Heat Treatment Cycles Research Articles

Microstructural Evolution and Room Temperature Mechanical Properties in Additively Manufactured Mar M 509 With Short Cycle Heat Treatment

Abstract The Co-based superalloy Mar M 509, known for its high-temperature oxidation and hot corrosion resistance, is processed via laser powder bed fusion (LPBF). Microstructure and mechanical properties of Mar M 509 in as-printed (As-P) and heat-treated (HT) states are compared based on two build orientations (longitudinal (L) and transverse (T)) to establish structure-property links with heat treatment. The As-P condition displays a distinct cellular microstructure (500–600 nm) with 50–60 nm carbide particles adorning cell boundaries. Longitudinal (L) build has columnar grains (8–35 μm along the major axis) with a grain aspect ratio of 4, while transverse (T) orientation exhibits equiaxed, bimodal microstructure (5–10 μm and 15–25 μm grain sizes). Strong <001> texture is noted in L. Mechanical properties at room temperature differ between L and T; T (569 ± 12 HV) has 15% higher hardness compared to L (489 ± 18 HV) and 34% higher 0.2% yield strength (YS), but 30% lower elongation than L. Post a short heat treatment cycle at 1250 °C, weld bead structure and cell boundaries break down. Both L (25–33 μm along the major axis) and T orientations (5–42 μm) experience grain growth, and carbides coarsen (250–350 nm). Post-heat treatment, dislocation density decreases, indicating recrystallization; lattice parameter of matrix reduces, implying solute depletion contributing to carbide enrichment. Yield strength drops from 860 MPa to 740 MPa in L and from 1150 MPa to 840 MPa in T, with ductility rising from 14% to 23% in L.

Development of martensitic/bainitic rolled homogeneous bulletproof (RHBP) steel sheets for civilian applications

ABSTRACT Bulletproof steel sheets are used for civilian applications like armouring cash money transfer vehicles and bank front points. The present work is dealing with controlled hot-flat rolling and post heat treatment cycle to develop rolled homogeneous bulletproof steel sheets for civilian applications. The steel alloy contains 0.3% C beside 0.93% Cr, 1.76% Ni and 0.62% Mo. The steel is additionally micro-alloyed with 0.004% B. 80 mm thickness steel slabs were controlled forged and followed by hot-flat rolling into 3 mm thickness sheets. A post heat treatment cycle was carried out for creation of bainite beside the martensitic structure to secure ballistic perforation resistance. Strips treated for 10 min. holding in a salt bath at 400 °C followed by water quenching contain 44% bainite and 56% martensite, possessing 388 HB, and present 1647 MPa σu and 1610 MPa σy. In addition to bainite and martensite phases, the microstructure contains ε-carbides measuring 600–800 nm. The ɛ-carbides were found embedded in the bainite aggregates and between the martensite laths. The sheets treated for 5 min. holding in the salt bath followed by water quenching showed up the highest value of σy indicating the highest resistance perforation among the others.

The effect of Sr2+ and thermal treatment conditions on electrochemical characteristics of Li3OCl solid electrolyte in batteries

Lithium-rich oxychloride anti-perovskite solid electrolytes have the potential to be promising for the next generation of batteries. Here, the influence of Sr2+ doping and heat treatment regime on the structure evolution, microstructure, and electrochemical properties of Li3–2xSrxOCl prepared by the hydrothermal route was reported. This research shows that both strategies of doping Sr2+ and the thermal treatment conditions lead to increase ionic conductivity. The lithium-ion conductivity increased with doping Sr2+ content and reached a maximum by x = 0.007 substitution of Sr2+ for Li+. A novel sintering approach is employed with ceramic elements for faster heat treatment that provids more homogeneity with a lower penetration barrier for lithium ions to diffuse in comparison with regular sintering in a furnace. Through this technique, the Li3-2×0.007Sr0.007OCl composition shows a high ionic conductivity of 0.38 × 10−3 S cm−1 at 60 °C. According to differential scanning calorimetry (DSC) measurements, the secondary hydroxide phases were found to have been transformed into the Li3-2xSrxOCl phase upon the first heat treatment cycle. Furthermore, the chemical stability of the Li/Li3-2×0.007Sr0.007OCl/Cu electrochemical cell was verified by cyclic voltammetry at 130 °C during lithiation/delithiation in a wide potential window (−1 to 8 V). The total impedance of the interfaces (RB) and the impedance of the bulk electrolyte (RA) in Li/ Li3-2×0.007Sr0.007OCl /NCM half-cell were obtained at 3690 and 723 Ω, respectively. The electrochemical stability of the Li3-2×0.007Sr0.007OCl electrolyte in contact with lithium metal is confirmed by chronopotentiometry test during charge/discharge conducted for 216 h at 60 °C. Moreover, the charge/discharge performance of Li/ Li3-2×0.007Sr0.007OCl/NCM half-cell showed an initial discharge capacity of 163 mAh/g and a discharge efficiency of 80 % after 20 cycles.

Effect of Oxygen Content on Microstructure and Mechanical Properties of Additive Manufactured Ti-6Al-4V

Titanium alloys are used in many fields including military, aerospace, and biomedical because of their excellent specific strength, corrosion resistance, and biocompatibility. Recently, much research has been focused on addressing the disadvantages of conventional manufacturing methods, including reducing material and energy waste when manufacturing titanium alloy parts by additive manufacturing methods. However, due to rapid cooling, during the additive manufacturing process the material develops acicular and lamellar microstructures, and despite high strength, those features are detrimental to ductility and toughness, as compared with conventionally manufactured alloys. As a result, numerous studies have sought to obtain an equiaxed microstructure through heat treatment. However, the developed heat treatment processes are quite complex, and involve several heat treatment cycles, making such processes economically unfavorable. To overcome these limitations we suggest a different approach to obtaining an equiaxed structure in 3D-printed titanium alloy, by controlling the oxygen level. The present study analyzed the globularization behavior of Direct Energy Deposited (DED) Ti-6Al-4V alloy as a function of oxygen content and a simple heat treatment. The microstructure was globularized through oxygen level control and furnace cooling to compensate the disadvantages in the mechanical properties of additive manufactured alloys.

Effect of solutionizing and double ageing treatments on the microstructural characteristics and tensile properties of Inconel 718 welds

Inconel 718 joints were fabricated using the pulsating direct current gas tungsten arc welding (PDCGTAW) and laser-arc hybrid welding (LAHW) techniques using Ni-Cr-Mo-rich filler. A cyclic post-weld heat treatment (PWHT) comprising 980 °C/20 min/air cooling; followed by two-stage ageing 720 °C/8 h/furnace cooling and 620 °C/8 h/air cooling was performed on both joints. The liquation cracking was noticed on the heat-affected zone (HAZ) of the LAHW joints. Microstructural characteristics were assessed on the joints in both as-welded (ASW) and PWHT conditions. Regardless of the joints, the coarse Mo-rich segregates were noticed in the fusion zone (FZ) in the ASW conditions. On the contrary, the partial dissolution of MC, precipitation of δ-Ni3Nb needles and the coarsening of strengthening precipitates were observed on LAHW and PDCGTAW joints after subjecting to cyclic PWHT respectively. Tensile studies corroborated that there is a considerable enhancement in the tensile and 0.2% offset yield strengths of LAHW joints after being exposed to PWHT, despite the formation of HAZ liquation cracking.

Influence of the heat treatment on the mechanical and tribological properties of cryogenically treated AISI M2 steel

Cryogenic treatments are known to increase the wear resistance and toughness of steel. However, the literature reports some contradictory results. This work aims to investigate and determine the optimum cryogenic parameters combined with cycles of quenching and tempering of AISI M2 tool steel to increase wear resistance and toughness. Charpy specimens were austenitized (1185 °C, 1200 °C or 1215 °C), quenched to room temperature, cryogenically treated to −196 °C (20 h or 28 h), and subjected to double tempering (520 °C, 535 °C or 550 °C). A Taguchi L9 DoE approach was used to determine the optimum set of heat treatment cycles. Mechanical and tribological properties were investigated in terms of wear, impact, hardness, and microhardness tests. The results show that austenitization temperature (AT), tempering temperature (TT), and cryogenic treatment (CT) time are interdependent. Depending on the AT and TT parameters used, CT can improve, have little influence, or even deteriorate the mechanical and tribological properties of AISI M2 steel. The amount of retained austenite after CT ranged from 4.3 to 7.6%. Cryogenic treatment improves wear resistance and impact toughness simultaneously.

Effect of nickel to cobalt ratio on mechanical properties of tungsten heavy alloys

The present study investigates the effect of Ni and Co on the mechanical properties of the W-Ni-Co alloy system. The alloys, based on 90 % W with varying Ni: Co ratios of 6:4, 7:3 and 8:2 (90W-6Ni-4Co, 90W-7Ni-3Co and 90W-8Ni-2Co) were processed through liquid phase sintering followed by a cyclic heat treatment (6 cycles). Thermomechanical treatment was carried out to improve strength and toughness levels. Quantitative metallography was employed to extract microstructural parameters including the volume fraction of the matrix phase, tungsten particle size, precipitate volume fraction and the contiguity of the W particles. The microstructural details at a finer scale were studied using scanning and transmission electron microscopy. Mechanical properties, such as tensile strength, elongation, hardness and impact toughness were also evaluated. The study shows that increasing the Ni: Co ratio decreases the precipitation of W in the matrix, which affects the mechanical behaviour of the alloys. The alloys having 7:3 and 8:2 Ni to Co ratio were additionally processed with an increased number of cycles (8 cycles) in the cyclic heat treatment to enhance the precipitation of fine W particles in the matrix. Since the alloy 6:4 showed brittle behaviour, it was not included for 8 cycles heat treatment study. The alloy with Ni to Co ratio of 8:2 resulted in superior combination of mechanical properties.

Improving Forging Outcomes of Cast Titanium Aluminide Alloy via Cyclic Induction Heat Treatment

The objective of this research was to improve the forging outcome of peritectic solidifying cast titanium aluminide (TiAl) 4822 alloy (Ti-48Al-2Nb-2Cr at.%) in hot isostatic pressed and homogenised (HH) condition using cyclic induction heat treatment (CHT). This study adds to research around CHT for TiAl alloys by applying industrially relevant induction heating to conduct five heating cycles at the single αphase temperatures (1370 °C) necessary for grain refinement. Two cooling rates were explored in each cycle, air cooling (ACCHT) and controlled furnace-like cooling (FCCHT), without returning to room temperature. Samples were assessed at each stage in terms of their morphologies, lamellar grain size and content, as well as phase and dynamic recrystallised fraction, and subsequent primary and secondary compression behaviour with uniaxial isothermal compression. The FCCHT process resulted in a homogeneously refined fully lamellar microstructure, and ACCHT, in a heterogeneous microstructure consisting of lamellar and feathery γ (γf) at differing fractions across the piece, depending on the cooling rate compared with HH. The results show that CHT improved forging outcomes for both compression stages investigated, resulting in uniform compression samples with higher volumes of dynamic recrystallised material compared with the instability seen with the compression of HH material.

Onset of Mechanical Degradation due to Transverse Compressive Stress in Nb3Sn Rutherford-Type Cables

In the framework of the High Field Magnets program at CERN, lower limits of degradative mechanical loads on reacted and impregnated Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn Rutherford-type cables are being studied with the goal of optimizing the pre-stress applied at room temperature on the accelerator magnets. Coils are loaded during assembly, for instance when transverse load is applied by using a collaring press or the bladder and key technique. Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn is susceptible to transverse compressive stress, which is the loading condition investigated for this study. The purpose of this work is to identify the maximum pressure that can be applied, at room temperature, to reacted cables without generating cracks in the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn subelements and to quantify the crack propagation with increasing pressure. Metallographic techniques are used to examine the onset and evolution of damage in the transverse planes of impregnated double-stacked Rutherford-type cable specimens with increasing stress. The damage pattern observed at subelement, strand and cable levels is described. Two reaction heat treatment cycles are analyzed.

Mechanical Properties of 6061 Aluminum Alloy under Cyclic Tensile Loading

During the service process of an aluminum alloy structure, its complex deformation zone experiences repeated loading problems such as repeated tension, compression, bending and reverse bending. At the same time, the cyclic loading and heat treatment process also have a certain impact on the mechanical properties of aluminum alloy extruded tubes. Therefore, the study of heat treatment process parameters has important engineering and practical value for the mechanical properties of aluminum alloy extrusion tubes under cyclic loading conditions. The experiment takes 6061-T6 aluminum alloy extruded tubes as the research objects. In the study, heat treatment and cyclic tensile tests were carried out on 26 aluminum alloy specimens to study the effects of different heat treatment parameters (such as heating temperature, holding time, and cooling method) on the stress–strain hysteresis curves, stress characteristics, hysteretic energy, skeleton curves and failure characteristics of the alloy under the same loading system. In addition, different cyclic tensile tests were carried out on 20 aluminum alloy samples without secondary heat treatment to discuss the effects of different cyclic loading regimes on the mechanical properties of the alloy. The research results indicate that the effect of heating temperature on the cyclic loading performance of the alloy is greater than that of the holding time, and the effect of the cooling method on the cyclic loading performance of the alloy is not obvious. A cyclic tensile loading regime has a significant impact on the strength, elongation and hysteresis energy of the alloy. The hysteretic behavior of the alloy during cyclic tensile loading depends on the applied stress level and loading history. As the number of cycles increases, the shape of the hysteresis curve tends to be stable, but there is no monotonic relationship between the number of cycles loaded and the hysteresis energy.

Investigation of Microstructure and Properties of Ultra-High Strength Steel in Aero-Engine Components following Heat Treatment and Deformation Processes

C250 steel, renowned for its remarkable strength and toughness, is extensively utilized in the aerospace industry for manufacturing critical components. This research investigates the microstructure and properties of forgings produced through different heat treatment temperatures, aging durations, and thermal cycling intervals. The results demonstrate that the samples were compressed at 1050 °C followed by air-cooling using the conventional maraging treatment. For the cycle heat treatment, temperature was maintained at 1050 °C and cycled 1–2 times, with a heat preservation period of 1 h and subsequent water-cooling. Solution heat treatment at a temperature of 1050 °C, aging for 5 h, and then air hardening were performed to achieve the best forging hardness. Interestingly, the solution time under age hardening conditions had no significant effect on the grain size but had a significant effect on the hardness of martensitic aged steel.

Production of Nanostructured Fasteners with High Shear and Fatigue Strength for Using in Aircraft Components

Multi-axis forging (MAF) and cyclic heat treatment are among the most widely used and easy to apply grain refinement methods. In this study, micro-alloyed steel samples were first subjected to MAF treatments at 880° C. Microstructural analysis showed that the average grain size, which was 13.2 µm initially, decreased to 11.2 µm application of the MAF. As a second-grain refinement technique, cyclic heat treatment was used. Samples were subjected to 1, 3, 5, 7, and 10 cyclic quenching. With this method, it was observed that the average grain size decreased to 2,3 µm. The mechanical tests showed that the second MAF process increased the yield and tensile strength of the material by about 16% while decreasing the elongation by %2. These tests also presented cyclic quenching increased tensile strength of the samples after the first application.

Time resolved insights into abnormal grain growth by in situ synchrotron measurements

Large oligo-crystalline or single-crystalline metallic materials are of great interest for numerous applications, and a recently developed strategy for promoting abnormal grain growth induced by a cyclic heat treatment opens up new opportunities to manufacture single crystals with a size of several centimeters. So far, the entire available knowledge on this kind of abnormal grain growth has been elaborated based on time discrete observations and, thus, detailed insights into the interplay of elementary mechanisms are still lacking in open literature. The present study reveals time resolved insights into this kind of abnormal grain growth for the first time. It was possible to break down the influence of the individual heat treatment phases by in situ synchrotron high energy X-ray diffraction analysis during cyclic heat treatment. The results obtained not only help to gain a deep understanding of the abnormal grain growth mechanisms, they will also be the basis for an adjustment of the cyclic heat treatment process to improve its efficiency and to eventually obtain even larger single crystals.

Effect of Heat Treatment on the Microstructure and Mechanical Properties of 18Ni-300 Maraging Steel Produced by Additive-Subtractive Hybrid Manufacturing.

The present work investigates the effectiveness of two heat treatment cycles-solution treatment + aging (STA) and direct aging (DA)-on optimizing the microstructure and enhancing the mechanical properties of 18Ni-300 maraging steel (300 MS) produced by additive-subtractive hybrid manufacturing (ASHM). The STA treatment led to a fully martensitic microstructure with minor remnants of the cellular substructures associated with the solidification conditions in ASHM. DA resulted in some reverted austenite and partial dissolution of the cellular morphologies into shorter fragments. Despite the contrasting microstructures, the tensile strength and the macro- and micro-hardness were comparable between STA and DA conditions. By contrast, the potential for improving the ductility was higher with the DA heat treatment. This is attributed to the higher reverted austenite content in the samples treated by DA, i.e., up to a maximum of 13.4% compared to less than 3.0% in the STA samples. For the DA sample with the highest reverted austenite content of 13.4%, the highest local and global fracture strain values of 30.1 and 5.9 ± 0.6% were measured, while the respective values were 23.4 and 4.4 ± 0.1% for the corresponding STA sample. This work suggests that DA of 300 MS produced by ASHM is sufficient to achieve comparable hardness and tensile strength to STA, whilst maintaining reasonable ductility. Avoiding the solution treatment cycle, with its appreciably higher temperatures, could benefit the dimensional stability and surface quality that are important for ASHM of 300 MS parts.

Investigation on Microstructure, Hardness and Fracture Energy of AISI H13 Hot Work Tool Steel by Cyclic Heat Treatment

Investigation on Microstructure, Hardness and Fracture Energy of AISI H13 Hot Work Tool Steel by Cyclic Heat Treatment

Minimizing intermetallic Laves phase evolution and enhancing precipitation strengthening of Superalloy-718 joints using InterPulse magnetic arc constriction and high frequency pulsation

The techniques of InterPulse magnetic constriction and high frequency pulsation of arc in gas tungsten arc welding (GTAW) were deployed to minimize the detrimental intermetallic laves phase evolution in fusion zone (FZ) of Superalloy-718 welds and enhance the precipitation strengthening of joints. The Superalloy-718 joints were developed by using an advanced variant of GTAW popularly known as InterPulse gas tungsten constricted arc welding (IP-GTCAW). The joints were subjected to the post weld heat treatment (PWHT) cycles of direct aging (DA), solution annealing at 980 °C and 1065 °C followed by aging (980STA and 1065STA) respectively. The tensile properties and hardness of joints were evaluated. The microstructures of joints were analyzed using optical (OM) and scanning electron microscopy (SEM). The elemental analysis of Laves phase and dendrite core of FZ of joints was performed by using energy dispersive spectroscopy (EDS). The tensile fractured surfaces were analyzed using SEM. Results showed that the Superalloy-718 joints developed using IP-GTCAW showed better response to PWHT than GTAW joints because of the greater refinement of FZ microstructure and evolution of finer and discrete Laves phases in FZ. The 980STA joints exhibited superior tensile properties than DA and 1065STA joints. It is correlated to the greater dissolution of Laves phases in nickel austenitic (γ) matrix of FZ resulting in more niobium (Nb) accessible for the precipitation strengthening of FZ. The 1065STA joints showed slightly higher hardness of FZ than 980STA joints because of the almost complete dissolution of Laves phases in FZ. However, the tensile properties of 1065STA joints are lower than 980STA joints because of severe grain growth in FZ and BM. The evolution of microvoids at the interface of Laves phase/weld matrix leading to the development and coalescence of microcracks in FZ on tensile loading is the main mechanism responsible for the premature fracture of joints.

Microstructure and rolling contact fatigue behavior of martensitic nodular cast irons with different graphite sizes and densities

AbstractMartensitic nodular cast irons (MNCIs) with different graphite sizes and number densities are produced through continuous casting and cyclic heat treatments to refine their microstructures and to obtain a hardness of 60 HRC, and their rolling contact fatigue (RCF) properties, fractography, and subsurface microstructures have been investigated in the present study. RCF tests show that the RCF life is mainly determined by the applied Hertzian stress. Based on the calculation of characteristic parameter size and the analysis of stress concentration factor, it is found that the lower nodularity rate of graphite brings about the larger stress concentration at matrix‐graphite interfaces. This stress concentration induces a debonding of the matrix and graphite, cracks, deformation, and carbon redistribution in a local volume, which results in the formation of a white etching area. The present study indicates that MNCIs are a potential material for lubricant‐free bearings after chemical homogenization, microstructure refinement, and hardness enhancement.

Carbon Fiber Reinforced Plastics Based on an Epoxy Binder with the Effect of Thermally Induced Self-Repair

The authors have proposed the novel approach for evaluation of the self-healing effect in carbon fiber reinforced plastics (CFRP) on micro- and macro samples, using the dynamic mechanical analysis (DMA) and the double-cantilever beam delamination methods, respectively. A modified epoxy resin with a self-healing effect was used as the matrix for carbon plastics. The flexural modulus E’ of microsamples with delamination and the specific delamination energy (crack resistance) GIR of macrosamples with a given initial crack were chosen as criteria for evaluating the self-healing of carbon plastics. The sensitivity of the E’ and GIR parameters to the applied initial crack is shown. The value of the elastic modulus E’ with the initial crack can be reduced up to two times compared to the E’ values for the control materials, depending on the length of the initial crack. The degree of recovery of E’ for CFRP with a microcrack varies from 91 to 118%. A high degree of healing could be achieved in 48 h. The GIR value of CFRP samples with a given macroseparation after heat treatment is 7% of the initial GIR value (0.7 kJ/m2). Recovery of delaminations for microsamples is more efficient than for macrosamples. The study of CFRP cracks by X-ray tomography before and after self-healing showed that the crack “overgrows” during the heat treatment cycle, and the defects (pores) formed during the manufacture of the sample decrease in size.

Reducing time and cost of the heat treatment post-processing of additively manufactured Ti6Al4V

The unique microstructure of Ti6Al4V produced through laser powder bed fusion (LPBF) displays high tensile strength with low elongation in the as-built state. To obtain desired material properties which comply with standards for Ti6Al4V products, further post-processing is needed. Conventional practice prescribes that the required tensile properties can be achieved with time-consuming post-processing heat treatment of LPBF parts, most of which use air or water quenching that causes surface modification, such as oxidation, when applied to Ti6Al4V. The aim of this work was to develop an alternative heat treatment process that would retain the benefit of additive manufacturing (AM), but reduce the related processing time and cost, while still complying with the part property specifications. A new heat treatment cycle was hypothesized and tailored, resulting from a critical analysis of current practice found in literature and practised in the Centre for Rapid Prototyping and Manufacturing (CRPM) of the Central University of Technology, Free State, based on the need for a quick and cost-effective cycle. By comparing the obtained material properties and microstructure with those of other common heat treatments, the cycle was found to be faster, lasting only four hours in total, and the material properties were superior to those of existing cycles. A high yield stress of 987 MPa was obtained while retaining a good elongation of 16 %.

Short-time induction heat treatment of high speed steel AISI M2: Laboratory proof of concept and application-related component tests

High-speed steels (HSS) such as AISI M2 are generally used for cutting and forming tools or broaches with the highest demands on load bearability and hot-hardness. The requirements for high-speed steels are, on the one hand, extremely high hardnesses of over 800–900 HV and, on the other hand, the thermal stability of the microstructure up to 500 °C. This combination is achieved by complex precipitation-hardenable iron-based alloys. Common heat treatment practices for high-speed steels use salt baths or vacuum ovens, which are energetically unfavorable and require subsequent cleaning. The study consists of short-time hardening and short-time tempering on a laboratory scale by means of dilatometry and thereupon an implementation in an industrial induction hardening machine. The laboratory scale experiments demonstrate the feasibility of bringing sufficient alloying elements into solution by means of several heat treatment cycles despite short holding times of 1 s and less. Combined tests of short time hardening and tempering show a hardness of up to 850 HV10 that are competitive with the conventional heat treatment route. First attempts to transfer the process to an industrial setup exhibit some peculiarities in the process control, but reveal promising results with a surface hardness of up to 750 HV10.