses on enhancing the microstructure and mechanical properties of 0.3% C-Cr-Mo-V steel by gas tungsten arc welded (GTAW) joints, utilizing post-weld heat treatment and cooling strategies (PWHTCS). A systematic experimental approach was employed to ensure a defect-free weld through dye penetrant testing (DPT) and X-ray radiography techniques. Subsequently, test specimens were extracted from the welded sections and subjected to PWHT protocols, including hardening, tempering, and rapid quenching using air and oil cooling (AC and OC, respectively) mediums. Results show that OC has enhanced tensile strength and hardness while simultaneously maintaining and improving ductility, ensuring a well-balanced combination of strength and toughness. Fractography analysis revealed ductile fracture in AC samples, whereas OC weldments exhibited a mixed ductile–brittle fracture mode. Thus, the findings demonstrate the critical role of PWHTCS, with OC, as an effective method for achieving enhanced mechanical performance and microstructural stability in high-integrity applications. Full article

Quench–polish–quench (QPQ) nitro-carburizing of AISI 4140 steel in a salt bath was performed in this study. Nitro-carburizing in a salt bath enhanced the formation of Fe-nitride on the outer surface layer. Moreover, the oxidizing treatment formed a thin oxide layer decorated on the outermost part of the QPQ-treated sample. The dense compound layer formed after nitro-carburizing in a salt bath consisted of refined granular Fe₃N and transformed to Fe₂N after post-oxidation treatment. Micro-shot peening (MSP) was adopted before QPQ treatment to increase the treated steel’s fatigue performance. The results indicated that MSP slightly increased the thickness of the compound layer and harden depth, but it had little effect on improving the fatigue strength/life of the QPQ-treated sample (SP-QPQ) compared to the non-peened one (NP-QPQ). A deep compressive residual stress (CRS) field (about 200 μm) and a hard nitrided layer showed a noticeable improvement in the fatigue performance of the QPQ-treated ones relative to the 4140 substrates tempered at 570 °C. The ease of slipping or deforming on the substrate surface was responsible for its poor resistance to fatigue failure. The cracking and spalling of the brittle surface layer were the causes for the fatigue crack initiation and growth of all of the QPQ-treated samples fatigue-loaded at/above 875 MPa. It was noticed that fatigue crack initiation at the subsurface inclusions was more likely to occur in the SP-QPQ sample fatigue-loading at 850 MPa or slightly above the fatigue limit. Full article

Martensitic stainless steel (MSS) is widely used in several parts of automobiles where high strength, hardness, and corrosion resistance are required. However, the metastability of retained austenite can transform into martensite under severe deformation, adversely affecting material properties. Cryogenic treatments (CTs) have been extensively employed in iron-based alloys for fastener application due to their advantageous effect. This study explores the heat treatment processes applied to 13Cr-2Ni-2Mo martensitic stainless steel (MSS), including austenitizing, cryogenic treatment, and tempering cycles. Cryogenic treatment at (−150 °C) for varying durations, followed by tempering at 200 °C for 2 h, and the impact of post-cryogenic tempering at 200 °C for different tempering duration on the microstructure and mechanical properties were evaluated. Experimental results indicate that the sample quenched at 1040 °C for 2 h (CHT) contains lath martensite, retained austenite, δ-ferrite, and undissolved carbide precipitation. Compared to as-quenched samples, hardness decreased by 5.04%, 7.24%, and 7.32% after tempering for 2 h, 5 h, and 10 h, respectively. Extending cryogenic durations to 2 h, 12 h, and 20 h promoted nucleation of a mixture of M₃C and M₂₃C₆ small globular carbides (SGCs) and grain refinement but resulted in hardness reductions of 5.04%, 5.32%, and 8.36%, respectively. The reduction in hardness is primarily attributed to a decrease in solid solution strengthening and promoted carbide coarsening. Full article

The effects of laser powder bed fusion (LPBF) parameters, such as power (200 to 350 W) and scan speeds (from 200 to 2000 mm/s), on the microstructure and mechanical properties of high-strength, low-alloy (HSLA) AISI 4340 steel were examined. A wide range of volumetric energy density (VED) between 93 and 162 J/mm³ produced samples with relative densities greater than 99.8%. The optimal parameter set was identified with laser power = 200 W, scan speed = 600 mm/s, hatch spacing = 0.12 mm, and slice thickness = 0.03, corresponding to VED = 92.6 J/mm³. Scanning electron microscopy revealed a predominantly martensitic microstructure for all processing parameters examined, although X-ray diffraction revealed the minor presence of retained austenite within the as-fabricated 4340 steel. Using the optimized LPBF parameters, the as-fabricated 4340 steel exhibited a yield strength of 1317 MPa ± 16 MPa, ultimate tensile strength of 1538 MPa ± 22 MPa, and 18.6 ± 1% strain at failure. These are similar to wrought 4340 steel quenched and tempered between 400 and 600 °C. Full article

The solid phase transformation of martensitic steel during heat treatment will affect the stress and temperature. Previous residual stress prediction models ignore the effect of phase transition on residual stress. In order to predict residual stress accurately, a residual stress calculation method considering solid phase transition was presented. The measures to reduce residual stress in quenching medium, cooling rate, and the starting temperature and tempering temperature of the martensitic transformation were studied. The experimental results show that residual stress decreases after air cooling. In a certain range, residual stress can be reduced during heat treatment by decreasing the cooling rate and the martensite start temperature. The recommended tempering temperature is 380 °C. Full article

Low-cost and low-alloy dual-phase (DP) steel with a tensile strength (TS) above 1000 MPa and high ductility is in great demand in the automobile industry. An approach to using a medium-carbon and fibrous DP structure for developing such new DP steel has been proposed. The microstructure and mechanical performance of fibrous DP steel obtained via partial reversion from martensite in Fe-C-Mn-Si low-alloy steel have been investigated. The TS of the as-quenched DP steel is above 1300 MPa, while the total elongation is less than 6%. The total elongation was increased to above 13%, with an acceptable loss in TS by performing additional tempering. The fibrous tempered-martensite/ferrite DP steel exhibits an excellent balance of strength and ductility, surpassing the current low-alloy DP steels with the same strength grade. Plate-like or quasi-spherical fine carbides were precipitated, and the relatively high-density dislocations were maintained due to the delay of lath recovery by the enrichment of Mn and C in martensite (austenite before quenching), contributing to the tempering softening resistance. In addition, nanotwins and a very small amount of retained austenite were present due to the martensite chemistry. High-density dislocations, fine carbide precipitation, and partially twinned structures strengthened the tempered martensite while maintaining relatively high ductility. Quantitative strengthening models and calculations were not included in the present work, which is an interesting topic and will be studied in the future. Full article

The metal injection molding (MIM) manufacturing process has made relevant advances for applications in components with complex geometries, small dimensions, and high production volumes. New technologies such as hot isostatic pressing (HIP), uniform polymer extraction, and sintering with reduced temperature variations improve metallurgical and mechanical properties. However, there are still knowledge gaps in understanding these technologies and the behavior of catalytic low-alloy steels obtained by the MIM process and cyclic applications. This study aims to analyze the behavior of Catamold 100Cr6 steel subjected to quenching and tempering heat treatment in different microhardness ranges and the effect of compressive stresses on the samples obtained by polishing using ceramic microchips. The samples were characterized using optical microscopy, scanning electron microscopy, an EDS microprobe, and X-ray diffraction and subjected to elastic return cycling and an experimental device developed to apply a 19° bending angle. The findings show a significant increase in fatigue life due to the compressive stresses (up to—430 MPa) generated by the reduction in retained austenite and surface plastic microdeformation, indicating the effectiveness of 100Cr6 Catamold steel in cyclic applications. Full article

To investigate the effect of tempering temperature on the hardness and its underlying mechanisms in 14Cr12Ni3Mo2VN martensitic stainless steel after high-frequency induction quenching (HFIQ), the microstructure, energy-dispersive spectroscopy (EDS) of precipitated particles, residual austenite, residual stress, and microhardness of the material tempered at different temperatures were examined and analyzed. The results reveal that a secondary hardening phenomenon occurs during the tempering process in 14Cr12Ni3Mo2VN martensitic stainless steel. Overall, with increasing tempering temperature, the microhardness initially decreases slightly, then rises to a secondary hardening peak, and finally drops rapidly. The secondary hardening peak corresponds to a tempering temperature of approximately 440 °C, with a microhardness of about 483 HV_0.1. The secondary hardening phenomenon is likely attributed to the dispersion strengthening caused by the precipitation of alloy carbides during tempering. The precipitation and coarsening of carbides reduce lattice distortion and solid solution strengthening, while the release of residual stress diminishes stress-induced strengthening. Additionally, the decomposition of the martensitic structure leads to the formation of ferrite and carbides. The combined effects of these factors result in a decrease in hardness. Full article

With the rapid development of long-distance transmission pipelines for oil and natural gas, pipeline steel is continuously evolving towards higher pressure, larger diameter, and thicker wall thickness. Many extensive studies and research have been conducted on X70 pipeline steel produced through traditional processing routes. This study focuses on Nb-V-Ti-N microalloyed steel with different nitrogen contents, systematically investigating the variations in microstructure and tensile properties after quenching and tempering processes. The results indicate that after quenching treatment, when the nitrogen content of the tested steel is 0.0020 wt%, its primary microstructure consists of granular bainitic ferrite (GBF), acicular ferrite (AF), and residual M/A (martensite/austenite) components. As the nitrogen content increases, the contents of acicular ferrite and M/A constituents gradually rise, while granular bainitic ferrite correspondingly decreases. After tempering treatment, the microstructure of the tested steel transforms into granular bainitic ferrite, acicular ferrite, and carbonitrides. Notably, with the elevation of nitrogen content, the number of high-angle grain boundaries in the microstructure significantly increases. Meanwhile, the mean equivalent diameter (MED) defined by the misorientation angle (MTA) ranging from 2 to 15° and the dislocation density (ρ) exhibit an overall decreasing trend. Both of these factors contribute significantly to yield strength, resulting in a gradual increase in yield strength (YS) as the nitrogen content rises. Additionally, the study finds that as the nitrogen content increases, the size of precipitated particles continuously enlarges, and their proportion in the microstructure gradually increases. This discovery provides important theoretical basis and practical guidance for further optimizing the microstructure and mechanical properties of X70 pipeline steel. Full article

JIS SCM440 steel is commonly used in precision parts after induction-hardening heat treatment. The fatigue behavior of induction-hardening parts largely depends on the combination of hardening depth and the magnitude and distribution of hardness and compressive residual stress. Therefore, it is necessary to determine the effects of different prior microstructures on the properties of JIS SCM440 steel after induction hardening. In the present study, the effects of prior microstructure (including spheroidized, annealed, normalized, and quenched and tempered) on the microhardness, hardening width, and residual stress of the induction-hardened specimens are investigated. The experimental results showed that the distribution behavior of residual stress in the hardened zone and heat-affected zone is due to the temperature gradient of the induction-hardening treatment. The hardened center appeared as compressive residual stress due to the martensitic transformation, which was accompanied by volume expansion. On the contrary, tensile residual stress will be generated in the heat-affected zone of incomplete phase transformation. The prior microstructure can affect the residual stress magnitude and distribution of microhardness and residual stresses due to the content of the cementite dissolved into the austenite at high temperatures. The difference in the carbon content of martensite after quenching will result in obvious differences in properties. The induction-hardened specimens with a normalized prior microstructure have the highest residual tensile stress in the heat-affected zone. The maximum residual tensile stress was 371 MPa in the heat-affected zone. The induction-hardened specimens with a quenched and tempered prior microstructure have the deepest hardening depth and widest residual compressive stress distribution range. The highest microhardness was 764 HV_0.3, while the maximum residual compressive stress was −752 MPa. Full article

The 2200 MPa low-alloy ultra-high-strength steels microalloyed with Ti (Ti-steel) and Ti and Nb (TiNb-steel) were quenched at 860 °C and tempered at 180 °C, and the mechanical properties, microstructure, and precipitated phase were studied by SEM, TEM, and physicochemical phase analysis. The results of mechanical properties showed that the TiNb-steel had higher strength than the Ti-steel, with a yield strength of 1746 MPa and 1802 MPa and a tensile strength of 2198 MPa and 2232 MPa, respectively. The TiNb-steel had a finer structure than the Ti-steel, with the effective grain sizes being 0.86 μm and 1 μm, respectively. The TiNb-steel had more MC-type carbides than the Ti-steel, and the MC-type carbide contents were 0.24 wt.% and 0.19 wt.%, respectively. The reason for the higher yield strength of the TiNb-steel is that it has higher strengthening effects due to finer grains, a higher density of dislocation, and more precipitation. The reason for the higher tensile strength of the TiNb-steel is that it has a higher coefficient of variable hardening K and a higher strain hardening index n. Full article

The Jiles–Atherton (J–A) model has seen extensive use for modelling the hysteresis behaviour of ferromagnetic materials due to its computational efficiency, simplicity of use, and small number of physically related parameters. However, in this work, the application of the J–A model to hysteresis curves obtained from experimental measurements for as-quenched and quenched-and-tempered engineering steels is considered. It has been demonstrated that the current form of the J–A model is not capable of representing certain observed features in the obtained hysteresis curves of these steels, in particular, the rapid narrowing of the loops seen for as-quenched steels and the sharp corners seen for quenched-and-tempered steels. This work has shown that a superior fit to the major loops for such steels can be obtained by applying Gaussian variations with respect to the applied magnetic field to the model parameters. The findings are supported by experimental results from engineering steels used in the oil and gas industry. Full article