Cooling process and mechanical properties design of hot-rolled low carbon high strength microalloyed steel for automotive wheel usage

For the purpose of developing Nb–V–Ti microalloyed, hot rolled, high strength automotive steel for usage in heavy-duty truck wheel-discs and wheel-rims, appropriate cooling processes were designed, and microstructures and comprehensive mechanical properties (tension, bending, hole-expansion, and Charpy impact) of the tested steels at two cooling schedules were studied. The results indicate that the steel consists of 90% 5 μm polygonal ferrite and 10% pearlite when subjected to a cooling rate of 13 °C/s and a coiling temperature of 650 °C. The yield strength, tensile strength, and hole-expansion ratio are 570 MPa, 615 MPa, and 95%, respectively, which meet the requirements of the wheel-disc application. The steel consists of 20% 3 μm polygonal ferrite and 80% bainite (granular bainite and a small amount of acicular ferrite) when subjected to a cooling rate of 30 °C/s and a coiling temperature of 430 °C. The yield strength, tensile strength, and hole-expansion ratio are 600 MPa, 655 MPa, and 66%, respectively, which meet the requirements of the wheel-rim application. Both the ferrite–pearlite steel and ferrite–bainite steel possess excellent bendability and Charpy impact property. The precipitation behavior and dislocation pattern are characterized and discussed.     

Correlation between microstructure and tensile properties in powder metallurgy AZ91 alloys

The ultrafine-grained (0.3–1.3 μm) AZ91 alloys, which were fabricated by powder extrusion in the range of 200 to 350 °C and subsequent aging at 100 °C for 8 h, exhibit a remarkable yield stress of 360–478 MPa and moderate tensile elongations of 6–8%. A composite structure was developed after extrusion with uniform β (Mg₁₇Al₁₂) particles dispersed in magnesium matrix. The extrusion temperature has an indirect role on yield stress since partial dissolution of β particles induced by high extrusion temperature fails to retard grain growth. Moreover, the strength was further enhanced by the formation of nano-scale precipitates during artificial aging. The high strength could be attributed to a combination effect of grain refinement, particle reinforcement and precipitation hardening.     

Influence of interface microstructure on the mechanical properties of titanium/17-4 PH stainless steel solid state diffusion bonded joints

In the present study, titanium was diffusion bonded to a type 17-4 precipitation hardening stainless steel in vacuum at different temperatures and times. Bonded samples were characterized using light microscopy, scanning electron microscopy (SEM) and X-ray diffraction technique (XRD). The inter-diffusion of the chemical species across the diffusion interface was evaluated by electron probe microanalysis (EPMA). Up to 850 °C for 60 min, FeTi phase was formed at the diffusion interface; however, α-Fe + λ, χ, Fe₂Ti and FeTi phases and their phase mixtures were formed above 850 °C for 60 min and at 900 °C for all bonding times. The maximum tensile strength of ∼342.4 MPa and shear strength of ∼260.3 MPa along with 12.8% elongation were obtained for the diffusion couple processed at 950 °C. The thicknesses of different reaction products at the bond interface play an important role in determining the mechanical properties of the joints. The residual stress of the bonded joints increases with the increases in bonding temperatures and times.     

Tensile properties of hot rolled Mg–3Sn–1Ca alloy sheets at elevated temperatures

Mechanical behavior of hot rolled Mg–3Sn–1Ca (TX31) magnesium alloy sheets were studied in the temperature range 25–350 °C. The microstructure of the alloy consisted of the eutectic structure of α-Mg + Mg₂Sn and a dispersion of needle-like CaMgSn. The highest room-temperature ductility of 18% was obtained by hot rolling of the cast slabs at 440 °C, followed by annealing at 420 °C. The high temperature tensile deformation of the material was characterized by a decrease in work hardening exponent (n) and an increase in strain rate sensitivity index (m). These variations resulted in respective drops of proof stress and tensile strength from 126.5 MPa and 220 MPa at room temperature to 23.5 MPa and 29 MPa at 350 °C. This was in contrast to the ductility of the alloy which increased from 18% at room temperature to 56% at 350 °C. The observed variations in strength and ductility were ascribed to the activity of non-basal slip systems and dynamic recovery at high temperatures. The TX31 alloy showed lower strength than AZ31 magnesium alloy at low temperatures, while it exhibited superior strength at temperatures higher than 200 °C, mainly due to the presence of thermally stable CaMgSn particles.     

Structure and properties of solid state diffusion bonding of 17-4PH stainless steel and titanium

Abstract

Solid state diffusion bonded joint between titanium and 17-4 precipitation hardening stainless steel was carried out in the temperature range of 800–1050°C in steps of 50°C for 30 min and also at 950°C for 30–180 min in steps of 30 min under a uniaxial pressure of 3·5 MPa in vacuum. Bonded samples were characterised using light microscopy, field emission scanning electron microscopy and X-ray diffraction technique. Up to 850°C for 30 min, FeTi phase was formed at the diffusion interface; however, α-Fe+λ, χ, Fe₂Ti and FeTi phases and phase mixtures were formed above 850°C for 30 min and at 950°C for all bonding times. Maximum tensile strength of ～326 MPa, shear strength of ～254 MPa and impact toughness of ～24 J were obtained for the diffusion couple processed at 1000°C for 30 min and 30–180 min time interval at 950°C, and maximum tensile strength ～323 MPa, shear strength ～243 MPa and impact toughness of ～22 J were achieved when bonding was processed for 120 min. The residual stress of the bonded joints increases with the increase in bonding temperatures and times.     

Experimental investigation of a thermoacoustic refrigerator driven by a standing wave twin thermoacoustic prime mover

The objective of this study is to analyse the performance of thermoacoustic refrigerator (TAR) measured in terms of hot end temperature and temperature difference across refrigerator stack with two different spacing namely 0.4 mm and 0.8 mm and stack used in refrigerating section was made of low thermal conductivity materials namely Mylar sheet and photographic film & the experiments were carried out at 1 MPa pressure using helium as working fluid. High powered acoustic wave with frequency of 460 Hz and pressure amplitude of $?$0.07 MPa was obtained from twin thermoacoustic prime mover (TAPM) and this acoustic wave produced temperature difference of 16 °C across the Mylar sheet stack made of 0.4 mm spacing in refrigerator section. From this study, it has been inferred that twin TAPM can act as efficient drive for TAR.     

Impact of pre-ageing on age hardening response of twin-belt cast AlMg1SiCu sheet

The potential of twin-belt cast (TBC) AlMg1SiCu sheet for structural automotive applications was investigated with a particular emphasis on the impact of pre-ageing on its age hardening response. The optimum T6 process for the TBC AlMg1SiCu sheet is identified to be a water-quench from the solution heat treatment at 540 °C and a subsequent ageing treatment at 180 °C. This process gives hardness values as high as 120 HV within several hours when ageing at 180 °C is performed shortly after the solution treatment. The age hardening capacity is impaired, however, when the sheet is stored at room temperature before the artificial ageing cycle. Pre-ageing at 100 and 140 °C is effective in improving the age hardening response of the naturally aged 6061 sheet. Pre-ageing suppresses natural ageing and clustering activities and gives lower T4 yet a much higher T6 hardness.     

Effect of aluminum on precipitation hardening in Cu–Ni–Zn alloys

The effect of aluminum on the precipitation hardening of Cu–Ni–Zn alloys with varying aging temperatures and times was investigated in this article, in the hope to achieve better mechanical properties. Vickers hardness, tensile, and electrical conductivity tests were carried out to characterize the properties of the Cu–Ni–Zn alloys with or without an addition of aluminum subjected to different aging treatments. The results show that an addition of 1.2 wt% aluminum can play an influential role in the precipitation hardening of the Cu–Ni–Zn alloys. For example, it can increase the peak hardness from 58 Hv for the solution-treated Cu–10Ni–20Zn alloy to 185 Hv for the solution-treated Cu–10Ni–20Zn–1.2Al alloy during aging at 500 °C. The yield strength, tensile strength, and electrical conductivity of the Cu–10Ni–20Zn–1.2Al alloy subjected to suitable treatments under prior cold-rolled and aged conditions can reach 889 MPa, 918 MPa, and 10.96% IACS, respectively, being much higher than those of the relevant alloy without aluminum and comparable to those of the Cu–Be alloys (C17200 and C17510). According to the transmission electron microscope observations, it was found that formation of nanosized precipitates with the L1₂-type ordered lattice results in precipitation hardening, and an orientation relationship of [011]_\textp//[011]_\textm [011]_{\text{p}}//[011]_{\text{m}} and (100)_\textp//(200)_\textm (100)_{\text{p}}//(200)_{\text{m}} exists between the precipitates and the α-Cu matrix.     

Effect of tempering temperature on the microstructure and properties of ultrahigh-strength stainless steel

The microstructure, precipitation and mechanical properties of Ferrium S53 steel, a secondary hardening ultrahigh-strength stainless steel with 10% Cr developed by QuesTek Innovations LLC, upon tempering were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and tensile and impact tests. Based on these results, the influence of the tempering temperature on the microstructure and properties was discussed. The results show that decomposition occurred when the retained austenite was tempered above 440 °C and that the hardening peak at 482 °C was caused by the joint strengthening of the precipitates and martensite transformation. Due to the high Cr content, the trigonal M₇C₃ carbide precipitated when the steel was tempered at 400 °C, and M₇C₃ and M₂C (5–10 nm in size) coexisted when it was tempered at 482 °C. When the steel was tempered at 630 °C, M₂C and M₂₃C₆ carbides precipitated, and the sizes were greater than 50 nm and 500 nm, respectively, but no M₇C₃ carbide formed. When the tempering temperature was above 540 °C, austenitization and large-size precipitates were the main factors affecting the strength and toughness.     

Yield strength enhancement of martensitic steel through titanium addition

Yield strength enhancement for martensitic steel fabricated by vacuum induction melting is investigated. It is found that the addition of Ti can improve the yield strength property of the martensitic steel, which can be attributed to increase in precipitation hardening from formation of TiC precipitates in the martensitic matrix. Moreover, the yield strength can be further enhanced by tempering and reheat quenching process, which can be ascribed to the formation of a superfine sized (~8 μm) grains and large amount of freshly nano-sized (1–10 nm) precipitates in the final martensitic structure for martensitic steel containing Ti. The experimental and theoretical results on the contribution of TiC precipitates to hardening of the martensitic steel are in excellent agreement, showing that the precipitation hardening of 188 MPa caused by TiC precipitates is the main reason why the yield strength for martensitic steel is enhanced via titanium addition.     

Effects of heat treatments on the microstructure and mechanical properties of Rene 80

Effects of heat treatments on room temperature mechanical properties and stress-rupture properties of Rene 80 have been investigated. The microstructures were analyzed by optical microscope and scanning electron microscope after each step of heat treatments. With the decrease of aging temperature, the average size of γ′ phase decreases, but the volume fraction of γ′ phase increases. The lower aging temperature suppresses the growing of the coarse γ′ particles, but facilitates the growth of the fine γ′ particles. After the optimum heat treatment, the ultimate tensile strength and yield strength are respectively higher than 1040 MPa and 950 MPa, the stress-rupture life at 871 °C/310 MPa is higher than 170 h with excellent ductility. The improved tensile strength and stress-rupture life are primarily due to the increased volume fraction of γ′ phase. The borides precipitate at grain boundaries at about 913 °C. The primary MC is found to decompose into M₆C at about 873 °C and M₂₃C₆ at 840–873 °C at grain boundaries. The precipitate of the carbides may partly contribute to the improved mechanical properties.     

Ultra-high-temperature tensile properties and fracture behavior of ZrB2-based ceramics in air above 1500 °C

Nearly fully dense ZrB₂–SiC–graphite composites were fabricated from commercially available powder at 1900 °C by hot pressing. The tensile strength of ZrB₂-based ceramics was measured in air up to 1750 °C, which is the first reported tensile strength measurement in air above 1500 °C. A mechanical testing apparatus capable of testing material in ultra-high temperature under air atmosphere was built, evaluated, and used. Tensile strength was measured as a function of temperature up to 1750 °C in air. The respective average values of the tensile strength measured at 1550 °C, 1650 °C, and 1750 °C are 58.4, 44.8, and 21.8 MPa, which are 49.4%, 37.9%, and 18.4% of their room-temperature strength (118.2 MPa), respectively. Moreover, the tensile fracture behaviors and mechanism of ZrB₂-based ceramics at different testing temperatures were discussed based on microstructure characterization.     

Diffusion bonding of titanium alloy to micro-duplex stainless steel using a nickel alloy interlayer: Interface microstructure and strength properties

In the present study, diffusion bonding of titanium alloy and micro-duplex stainless steel with a nickel alloy interlayer was carried out in the temperature range of 800–950 °C for 45 min under the compressive stress of 4 MPa in a vacuum. The bond interfaces were characterised by scanning electron microscopy, electron probe microanalyzer and X-ray diffraction analysis. The layer wise Ni₃Ti, NiTi and NiTi₂ intermetallics were observed at the nickel alloy/titanium alloy interface and irregular shaped particles of Fe₂₂Mo₂₀Ni₄₅Ti₁₃ was observed in the Ni₃Ti intermetallic layer. At 950 °C processing temperature, black island of β-Ti phase has been observed in the NiTi₂ intermetallics. However, the stainless steel/nickel alloy interface indicates the free of intermetallics phase. Fracture surface observed that, failure takes place through the NiTi₂ phase at the NiA–TiA interface when bonding was processed up to 900 °C, however, failure takes place through NiTi₂ and β-Ti phase mixture for the diffusion joints processed at 950 °C. Joint strength was evaluated and maximum tensile strength of ∼560 MPa and shear strength of ∼415 MPa along with ∼8.3% ductility were obtained for the diffusion couple processed at 900 °C for 45 min.     

Einfluss von Temperatur und Vorverformung auf das plastische Werkstoffverhalten von modernen Karosseriestählen

Influence of temperature and prestraining on the plastic material behaviour of modern sheet steels for autobody applications Within the scope of a common research project of the automotive and steel industry, characteristic values describing the plastical behaviour of 20 sheet steels have been determined. In detail, quasistatic tensile tests at the testing temperatures ‐40 °C, 23 °C and 100 °C were carried out to obtain flow curves for the as delivered materials as well as for steels after a defined prestraining or heat treatment. Additionally, sheet metal testing led to forming limit diagrams and limiting drawing ratios including the working ranges for deep drawing. The results of the tensile tests showed significant differences between steel groups with regard to their strain hardening behaviour, which can be described by the ratio of yield and tensile strength R_p0,2/R_m or the Θ_IV‐value, and their temperature sensitivity. Within one steel group, consisting of steels with similar strain hardening behaviour, it might be possible to determine flow curves of one steel in a defined condition in order to calculate the flow curves of other steels with different strength. An advantage would be a lesser number of experimental tests which have to be performed in order to supply reliable input data for numerical material and component modelling.     

Precipitation evolution and creep strength modelling of 25Cr20NiNbN austenitic steel

25Cr-20Ni-Nb-N is a high strength and oxidation-resistant austenitic stainless steel intended for Ultra-Supercritical (USC) power plants. In this work, the precipitation evolution, and creep strength at 650 and 750°C for up to 100?000?h are predicted. Six precipitates are considered in the thermokinetic calculation by MatCalc: M₂₃C₆, η (Cr₃Ni₂SiN), σ, G, Z, Nb(C,N). For the creep strength prediction, three hardening mechanisms are taken into account: dislocation, precipitation, and solid solution hardening. Both matrix composition and precipitation evolution, calculated with MatCalc, are used for modelling the precipitation and solid solution hardening. It is found that the dislocation hardening, followed by precipitation hardening gives the largest contribution to the creep strength. The most important precipitates strengthening phases are found to be Z-Phase and Nb(C,N), which are nucleated at the dislocations. The model for the creep rate can represent how the creep exponent is raised with increasing applied stress and reduced temperature.     

Hexagonal martensite decomposition and phase precipitation in Ti–Cu alloys

The mechanical behavior of Ti–Cu alloys can be improved by controlling Ti₂Cu precipitation. In eutectoid alloys, such precipitation can be achieved by the decomposition of martensite in response to aging heat treatment. The purpose of this work is to discuss the evolution of precipitates during the decomposition of hexagonal martensite in Ti–Cu alloys. First, samples with near-eutectoid compositions were prepared in an arc furnace equipped with a non-consumable tungsten electrode and water-cooled copper hearth under a high purity argon atmosphere. After chemical homogenization at a temperature in the beta field, the samples were water-quenched and examined by differential scanning calorimetry and high-temperature X-ray diffraction. The results indicate that rapidly quenched near-eutectoid Ti–Cu alloys present Ti₂Cu precipitates. Regardless of the cooling rate applied, such precipitation is unavoidable. No evidence of beta phase stabilization was found in the rapidly quenched samples. Precipitation temperatures of coherent and incoherent phases of 415 °C and 550 °C, respectively, were determined from the differential scanning calorimetry measurements. Ti₂Cu precipitation was examined in situ by high temperature X-ray diffraction experiments. The total decay of martensite was found to occur above 575 °C. Vickers hardness testing of aged samples revealed a correlation between phase precipitation and hardening.     

Influence of secondary carbides precipitation and transformation on the secondary hardening of laser melted high chromium steel

The influence of secondary carbides precipitation and transformation on the secondary hardening of laser melted high chromium steels was analyzed by means of scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The microstructure of laser melted high chromium steel is composed of austenite with supersaturated carbon and alloy elements and granular interdendritic carbides of type M₂₃C₆. Secondary hardening of the laser melted layer begins at 450 °C after tempering, and the hardness reaches a peak of 672HV at 560 °C and then decreases gradually. After tempering at 560 °C, a large amount of lamellar martensite was formed in the laser melted layer with a small quantity of thin lamellar M₃C cementite due to the martensitic decomposition. The stripy carbides precipitating at the grain boundaries were determined to be complex hexagonal M₇C₃ carbides and face centered cubic M₂₃C₆ carbides. In addition, the granular M₂₃C₆ carbides and fine rod-like shaped M₇C₃ carbides coexisted within the dendrites. As a result, the combined effects of martensitic transformation, ultrafine carbide precipitations, and dislocation strengthening result in the secondary hardening of the laser melted layer when the samples were tempered at 560 °C.     

High strength Al–Al2O3p composites: Optimization of extrusion parameters

Composite aluminium alloys reinforced with Al₂O_3p particles have been produced by squeeze casting followed by hot extrusion and a precipitation hardening treatment. Good mechanical properties can be achieved, and in this paper we describe an optimization of the key processing parameters. The parameters investigated are the extrusion temperature, the extrusion rate and the extrusion ratio. The materials chosen are AA 2024 and AA 6061, each reinforced with 30 vol.% Al₂O₃ particles of diameter typically in the range from 0.15 to 0.3 μm. The extruded composites have been evaluated based on an investigation of their mechanical properties and microstructure, as well as on the surface quality of the extruded samples. The evaluation shows that material with good strength, though with limited ductility, can be reliably obtained using a production route of squeeze casting, followed by hot extrusion and a precipitation hardening treatment. For the extrusion step optimized processing parameters have been determined as: (i) extrusion temperature = 500 °C–560 °C; (ii) extrusion rate = 5 mm/s; (iii) extrusion ratio = 10:1.     

Effects of Ag addition on the microstructure and thermal stability of 6156 alloy

The effects of Ag addition on the microstructure and thermal stability of 6156 Al–Mg–Si–Cu alloy were investigated by means of hardness measurement, tensile tests, differential scanning calorimetry, and transmission electron microscopy. The results showed that addition of small amount of Ag to 6156 alloy did not change the precipitation sequence mainly β″ and Q′ strengthening phase but slightly accelerated the age-hardening rate and increased peak hardness at the same aging condition. The tensile properties enhanced about 30 MPa at the room temperature or thermal exposure at lower temperature (<100 °C). With the exposed temperature and time increased to 150 °C for 1000 h, almost no difference between the Ag-containing and Ag-free alloys. When exposed at 200 °C, the tensile strength of Ag-containing alloy became lower than that of Ag-free alloy because of the coarsening precipitations in matrix and boundary observed by TEM observed. For both alloys, thermal exposure at temperatures 100 °C for long time periods had no significant effect on tensile properties. Loss in strength was small and large with prolonging the exposure time at 150 and 200 °C, respectively.     

Flexural behaviour of CNT-filled glass/epoxy composites in an in-situ environment emphasizing temperature variation

The paucity of structural defects in carbon nanotube (CNT) with unrivalled mechanical properties has always posed an interest to material scientists for its potential incorporation in soft polymer resins to achieve superior mechanical stability. Present investigation focuses on the assessment of flexural behaviour of glass/epoxy (GE) and multiwalled carbon nanotubes (MWCNT) embedded glass/epoxy (0.3 wt. % of epoxy) (CNT-GE) composites at different in-service environmental temperatures. In-situ 3-point bend tests were performed on GE and CNT-GE composites at −80 °C, −40 °C, room temperature (20 °C), 70 °C and 110 °C temperatures at 1 mm/min crosshead speed. The results revealed that at 110 °C temperature, the flexural strength of GE and CNT-GE composites was significantly decreased by 67% and 81% respectively in comparison to their strength at −80 °C temperature. Similarly, 38% and 77% decrement in modulus was noted for GE and CNT-GE composites respectively. Dynamic mechanical thermal analysis (DMTA) was carried out in the temperature range of −100 °C to 200 °C to correlate the mechanical and thermo-mechanical response of both the material systems. Addition of 0.3 wt. % MWCNT in GE composite resulted in lowering of glass transition temperature (T_g) by 12 °C. Furthermore, to understand various possible deformation and failure mechanisms, the post failure analysis of the fractured specimens, tested at different temperatures, was carried out using scanning electron microscope (SEM). The critical parameters needed during designing composite structures were calculated and modelled using Weibull constitutive model.