Hot Working Process Research Articles

Mechanical Properties of High‐Speed Steel AISI M50 Produced by Laser Powder Bed Fusion

The microstructure and mechanical properties of high‐speed steel AISI M50 (80MoCrV42‐16, Mat. Nr. 1.3551), produced by laser powder bed fusion (LPBF), are analyzed. The mechanical properties in hardened and tempered condition are characterized by hardness, fatigue strength, and toughness and compared with the properties of conventionally produced samples. Moreover, the effects of an additional posttreatment by hot isostatic pressing (HIP) on the microstructure and mechanical properties are investigated. Dilatometric testing is used to investigate the influence of the different initial microstructures on hardening. All heat‐treated samples expose a fine martensitic microstructure with high hardness. The conventionally produced samples show a band‐like orientation of carbides due to the production by vacuum induction melting and vacuum arc remelting followed by a hot working process. This carbide structure is bypassed by the rapid cooling in the LPBF process. The LPBF samples show a comparable hardness after hardening and tempering to the conventionally produced material. In heat‐treated state, the LPBF samples show a low fatigue strength. Posttreatment by HIP included in the heat‐treatment chain significantly increases the fatigue strength. Nevertheless, the fatigue strength is still lower compared with the reference material. Both LPBF grades show a low toughness compared with the reference material.

Characterization of hot deformation behavior of 30Si2MnCrMoVE low-alloying ultra-high-strength steel by constitutive equations and processing maps

Isothermal compression tests of as-forged 30Si2MnCrMoVE low-alloying ultra-high-strength steel were carried out on a Gleeble 3500 thermal simulator at the deformation temperatures of 950–1150 °C and strain rates of 0.01–10 s−1. Based on the classical stress–dislocation density relationship and the kinematics of the dynamic recrystallization, the constitutive equations of the work hardening dynamical recovery period and dynamical recrystallization period were developed by using the work hardening curve and Avrami equation, which shows good agreement with the experimental value. Processing maps at the strain of 0.90 were constructed based on dynamic material model and were analyzed combined with microstructure observation under different conditions. The optimum parameter based on the processing maps was obtained and verified by a supplementary experiment. The power dissipation maps and instability maps at strains of 0.05–0.90 were also constructed, and the evolution law was analyzed in detail. The established constitutive equation and hot processing maps can provide some guidance for hot working process.

Microstructural evolution and Mechanical Properties of a Newly Developed Ti2AlNb-based alloy

The effects of the microstructure on the tensile and creep properties of the alloy at room temperature and high temperature were investigated by controlling the microstructures of the alloy by different hot working processes. It is found that the lath microstructure obtained by forging in B2 single phase zone has high tensile strength. The tensile strength is 1188 MPa at room temperature and 950 MPa at high temperature. The equiaxed structure obtained by forging in O+B2 phase region has the characteristics of high plasticity, creep resistance and low tensile strength. The elongation at room temperature is 9.0%, and the elongation at high temperature is 36%. The ambient temperature, high temperature tensile properties of the dual microstructure obtained by forging in the three-phase zone of α2+O+B2 are between the lath and the equiaxed microstructure.

Hot workability of Co–Fe–Mn–Ni–Ti eutectic high entropy alloy

Multicomponent Co25Fe25Mn5Ni25Ti20 eutectic high entropy alloy (EHEA), synthesized by vacuum arc melting followed by suction casting method resulted in two types of solid solution phases (fcc CoFeNi-rich (α) and bcc Ti-rich (β)) and Ti2(Ni, Co) type Laves phase. The hot deformation behaviours have been investigated at a temperature ranging from 1073 to 1273 K and different strain rates (10−3,10−2,10−1and 1 s−1). The optimum hot workability conditions of EHEA lies in temperature range 1073–1273 K and strain rate range 10−3 s−1-10−1.6 s−1 as well as temperature 1130–1225K and strain rate 10−0.5-1 s−1. The constitutive relation is established to understand the plastic deformability at high temperature. Finite element simulation has also been used to predict the deformation behaviour during the hot working process.

Deformation behaviour and microstructural evolution during the hot compression of Ti-5Al4Zr8Mo7V alloy

The hot deformation behaviour and microstructural evolution of a metastable β titanium alloy Ti-5Al4Zr8Mo7V at deformation temperatures of 775, 795, 815 and 835 ℃ with true strains of 0.25, 0.50 and 1.00 over a strain rate range of 0.001-1 s−1 were investigated to optimize the hot working process window. An Arrhenius type constitutive equation is obtained to describe the hot deformation characteristics. The flow stress behaviour and the deformation thermal activation energy of 187 kJ/mol indicate dynamic recovery of the alloy. Based on the hot compression data, the processing maps at strains of 0.25, 0.50 and 1.00 are constructed to characterize the hot workability. Microstructural investigation confirms the occurrence of flow localization at the predicted unstable region. Whereas, globularization of α phase, dynamic recovery and dynamic recrystallization of β phase are the main dynamic softening mechanisms. The results show that the hot workability of Ti-5Al4Zr8Mo7V alloy is better at higher deformation temperature, higher strain, and lower strain rate, in which case the original microstructure is replaced by the equiaxed α phase and recrystallized β phase.

Processing maps for hot working of HSLA pipeline steel

The hot deformation features of HSLA pipeline steel are researched in the temperature from 800 °C–1100 °C and strain rate from 0.001–10 s−1 using isothermal compression tests. According to the variation of power dissipation efficiency with the temperature and strain rate, the hot working process map of HSLA pipeline steel was obtained. The power dissipation map of the HSLA pipeline steel exhibits three domains. The first area exists in the temperature from 885 °C–1100 °C and strain rate from 0.01–1 s−1. The second domain occurs in the temperature from 800 °C–860 °C and strain rate from 0.012–1 s−1. The third area occurs in the temperature from 800 °C–825 °C and strain rate from 0.001–0.008 s−1. When the specimen was deformed at higher temperature and lower strain rate, the efficiency of the power dissipation is more than 0.3, which is considered suitable for DRX to occur and may be considered a feasible condition for hot working.

Microstructure Evolution of the Semi-Macro Segregation Induced Banded Structure in High Strength Oil Tubes During Quenching and Tempering Treatments.

C110 oil well casing tubes should have high strength and corrosion resistance which is commonly used for deep wells operation containing corrosive media. In this paper, the microstructure evolution of a kind of semi-macro segregation originated banded structure in casing tubes is studied under different heat treatments. It is shown that the characteristics of the banded structure will change significantly in subsequent hot working and heat treatment processes. For the hot-rolled ones, the banded structure is composed of pearlite plus bainite. After quenching, it evolves into martensite band with high concentration solute elements. Finally, the banded structure will change into a carbide banding under the following tempering process. The temperature and cooling rate of the tempering practice show an obvious effect on the final band structure. To improve anti-SSC (sulfide stress corrosion cracking) performance, the favorable QT (quenching and tempering) practice for C110 steel should be a higher tempering temperature and a quicker cooling rate, from which the banded structure defects can be decreased together with an obvious improvement of the tube wall hardness uniformity.

Examination of Formability Properties of 6063 Alloy Extruded Profiles for the Automotive Industry

Bendability is a crucial property of automotive parts, which describes the ability of extruded profiles to be formed to shape, without the appearance of discontinuities that will have an adverse effect on the mechanical properties and their energy absorption capacity (crashworthiness). Anisotropic behavior exhibited by Al alloy extrusions has been documented due to the nature of the hot working process. In the present article an attempt is made in order to determine the main factors influencing bending properties in longitudinal and transverse to extrusion direction, which are also related with anisotropic behavior. Mechanical testing, microscopic examination and finite element analysis were applied in the frame of the current investigation. The findings were indicative that grain morphology and orientation (texture), especially on the surface zone of the extruded profiles, as well as morphology and distribution of the constituent particles are strongly related with formability.

Investigation on Grain Boundary Character Distribution During Dynamic Recrystallization of Austenitic Stainless Steel During Hot Deformation

Abstract The phenomenon of dynamic recrystallization (DRX) is central to the hot working and thermomechanical processing of austenitic stainless steel. This study aims to relate two distinct parameters that influence DRX: the content of interstitial atoms, such as nitrogen, and character of grain boundaries in the steel. 316LN austenitic stainless steel has been subjected to thermomechanical processing at 1,423 K through uniaxial compression at different strain rates. At these, the microstructure is fully recrystallized. The nature, character, and growth of DRX grains are studied using Electron Backscatter Diffraction (EBSD). EBSD is used to determine grain boundary character distribution and the Effective Grain Boundary Energy (EGBE) parameter. The evolution of EGBE and its relation to grain growth are analyzed. The concomitant development of special low-energy boundaries is correlated with the interstitial content in the steel, thus linking interstitial content to EGBE. The correlations so made are tested by examining the corrosion resistance of different boundaries formed at different deformation conditions.

Deformation behavior and microstructure evolution of titanium alloys with lamellar microstructure in hot working process: A review

Titanium alloys have been widely used in many industrial clusters such as automotive, aerospace and biomedical industries due to their excellent comprehensive properties. In order to obtain fine microstructures and favorable properties, a well-designed multi-step thermomechanical processing (TMP) is critically needed in manufacturing of titanium components. In making of titanium components, subtransus processing is a critical step to breakdown lamellar microstructure to fine-structure in hot working process and thus plays a key role in tailoring the final microstructure and properties. To realize this goal, huge efforts have been made to investigate the mechanisms of microstructure evolution and flow behavior during the subtransus processing. This paper reviews the recent experimental and modelling progresses, which aim to provide some guidelines for the process design and microstructure tailoring for titanium alloy research community. The characteristics of the initial lamellar microstructure are presented, followed by the discussion on microstructure evolution during subtransus processing. The globularization of lamellar α is analyzed in detail from three aspects, i.e., globularization mechanism, heterogeneity and kinetics. The typical features of flow behaviors and the explanations of significant flow softening are then summarized. The recent advances in modelling of microstructure evolution and flow behaviors in the subtransus processing are also articulated. The current tantalized issues and challenges in understanding of the microstructure evolution and flow behaviors of the titanium alloys with lamellar microstructure are presented and specified in future exploration of them.

Torsion Failure Analysis of Galvanized Steel Wires for Transmission and Distribution Lines Based on ANSYS Numerical Simulation

This study provides a detailed failure analysis of galvanized steel wires (3mm in diameter) for a 35 kV transmission and distribution line, which was carried out by combining the conventional material analysis methods with the finite element method (FEM). It is found that the failed material had good plasticity (5% in elongation), and under the soft torsion loading condition (0.75 in stress state soft coefficient), the ductile fracture should occur on the material. Additionally, the theoretical number of torsions calculated by the FEM was 26.2 times, while the actual number of torsions achieved by the test was only 2.2 times and the local fracture surface exhibited brittle fracture characteristics. The results showed that the local torsion brittle fracture of the material with good plasticity was caused by triaxial stress in the torsion condition, which led to the formation of cavity in the pulling stress area in the material’s center, and the finite element calculation results indicated that the stress state soft coefficient at the cavity was distributed between 0.31 and 0.38, and the stress concentration at the cavity was more than twice the normal value. Besides, the Widmanstatten structure formed as the improper hot working process is the corresponding structural reason.

Hot working process optimization of Fe–20Mn–19Cr–0.5C–0.6N steel by activation energy, power dissipation and microstructural evolution

Hot deformation behavior of an Fe–20Mn–19Cr–0.5C–0.6N high-nitrogen austenitic steel has been studied by isothermal compression tests in deformation temperature range of 800–1200 °C and strain rate range of 0.01–10 s−1. Results indicate that the Fe–20Mn–19Cr–0.5C–0.6N steel has high deformation resistance due to strong hindering effect on dislocation moving by nitrogen-induced lattice misfit. The twinning-induced plasticity effect is gradually suppressed with the increase in deformation temperature, and high-temperature plastic deformation mechanism by twinning is gradually replaced by dislocation planar slip. The deformation resistance is up to 343 MPa at deformation conditions of (1000 °C, 0.01 s−1), which is over 100 MPa higher than that in martensitic steel and 50 MPa higher than that in austenitic steel. Besides, value of deformation activation energy for the Fe–20Mn–19Cr–0.6N steel is up to 784 kJ mol−1. Power dissipation efficiency is lower than 0.13, while hot processing map exhibits a very wide range of working area. The optimum hot working process obtains at deformation temperature range of 950–1200 °C and strain rate range of 0.01–10 s−1, when deformation activation energy is less than 662.6 kJ mol−1, power dissipation efficiency exceeds 0.22, dynamic recrystallization fraction is over 46.1% and microstructures are without instable characteristics.

A multiple synchronous imaging method for strong illuminants induced during a hot working process

Strong illuminants that often appear during a hot working process are closely related to the processing. To further reveal the characteristics of strong illuminants through visual observation, the multiple synchronous imaging method for a strong illuminant is proposed in this paper. The strong illuminant-emitted light is reflected and transmitted multiple times between the two non-parallel reflective attenuators. Each time, the light transmits through the second attenuator, and is recorded using a camera. A series of images of illuminants are thus attained. The angle between the two attenuators determines the position of each image; the light transmittance of the attenuator decides the brightness of the images; the number of reflections/transmissions determines the sizes of the filtered-out low light intensity areas. By observing the strong illuminants generated during the hot working process using this method, the morphologies of illuminants with different light intensity areas can be obtained, which is of great significance to understand the hot working process and to optimize the processing in future studies.

Microstructure Evolution Modeling and Simulation for Dynamic Recrystallization of Cr12MoV Die Steel During Hot Compression Based on Real Metallographic Image

In this work, microstructure evolution of dynamic recrystallization (DRX) behavior in Cr12MoV die steel was investigated via experiments and simulations systematically. Firstly, hot compression tests were performed to obtain the true stress–strain curves. Based on the experimental results, the flow stress model was established, and Avrami equation was developed to model the kinetics of DRX. Then, the cellular automaton (CA) model was established to describe DRX behavior. In order to obtain more accurate simulation results, a microstructure enhancement, extraction and conversion program based on fingerprint image enhancement algorithm was developed to generate real initial microstructure which could be directly used in CA simulation. With real initial microstructure generation, good agreement between simulated and experimental results was achieved, indicating the high accuracy of the established CA model. Finally, the CA model was employed to investigate the hot deformation behavior of Cr12MoV die steel under multiple thermomechanical conditions. It could be found that a lower strain rate was beneficial to the occurrence of DRX. When the strain rate was beyond 1.0 s−1, the DRX fraction was very small. This work would provide a significant guidance to optimize the hot working process of Cr12MoV die steel or some other similar steels.

Mechanisms of DRX nucleation with grain boundary bulging and subgrain rotation during the hot working of nickel-based superalloys with columnar grains

Nickel-based superalloys were thermomechanically processed at temperatures of 1050–1170 °C and strain rates of 0.001–10 s−1 to study the hot-deformation behavior and dynamic recrystallization (DRX) of the columnar structures. Electron backscatter diffraction (EBSD) was employed to characterize the DRX microstructure of the columnar grains during the thermomechanical processing of the alloys. A linear relationship between the critical strain-hardening rate and critical stress was derived and validated. The critical conditions for DRX were directly determined from the corresponding peak values, which could be used to effectively predict the critical conditions for DRX to occur. It was determined that discontinuous-DRX (DDRX) and continuous-DRX (CDRX) both have an important role in the microstructural evolution. During deformation of the columnar grains, the DRX nuclei first developed at the serrated grain boundaries (GBs) via grain boundary bulging. Subsequently, the continuous misorientation accumulation led to subgrain rotation, which accelerated nucleation within the deformed grains. Owing to DDRX, the DRX nuclei tended to develop at the GBs parallel to the loading direction, leading to the heterogeneous distribution of the newly formed, fine DRX grains. The findings of this study provide a basis for the precise control of the microstructures of nickel-based superalloys with columnar grains during the hot-working process, and therefore, could be used to tailor their properties.

Physical Modelling of the Ball-Rolling Processes

The objective of the article was to present the state of the problem of physical modelling of the hot-working processes with plasticine as the model material. It was stated that the aforementioned method can prove helpful in analyzing complex plastic forming processes such as cross rolling and helical rolling of balls. In order to confirm this hypothesis, an attempt at forming steel balls with diameters of 40 mm (cross rolling) and 57 mm (helical rolling) under laboratory conditions was made. Further on, these processes were conducted in model form using special model rolling mills and 3D printed acrylonitrile butadiene styrene (ABS) tools. The comparison of the test results regarding shape and manufacturing accuracy, as well as force parameters, confirmed the validity of using physical modelling in the investigation of the process of cross rolling and helical rolling of balls.

Physical-based constitutive model considering the microstructure evolution during hot working of AZ80 magnesium alloy

A physical-based constitutive model was developed to model the viscoplastic flow behavior and microstructure evolution of AZ80 magnesium alloy during the hot working process. The competing deformation mechanisms, including work hardening, dynamic recovery, and dynamic recrystallization, in an isothermal compression environment were considered in the model. The internal state variables, including the normalized dislocation density and recrystallized volume fraction, were incorporated into the model to articulate the microstructure evolution during hot deformation. The kinetic condition critical for dynamic recrystallization, considering the effects of the deformation temperature and strain rate, was obtained by employing both the Poliak-Jonas criterion and Zener-Hollomon parameter. Microstructure observations indicate that the recrystallized volume fraction increases with decreasing Z parameter at constant strain, which is consistent with the predicted kinetics model. Based on the developed model, a good correlation was also obtained between the predicted and experimental flow stress. The results indicate a good predictability of the model in describing the hot deformation behavior and microstructure evolution of AZ80 magnesium alloy.

Rolling paths design assisted by target-temperature driven intelligent FE simulation of radial-axial ring rolling

The microstructures of hard-to-deform materials such as titanium alloy are very sensitive to temperature change in hot working process. During ring rolling process, unreasonable rolling paths will lead to drastic temperature change in local region of ring, thus damaging the microstructure and performance of rolled ring. This work proposes a method for designing the rolling paths which could accurately control the ring temperature by target-temperature driven intelligent FE simulation. The main idea of target-temperature driven intelligent simulation is introduced. An intelligent 3D-FE model for TA15 titanium alloy ring rolling is established under ABAQUS/Explicit environment. The rolling paths under different initial conditions are obtained by intelligent FE simulations. The influence rule of initial conditions on rolling paths is revealed. The temperature control effects and change under different initial conditions are discussed. Considering the temperature control effects, a feasible range of initial ring temperature is suggested. Using the proposed method, the quick and accurate design for the rolling paths in radial-axial ring rolling process is realized. It is of great significance for the design and optimization of rolling paths and the accurate regulation of ring temperature in actual production.

Evolution of activation energy during hot deformation of Al–15% B<sub>4</sub>C composites containing Sc and Zr

During hot deformation, the activation energy, Q, is an essential parameter that indicates the difficulty level in the hot working processing. The evolution of the activation energies of three Al–15% B4C composites (the base material, S40 with 0.4% Sc and SZ40 with 0.4% Sc and 0.2% Zr) was investigated using high-temperature flow stress data based on a revised Sellar’s constitutive equation. The microstructure evolution during hot deformation was characterized using a transmission electron microscope. The calculated activation energy maps reveal that the activation energy during hot deformation was related to the microstructure change in addition to deformation conditions. For the base composite, the variation of the activation energy was small because the microstructure barely changed during deformation. For the Sc and Zr containing composites (S40 and SZ40), dynamic precipitation occurred at high deformation temperature and the activation energy map can be divided in two regions. The activation energy decreases with an increase of deformation temperature to the minimum level in the region I where the composites were in the solid solution condition. It follows by an increase with increasing temperature in the region II where dynamic precipitation occurred. Based on the combination of the activation energy map with the flow instability zone, the optimum hot workability of a composite in term of excellent processability was proposed at the domain where the less energy of hot deformation was required.

THE EFFECT OF ROLLING CYCLES AND SETTING DIRECTION OF THE REINFORCEMENT ON ALUMINUM BASED COMPOSITE BY REPETITIVE PRESS ROLL FORMING (RPRF) METHODS

Development of technology shown rapid progress in various of structural applications especially on advanced of component in industry. Using of aluminum as a matrix on composite fabrication by addition ceramic as reinforcement is the better solution, for expected to improve the mechanical properties on materials-based composites. The process carried out was combining SiC particles with an aluminum (AA1100) as a matrix with variations number of rolling cycle’s: 2, 4 and 6 as direction. Preparation of SiC particles with transverse, cross section and mix variations through a hot working process with the technology of severe plastic deformation (SPD) using the method of repetitive press roll forming (RPRF). The results obtained was mechanical properties increased when the roll cycles high with mix of variations in the direction of SiC arrangement resulting in a wider distribution of SiC into the matrix AA1100. Rolled process by cycle 6 and the direction of the SiC mix resulted in higher hardness of 55.54 HV, tensile strength was 90.46 MPa, porosity 0.33% density 1.831 gr/cm 3 and SiC distribution 23.19%. Increasing the number of rolling cycles, the thickness of the interface for the formed layer and the size of SiC particles will decrease due to SiC particles scattered in most aluminum matrices, which can increase the hardness value.