Change In Yield Strength Research Articles

Characteristics of surface and subsurface of formed thread parts by axial-infeed thread rolling process

The thread rolling process has been widely applied to manufacture high-performance thread parts. In this process, the evolutions of surface and subsurface are frequently introduced, which affect the working performance of manufactured parts. In this study, an axial-infeed thread rolling process (ATRP) is employed, and the macro-meso surface characteristics under different lubrications and operating conditions are investigated. Moreover, the distributions of microstructure and hardness on the subsurface of formed tooth are analyzed in detail, along with the study of stress state and yield strength change. It is found that the MoS2 grease is more effective in reducing the surface roughness and defects than the lubrication oil and water-base graphite during the ATRP process. Increasing rolling speed improves the quality of surface morphology and can reduce the surface roughness. On the subsurface of bottom and flank, intensive shear stress occurs in a narrow region, resulting in the elongation and refinement of the grains and increasing the low angle grain boundary fraction. Based on the grain size and plastic strain, the yield strength is predicted. The maximum yield strength and hardness on the bottom of formed tooth are improved by 41.2% and 39.4%, respectively.

Local deformation mechanisms in metal systems with a tailored grain size distribution

A large change in yield strength and ductility takes place in metals with a recrystallized average grain size in the near-micrometer grain size regime. This has been utilized to explore the influence of grain size heterogeneity on plastic deformation for recrystallized grains in aluminum prepared by spark plasma sintering, where tailoring of the grain size distribution and spatial arrangement can be achieved by selecting powder sizes and mixing conditions prior to sintering. The spatial variation of plastic deformation has been explored by digital image correlation (DIC), either at the sample scale or at the grain scale. For the latter colloidal SiO2 markers also allow repeated collection of electron backscatter diffraction data over the same area during in-situ deformation, proving correlated information on local displacement gradients and crystal lattice rotations. For samples prepared from well-mixed powders only small deviations from a rule-of-mixtures in the yield strength are observed. A mesoscale pattern of strain distribution is observed from DIC investigations where this is related to, but not fully consistent with, the spatial arrangement of fine and coarse grains. In the case of macroscopically layered arrangements of coarse and fine grains, deformation is dominated by development of local strain concentrations, which take place preferentially in the fine grain regions.

Deformation evolution depending on grain boundary type and hydrogen concentration in nickel investigated by molecular dynamics simulation

The impacts of hydrogen concentration on tensile deformation in the nickel bicrystals with different typical grain boundaries (GB) were investigated by molecular dynamics simulation. The deformation behavior was dependent on the GB type and hydrogen concentration. A critical hydrogen concentration was obtained from a drastic change of the theoretical yield strength. Below critical concentration, hydrogen increased the yield strength and the atomic rearrangement was effectively hindered due to uniform distribution of hydrogen. Above critical concentration, the nickel-hydride formed and caused a sharp decrease in yield strength, which was independent of the GB type.

Thermal Aging Effects on the Yield and Tensile Strength of 9Cr-1Mo-V (Grade 91)

Abstract The long-term exposure of 9Cr-1Mo-V (Grade 91) steel to elevated temperature can have a significant effect on reducing its yield and tensile strength. The yield and tensile strength changes, in turn, have potential implications to the assurance of the integrity of components constructed with this material over their design or intended lifetime. The ASME Boiler & Pressure Vessel Code (BPVC), Section III, Division 5 (III-5, high temperature reactors) provides tabulated reduction factors for Grade 91 yield and tensile strength as a function of exposure temperature and time up to 300,000 h. ASME BPVC III-5 s intent to extend these factors to an exposure duration of 500,000 h, the lack of available historic information to support the existing factors, and the recent development of a physics-based prediction model for ASME BPVC application are prime motivation for this study. This paper describes results of the conventional time-temperature Hollomon–Jaffe parameter, strength reduction ratio prediction method using an updated, extensive Grade 91 unaged and related aged material strength database. The method, previously used by Oak Ridge National Laboratory in its evaluation of Grade 91 and likely used in development of the existing BPVC III-5 reduction factors, provides strength reduction ratio predictions useful for general component fitness-for-service assessments and for computing BPVC III-5 reduction factors as defined. Specific reduction factors to 500,000 h at 650 °C applicable to ASME BPVC III-5 are computed from the strength reduction ratios.

Change in Mechanical Properties During Drawing of Wire from Ni 99.6

This paper presents the results of a study of changes in mechanical properties, namely tensile strength, yield strength and elongation when drawing a wire from NP2 nickel (Ni 99.6). The wire samples taken along the drawing route were tested for tension. Based on the test results obtained, graphs of changes in tensile strength, yield strength and elongation were plotted depending on cold work. The approximation of the graphs made it possible to obtain equations for the change in tensile strength (UTS), yield strength (0.2 US) and elongation (δ) depending on cold work (ɛ∑). The obtained dependences are recommended to be used to predict the mechanical properties of nickel wire during drawing and when developing software for designing resource-saving drawing routes.

Microstructural evolution during tempering and intrinsic strengthening mechanisms in a low carbon martensitic stainless bearing steel

Herein, the microstructural evolution and corresponding change in tensile properties in a low carbon martensitic stainless bearing steel during tempering were studied. The results showed that there was no δ-ferrite in the studied steel after austenitizing at 1000∼1100 °C, and the volume fraction of austenite increased with increasing the tempering temperature from 470 °C to 590 °C due to the formation of reverted austenite. According to the EDS measurements, the reverted austenite was rich in C, Cr and Mo, other than previously assumed Co and Ni. On top of that, a considerable number of nanosized Mo-rich carbides with globular and rod-like morphologies were found after tempering at 530 °C, and further coarsened after tempering at 590 °C. Along with the change of phase constituents, the dislocation density decreased, and martensite block width increased as the tempering temperature increased. All the evolution of microstructures jointly contributed to an initial increase and then a decrease in the strength, and various strengthening mechanisms contributed to the change of yield strength were quantitatively analyzed and discussed in the study.

Numerical and Experimental Study of an Industrial Case for Grain Size Evolution in Bainitic Steel in Controlled Hot Forging and its Influence on Mechanical Performance

Controlling the recrystallization is an important way to reach grain size refinement and outstanding strength and toughness on alloy metals. This study sets out the application and investigation of mathematical microstructure modeling of a newly designed bainitic steel for hot forging industrial applications. The macro-scale model was used to observe and predict the austenitic grain size behavior during the controlled forging of a gear. Arrhenius grain growth kinetic and recrystallization model for a new class of bainitic steel was established for the given strain rate ranges and temperatures. This model was calibrated through microscopic analysis and used to simulate the unpublished constants of low alloyed bainitic forging steel DIN 18MnCrSiMo6-4 microstructure module using DEFORM® commercial finite element code. The increased temperature due to the adiabatic effect was investigated by numerical analysis, demonstrating its influence on grain coarsening. Local tensile test and Charpy-V notch were compared at different industrial hot forging temperatures and local plastic strain. Changes in yield strength and ductility have demonstrated the grain size influence on the processing parameters. The employed numerical model was an efficient tool to predict and present an alternative path to develop robust industrial forging using semi-empirical models.

Development of biaxial tensile testing for porous polymer membranes

Porous polymer membranes are typically polymeric materials that consist of many pores and complex network structures. As a result of these features, the membranes sometimes undergo anisotropic elastoplastic deformation. Hence, it is important to investigate the deformation behavior of these materials, including anisotropic deformation under multiaxial loading. In this study, we developed a new biaxial tensile testing machine for porous polymer membranes to evaluate their deformation behavior under biaxial tension and to understand the deformation mechanisms from their microstructure. First, uniaxial tensile tests were conducted to investigate changes in Young's modulus and yield strength for different tensile directions. These tests showed that the materials underwent anisotropic deformations. Next, we developed a novel biaxial tensile testing protocol for the membranes. It is usually difficult to grip the specimens and apply uniform deformations, as the membranes have low deformation rigidity and are very thin. Thus, a gripping component was developed to enable uniform tensile deformations to be applied. Stress–strain curves for each loading axis were obtained by changing the applied strain ratio. A yield surface based on the results was created to investigate the deformation behavior under biaxial tensile loading. We also investigated the deformation mechanisms of the porous microstructure via SEM observation.

Research on high-temperature mechanical properties of wellhead and downhole tool steel in offshore multi-round thermal recovery

Abstract High-temperature tensile tests at 25, 150, 250, and 350°C were carried out on 30CrMo, 42CrMo, 1Cr13, and 304 steels. The changes in tensile strength, yield strength, elongation, and area reduction ratio with temperature were determined. By analyzing the fracture morphology and the relationship between strength and hardness, the influence of high-temperature mechanical properties on crack sensitivity and the mechanism of crack formation is discussed. Experimental results indicated that both the tensile and yield strengths of the four steels gradually decrease with the increase in temperature. The yield ratios of 30CrMo, 42CrMo, 1Cr13, and 304 steels are, respectively, 0.71–0.77, 0.79–0.86, 0.84–0.88, and 0.33–0.40 which shows that among the four steels, 304 has the best ductility, while 1Cr13 has the worst ductility. As for the four steels, the values of reduction ratio of area are greater than 60%, except for 42CrMo which is slightly lower than 60% at 150 and 250°C, indicating that the four steels have low crack sensitivity within the test temperature range. Ductile fracture is the main fracture mechanism for 30CrMo, 42CrMo, and 304 steel, whereas brittle fracture is predominant for 1Cr13. There is a linear regression relationship between the strength and hardness at different temperatures. The obtained linear regression relationship can be used to predict and estimate the strength of 30CrMo, 42CrMo, 1Cr13, and 304 steels at different temperatures according to the hardness results.

Rheological modelling of high‐temperature stationary creep tests of Grade S275JR steel

AbstractThe paper presents a new validation study of a unified rheological model, in reproducing creep strains from a recent experimental study performed on grade S275JR coupons. The objective of this research is to further test the modelling capabilities of the proposed rheological model and its validity for application in general structural fire engineering analysis. The rheological model is composed of two serially connected Kelvin‐Voight elements, used to represent the material's sensitivity to strain‐rate‐governed changes of yield strength, temperature and heating rate. The paper presents the performance of the model in stationary‐creep conditions.

Probability-Dependent Precipitation Strengthening Effect of Anisotropic Precipitate in Al-Mg-Si Alloy Produced by T6 Heat Treatment

This study proposed a constitutive equation to predict the change in yield strength according to the behavior of β″ metastable precipitates, which have a profound effect on strength among materials precipitated during the T6 heat treatment of Al-Mg-Si alloy. The β″ precipitate is a metastable phase before it becomes a β (Mg2Si) precipitate, and is distributed in the form of nano-scale rods in the aluminum alloy matrix. Existing precipitation strengthening models assume the shape of the precipitate to be spherical, and in that case the equation that depends on the Orowan mechanism with the average precipitate size and distribution should dominate. However, precipitates are formed in various shapes and sizes by anisotropic growth. In particular, rod-shaped precipitates are not suitable for the existing precipitation strengthening model. In this study, an Al-Mg-Si alloy was fabricated by gravity casting followed by T6 heat treatment. The new precipitation strengthening effect equation proposes that the β″ precipitate affects yield strength during plastic deformation of the Al-Mg-Si alloy. The proposed precipitation strengthening effect equation probabilistically considers the Critical Resolved Shear Stress (CRSS), which varies depending on the angle between the dislocation and the precipitate, when the dislocation passes through a rod-shaped precipitate.

Understanding Zones of Molecular Dimension in Poly (Lactic Acid) Composites through Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy: Correlation with Tensile Yield Test Measurements

Abstract Multiwalled carbon nanotubes (MWCNTs) filled poly (lactic acid) composites (MWCNTs PLAC) and hybrid poly (lactic acid) composites (HPLAC) filled with hybrid fillers, i.e., combining variable weight percentage of MWCNTs (0.2, 0.4, 0.6, 0.8, and 1.0 wt. %) and fixed weight percentage of carbon black (CB) (2.5 wt. %) were prepared using oligomeric dispersion. This article focuses on the use of attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy to analyze the intermolecular interactions and zones of molecular dimension within polymer composite materials. For the first time percent transmittance (%T) and transmittance area (A) of peaks from ATR-FTIR spectra were corroborated with the zones of molecular dimension within MWCNTs PLAC and HPLAC. Tensile measurements were performed on test specimens of MWCNTs PLAC and HPLAC, and the numerical values of tensile yield strength were determined. The numerical values of tensile yield test measurements of MWCNTs PLAC and HPLAC were then compared with the %T and A of MWCNTs PLAC and HPLAC ATR-FTIR spectra, which presented a new way to qualitatively analyze the zones of molecular dimension and their role toward the change in the tensile yield strength of the MWCNTs PLAC and HPLAC.

The influence of post-weld tempering temperatures on microstructure and strength in the stir zone of friction stir welded reduced activation ferritic/martensitic steel

Abstract Reduced activation ferritic/martensitic (RAFM) steels are among the most competitive candidates of structural materials for nuclear fusion reactors, due to their superior comprehensive properties. Friction stir welding (FSW) was investigated in joining RAFM steel, considering its potential advantages in obtaining an optimal microstructure and mechanical properties of welded joint. To evaluate the feasibility of FSW in joining RAFM steel, an in-depth understanding of the microstructure-property relationships for friction stir welded joints of RAFM steel is necessary. In this research, the quantitative relationships between microstructural evolution and tensile properties in the stir zone (SZ) of friction stir welded RAFM steel after post-weld tempering treatment (PWTT) were systematically studied. Three different post-weld tempering temperatures namely 720 °C, 760 °C, and 800 °C were adopted. Then the uniaxial tensile properties were tested at room temperature and 550 °C, respectively. Electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and the Thermo-Calc Calphad software were adopted to systematically investigate the microstructural evolution. Martensite lath width, precipitate number density, equilibrium solid solubility of alloying elements in the matrix, and geometrically necessary dislocation (GND) density were analyzed quantitatively. With the results obtained, we assessed the contribution of each strengthening mechanism to the 0.2% offset yield strength. According to the effective inter-barrier spacing theory, a microstructure-sensitive yield strength model was obtained to well predict the change in yield strength at different conditions. Finally, the results calculated by equivalent strengthening effect indicated that the crucial microstructure determining the yield strength of the SZ for RAFM steel after PWTT is the high density of dislocation substructures.

The Effects of Operational, Testing, and Material Characterizations on the Change in Yield Strength From Plate to Pipe

Abstract The rapid growth of energy consumption has increased the demand of pipelines for the transportation of oil and gas. UOE is one common technique for manufacturing pipes, and its pipe products are evaluated by tensile tests after cutting and flattening the specimen. The mechanical performance of the pipe sample has a direct influence on their application and a significant economic impact on manufacturers. There is often a change in the yield strength from plate to pipe. This work sets out to provide a critical knowledge base for this change, with emphasizing the important influence of the plate mechanical properties on the pipe so that the quality of pipe can be further ensured. In the work, first, the historical data of the pipe yield strength were collected and plotted to provide an overall insight and a quantitative review on change of the pipe yield strength. Second, an experimentally validated finite element model of the pipe forming and mechanical testing was explained. Finally, using the validated Finite Element model, a parametric study was conducted to determine the individual role that each plate material parameter plays on changing the pipe yield strength. We found that the pipe yield strength can differ significantly by varying the plate properties. This work sheds lights on the desired plate mechanical properties to optimize the final pipe yield strength.

Significance of Finite Element Models and Solid-State Phase Transformation on the Evaluation of Weld Induced Residual Stresses

In the present study, different finite element (FE) models were prepared to investigate weld induced residual stresses in thick multi-pass butt welded joint of SA516 Gr. 70 plates. Both 3D and 2D full geometry models and their axisymmetric half models were taken into consideration. The competence of these FE models on the accuracy of predicting residual stress distribution across the weld cross-section was investigated by comparing it with the experimental results. Blind hole drilling technique and deep hole drilling technique were employed to evaluate the surface and through-thickness residual stress distributions, respectively. In addition, the change in volume and yield strength of weld material due to austenitic phase transformation was also incorporated in the material modeling to observe the effect of solid-state phase transformation (SSPT) on the evaluation of residual stresses. Computed residual stresses obtained from different FE models indicate that the 3D FE models procured the best accuracy compared with the experimental results. On the other hand, 2D models can save a significant amount of computational time with reasonable accuracy. Incorporation of SSPT in the 3D FE full model exhibited a better agreement of predicted results with the experimental measurements.

Dynamic Strength of Copper at High Pressures Using Pressure Shear Plate Experiments

Measurement of strength at high pressures is critical to understanding the behavior of materials subjected to extreme loading conditions. Recent developments have extended the pressure shear plate impact (PSPI) technique to the high-pressure regime. Such experiments provide a unique opportunity to extract the complete stress–strain behavior of materials at relatively high pressures. The modified technique includes a new fiber-optic heterodyne transverse velocity interferometer system and new analysis methods to account for the inelastic response of the anvil plates. In this study, PSPI experiments are conducted on oxygen-free high conductivity (OFHC) copper at pressures ranging from 10 to 43 GPa and at strain rates of ~ 105 s−1. Complete stress–strain curves of copper are obtained using a hybrid methodology that relies on numerical simulations and particle velocity records gathered in these experiments. Experiments are conducted at similar pressures and strain rates using two types of anvils that undergo different levels of plastic strains: tool steel and tungsten carbide, to check the robustness of the hybrid method. The behavior of copper at these high pressures and strain rates is compared with previous literature data, and the effect of pressure and strain rate on the strength behavior of copper are discussed. It was observed that the rate of change in yield strength with pressure was 2.5 times the rate of change of shear modulus with pressure. The strength models that predict the response of copper at these extreme conditions are also examined and validated.

Suppression of the annealing-induced hardening effect in ultrafine-grained Al at low temperatures

The influence of decreased temperatures of mechanical tension on the effect of annealing-induced hardening (AIH) in ultrafine-grained (UFG) aluminum of commercial purity is revealed and discussed for the first time. The UFG structure was formed by high-pressure torsion processing. The dependences of changes in yield strength, tensile strength and ductility, caused by annealing, on the deformation test temperature were experimentally obtained. The critical deformation temperature was revealed at which the AIH effect is completely suppressed. A theoretical explanation of this phenomenon is suggested.

정착부 구속철근의 항복강도 및 간격 변화에 따른 PSC 정착장치 하중전달성능 평가

정착부 구속철근의 항복강도 및 간격 변화에 따른 PSC 정착장치 하중전달성능 평가

Contact Yield Initiation and Its Influence on Rolling Contact Fatigue of Case-Hardened Steels

AbstractStress distributions and plastic deformation zones are factors directly influencing the fatigue life of components under cyclic contact. An effective approach to improving the resistance of a steel to contact fatigue failure is surface hardening, which builds gradient yield strength from the surface of the steel to the bulk. When using the distortion energy theory as the criterion to identify failure initiation for a case-hardened steel, contact yield starts in the subsurface wherever the von Mises stress reaches the local material strength, rather than at the point of the maximum von Mises stress in the subsurface. If the yield strength changes from the surface to the bulk following a straight line, the location of yield initiation should occur at the tangency of the strength line and the von Mises stress curve. Analyses on circular, rectangular, and elliptical contacts are presented to reveal the locations of contact yield initiation for such case-hardened steels subjected to rolling contact stresses, for which the influence of friction can be ignored. A group of formulas relating contact yield initiation, in terms of the critical pressure, location of the first yield, and plasticity index (transition to plasticity) to case-hardening parameters, such as the case slope, the minimum case depth, and surface and bulk strengths, are derived to facilitate contact element designs using case-hardened materials. The results are applied to examine the rolling contact behaviors of several case-hardened steels, and the data correlation suggests that their rolling contact fatigue lives are related to a nondimensional case-hardening slope besides external loading.

Failure analysis of structural steel subjected to long term exposure of hydrogen

Embrittlement of steel due to its exposure to hydrogen is a well-established phenomenon, which diminishes its ductility and toughness. However, there is limited research on how hydrogen, which evolves from the corrosion process, affects the yield strength of mild steel in the long-term. This paper presents the results obtained from a long-term investigation on hydrogen evolving from a corrosion reaction, which causes embrittlement of mild steel. Both mechanical tests and microstructural analyses on corroded steel specimens are performed at three different intervals for one year. Time-dependent relations for predicting the corrosion rate and its subsequent hydrogen release were derived by analysing the test results. Moreover, relations for predicting the change in yield strength as a function of hydrogen content, corrosion rate and compositional element change were developed, along with one single equation considering all these factors. Furthermore, fractography analysis was performed to observe HE effect and reasons for the decline in yield strength. The analysis revealed hydrogen induced micro cracks, micro pores, intergranular cracks, grains deformation and hydrogen blisters.