SA508-3 Steel Research Articles

Prediction and formation mechanism of serrated chips in cutting of SA508–3 steel under enhanced cooling and lubrication environments

The formation of serrated chips is the typical characteristic of high-speed machining. In this paper, a new prediction method based on Williams’ model was presented to give the critical cutting condition for serrated chips formation. The serrated chips formation mechanism under different cooling conditions was analyzed by finite element method. Results indicated that the cutting forces abruptly reduces when the serrated chip forms, the chips serrated degree (Gs) under various cooling conditions (i.e., dry, minimum quantity lubrication (MQL), supercritical CO2 (scCO2), and scCO2-MQL) was described by the following order: scCO2(Gs)＞scCO2-MQL(Gs)＞MQL(Gs)＞dry(Gs). The adiabatic shear owing to the cutting heat and crack initiated from the chip free surface under scCO2 cooling promoted the serrated chips formation.

Effect of milling parameters on machinability of SA508-3 steel in high-speed milling with uncoated and coated carbide tools

SA508-3 steel is popularly used to produce core unit of nuclear power reactors due to its outstanding ability of anti-neutron irradiation and good fracture toughness. Additive forging is a new technology for manufacturing SA508-3 steel forgings. However, the production efficiency and interface bonding quality of heavy forgings are respectively limited by the processing efficiency and surface quality of substrates in the additive forging process. High-speed milling technology is an effective method for improving machining efficiency and quality. Unfortunately, only a few studies on the milling of SA508-3 steel have been reported. In this study, we studied high-speed milling of SA508-3 steel and compared the cutting performances of uncoated, titanium aluminum nitride (TiAlN)-coated, and Al2O3-coated carbide tools. The tool life and cutting force were evaluated using various milling parameters under dry milling conditions. The wear modes and mechanisms were also investigated. The results show that adhesive wear occurs more frequently in the uncoated carbide tool, whereas coating flaking is predominant in the Al2O3- and TiAlN-coated carbide tools. Furthermore, the Al2O3-coated carbide tool showed better cutting performance than the TiAlN-coated and uncoated carbide tools considering the tool life and surface quality. The tool life of the Al2O3-coated carbide tool reached 200 min and the removed workpiece material was 182 × 103 mm3 under the blunt tool criteria. The study of tool life and wear behavior based on the practical cutting experiments contribute to the improvement of the milling quality and provides a theoretical basis for tool material selection and process optimization in milling SA508-3 steel.

The dynamic compressive behavior and constitutive model of SA508-3 steel at high strain rates under elevated temperatures

The dynamic compressive behavior and constitutive model of SA508-3 steel at high strain rates under elevated temperatures

Dynamic fracture behavior of SA508-3 steel for nuclear power equipment under medium-and low-loading rates

Abstract The heat treatment of SA508-3 steel was carried out by means of quenching and manual aging. The microstructure and mechanical properties of the prepared SA508-3 steel were analyzed and tested. The results show that the main composition of the matrix of SA508-3 steel is bainite, and the main composition of the second phase is alloy cementite containing Fe, Mn and C. The second phase is distributed at both the matrix and grain boundaries. The second phase can prevent the dislocation from moving by fixing the dislocation, and the second phase at the grain boundary can strengthen the matrix by hindering the grain boundary movement. The stress-strain curves of SA508-3 steel under different loading rates show that when the strain rate is greater than 0.5 m s−1, the fracture mode of the steel is brittle fracture, and when the strain rate is less than 0.5 m s−1, the fracture mode of the steel is ductile-brittle bonding fracture. The second phase of the crack first diffuses to the grain boundary, reducing the strength of the grain boundary. When the loading rate is high, the second phase at the grain boundary cannot diffuse in time, and the material undergoes transgranular fracture and intergranular fracture.

Effect of Cooling Rate on Phase Transformation and Strain Response of SA508-3 Steel by Numerical and Experimental Study

Abstract The key to accurately predict welding residual stress is to explore the solid-phase transition law of SA508-3 steel and establish the phase transition model under continuous cooling conditions. The effects of cooling rate on phase transformation and strain response of SA508-3 steel were investigated in this paper. The established programs accurately predict the evolution of microstructure at different cooling rates, the relationship between the bainite volume fraction and temperature is fitted by a modified bainite transformation model and is validated by optical microscopy and dilatometric test. The strain evolution of SA508-3 steel is predicted at different cooling rates and is verified by the dilatometric test. In addition, the effects of bainite transformation and martensite transformation on the strain response of SA508-3 steel are intensively discussed. This work casts light on the simulation of metallurgical effects at different cooling region during welding.

The Dynamic Compressive Behavior and Constitutive Model of Sa508-3 Steel at High Strain Rates Under Elevated Temperatures

The Dynamic Compressive Behavior and Constitutive Model of Sa508-3 Steel at High Strain Rates Under Elevated Temperatures

Mechanism of Improving the Impact Toughness of SA508-3 Steel Used for Nuclear Power by Pre-Transformation of M-A Islands

Mechanism of Improving the Impact Toughness of SA508-3 Steel Used for Nuclear Power by Pre-Transformation of M-A Islands

Effects of hot deformation and heat treatment on grain refinement for as-cast and spray formed SA508-3 steel

In order to identify quantitatively the different effects of hot deformation and heat treatment processes on the grain size of as-cast and spray formed SA508-3 steel, two kinds of billets were produced respectively. Samples were machined, forged and heat-treated in sequence under the same conditions for both billets. Then, the corresponding microstructures were observed and analysed for each process. The experimental results demonstrate that both hot deformation and heat treatment are beneficial to refine the grains of the as-cast and spray formed billets. Furthermore, the grains of spray formed samples are much finer and more uniform than that of as-cast ones. It indicates that the spray forming process is more competitive for grain refinement than the conventional casting. However, the existence of voids in the spray formed is detrimental for the mechanical properties, especially the elongation. Therefore, not only the spray forming process but also the hot deformation parameters should be further improved to densify the microstructures and guarantee the complete closure of voids.

Effects of hot deformation and heat treatment on grain refinement for as-cast and spray formed SA508-3 steel

Effects of hot deformation and heat treatment on grain refinement for as-cast and spray formed SA508-3 steel

Finite Element Implementation of a Temperature-Dependent Cyclic Plastic Model for SA508-3 Steel

A new temperature-dependent cyclic plastic model, combining the nonlinear cyclic softening and kinematic hardening rules is established for a nuclear material of SA508-3 steel. A modified isotropic hardening rule is proposed to capture the temperature-dependent cyclic softening, and a modified kinematic hardening rule is established to improve the prediction of the ratcheting behavior by introducing an exponential function related to the accumulated plastic strain. The stress is updated by the radial return mapping algorithm based on the backward Euler integration. A new expression of consistent tangent modulus for the equilibrium iteration is derived, and then the proposed model is implemented into the finite element software ABAQUS by using the user material subroutine (UMAT) to simulate the temperature-dependent ratcheting behaviors of SA508-3 steel. Finally, the ratcheting evolutions of notched bars at elevated temperature are obtained by uniaxial stress-controlled cyclic tests, and the nonuniform strain fields on the surface of plates with a center hole is measured by using the digital image correlation (DIC) technology. Comparisons between experimental and simulated results of a material point and structural examples show that the implemented model can provide reasonable predictions for ratcheting behaviors and nonuniform strain fields of structures at different temperatures for SA508-3 steel.

Precise Determination of Trace Hydrogen in SA508-3 Steel for Nuclear Reactor Pressure Vessels

Hydrogen is one of the most detrimental elements in steel, therefore it must be strictly controlled, especially in the ferritic steel SA508-3 used in nuclear power plant, due to its persistent exposure to irradiated environment. However, the tests of hydrogen concentration in current industry have provided usually unstable and sparse results, which seriously perplexes practitioners. In the present study, tested hydrogen contents of a series of specimens with different machining and cleaning conditions were compared. Experimental and theoretical investigations showed that differences in surface contamination can significantly influence hydrogen analysis result, and the measured hydrogen content is the combination of the inherent hydrogen in the steel and the adsorbed hydrogen on the surface. Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIM) was then used to determine the hydrogen concentration gradient from the surface to the base of the SA508-3 steel, with the aim of confirming the existence of surface hydrogen. The result demonstrated that, if the specimen surface is in a rough and polluted state, the count intensity of hydrogen on the specimen surface, which qualitatively reflects the content of surface hydrogen can be more than 4 times that in the base of the specimen. Finally, on the basis of the analysis of tested results, a pre-treatment method combining a certain machining speed and ultrasonic cleaning is proposed in order to obtain a stable and reliable hydrogen analysis result. Furthermore, a mass of practical industry data is used to confirm the effectiveness of this pre-treatment method.

Ratcheting Behavior of SA508-3 Steel at Elevated Temperature: Experimental Observation and Simulation

A series of monotonic uniaxial tensile tests, strain-controlled and stress-controlled cyclic tests of SA508-3 steel were conducted from 25 to 350 °C. Results showed that the steel exhibited temperature-dependent cyclic softening characteristic and obvious ratcheting behavior, and dynamic strain aging was observed in the range of 250–350 °C. Based on experimental observations, a temperature-dependent cyclic plastic constitutive model was proposed by introducing the nonlinear cyclic softening and kinematic hardening rules, and the dynamic strain aging was also considered into the constitutive model. Comparisons between experiments and simulations were carried out to validate the proposed model at elevated temperature.

Multi-scale simulation for the forming of a heavy vessel head considering the evolution of defects and microstructure

The head of nuclear pressure vessel is a key component to guarantee the safety of nuclear power plant, so it is necessary to improve its mechanical properties during manufacturing. In the practical production, due to the huge size of the ingots from which the head is manufactured, coarse grains and voids are common defects existing in the material. Furthermore, cracks may appear in the forming process. It is highly demanded that the forming process must be properly designed with suitable parameters to compact the voids, to refine and homogenize the grains and to avoid cracks. Therefore, the research on the evolution of internal voids, grain size and cracks is very important to determine the forming process of huge components. SA508-3 steel is the material to manufacture the head of pressure vessel in the nuclear island. In the previous studies, we have separately built models to evaluate the evolution of internal voids, grain size and cracks during the hot forming process for SA508-3 steel. This study integrates the models for multi-scale simulation of the forging process of the head of nuclear pressure vessel in order to control the quality of the forgings. Through the software development, the models are integrated with a commercial finite element code DEFORM. Then, the extended forging and final forging processes of the head are investigated, and some appropriate deformation parameters are recommended.

Mathematical Modeling of High-Temperature Constitutive Equations and Hot Processing Maps for As-Cast SA508-3 Steel

The hot deformation behavior and hot workability characteristics of as-cast SA508-3 steel were studied by modeling the constitutive equations and developing hot processing maps. The isothermal compression experiments were carried out at temperatures of 950°C, 1050°C, 1150°C, and 1250°C and strain rates of 0.001 s−1, 0.01 s−1, 0.1 s−1, and 1 s−1 respectively. The two-stage flow stress models were established through the classical theories on work hardening and softening, and the solution of activation energy for hot deformation was 355.0 kJ mol−1 K−1. Based on the dynamic material model, the power dissipation and instability maps were developed separately at strains of 0.2, 0.4, 0.6 and 0.8. The power dissipation rate increases with both the increase of temperature and the decrease of strain rate, and the instable region mainly appears on the conditions of low temperature and high strain rate. The optimal hot working parameters for as-cast SA508-3 steel are 1050–1200°C/0.001–0.1 s−1, with about 25–40% peak efficiency of power dissipation.

The coarsening effect of SA508-3 steel used as heavy forgings material

SA508Gr.3 steel is popularly used to produce core unit of nuclear power reactors due to its outstanding ability of anti-neutron irradiation and good fracture toughness. The forging process takes important role in manufacturing to refine the grain size and improve the material properties. But due to their huge size, heavy forgings cannot be cooled down quickly, and the refined grains usually have long time to grow in high temperature conditions. If the forging process is not adequately scheduled or implemented, very large grains up to millimetres in size may be found in this steel and cannot be eliminated in the subsequent heat treatment. To fix the condition which may causes the coarsening of the steel, hot upsetting experiments in the industrial production environment were performed under different working conditions and the corresponding grain sizes were measured and analysed. The observation showed that the grain will abnormally grow if the deformation is less than a critical value. The strain energy takes a critical role in the grain evolution. If dynamic recrystallization consumes the strain energy as much as possible, the normal grains will be obtained. While if not, the stored strain energy will promote abnormal growth of the grains.

Numerical Simulation of Microstructure Evolution for SA508-3 Steel during Inhomogeneous Hot Deformation Process

Based on hot compression tests by a Gleeble-1500D thermo-mechanical simulator, the flow stress model and microstructure evolution model for SA508-3 steel were established through the classical theories on work hardening and softening. The developed models were integrated into 3D thermal-mechanical coupled rigid-plastic finite element software DEFORM3D. The inhomogeneous hot deformation (IHD) experiments of SA508-3 steel were designed and carried out. Meanwhile, numerical simulation was implemented to investigate the effect of temperature, strain and strain rate on microstructure during IHD process through measuring grain sizes at given positions. The simulated grain sizes were basically in agreement with the experimental ones. The results of experiment and simulation demonstrated that temperature is the main factor for the initiation of dynamic recrystallization (DRX), and higher temperature means lower critical strain so that DRX can be facilitated to obtain uniform fine microstructure.

Model for predicting ductile fracture of SA508-3 steel undergoing hot forming

On the basis of the tensile tests that were conducted on a Gleeble thermomechanical simulator at various temperatures and strain rates, the fracture behaviour of SA508-3 steel was investigated, and a model for predicting the occurrence of ductile fracture during hot deformation was established. In order to determine the critical fracture strain ϵf that initiates the necking of the material, a ‘virtual’ part of the specimen was created according to the constant volume condition to replace the necking volume of the specimen. Experimental results indicated that ϵf depends on temperature and strain rate, and subsequently, a model of ϵf was established. In conjunction with Oyane’s fracture criterion, an evaluation method for predicting the fracture of SA508-3 during hot forming was reproduced. The model was verified by upsetting tests, and good coincidence was obtained. As an application, it was applied to a simulation of a forging stretching process, and the calculated results showed the model has preferable predication performance.

The Effect of Quenching Media on the Microstructure and Mechanical Properties of SA508-3 Steel

Three different quenching media (water, oil and air) were used to compare the effect of cooling rate on the microstructure and mechanical properties of SA508-3 steel. The result has demonstrated that the microstructure for water-quenched specimen is the mixture of martensite and lower bainite, for oil-quenched specimen is the mixture of upper bainite，lower bainite and a little martensite, while for air-cooled specimen is mostly granular bainite. The product of water and oil Q&T was tempered martensite with qualified mechanical properties. The air-cooled granular bainite was translated to massive and allotriomorphic ferrite during tempering, which had poor mechanical properties. To avoid the formation of granular bainite, it is necessary to increase the cooling rate to above 5°C/s.

Modeling flow stress constitutive behavior of SA508-3 steel for nuclear reactor pressure vessels

Based on the measured stress–strain curves under different temperatures and strain rates, a series of flow stress constitutive equations for SA508-3 steel were firstly established through the classical theories on work hardening and softening. The comparison between the experimental and modeling results has confirmed that the established constitutive equations can correctly describe the mechanical responses and microstructural evolutions of the steel under various hot deformation conditions. We further represented a successful industrial application of this model to simulate a forging process for a large conical shell used in a nuclear steam generator, which evidences its practical and promising perspective of our model with an aim of widely promoting the hot plasticity processing for heavy nuclear components of fission reactors.

The Influence of Composition Segregation to the Microstructure and Mechanical Properties of SA508-3 Steel

A large scale ingot was dissected to study the segregation law of the elements. The influence of composition segregation to the microstructure and mechanical properties of SA508-3 steel was studied by the comparison of three positions on the ingot. Two precipitated phases were approved to be alloyed cementite and molybdenum carbide (Mo2C). It has demonstrated that the middle and upper parts of the ingot were almost the same in compositions except for a slightly difference in the carbon content. The upper part with more carbon included has relatively more carbide precipitation after the performance heat treatment. The bottom part of the ingot has the lowest carbon and molybdenum content, while the reduction in the amount of precipitated carbide was not observed. On the contrary, fine needle-like Mo2C are extensively distributed in the matrix of the bottom part besides a variety of coarse cementite rods. And the data of Energy Dispersion Spectrum (EDS) mapping has suggested that precipitation of Mo2C tends to bring the segregation of impurities. Many large inclusions were found in the bottom part of the ingot, which were considered to be the main reason for the strength loss of this area.