High-strength Structural Steel Research Articles

Tensile Coupon Testing and Residual Stress Measurements of High-Strength Steel Built-Up I-Shaped Sections

High strength structural steels (with yield stresses greater than 65 ksi) may have notably different material characteristics when compared to structural steels conventionally used in building construction [i.e., ASTM A992/A992M (2022) or A572/A572M Gr. 50 (2021)]. This paper presents findings from an experimental program that investigated the material characterization of ASTM A656/A656M Gr. 80 (2024) plate steel. The results obtained were compared to conventional ASTM A572/A572M Gr. 50 steel. Two types of testing were performed for this work: tensile coupon testing and residual stress testing. The tensile coupon testing was carried out for both the A656/A656M Gr. 80 and A572/A572M Gr. 50 plate material. The A656/A656M Gr. 80 plate material showed more variation between the two different plate thicknesses in both mechanical behavior and microstructure due to differences in steel production. The 0.375 in. thick plate exhibited a clear yield plateau with an ultimate/yield stress ratio similar to the Gr. 50 material. In contrast, the 0.5 in. plate did not have a yield plateau and reached lower ultimate strain. The residual stress testing was performed using a sectioning technique for one A572/A572M Gr. 50 and five A656/A656M Gr. 80 built-up sections that were fabricated from 0.5 in. and 0.375 in. plate material. Residual stresses obtained from measurements were compared to previously published predictive models. The ECCS model and BSK99 models were found to be reasonable predictors of residual stresses for all specimens except the one section fabricated from 0.5 in. thick Gr. 80 plate. When comparing the Gr. 50 and Gr. 80 specimens of the same cross-sectional geometry, the residual stresses were similar, implying that cross-sectional geometry is more prevalent than the nominal yield stress in determining residual stresses in built-up I-sections.

Fatigue lifetime predictions of notched specimens based on the critical distance and stress concentration factors

The theory of critical distances is used for evaluation of effects of notches on load-bearing capacity of notched components under static or cyclic fatigue loading. The theory can also be used to predict the fatigue life of notched components. One of the assumptions for a reliable prediction is that the critical distance is independent of the notch geometry and its stress concentration level. The study shows that the critical distance depends not only on the number of cycles to failure (under fatigue loading), but also on the notch radius. In this case, direct use of the critical distance leads to unreliable predictions. The study quantifies the relation between the model notch (from which the critical distance is determined) and the predicted notch by means of the ratio of their stress concentration factors. The predictions modified by the ratio of stress concentration factors provide results that are in satisfactory agreement with the experimental data. The predictions were made and verified on the basis of fatigue data measured on two stainless steels 1.4306 and 1.4307, and high strength structural steel S690QL. Smooth and notched specimens were tested on an ultrasonic fatigue machine in a symmetrical tension–compression mode. The results were evaluated in the regions of high cycle and very high cycle fatigue.

Cyclic Void Growth Model Parameter Calibration of Q460D Steel and ER55-G Welds after Exposure to High Temperatures

When high-strength steel is heated to high temperatures and then cooled naturally, its ductility decreases. In earthquake-prone areas, it is necessary to evaluate the ultra-low cycle fatigue fracture (ULCF) behavior of high-strength steel structures after a fire if these structures are used continuously. However, the ULCF fracture model of high-strength steel subjected to high temperatures followed by natural cooling has not been deeply studied. In view of this, twelve notched, round bar specimens fabricated from Q460D steel and ER55-G welds were heated to 900 °C followed by natural cooling and then cyclic loading experiments and finite element analyses (FEA) were performed on these specimens. The fracture deformation obtained from the experiments was used in the FEA to calibrate the damage degradation parameter of a Cyclic Void Growth Model (CVGM) of Q460D steel and ER55-G welds under this condition. The calibrated values were 0.30 and 0.20, respectively. The calibrated CVGM was employed to predict the number of cycles and the force and displacement at the fracture moment of the notched round bar specimens. The predicted results aligned closely with the experimental results, indicating that CVGM is effective in predicting the fracture of Q460D steel and ER55-G welds following exposure to 900 °C and subsequent natural cooling.

Comparison of fatigue crack growth design curves on GMAW and EBW joints of high strength steels

There is a growing demand in the industrial sector for the use of high-strength structural steels (HSSSs), which can achieve a significant weight reduction in structures. These structural steels are usually produced by quenching and tempering (Q + T) or thermomechanical treatment (TM), and their applications in welded structures pose several challenges for the users. In industrial practice, gas metal arc welding (GMAW) is basically the most commonly used fusion welding process, which has a relatively high heat input. However, at HSSSs, there is a need for low heat input but, at the same time, productive welding processes. High-energy density welding processes, e.g., electron beam welding (EBW), offer a unique opportunity to weld these steels. The widespread use of HSSSs is also hampered by the fact that the benefits of high strength can be exploited primarily under static loading. At the same time, different welded structures made of HSSSs are often subjected to cyclic loading, and possible weld defects and material discontinuities are major risks in this case. During our experiments, GMAW and autogenous EBW processes were applied to make welded joints from S960 Q + T and TM structural steels. The fatigue resistance of the welded joints was characterized by fatigue crack growth (FCG) tests, considering the increased crack sensitivity of HSSSs. A statistical approach was followed both in the design of the experiments and in the evaluation of their results. Based on the test results fatigue crack propagation design curves were determined for the investigated GMAW and EBW welded joints. The design curves were compared with each other, with design curves of lower strength material (S690QL) and with the recommended fatigue crack growth laws of BS 7910.

A numerical model simulating cyclic behavior of high-strength steel

High-strength steel (HSS) can effectively and economically design structural members that withstand large forces induced by extreme events such as earthquakes. To evaluate the seismic performance of HSS members and structures using nonlinear finite element (FE) analyses, using an accurate material model is important, which can simulate the inelastic cyclic behavior of the HSS. The peculiar material behavior of HSS needs to be considered in the model. This study used a combined hardening model to construct the material model. The configuration of the model and the constituent model parameter values are determined to precisely simulate the low-cycle fatigue (LCF) behavior of HSS. An efficient particle swarm optimization (PSO) algorithm is used to determine parameter values with LCF test data of 54 individual HSS coupons. In additions, to conveniently determine the model parameter values with only monotonic tensile test data instead of conducting expensive LCT tests, empirical equations are proposed. It is shown that the LCF curves of HSS can be accurately simulated using the constructed material model with the proposed equations.

Strain rate-temperature effects and constitutive models for Q690D QT steel

This paper investigates the mechanical properties of the high-strength structural steel Q690D QT (Quenched and Tempered) over a wide temperature range and a wide strain rate range. Firstly, a series of quasi-static and dynamic tests adopting the universal testing machine and Split Hopkinson Pressure Bar were carried out to investigate the mechanical properties at strain rates from 0.00025 s−1 to 5000 s−1 and temperatures from room temperature (20 °C) to 700 °C. Stress-strain curves and dynamic yield strength increase factors were extracted and investigated. Test results show that Q690D QT exhibits different strain rate effects at different temperatures, more significant at the temperatures of 600 and 700 °C, as well as the high-temperature softening effect. Dynamic strain aging effect is identified at temperatures between 300 − 600 °C for plastic strain rates of 800 s−1 and 2300 s−1, nevertheless, this effect is observed at a higher temperature range of 500–700 °C for higher plastic strain rates of 4000 s−1 and 6400 s−1. From the analysis, the adiabatic thermosoftening effect cannot be ignored when the plastic strain rates reach 4000 s−1. Finally, an improved form of the Johnson-Cook model is proposed for capturing the adiabatic temperature effects at different temperatures and validated by the test data. A parameter dynamic temperature factor is proposed to describe the comprehensive effect of strain rate, test temperature, and adiabatic thermosoftening.

Revealing the mechanism of crack initiation and propagation in CGHAZ of S690QL welded joints through low-temperature impact tests with gradient setting of pendulum angle

As the most brittle areas of welded S690QL high-strength steel structures, coarse-grained heat-affected zone (CGHAZ) was crucial for the structural integrity of components. This paper designed an innovative low-temperature impact testing method with a gradient setting of pendulum angle to investigate the micro-mechanism of crack initiation and propagation in the CGHAZ of S690QL welded joints. The results reveal that continuous blocky M−A constituents were distributed along prior austenite grain boundaries (PAGBs), leading to stress concentration and crack initiation. During low-temperature impact processes, cracks propagate in a transgranular mode along {100} or {110} crystal planes. Due to the interaction between compressive stress and the crack tip, regions with high kernel average misorientation (KAM) such as PAGBs can divert the crack. The coarsening of prior austenite grains in the CGHAZ leads to a diminished hindering effect of PAGBs on crack propagation. The findings elucidate the complex relationship between microstructure and impact toughness of CGHAZ, providing insights into crack behavior and mechanisms in CGHAZ.

Assessment of fatigue crack growth resistance of newly developed LTT alloy composition for the repair of high strength steel structures

Tensile residual stress (TRS) is a well-known factor that deteriorate the integrity of welded joints. Fatigue failure is accelerated by the existence of TRS introduced during the welding process. There have been efforts in the last two decades to develop filler alloys that can reduce TRS by introducing compressive residual stress (CRS) to oppose the TRS in high strength steel welded joints. These works are based on the theory of austenite (γ) to martensite (α’) transformation and the filler is often called a low transformation-temperature (LTT) alloy. Many studies have reported that the fatigue strength (FS) of weld joint made with LTT alloy is many times better than that of the conventional fillers. It is reported to be particularly useful in the repair of high strength steel structures. However, studies on the fatigue crack growth (FCG) behaviour of these LTT alloys is scarce. In this work, we developed Fe-CrNiMo based LTT weld metal composition, assessed its FCG behaviour and compared the results with that of a conventional welding wire (ER70S-6). It is found that ER70S-6 weld metal obtained under relatively fast cooling is extremely tough, but the associated heat affected zone (HAZ) has poor resistance to FCG which obscured the benefit of the tough weld metal. High heat input or condition that results to slow cooling of the ER70S-6 weldment deteriorates its resistance to FCG. Unfortunately, despite its low martensite start temperature of 231±7 and the anticipated beneficial effect of induced CRS, the LTT alloy studied had the lowest FCG resistance. The LTT alloy appears to have an intrinsic microstructural feature or a ‘fault line’ that reduced its resistance to FCG. While the LTT alloy weld metal has poor resistance to FCG, the associated HAZ resisted FCG more than the HAZ associated with ER70S-6 weld metal. It is observed that aligning the ER70S-6 weld metal perpendicular to the crack front produced the highest resistance to fatigue crack initiation and propagation. In the case of ER70S-6, it is believed that the weld metal induced a CRS at the notch tip which resulted to the high fatigue resistance. In the case of the LTT alloy, perpendicular alignment of the weld metal produced slight improvement.

Synchrotron diffraction residual stresses studies of electron beam welded high strength structural steels

In recent years, there's been a strong focus on enhancing high-strength structural steels (HSSSs), making them tougher, stronger, lighter, and more weldable. However, these steels face challenges like cold cracking sensitivity (CCS) and reduced toughness with higher heat input. Therefore, exploring HSSSs with advanced welding techniques like electron beam welding (EBW) is crucial. EBW known for its innovation and precision, is gaining popularity in HSSSs applications for vehicles, construction industries etc. offering advantages like narrow welds, reduced heat-affected zone (HAZ) and low distortion. It is essential to understand the influence of residual stresses (RS) in HSSSs which ultimately affect structural integrity and fatigue life. Using EBW on HSSSs, coupled with synchrotron XRD for residual stress studies, has great potential to advance this research significantly. In this work, a 15-mm-thick EB-welded joint of S960QL and S960 M steels was used to study the RS. At an energy level of 20 keV, RS data from the synchrotron XRD method and its result on the top of welded joints for both types of steel were compared. S960 M steel showed a longitudinal tensile stress of 250 MPa at the weld toe, approximately 1.8 times higher than the 138 MPa observed in S960QL steel under similar conditions. In transverse stress, S960QL exhibits a higher stress of 279 MPa compared to S960 M 46 MPa at the weld toe, approximately 83% lower. The synchrotron method measured 76% lower LRS for S960QL compared to conventional XRD, while S960 M exhibited 55% lower LRS using the same approach.

Machine-learning-assisted design of high strength steel I-section columns

High strength steel has been attracting attention in the building industry due to its superior mechanical properties. The accurate design of high strength steel structures is crucial to boost its wide application. In this paper, an accurate and unified design approach for high strength steel I-section columns with different material grades, boundary conditions, geometric dimensions (including cross-section sizes and member lengths) and failure modes is proposed based on machine learning. Firstly, 871 experimental and numerical data were collected from the literature to establish a database. Then, seven machine learning algorithms, including Decision Tree, Random Forest, Support Vector Machine, K-Nearest Neighbour, Adaptive Boosting, Extreme Gradient Boosting and Categorical Boosting, were applied to establish machine learning regression models to predict buckling resistances of high strength steel I-section columns. The model performance was then evaluated through statistic indices, with the evaluation results indicating that the Categorical Boosting trained model yields the highest level of accuracy. Based on the data in the collected database, the regression model trained by Categorical Boosting and existing codified design provisions, as given in the European code and American specification, were assessed and compared. The European code and American specification were found to yield scattered and inaccurate failure load predictions, while the Categorical Boosting trained model led to substantially more accurate and consistent failure load predictions for high strength steel I-section columns with different material grades, boundary conditions, geometric dimensions and failure modes.

Unified machine-learning-based design method for normal and high strength steel I-section beam–columns

High strength steel is regarded as a promising construction material due to its superior mechanical properties. However, the codified failure load predictions for high strength steel welded I-section beam–columns are not accurate in some cases owing to the lack of relevant design codes for high strength steel structures. In addition, current design codes ignore the interaction effect of the cross-section plate elements, leading to the inaccuracy of codified predictive ultimate resistances for both normal and high strength steel beam–columns. In this paper, a unified and accurate design method was proposed based on machine learning for both normal and high strength steel welded I-section beam–columns failing by different failure modes. Firstly, a total of 812 experimental and numerical data on welded I-section beam–columns with various steel grades, geometric dimensions, including cross-section dimensions and member lengths, and failure modes, including global buckling, local buckling and local–global interactive buckling were collected to establish a database. Seven machine learning algorithms, including Decision Tree, Random Forest, Support Vector Machine, K-Nearest Neighbour, Adaptive Boosting, Extreme Gradient Boosting and Categorical Boosting were applied to develop regression models to predict failure loads. Based on the established database, each machine learning model was then trained and key hyperparameters were optimised. The model performance was evaluated through a series of statistical indicators and the feature importance analysis, and the evaluated results indicated that the XG-trained regression model had the highest level of accuracy. Finally, the predictions given by the XG-trained regression model were compared with those of the existing codified design provisions, as given in Eurocode and American codes. The evaluation results indicated that the codified predictive bearing capacities of I-section beam–columns were scattered and inaccurate, while the XG-trained regression model provided substantially improved failure load predictions.

Seismic behavior of Q620 high-strength steel welded Box-section columns: Experimental tests, analysis, and model

This study experimentally and numerically investigates the seismic behavior of high-strength steel (HSS) welded Box-section columns. The HSS structures have made rapid development in various types of building structures. However, the high yield-to-tensile ratio of HSS limits its application in seismic design. To address this gap, a new type of HSS called Q620 has been developed by Hebei Iron and Steel Co., Ltd. (HISCO). A total of three specimens were designed by this seismic-resistant Q620 HSS material and tested under cyclic load. The main failure modes included local buckling of the bottom section of the column, and obvious flexural deformation led to fracture of the specimen. Additionally, the nonlinear kinematic hardening model was utilized to accurately establish the finite element (FE) model for the HSS welded Box-section columns. Based on the validated FE model, an extensive parametric analysis (e.g., width-to-thickness ratio, axial compression ratio, and slenderness ratio) was conducted to evaluate the seismic behavior of the Q620 HSS welded Box-section columns. The results indicated that the Box-section columns exhibited favorable hysteretic behavior and notable plastic deformation capacity. A higher width-to-thickness ratio resulted in a faster decline in stiffness and a more pronounced advancement of damage. The ductility and energy dissipation capacity of Box-section columns were generally decreased with increasing axial compression ratio or slenderness ratio. A newly simplified hysteretic model was proposed, which was in good agreement with the experimental results.

Post-fire mechanical properties and constitutive model of Q690 high-strength structural steel

Fire hazard is one of the main threats to the integrity of steel structural buildings. Developing an accurate post-fire constitutive model for structural steel is crucial for enhancing the failure assessment of steel structures exposed to fire. In this study, the post-fire mechanical behavior of Q690 high-strength structural steel is investigated. Eighteen round bar specimens made of Q690 steel were heated with temperatures ranging from 20 °C to 900 °C. Tensile tests were conducted on these specimens to obtain the post-fire stress-strain curves and associated mechanical parameters of the material. To accurately characterize the post-fire behavior of Q690 steel, a temperature-dependent elastoplastic constitutive model combined with its numerical implementation algorithm is proposed. This model consists of four components: a temperature-dependent elasticity criterion, the von Mises yield criterion, an associated flow rule, and a temperature-dependent isotropic hardening law. The proposed constitutive model has a notable feature, i.e. the material parameters in the model can be defined as a function of temperature according to the test results. As a result, when utilizing this model for post-fire assessments of steel members, only three routine parameters, i.e. elastic modulus at room temperature, Poisson's ratio, and the temperature experienced by the material, are required in the numerical simulation to generate the material’s stress-strain relationships. To verify the proposed constitutive model, the new model was introduced into the numerical simulations of the tested round bar specimens after exposure to various temperature conditions. Both the load-displacement curves and local deformation of each round bar specimen obtained from the numerical simulations are compared with the test results to validate the accuracy of the new model. The constitutive model presented in this study provides a useful tool for assessing the post-fire behaviors of steel components.

Life-cycle seismic performance of Q690 steel columns with H-section under various bi-directional cyclic loading paths

The ageing effect in the life-cycle assessment should be considered for the high-strength steel structures constructed in corrosive environments and high-seismicity zones. Additionally, the structures can suffer multi-dimensional seismic excitations because of the spatial effect of ground motions. Therefore, this study aims to investigate the impact of different loading protocols on the mechanical properties of Q690 steel columns with H-section during the whole life-cycle. Firstly, numerical methods considering time-varying corrosion process are used to simulate the ageing steel columns. After that, the nonlinear hysteretic behaviors of the ageing steel columns under different loading protocols are investigated. The factor of resistance degradation (the degree of degradation of ultimate resistance) is employed to quantify the effect of loading protocols on ageing steel columns. Finally, the biaxial ultimate interaction curve considering the ageing effect and a practical design process are proposed. The findings demonstrate that the steel columns exhibit different mechanical properties under various loading protocols, and the coupling direction of bi-directional displacement loads influences the degree of degradation of ultimate resistance. As service time increases, the factor of resistance degradation of steel columns gradually reduces by up to 40%. The proposed biaxial ultimate interaction curve has acceptable accuracy. These findings emphasize the importance of employing the unfavorable loading protocols in conducting the seismic design of the steel columns in different life-cycle stages.

The microstructure and properties of high-strength structural steel for fruit auxiliary picking equipment

The microstructure and mechanical properties of high-strength structural steel used in the fruit-picking equipment were studied by using a metallographic microscope, a tensile tester, and an impact toughness tester. The results revealed that the structure of the experimental steels was a mixed microstructure of ferrite and pearlite, with an average grain size of ferrite of 12 μm. The yield strength was between 515-540 MPa and the tensile strength was 635-645 MPa. The impact toughness at room temperature reached over 200 J. The physical properties of the studied steels completely fulfilled the requirements of the high-strength structural steel for fruit auxiliary picking equipment. The new method cut down the tempering heat treatment process and decreased production costs.

Comparative study about the results of HAZ physical simulations on different high-strength steel grades

With continuous improvements, structural steels are available in even higher strength grades above 1000 MPa yield strength. As the great majority of these steels are used in welded structures, their weldability needs to be taken into account. Several factors can cause difficulties during welding of these steels, but in this paper the softening behavior and the toughness characteristics of the heat-affected zone (HAZ) are examined. As the critical parts of the HAZ in a real welded joint are relatively small, their investigating ability is limited. However, the physical simulation provides a way of evaluating specimens made from a given material to produce the specified HAZ areas in a suitable size range for subsequent testing. In this research work, three different strength categories of high-strength structural steels (with yield strength of 960 MPa, 1100 MPa, and 1300 MPa) are investigated by physical simulation. In the case of different technological variants of gas metal arc welding (GMAW) process, the effect of the cooling time t8/5 is investigated in different HAZ subzones considered to be critical. The thermal cycles were determined according to the Rykalin 3D model. The investigated cooling times were t8/5 = 5 s, 15 s, and 30 s. The properties of the selected coarse-grained, intercritical and intercritically reheated coarse-grained zones are analyzed by laser scanning microscope, scanning electron microscope, hardness test, and instrumented Charpy V-notch impact toughness test. Furthermore, additional investigation like JMatPro calculations, electron backscatter diffraction measurements, and prior austenite grain size calculation were carried out. As a result of the tests, the investigated heat-affected subzones indicated higher sensitivity to the welding heat input compared to conventional structural steels. Overall, the results of the tests show that the application of shorter t8/5 cooling time can be beneficial for the investigated high-strength steel grades, since significant toughness reduction and the risk of softening occur in the whole cooling time range.

Plasma wire arc additive manufacturing and its influence on high-carbon steel substrate properties

Steels with a high carbon content are considered to be difficult to weld. High carbon equivalent indicates not only problems when joining such steels but also when using them as substrate in additive manufacturing (AM). In this study, the possibility of manufacturing a high-strength steel structure on a high-carbon steel substrate using plasma wire arc additive manufacturing (PWAAM) is demonstrated. This study deals with the thermal history and its effects on the substrate resulting from a multilayer build-up. In-substrate temperature measurements as well as metallographic and hardness measurement will provide an understanding of the influence of the process on the substrate. Additionally, a local pre-heating concept using penetration depth induction (PDI) is considered. PDI also enables energy rearrangement in which energy is removed from the AM process and introduced directly into the substrate by PDI. These variants of the preheating concept are also being investigated concerning their effects on the substrate and its properties. A comparison of multilayer buildup without and with PDI is intended to show the differences but also the possibilities that can be achieved with the PDI approach. The investigations carried out are intended to create the basis for a quality AM structure on a high-carbon steel substrate as well as optimized properties of the substrate. It is also shown that the transition zone between the substrate and the AM structure is not trivial for such a material combination.

Mechanical properties of corner material in cold-formed steel structure: From normal strength to high strength

Cold-forming effect varies from normal strength steel to high strength steel due to the different chemical compositions and mechanical properties of parent materials. To date, research studies on material characterisation of cold–formed steels have been mainly focusing on the normal strength structural steel, while systematic research on high strength structural steel remains scarce. In this paper, an experimental investigation into changes in mechanical properties of high strength steel undergoing various levels of plastic deformation is firstly presented. The test results have been combined with other available high strength steel data, and compared to the normal strength steel data. Of the analysis results indicated, unified predictive equations, modified from Karren’s model in 1967 can predict the strength enhancement behaviours of normal strength and high strength steel, utilising the ultimate-to-yield strength ratio of parent materials fu,f/fy,f and inner radius-to-thickness ratio after cold–forming ri/t. Moreover, the decreases in the Young’s modulus, ultimate strain and elongation at fracture of normal strength and high strength steel after cold-forming can be predicted with no further grouping required. Statistical evaluation results confirm the accuracy of the proposed predictive expressions. This paper extends the scope of the strength enhancement equation in current north American cold-formed steel design code AISI S100–16 to high strength steels with grade up to 960 MPa, and provides key predictive material equations that can be further used in the design or numerical modelling of cold-formed steel structures.

Evolutions of temperature field and stress field in narrow gap oscillating laser welding process based on equivalent heat source

With the increased demand for high-performance thick high-strength steel structures in high-end offshore equipment, narrow gap oscillating laser welding with the advantage of high power density and satisfied penetration depth has attracted considerable attention. A thermal-elastic-plastic finite element model was established to calculate the narrow gap oscillating laser welding process of 30-mm-thick high-strength steel, which requires a large amount of computational time cost. To increase the computational efficiency, two equivalent heat source models were founded to replace the laser oscillation. The temperature field and stress-strain field were analyzed. The accuracy of the equivalent heat source model was verified from the weld seam profile and residual stress distribution curves. Results showed that the simulated results were in good agreement with the actual results. With the number of welding passes increasing, the cross-section shape of the molten pool changed from an elongated shape to an arc shape, and the groove gap on the top surface was reduced from 4.82 mm before welding to 3.18 mm after welding. The maximum equivalent residual stress, with the value of 669 MPa, was distributed in the middle and upper parts of the weld seam as well as the root. The longitudinal residual stresses exhibited tensile stresses in the weld seam zone. Due to the upward bending of the workpiece, the transverse residual stresses showed compressive stresses on the top surface of the weld seam and maximum tensile stresses with the value of 806 MPa at the bottom of the weld seam.

The Effect of Cellulose Nanocrystal-Based Nanofluid on Milling Performance: An Investigation of Dillimax 690T.

Machining high-strength structural steels often requires challenging processes. It is essential to improve the machinability of such materials, which are frequently needed in industrial manufacturing areas. Recently, it has become necessary to enhance the machinability of such materials using different nanopowders. In this study, different cooling/lubricating (C/L) liquids were prepared with cellulose nanocrystal (CNC) nanopowder. The aim was to improve the machinability properties of Dillimax 690T material with the prepared CNC-based cutting fluids. CNC nanopowders were added to 0.5% distilled water by volume, and a new nanofluid was produced. Unlike previous studies, base synthetic oil and CNC-based cutting fluid were sprayed on the cutting area with a double minimum quantity lubrication (MQL) system. Machinability tests were carried out by milling. Two different cutting speeds (Vc = 120-150 m/min), two different feed rates (f = 0.05-0.075 mm/tooth), and four different C/L environments (dry, MQL oil, CNC nanofluid, MQL oil + CNC nanofluid) were used in the experiments. In the study, where a total of 16 experiments were performed, cutting temperature (Tc), surface roughness (Ra), tool wear (Vb), and energy consumption results were analyzed in detail. According to the test results, significant improvements were achieved in the machinability properties of the material in the experiments carried out using CNC nanofluid. In particular, the hybrid C/L environment using MQL oil + CNC nanofluid improved all machinability metrics by over 15% compared to dry machining. In short, using CNC nanopowders offers a good milling process of Dillimax 690T material with effective lubrication and cooling ability.