Thermomechanical Heat Treatment Research Articles

Annealing Phenomena in the Electroformed Invar FMM

In the OLED display industry, Invar(Fe-36%Ni) exhibiting anomalous low thermal expansivity is commonly used as a material for the fine metal mask (FMM), which guides the evaporated diode materials through its small holes onto the correct positions of the substrate glass. We are investigated that the coefficient of thermal expansion (CTE) as a function of the alloy composition before and after annealing in the Fe-Ni alloys consisting of the α(BCC) and γ'(FCC)phases fabricated by using an electroforming method. The CTE-composition curve of the electroformed Fe-Ni alloys was measured to be different from that of the pyrometallurgically produced Fe-Ni alloys. The electroformed Invar can perform low CTEs via thermomechanical heat treatment where the a phase and γ′ phase in the as-deposited state are transformed into the γ″(FCC) phase. During annealing of the electroformed Invar, the as-deposited texture with the <110>α//ND + <100>γ′//ND components was changed into the the major <111>//ND and minor <100>//ND components. This texture evolution occurring during annealing has been explained in terms of a combined effect of abnormal grain growth (AGG) and the α → γ″ (or α → γ'→ γ″) phase transformation.

Grain refinement and mechanical properties improvement of additively manufactured Al-Cu alloy through pre-deformation in thermo-mechanical treatment

Inferior mechanical properties induced by porosity and coarse grains of wire-arc additively manufactured Al-Cu alloy restrict its industrial application. The inter-layer cold rolling, conventional heat treatment (T6) and thermo-mechanical heat treatment (T8) was applied to Al-Cu alloy, and the effect of pre-stretched strain in T8 treatment was investigated. The pre-stretched strain induces equal increments of hardness and YS for both the as-deposited and cold-rolled conditions before aging treatment, while the increments for cold-rolled samples are larger after aging treatment. The cold-rolled sample shows better tensile properties than the as-deposited sample with the same pre-stretched strain in T8 treatment, proving that T8 treatment further increased the strengths compared to the T6 treatment. It shows the best tensile properties with the pre-stretched strain of 5 %, exceeding the properties of 2219-T87 plate. With the increased pre-stretched strain, the density of the θ′ phases significantly increases; while further increase to 10 % pre-stretched strain induces severe coarsening of θ′ phases. Therefore, an optimized pre-stretched strain leads to high-density and tiny dispersed θ′ phases, thus resulting in the best tensile properties. Grain refinement induced by inter-layer cold rolling and uniformly precipitated phases resulted from proper pre-stretched strain both contribute to the strengths enhancement. The strengthening mechanism of hybrid additive manufacturing and inter-layer cold rolling followed by T8 treatment was discussed.

Deformation behavior of the hybrid S-phase layer formed on a stainless-steel substrate by thermochemical heat treatment under mechanical loading

The hybrid S-phase layer was formed on the surface of an austenitic stainless steel by the thermomechanical heat treatment, which was carried out at a low temperature (475 °C). The thickness of the layer depends on the methane (CH4) gas content in the heat treatment chamber. The microstructure of the layer contains plenty of microstructural defects, such as slip bands, twins, stacking faults, and entangle dislocations, as confirmed by detail electron microscopy investigations. The S-phase layers were subjected to nanoindentation and nanoscratching to investigate their micro-mechanical properties and deformation aspects; and compared against the substrate (bulk austenitic) material. This layer exhibit about 6 folds increase in hardness (11.65–14.65 GPa) to that of bulk austenitic (2.33 GPa), whereas the Young's modulus increased by 1.70 times (124.84–142.34 GPa for the S phase layers and 83.25 GPa for bulk material). This increase in hardness caused relatively less plastic deformation in the layer, compared to bulk austenitic under identical loading conditions. This induced different sub-surface deformation aspects, not only in terms of deformation depth, but also in microstructural aspects, as reviled through a transmission electron microscopy investigation on the residual nanoindentation imprint and scratch tracks.

Influence of Anatomical Root Factors on Cyclic Fatigue Resistance of Heat Treated NiTi Instrument: A Review

Endodontic treatments have been improved over the last decade by the use of nickel titanium (NiTi) alloys. NiTi instrument had shape memory and superelastic properties which made the instrument more flexible, so the treatment quality was enhanced by eliminating the procedural errors. Although the numerous benefits of the NiTi instruments, the fracture was the most frequent failure that may be happened during its use due to flexural (cyclic) or torsional fatigue. Various thermo-mechanical heat treatments have been applied to NiTi instruments to improve fracture resistance and clinical performance. Several parameters influenced the fatigue resistance of NiTi files, and the most important factor that affected the likelihood of NiTi instrument fatigue was the root canal shape. Data collected of 100 papers from the Goggle scholar and Scien ceDirect database. After excluding 60 unrelated papers, thi s review of 40 included related studies aims to evaluate an overview of the anatomical root-related factors affecting t he cyclic fatigue of heat-treated NiTi instruments. Concl uded that the separation rates of file increased when the radius of curvature decreases and when the angle of curvature increased. Canals with the double curvatures increase the risk of cyclic fatigue. Also, the resistance to cyclic fatigue of NiTi files was affected by the canal curvature posit ion, with a short failure life coronally. So even in the improve ment of wire metallurgy and designs of instrument, the anatomy of canal still impact on the cyclic fatigue and should be considered during preparation especially with th e new concept of minimally invasive endodontic treatmen t.

Effect of combined cold deformation and subsequent heat treatment on the microstructure, texture, mechanical properties and cementite spheroidization in AISI 1070 steel

In this study, the effects of thermomechanical heat treatments on two batches of commercial AISI 1070 steel were investigated. Both samples underwent cold rolling and annealing at 600 °C for 30 min, and their microstructure and crystallographic texture were analyzed using XRD, SEM observations, and EBSD measurements. Tensile testing was also conducted to evaluate the mechanical properties of the steels. The results showed that the lamella pearlitic microstructure of both samples was transformed into α-ferrite matrix with spheroidal cementite particles (Fe3C). Sample A exhibited a strong γ-fiber texture parallel to the rolling plane, with a typical cold-rolled texture and high anisotropic characterization dominated by (111)<uvw> orientation. Sample B, on the other hand, exhibited (111)[12¯1] and GOSS (110)<001> texture components with dominant (111) and (101) grain orientations along the rolling direction. While the yield stress and tensile strength were higher in sample A, sample B had better toughness and elongation properties, indicating that it was more ductile. These findings highlight the importance of controlling heat treatment processes to achieve desired microstructures and mechanical properties in steels and provide insights into the relationship between microstructure, crystallographic texture, and mechanical properties. The results of this research could be useful in optimizing the process and performance of steels in various industrial applications.

Investigation of A New Type of Aluminum–Magnesium Alloy with Bismuth Additions Subjected to Thermomechanical Heat Treatment

Investigation of A New Type of Aluminum–Magnesium Alloy with Bismuth Additions Subjected to Thermomechanical Heat Treatment

Phase-Field Simulation of Microstructure Formation in Gas-Atomized Al–Cu–Li–Mg Powders

Al-Cu-Li (2xxx series) powders for additive manufacturing processes are often produced by gas atomization, a rapid solidification process. The microstructural evolution of gas-atomized powder particles during solidification was investigated by phase-field simulations using the software tool MICRESS. The following topics were investigated: (1) the microsegregation of copper and lithium in the particle, and the impact of lithium addition on the formation of secondary phases in Al-2.63Cu and Al-2.63Cu-1.56Li systems, (2) the effect of magnesium on the nucleation and final mass fraction of T1 (Al2CuLi) growing from the melt, and (3) the effect of increased magnesium content on the T1 and S' (AlCu2Mg) phase fractions. It is observed that the addition of lithium into the Al-Cu system leads to a decrease in the solid solubility of copper in the primary matrix; consequently, more copper atoms segregate in the interdendritic regions resulting in a greater mass fraction of secondary precipitates. Our result agrees with findings on the beneficial impact of magnesium on the nucleation and precipitation kinetics of T1 precipitates in the conventional casting process with further thermomechanical heat treatments. Moreover, it is observed that the increase in magnesium from 0.28 wt.% to 0.35 wt.% does not significantly affect the nucleation and the amount of the T1 phase, whereas a decrease in T1 phase fraction and a delay of T1 formation are encountered when magnesium content is further raised to 0.49 wt.%.

Alternate Thermomechanical Heat Treatment Cycles for the Enhancement in Hardness and Tensile Properties of Commercial Grade AA6061

Numerous research incorporating precipitation hardening and/or strain hardening on Al-Mg-Si alloys has demonstrated improved mechanical properties in the alloys. However, there is a lack of research on how these processes can be combined efficiently to provide better strength and hardness for the alloys. The present study describes the effects of low temperature thermomechanical treatment (LTMT - combination of conventional age hardening and strain/work hardening) carried at two isothermal aging temperatures on the hardness and strength of the aluminium alloy 6061 (AA6061). Further, an innovative LTMT process is also proposed, which aims at further enhancing the mechanical properties of the alloy. The study revealed the increase in degree of deformation improved the strength and hardness of the alloy with reduced aging time. The effect of the degree of deformation, pre-deformation, and alternate deformation and aging cycles on the hardness and strength of the alloy was studied with the help of fracture surface analysis.

A short review on thermal treatments of Titanium & Nickel based alloys processed by selective laser melting

Selective laser melting (SLM) has been considered as a well-researched technology used to produce near net shape metallic components with superior mechanical and physiochemical characteristics. Recently SLM has recently been used for the production of components made by high-performance alloys such as Titanium and Nickel. However, the microstructural and physical anisotropy along with the defects have negative impact on the performance and behavior of SLM-produced alloys. Therefore, to improve the material characteristics, extra attention must be paid to the source(s), identification, and removal of these deviations. Thermal heat treatments are often considered as an effective approach to deal with aforementioned issues. stress-relieving, quenching/hardening, annealing/recrystallization, and thermo-mechanical heat treatments are among prominent procedures adopted for the post processing of SLM fabricated metallic components and hence considered in this article. Since these procedures have substantial impact on the microstructure and mechanical behavior therefore an assessment of their influence and resulting changes is significantly important. Therefore, this short review provides the said knowledge on thermal treatments used to improve the SLM fabricated titanium and nickel-based alloys. This review can significantly help in designing and selecting appropriate post processing thermal treatments keeping in mind the expected outcome.

Integral Computer Model for Simulating Microstructure Evolution during Thermomechanical Processing and Heat Treatment of Steels and Predicting their Final Mechanical Properties

An integral computer model/program AusEvol Pro was developed to describe the evolution of steel microstructure during thermomechanical processing (hot rolling, forging), as well as subsequent heat treatment (normalization, tempering), and to evaluate the final mechanical properties (yield stress, tensile stress, elongation), hardness and impact toughness. The program implements a set of physically based models that allow quantitative description of all significant processes of steel structure formation with account of the effects of chemical composition both during thermomechanical processing and heat treatment. Calculations of the final mechanical properties are carried out using the developed models that take into account all physically meaningful contributions. The models created are verified both on the extensive database of our own experimental studies and on reliable data from literature for steels of various chemical compositions.

Microstructure and Tensile Properties of TRIP-Aided Steel Sheet Subjected to Hot-Rolling and Austempering

Various high strength steel sheets for weight reduction and safety improvement of vehicles have been developed. TRIP-aided steel with transformation induced plasticity of the retained austenite has high strength and ductility. Conventional TRIP-aided steels are subjected to austempering process after austenitizing. Generally, elongation and formability of TRIP-aided steel are improved by finely dispersed retained austenite in BCC phase matrix. The finely dispersed retained austenite and grain refinement of TRIP-aided steel can be achieved by hot rolling with heat treatment. Therefore, the improvement of mechanical properties of TRIP-aided steel is expected from the manufacturing process with hot rolling and then isothermal transformation process. In this study, thermomechanical heat treatment is performed by combining hot rolling and isothermal holding as the manufacturing process of TRIP-aided steel sheets. The complex phase matrix is obtained by hot rolling and then isothermal holding. Although the hardness of the hot rolled and isothermal held TRIP-aided steel is decreased, the volume fraction of retained austenite is increased.

Microstructure Characterization of Reversed Transformation in Cryogenically Rolled 22MnB5.

Hot stamping is a well-known process to produce structural automotive parts with an excellent strength-to-weight ratio. However, this process is more expensive due to the lower energy efficiency and operating cost of the traditional roller-hearth furnace. Additionally, lower ductility and toughness are commonly recognized as the main disadvantages of the current hot stamped ultra-high-strength parts. Refinement of austenite grains could be a profitable way to improve the strength of hot stamped parts. In this work, the evolution of reversed transformation in asymmetrically cryogenically rolled samples was studied in order to control the austenite. Thermomechanical simulation and heat treatment in the salt bath were used to investigate the reversed transformation process, and the typical microstructures were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Compared with symmetric prerolling, ferrite recrystallization could be remarkably inhibited by asymmetric rolling at the liquid nitrogen temperature (LNT) during the reheating process. Additionally, the nucleation of the austenite inner grains can also be promoted and the dynamics of the reversed transformation accelerated by asymmetric prerolling. Such phenomena might be very useful to refine the parent austenite grains before press hardening and enhance the new hot stamping strategy by partial fast reheating.

Melting and Thermomechanical Processing of High Strength 0.3%C–CrMoV (ESR) Steel

0.3%C–CrMoV steel were processed through electric arc furnace melting followed by electro slag refining. 2800 mm diameter class of rolled rings and 8 mm thick plates required for fabrication of solid rocket booster motorcase were realized. Tensile and fracture toughness properties were evaluated as a part of characterization of the steel. Low fracture toughness in initial melts was investigated using optical and scanning electron microscopy. Modifications in methods of alloying additions/processing were suggested and incorporated to achieve the desired mechanical properties in industrial scale melts. Process was also fine-tuned by incorporating additional thermomechanical working and heat treatment cycles to achieve the required mechanical properties. Hardening cycle of 925 °C for 1 h followed by oil quenching and tempering cycle of 505 °C for 2 h followed by oil quenching was found to result in optimum combination of mechanical properties. Repeatability in processing and consistency in achieving the mechanical properties of the steel at industrial scale was demonstrated by processing 24 melts of 6 tons each into large size rings and plates.

EIS Behavior of Experimental High-Strength Steel in Near-Neutral pH and Load Conditions

Two thermomechanical heat treatments were applied to a high-strength low carbon steel with an experimental chemical composition, and as a result two different microstructures were obtained. Steel A had a ferritic microstructure, and steel B had a bainitic-martensitic one. The corrosion behavior was reviewed at long times in samples without load by means of Electrochemical Impedance Spectroscopy (EIS) in a near-neutral pH (NNpH) environment. The results showed that the quantity and adherence of corrosion products on the sample surface at long times are different. Hence, the impedance was higher for steel B. Slow strain rate testing (SSRT) was applied to tempered samples of the two steels at 473 K, 673 K, and 873 K (200 °C, 400 °C, and 600 °C), and the corrosion behavior was acquired using EIS at the same time as the SSRT in NNpH conditions. This is a novel result because the tension samples were not electrically isolated from the rest of the load frame. The impedance for the ferritic steel was higher than the bainitic–martensitic one, while it slightly decreased for both steel over time. Tempering improved the corrosion resistance for steel A, while it was not modified for steel B. The corrosion behavior could be associated with the susceptibility of these steels to stress corrosion cracking. A transmission line model was proposed to show qualitatively the corrosion behavior of a crack in the steel, if there is a potential profile inside the crack. A hypothetical potential profile was acquired as well as different impedance behaviors based on electrochemical variables.

Studies on Creep Deformation and Rupture Behavior of 316LN SS Multi-Pass Weld Joints Fabricated with Two Different Electrode Sizes

Effect of electrode size on creep deformation and rupture behavior has been assessed by carrying out creep tests at 923 K (650 °C) over the stress range 140 to 225 MPa on 316LN stainless steel weld joints fabricated employing 2.5 and 4 mm diameter electrodes. The multi-pass welding technique not only changes the morphology of delta ferrite from vermicular to globular in the previous weld bead region near to the weld bead interface, but also subjects the region to thermo-mechanical heat treatment to generate appreciable strength gradient. Electron backscatter diffraction analysis revealed significant localized strain gradients in regions adjoining the weld pass interface for the joint fabricated with large electrode size. Larger electrode diameter joint exhibited higher creep rupture strength than the smaller diameter electrode joint. However, both the joints had lower creep rupture strength than the base metal. Failure in the joints was associated with microstructural instability in the fusion zone, and the vermicular delta ferrite zone was more prone to creep cavitation. Larger electrode diameter joint was found to be more resistant to failure caused by creep cavitation than the smaller diameter electrode joint. This has been attributed to the larger strength gradient between the beads and significant separation between the cavity prone vermicular delta ferrite zones which hindered the cavity growth. Close proximity of cavitated zones in smaller electrode joint facilitated their faster coalescence leading to more reduction in creep rupture strength. Failure location in the joints was found to depend on the electrode size and applied stress. The change in failure location has been assessed on performing finite element analysis of stress distribution across the joint on incorporating tensile and creep strengths of different constituents of joints, estimated by ball indentation and impression creep testing techniques.

Microstructure and mechanical properties of a high strength Cu-Ni-Si alloy treated by combined aging processes

High strength Cu-Ni-Si alloys were potential elastic conductive materials due to their high strength and high electrical conductivity. To address the trade-off between strength and electrical conductivity, a combined aging process has been developed to enhance the properties of a Cu-6.0Ni-1.0Si-0.5Al−0.15Mg-0.1Cr alloy after an initial thermo-mechanical heat treatment. The microstructure evolutions of the studied alloys were investigated by optical microscopy, scanning electron microscopy, and transmission electron microscopy. The mechanical and physical properties were evaluated through hardness measurements, tensile test and conductivity test. The results demonstrated that a hardness value of 381 HV, an ultimate strength of 1155.8 MPa, an elongation of 3.5%, and an electrical conductivity of 25.2%IACS were achieved after an appropriate combined aging process. Cold rolling led to the formation of a dense dislocation wall as a result of dynamic interactions among the dislocations and precipitates. During the second stage of aging, nanoscale precipitates formed in the first stage slowly coarsened and effectively pinned dislocations motion. The combined aging process increased the hardness and strength, without sacrificed the electrical conductivity. This work provided an effective route for balancing the strength and electrical conductivity of copper alloys.

Depletion zones and intergranular corrosion of superaustenitic stainless steel Alloy 926

Superaustenitic stainless steel Alloy 926 was evaluated after different isothermal annealing and thermomechanical heat treatments. The impact of two different quenching and tempering processes on sensitization was determined. Since Alloy 926 contains large amounts of molybdenum and chromium, precipitation of various intermetallic phases is strongly favored. Based on SEM and EF‐TEM, it was found that the size as well as the extent of elemental depletion zones is in good agreement with the degree of sensitization (DOS), independently of the applied corrosion test. Elemental depletion zones of two isothermally annealed conditions were obtained quantitatively and related to those of thermo‐mechanically rolled samples. Comparison of both annealed conditions revealed the different operating principles of Streicher test and DL‐EPR‐test. In consequence, morphology, distribution, and chemical composition of precipitates have a considerable impact on corrosion test results. Different microstructural parameters and their relevance on prediction of corrosion properties are pointed out and discussed accurately.

Microstructure and carbides characteristics of spray formed 9V high vanadium steels

To develop advanced high speed steel with high vanadium content, a kind of vanadium content 9% high speed steel in the form of circular ring was spray formed under certain thermal conditions. The carbides topography of as-cast and as-sprayed 9V steel were observed. The effects of post thermo-mechanical processing (hot isostatic pressing and hot forging) and heat treatment on the microstructure and carbides distribution were investigated. The spray formed 9V steel typically consists of fine, homogeneous and angular-shaped MC carbides with diameters ranging from 1 to 2 μm. Spray formed 9V steel shows an excellent hot workability following increase of forging ratio due to fine homogeneous carbides. After heat treatment, spray formed 9V steel is composed of MC carbides, martensite and retained austenite. The hardness value of heat treated materials is inversely proportional to the volume fraction of residual austenite. Fine dispersed MC carbides have significant influences on the dislocation motion of spray formed 9V steel.

Weldability assessment and high-temperature properties of advanced creep resisting austenitic steel X6CrNiNbN25-20 (DMV310N/HR3C)

The modern Ultra Super Critical Power Plants (USC PP) applying the 600 °C technology require advanced stainless steels in superheater/reheater systems in order to cope with the increased steam parameters. Different grades of stainless steels have been developed by increasing Cr-contents, alloying with stabilizing and precipitating elements as well as thermomechanical heat treatments resulting in high creep rupture strengths and improved oxidation/corrosion resistance. In the context of a collaborative research project, X6CrNiNbN25-20 (DMV310N/HR3C) has been investigated. The main focus of the research project was on characterization and weldability assessment. As a result, the base metal under investigation was compared with governing code cases and specifications. Base metal chemical composition, microstructures, mechanical properties, reheat cracking sensitivity, and hot ductility as well as creep rupture strengths have been investigated. A weldability assessment, including thermal simulation and welding procedure qualifications, has been performed to establish parameter windows for similar and dissimilar welding. Dissimilar welding between grade 92 and austenitic steel tubes has been performed. The project also took the opportunity to investigate the behavior of a recently developed gas tungsten arc welding (GTAW) P87 consumable for dissimilar welding. Cross-weld creep rupture tests have been conducted for both similar and dissimilar welding, while aging tests addressed microstructural stability.

Weldability assessment and high temperature properties of advanced creep resisting austenitic steel DMV304HCu

The modern (USC PP) applying the 600 °C technology require advanced austenitic stainless steels in superheater/reheater systems in order to cope with the increased steam parameters. Different grades of austenitic stainless steels have been developed by increasing Cr contents, alloying with stabilizing and precipitating elements as well as thermomechanical heat treatments resulting in high creep rupture strengths and improved oxidation/corrosion resistance. In the context of a collaborative research project, DMV304HCu (X10CrNiCuNb18-9-3) has been selected. The main focus of the research project was on characterization and weldability assessment. As a result, the base metal under investigation was compared with governing code cases and specifications. Base metal chemical composition, microstructures, mechanical properties, reheat cracking sensitivity, hot ductility as well as creep rupture strengths have been investigated. A weldability assessment, including thermal simulation and welding procedure qualifications, has been performed to establish parameter windows for similar and dissimilar welding. Dissimilar welding between Grade 92 and austenitic stainless steel tubes has been performed. The project also took the opportunity to investigate the behavior of a recently developed gas tungsten arc welding (GTAW) P87 consumable for dissimilar welding. Cross-weld creep rupture testing has been conducted for both similar and dissimilar welding, and aging tests addressed microstructural stability.