Thermomechanical Processing Parameters Research Articles

Knowledge-based artificial neural network model to predict the properties of alpha+ beta titanium alloys

In view of emerging applications of alpha+beta titanium alloys in aerospace and defense, we have aimed to develop a Back propagation neural network (BPNN) model capable of predicting the properties of these alloys as functions of alloy composition and/or thermomechanical processing parameters. The optimized BPNN model architecture was based on the sigmoid transfer function and has one hidden layer with ten nodes. The BPNN model showed excellent predictability of five properties: Tensile strength (r: 0.96), yield strength (r: 0.93), beta transus (r: 0.96), specific heat capacity (r: 1.00) and density (r: 0.99). The developed BPNN model was in agreement with the experimental data in demonstrating the individual effects of alloying elements in modulating the above properties. This model can serve as the platform for the design and development of new alpha+beta titanium alloys in order to attain desired strength, density and specific heat capacity.

Mechanisms of Thermo-Mechanical Process on Grain Boundary Character Distribution of 316L Austenitic Stainless Steel

Grain boundary engineering (GBE) was carried out on 316L austenitic stainless steel with Thermo-mechanical processing (TMP), which was performed by unidirectional compression and subsequent annealing. The effect of TMP parameters including the strain and annealing time on grain boundary character distribution (GBCD) and the corresponding mechanism was investigated in the study. The results showed that high fraction of low-Σ coincident-site lattice (CSL) grain boundaries (about 55%) associating with interrupted network of random boundaries was obtained through TMP of 5% cold compression followed by annealing at 1000 °C for 45 min. The fraction of low-Σ boundaries increased with increasing the annealing time under all the experiment strain, but the mechanisms were different between the low and medium above levels of strain. Grains rotation and reaction of migratory boundaries might be the reasons of low-Σ boundaries growth in the strain of 5% and in the strain greater than or equal to 10%, respectively.

Dissolution of Precipitates During Solution Treatment of Al-Mg-Si-Cu Alloys

A model combining classical diffusion-controlled dissolution equation for a single spherical particle and Johnson–Mehl–Avrami-like equation is used to deal with dissolution process for different kinds of precipitations (Si, Mg2Si, Q(Al1.9Mg4.1Si3.3Cu)) in Al-Mg-Si-Cu alloys. The results reveal that the dissolution time of precipitates increases with increasing their sizes and solute concentrations in the alloy matrix; for the same size and concentration, their dissolution times follow Si > Q(Al1.9Mg4.1Si3.3Cu) > Mg2Si. Two precipitates (Mg2Si and Al1.9Mg4.1Si3.3Cu) with a size of about 700 nm were obtained in a cold rolled Al-Mg-Si-Cu-Zn alloy, and the complete dissolution time is about 15 seconds, which is basically the same as the calculated time by the developed model. The theoretical prediction of dissolution time can be greatly used to design solution treatment and thermomechanical processing parameters of Al-Mg-Si-Cu alloys.

Influence of Thermomechanical Control Process on the Evolution of Austenite Grain Size in a Low-Carbon Nb-Ti-Bearing Bainitic Steel

On the basis of hot rolling practice, the effects of thermomechanical control process parameters on the evolution of austenite grain size before the deformation at non-recrystallization zone were investigated in detail. The inflections in the strain hardening rate versus true stress curves show that the dynamic recrystallization (DRX) has initiated for different deformation conditions studied in the present work. But the volume fractions of the equiaxed grains in the specimens which were immediately water quenched to room temperature after deformation are different from each other. Moreover, the main refinement mechanisms for different deformation conditions have been differentiated. It is interesting to note that the austenite grain size can be refined significantly with increasing the strain from 0.0 to 0.5 for different deformation temperatures. However, when the strain increases to 0.8, the austenite grain size cannot be further refined for the higher deformation temperature range, while the austenite grain size can be further refined for the lower deformation temperature range. In addition, the effect of strain rate on the austenite grain refinement is vigorous for the higher deformation temperatures. Moreover, the empirical equation to estimate the austenite grain size for different deformation parameters was established.

Development of High Nitrogen Stainless Steel for Cryogenic Applications

Austenitic stainless steels are extensively used as structural materials for various aerospace systems. Nitrogen containing stainless steels have special role due to their austenite stabilization tendency down to subzero temperatures, improved strength and resistance to sensitization. Primary processing of nitrogen containing cryogenic grade stainless steel 202 has been carried out through two different melting routes viz. (1). conventional melt route of electric arc furnace (EAF) melting followed by vacuum oxygen decarburization (VOD) & vacuum degassing (VD) and other one through (2). vacuum induction melting (VIM) followed by ESR. Chemical analysis and macrostructure analysis was carried out on the samples drawn from these billets. Homogenization and thermomechanical processing parameters were selected and the same were followed for the ingots made through both the melt routes. Mechanical properties evaluation (including tensile properties at subzero temperature of 77K) and micro structure characterization of the products realised from all the two melt routes were carried out. It is observed that, both the melt routes could result in achieving the required aerospace quality of alloy with respect to the chemical composition, metallurgical and mechanical properties. This paper confirms that any of the melt routes studied herein can be adopted according to availability of the facilities. The process development and characterization of the steels processed by conventional EAF+ VD & VOD and VIM+ESR melt routes is presented in this paper.

Processing and Characterization of Superinvar for Space Application

Material with ultra-low coefficient of thermal expansion (CTE) is required for mounting camera and other optical elements in satellite systems. Invar (64 Fe 36Ni) has been the work-horse material for this purpose. In recent years, modified version of conventional invar i.e. Superinvar with 5% cobalt (replacing 5% nickel) is being used to further bring down the errors in camera mountings due to thermal expansion. Processing of this alloy poses many challenges due to its requirement of ultra-low CTE. In the present work, melting and thermomechanical processing parameters were selected to meet the specified requirement of the alloy. The alloy was melted through vacuum induction melting process to obtain uniform and homogeneous chemistry and properties. Virgin raw material was used to achieve lowest carbon and manganese contents. Chemical composition thus obtained is found to be within the specification. Material was hot worked to refine the microstructure. Three different sizes of forged blocks were produced. Hot worked material was heat treated to obtain desirable and stable microstructure. Heat treatment cycle for stabilization was selected and used to retain carbon in the solution and minimize temporal growth. Mechanical properties (tensile strength and modulus of elasticity) and physical properties (CTE, thermal conductivity) were evaluated. Properties were found to be meeting the specification. It is observed that the material shows uniform single phase austenitic microstructure. The paper presents details of the process selection and challenges in processing of this alloy to obtain the targeted CTE < 0.6x10-6 per °C in the temperature range of 25°C to 150°C along with other desired mechanical properties.

Microstructure and Mechanical Behavior of High-Entropy Alloys

High-entropy alloys (HEAs) have generated interest in recent years due to their unique positioning within the alloy world. By incorporating a number of elements in high proportion, usually of equal atomic percent, they have high configurational entropy, and thus, they hold the promise of interesting and useful properties such as enhanced strength and alloy stability. The present study investigates the mechanical behavior, fracture characteristics, and microstructure of two single-phase FCC HEAs CoCrFeNi and CoCrFeNiMn with some detailed attention given to melting, homogenization, and thermo-mechanical processing. Ingots approaching 8 kg in mass were made by vacuum induction melting to avoid the extrinsic factors inherent to small-scale laboratory button samples. A computationally based homogenization heat treatment was given to both alloys in order to eliminate any solidification segregation. The alloys were then fabricated in the usual way (forging, followed by hot rolling) with typical thermo-mechanical processing parameters employed. Transmission electron microscopy was subsequently used to assess the single-phase nature of the alloys prior to mechanical testing. Tensile specimens (ASTM E8) were prepared with tensile mechanical properties obtained from room temperature through 800 °C. Material from the gage section of selected tensile specimens was extracted to document room and elevated temperature deformation within the HEAs. Fracture surfaces were also examined to note fracture failure modes. The tensile behavior and selected tensile properties were compared with results in the literature for similar alloys.

Manufacturing of High Entropy Alloys

High entropy alloys (HEAs) have generated interest in recent years due to their unique positioning within the alloy world. By incorporating a number of elements in high proportion they have high configurational entropy, and thus they hold the promise of interesting and useful properties such as enhanced strength and phase stability. The present study investigates the microstructure of two single-phase face-centered cubic (FCC) HEAs, CoCrFeNi and CoCrFeNiMn, with special attention given to melting, homogenization and thermo-mechanical processing. Large-scale ingots were made by vacuum induction melting to avoid the extrinsic factors inherent in small-scale laboratory button samples. A computationally based homogenization heat treatment was applied to both alloys in order to eliminate segregation due to normal ingot solidification. The alloys fabricated well, with typical thermo-mechanical processing parameters being employed.

Hot deformation behaviour of Ni–30Fe–C and Ni–30Fe–Nb–C model alloys

This research investigates the effect of thermomechanical processing (TMP) parameters (temperature and strain rate) and the addition of Nb and C on the dynamic recrystallisation (DRX) of austenite and on deformation activation energy using Ni–30Fe–Nb–C and Ni–30Fe–C model alloys. Plane strain compression tests were carried out in a Gleeble TMP simulator. Critical and peak strains and stresses were determined in the temperature range of 925–1150°C at strain rates of 0.01s−1, 0.1s−1 and 1s−1. Both critical and peak strains and stresses increase with a decrease in the deformation temperature, an increase in strain rate or the addition of Nb in the model alloy. The Zener–Hollomon parameter and deformation activation energy values have also been determined using the experimental parameters of DRX and an Arrhenius type equation. The deformation activation energy of Ni–30Fe–Nb–C and Ni–30Fe–C were calculated to be 419KJ/mol and 362KJ/mol, respectively. The volume fraction of dynamically recrystallised grains determined using an Avrami type model was in good agreement with metallographic results, except for the deformation temperature near the recrystallisation stop temperature. The delayed initiation and completion of DRX due to the addition of Nb was more pronounced at lower deformation temperatures.

Microstructure, mechanical properties and electrochemical behavior of a novel biomedical titanium alloy subjected to thermo-mechanical processing including aging

This paper presents the results of a study in which influence of different thermo-mechanical processing (TMP) parameters on the mechanical properties and electrochemical behavior of a new metastable β alloy Ti–20.6Nb–13.6Zr–0.5V (TNZV) was investigated. The TMP comprised of plastic deformation in β phase field (850°C) followed by solution heat treatments at below β transus temperature (650°C) and cooling at various rates in addition to aging. Depending upon the TMP conditions, a wide range of microstructures with varying spatial distributions and morphologies of equiaxed/elongated α, β phases were attained which allowed a wide range of mechanical and electrochemical properties to be achieved. The corrosion behavior of the alloy used in the present study was evaluated in a Ringer’s solution at 37°C using open circuit potential–time and potentiodynamic polarization measurements. The results of the study revealed that amongst different solution treated samples, water quenched (WQ) samples exhibited optimum results of elastic modulus, elongation, corrosion resistance as well as reasonable strength and hence it can be considered to be a potential candidate for biomedical applications.

Cover Picture: Materials and Corrosion. 2/2015

Cover:Grain boundary engineering of metastable 204Cu, 301, and 301LN austenitic stainless steels was studied. Thermomechanical processing parameters were obtained for each of the tested steel grades by varying cold rolling reduction and annealing temperature, and by analysing the acquired grain boundary character distributions in SEM with EBSD. A high fraction of about 80 % low‐Σ CSL boundaries was attained in all of the tested steels. Effect of the GBE‐treatments on sensitization and intergranular corrosion properties was studied with Streicher and DL‐EPR tests. GBE‐treating was found to beneficially decrease the rate of intergranular corrosion and sensitization in 204Cu and 301 steels, but in low‐carbon 301LN steel no improvement was observed. The cover photo shows highly twinned microstructure in a GBE‐treated 204Cu austenitic stainless steel. Twin boundaries and other low‐Σ CSL boundaries (Σ ≤ 29) are presented with red and random boundaries with black.More information can be found in: A. Forsström, J. Talonen, T. Saukkonen, H. Hänninen, Grain boundary engineering of metastable 204Cu, 301, and 301LN austenitic stainless steels to improve their sensitization resistance, Materials and Corrosion, 2015, 66, 95.

Active biodegradable sodium caseinate films manufactured by blown-film extrusion: Effect of thermo-mechanical processing parameters and formulation on lysozyme stability

Sodium caseinate-based edible/biodegradable films containing lysozyme were prepared directly by extrusion processes to obtain antimicrobial films. Thermoplastic pellets containing 1% (w/w) lysozyme were blown by an industrial-like blown-film-extruder to obtain thermoplastic thin films. Lysozyme stability depended mainly on processing temperature and glycerol content. A correct choice of these parameters allowed obtaining pellets and films with 40.8±0.5% and 26.4±0.6% lysozyme residual activity, respectively. Whatever the storage temperature (−20°C or room temperature), lysozyme activity of pellets remained constant for at least 5 weeks of storage. Antimicrobial activity against Micrococcus luteus was assayed in Tryptone Soya Broth (TSB): no bacterial growth was observed for 25h at 37°C in the presence of 0.05g.L−1 pellets, while a 2.4 log increase of bacterial population was observed in the presence of control pellets without lysozyme. Traditional properties of films such as their water sorption and mechanical properties were also investigated and proved that addition of lysozyme at a 1% concentration did not affect these properties as well as their ultimate aerobic biodegradability. The application potential of such edible antimicrobial films is discussed in relation with these properties.

Influence of annealing temperature on the baking response and corrosion properties of an Al–4.6 wt% Mg alloy with 0.54 wt% Cu

The effect of different thermo-mechanical processing parameters on the baking response, and intergranular corrosion (IGC) resistance of an Al–4.6wt% Mg–0.54wt% Cu alloy was investigated. It has been shown that the high temperature anneal at 530°C resulted in superior baking response of pre-deformed material after short annealing at 140–180°C, compared to the annealing at 280°C. Specimens pre-deformed 2% had positive baking response for both initial annealing temperatures, however, pre-deformation of 5% induced softening during the baking treatment of the specimens annealed at 280°C. Prior to sensitization of 7days at 100°C, specimens at all stages of thermo-mechanical treatment were IGC resistant. After the sensitization specimens that underwent annealing at 530°C remained IGC resistant, but specimens that underwent annealing at 280°C became IGC susceptible. Better corrosion properties and baking response are related to the higher Cu content in solid solution after annealing at 530°C than at 280°C.

Micro-hardness and Young's modulus of a thermo-mechanically processed biomedical titanium alloy

This paper presents a study on the influence of different thermo-mechanical processing (TMP) parameters on some required properties such as micro-hardness and Young`s modulus of a novel near beta alloy Ti-20.6Nb-13.6Zr-0.5V (TNZV). The TMP scheme comprises of hot working above and below beta phase, solutionizing treatment above and below beta phase coupled with different cooling rates. Factorial design of experiment is used to systematically collect data for micro-hardness and Young`s modulus. Validity of assumptions related to the collected data is checked through several diagnostic tests. The analysis of variance (ANOVA) is used to determine the significance of the main and interaction effects. Finally, optimization of the TMP process parameters is also done to achieve optimum values of the micro-hardness and Young`s modulus.

Microstructural Stability and Hot Deformation of γ–γ′–δ Ni-Base Superalloys

Nickel-base superalloys exhibit excellent high-temperature mechanical and physical properties and remain the first choice for structural components in advanced gas turbine engines for the aerospace propulsion and power generation applications. In response to the increasing demand for more efficient solutions and tighter requirements linked to gas turbine technologies, the properties of nickel-base superalloys can be improved by modification of their thermo-mechanical and/or compositional attributes. Recent investigations have revealed the potential use of ternary eutectic γ–γ′–δ Ni-base superalloys in advanced gas turbines due to high temperature mechanical properties that are comparable to state-of-the-art polycrystalline Ni-base superalloys. With properties largely dependent on microstructural strengthening mechanisms, both the composition and thermo-mechanical processing parameters of this novel class of alloys need to be optimized concurrently. The hot deformation characteristics of four γ–γ′–δ Ni-base superalloys with varying levels of Nb were evaluated at temperatures and strain rates between 1353 K and 1433 K (1080 °C and 1160 °C) and 0.01 to 0.001/s, respectively. Evidence of dislocation-based plasticity was observed following deformation at low temperatures and high strain rates, while high temperatures and low strain rates promoted superplasticity in these alloys. The extent of the microstructural changes and the magnitude of the cavitation damage which occurred during deformation was found to vary as a function of the alloy composition.

Effect of controlled rolling/controlled cooling parameters on microstructure and mechanical properties of the novel pipeline steel

The study of controlled rolling/controlled cooling process parameters which affect the microstructure and mechanical properties of a novel pipeline steel have been optimized by the orthogonal experiment with four factors and three levels in this paper. However, the parameters of thermo-mechanical control process (TMCP) optimized by the Gleeble-3500 hot simulator could not satisfy performance requirements of the X100 pipeline steel. In order to improve the performance of this steel, the influence of finish cooling temperature (FCT) on the microstructure and property is studied in detail. It is found that, as this steel is thermo-mechanically treated by this set of parameters (the start heating temperature, finish rolling temperature (FRT), FCT and cooling rate of 1,180 °C, 810 °C, 350 °C and 35 °C/s, respectively), the microstructures are mainly composed of granular bainite (GB) and acicular ferrite (AF). The effective grain sizes are below 20 µm; the steel reached the optimal balance between the strength and the toughness; the yield strength is 695 MPa; the tensile strength is 768 MPa; the elongation is 16.6 %; the impact energy is 262 J at room temperature. All indexes could meet the requirements of X100 pipeline steel.

Grain boundary engineering of metastable 204Cu, 301, and 301LN austenitic stainless steels to improve their sensitization resistance

Grain boundary engineering (GBE) of 304 and 316 austenitic stainless steels has been widely researched but studies on other grades of austenitic stainless steels are less common. In this study, application of GBE on metastable austenitic stainless steels 204Cu, 301, and 301LN was assessed. Thermomechanical processing parameters were obtained for each of the tested steel grades by varying cold rolling reduction and annealing temperature, and analyzing the acquired grain boundary character distributions (GBCD) in scanning electron microscope (SEM) with electron backscatter diffraction (EBSD). A high fraction of about 80% low‐Σ CSL boundaries was attained in all of the tested steels. Effect of the GBE‐treatments on sensitization and intergranular corrosion properties was studied with Streicher and DL‐EPR tests. GBE‐treating was found to beneficially decrease the rate of intergranular corrosion and sensitization in 204Cu and 301 steels, but in low‐carbon 301LN steel no improvement was observed.

Effects of initial microstructure and thermomechanical processing parameters on microstructures and mechanical properties of ultrafine grained dual phase steels

Dual phase (DP) steels was investigated using thermomechanical processes designed to obtain ultrafine/nanoferrite-carbide aggregates and martensite–ferrite duplex starting structures prior to intercritical annealing. The effects of processing parameters such as intercritical annealing temperature and time on the microstructural evaluations, mechanical properties, strain hardening behaviors and fracture mechanisms have been studied. The ferrite grain size and martensite volume fraction were depended on the initial microstructure and thermomechanical processing parameters. Ultrafine grained DP (UFG-DP) steel with an average grain size of about 2μm was achieved by short intercritical annealing of the 80% cold-rolled duplex microstructure. Tensile testing revealed an excellent strength–elongation balance (UTS×UE>110Jcm−3) in the DP steels. The new UFG-DP steels showed superior mechanical properties in comparison with the commercially used high strength steels. The variations of strength, elongation, strain hardening behavior and fracture mechanism of the specimens with thermomechanical parameters were correlated to microstructural features.

Effect of microstructural constituents on strength–toughness combination in a low carbon bainitic steel

A low carbon bainitic steel with varying strength was obtained through change in the thermo-mechanical processing parameters. In this regard, we elucidate the relationship between processing parameters, microstructure, and mechanical properties. The study underscores that the volume fraction of martensite–austenite (M–A) constituent is strongly governed by the rolling reduction ratio. The tensile strength was increased with increase in the volume fraction of lath bainite (LB) and M–A constituent, while the yield strength increased monotonically with increase in the volume fraction of LB, and was not sensitive to the volume fraction of M–A constituent. Acicular ferrite (AF) was responsible for high impact energy. Large blocky M–A constituent facilitated initiation of cracks at the M–A constituent/matrix interface, while small M–A constituent of size less than 1μm was helpful in improving toughness. The volume fraction of retained increased austenite with increase in rolling reduction ratio, which neutralized the negative effect of the increase in the volume fraction of M–A constituent on impact toughness, contributing to improvement in toughness.

Microstructure and mechanical properties of two Api steels for iron ore pipelines

This research compares the mechanical behavior of two API steels (X60 and X70) used in the longest pipeline in the world for the conveyance of iron ore. Tensile tests, Charpy impact tests, CTOD tests and fatigue crack growth tests are performed at ambient temperature. Metallographic examination showed a banded microstructure consisting of polygonal ferrite and pearlite in both steels, with smaller grain size and the presence of a small quantity of bainite in the X70 steel. All the mechanical tests revealed a ductile behavior for the two steels. The X70 steel is preferable for the pipeline project, due its better mechanical resistance, with no significant loss of fracture toughness and fatigue resistance. Its performance could be even better, if an appropriate combination of thermomechanical processing parameters were able to produce a microstructure with minor amount of pearlite, where acicular ferrite/bainite are present.