Miniature Tensile Specimens Research Articles

Implementation of miniature tensile specimens in mechanical properties assessment of directed energy deposited Ti-6Al-4V: As-built and heat treated

Within the last two decades, additive manufacturing (AM), a. k.a. 3D printing, has provided promising solutions for producing near-net-shape components with intricate geometries. From the material perspective, titanium alloys, one of humankind's most essential structural materials, are being considered the first candidate for AMed parts due to their unique characteristics in strength-weight-corrosion combinations. However, measuring the mechanical properties of designed geometry remains a challenge due to the ineffectiveness of conventional standard tensile specimens in assessing the site-specific and intricate geometries. In AM, the current approach often consists of evaluating standard-sized samples with the assumption that components with complex geometries possess comparable mechanical properties, even though they may have undergone different thermal processes in various situations. Hence, combining the microstructural characterization, this study aims to investigate the mechanical properties of direct energy deposition (DED) Ti-6Al-4V through a newly designed miniature tensile test (M-TT) specimen. The as-built specimen showed ultimate tensile strength (UTS) of ∼1305.3 MPa and elongation of ∼8.6 % with the mixed basketweave and α-colony microstructure. However, the DED Ti-6Al-4V specimen heat-treated at 850 °C exhibited the highest average elongation of ∼13.2 % and decent UTS of ∼1240.3 MPa with the α+β microstructure.

ASTM interlaboratory study on tensile testing of AM deposited and wrought steel using miniature specimens

An interlaboratory study, involving eigth international laboratories and coordinated by COMTES FHT (Czech Republic), was conducted to validate tensile measurements obtained using miniature specimens on additively manufactured (AM) components and artifacts. In addition to AM 316L stainless steel (316L SS), a wrought high-strength steel (34CrNiMo6V, equivalent to AISI 4340) was also used. Based on the results, a precision statement in accordance with ASTM E691 standard practice was developed, intended for inclusion in a proposed annex to the ASTM E8/E8M tension testing method. The primary outcomes of the study highlighted the agreement between yield and tensile strength measured from miniature and standard-sized tensile specimens. Furthermore, most tensile properties exhibited similar standard deviations, offering users insight into the efficacy of miniature specimen applications.

Influence of welding thermal cycles on properties of TMCP and Q&T steels evaluated by thermo-physical simulation

The welding process represents an unintentional, but unavoidable, heat treatment in the form of soft annealing or tempering, but also hardening during the cooling of the melt. Particularly in the case of high-strength fine-grained structural steels, this leads to critical states of the mechanical-technological properties of the base material. In order to investigate the influence of the heat input during welding on the resulting properties of the heat-affected areas, a thermo-physical simulation was carried out on a quenching and forming dilatometer Bähr DIL 805 A/D, considering low-alloyed quenched and tempered (Q&T) and low-alloyed thermo-mechanically controlled processed steels (TMCP) with yield strengths in the range of 500 to 960 MPa (S500MC, S700MC, S770QL, and S960QL). For this purpose, time–temperature cycles based on gas metal arc welding (GMAW) were simulated with different maximum temperatures (1200 °C; 1000 °C; 800 °C), representing the typical microstructural regions of the heat-affected zones (HAZ), and cooling times t8/5 (5 s; 12 s; 20 s; 25 s) on miniature tensile specimens. To evaluate the property changes of the characteristic HAZ, tensile tests, hardness measurements, and microstructural investigations were analyzed. The investigations illustrate the significant influence of heat input during the welding process on the resulting mechanical-technological properties and microstructure for both kinds of steel. It was demonstrated that all the steels investigated tend to soften with increasing cooling times. The investigated Q&T steels have a lower risk of falling below the strength of the untreated base material than the investigated TMCP fine-grained structural steels. The considerably pronounced softening also resulted in the minimum strength values not being achieved for certain cooling time ranges.

Use of miniaturized tensile specimens to evaluate the ductility and formability of dual phased steels for Rapid Alloy Prototyping

This work aims to investigate feasibility of using non-standard miniaturized tensile specimens (MTS) to characterize total elongation and formability behaviour of dual phase steels: to establish the scaling rules for MTS to predict a range of mechanical properties of steels obtained through rapid alloy prototyping (RAP) in the laboratory. Accurate measurement of these properties using miniature specimens allows the formability behaviour of materials produced using RAP, a process which can produce lab scale samples of < 100 grams. This allows the effects of multiple compositions and thermomechanical processing parameters on formability to be evaluated. For this study, the ability of miniature tensile specimens to characterize ductility, forming limits and r-values has been compared to properties measured using standard sized tensile specimens for the dual phase steels, DP600 and DP800. 5 different tensile specimens were investigated, with gauge lengths varying from 80 mm to 5 mm and samples were taken at different orientations to the rolling direction. Total elongation was evaluated against slimness ratio for each tensile sample using the Bertelle–Oliver method and good correlations were obtained, despite specimen thicknesses being below the critical value, and the tensile strength of DP800 exceeding the limit stated for the method. Forming limit curves (FLC) were predicted using the Keeler–Brazier method, based on tensile data obtained using the standard and non-standard tensile specimens These curves compared favourably to experimentally obtained FLCs, however the curve predicted using the miniaturized tensile specimen over-predicted the major strain. The anisotropy of the rolled sheet steels was characterized from r-values obtained from tensile tests performed at different orientations to the rolling direction. Values obtained from the miniature tensile test were comparable to those obtained from standard tests within the range of scatter. The results obtained through this study provide confidence in using the miniature tensile specimens to predict the formability of heterogeneous alloys such as DP steels, manufactured using the RAP process.

Development of a numerical simulation model for self-piercing riveting of additive manufactured AlSi10Mg

Laser additive manufacturing processes are used for the production of highly complex geometric structures due to their high geometric freedom. Additive manufacturing processes, in particular powder-based selective laser melting, are used to produce metallic additive manufactured components for the automotive and aerospace industries. Different materials are often joined together to realize sustainable lightweight construction. The production of such mixed construction joints is often realized using mechanical joining technology (e.g. self-piercing riveting). However, there is currently very little experience with the mechanical joining of metallic additive manufacturing components. Furthermore, there is insufficient knowledge about the effects that occur during the mechanical joining of additive manufacturing components. In this article, a method is presented to investigate the joinability of additively manufactured components with conventionally manufactured components using a numerical simulation of the self-piercing riveting process. For this purpose, the additive manufacturing materials are characterized experimentally, the simulation model is configured, and the joining process with additive manufacturing materials is represented in the numerical simulation. Furthermore, the influence of the building direction on the mechanical properties is shown using miniature tensile specimens. Besides the configuration of the simulation model, the influence of heat treatment on the self-piercing riveting process is presented.

Use of miniature tensile specimens for measuring mechanical properties in the steady-state and transient zones of Ti–6Al–4V wire-fed electron beam deposits

Recent developments in additive manufacturing (AM) have shown the importance of small-scale mechanical testing due to material availability issues, the microstructural variance within the complex build geometries, or thin-type features such as lattices. The current approach in AM is to test standard-sized coupons and postulate that a component with complex geometry has similar mechanical properties even though their thermal histories are completely different in many cases. In this research, small-scale tensile testing methodologies of AM parts made by electron beam wire-fed (EB-WF) deposited Ti–6Al–4V were investigated. Specifically, specimens extracted from well-established Ti–6Al–4V microstructural zones of transient and steady-state regions were compared to understand the effect of local microstructure on mechanical properties, which were evaluated through microhardness and miniature tensile testing. The average microhardness in the transient and steady-state zones are 321 Hv and 306 Hv, respectively. Although distinct microstructural features were observed in these zones, their tensile properties were similar. Both deposited zones had tensile yield and ultimate strength values ranging between 825-832 MPa and 880–907 MPa, respectively. The measured properties of the miniature specimens were comparable with E8 standard-sized specimens, higher than the ASTM F1108 cast and lower than the AMS 4911P wrought Ti–6Al–4V standards. Deformation and fractography analysis were used to correlate microstructure and property relationships for different α lath morphologies. Basket-weave morphology in the transient zone specimen showed profound grain splitting by basal slip which led to microstructural refinement and hindered softening during plastic flow.

A Comparative Study on Representativeness and Stochastic Efficacy of Miniature Tensile Specimen Testing

Abstract In this article, a miniature dog bone tensile coupon design was tested against the existing ASTM standard specimen design. Specimens were prepared from commercially sourced austenitic stainless steel 304 alloy, and a defect-ridden additively manufactured 304L alloy was studied. By utilizing a tensile specimen design that is 1/230th volume of the smallest ASTM E8-04(2016), Standard Test Methods for Tension Testing of Metallic Materials, dog bone specimen, coupled to a digital image correlation (DIC) setup, case studies were performed to compare tensile property measurements and strain field evolution. Whereas yield strength measurements were observed to be similar, post-yield, the ultimate strength measurements and ductility measurements from the miniature specimens were observed to be higher than the ASTM specimen design. Although the strength measurements were comparable, the strain evolution was found to differ in the miniature specimens. Studies to assess effects of varying thickness and defect population were also pursued on the miniature tensile specimen. From the DIC strain field estimations, the peak local strain values at ultimate tensile strength were observed to be increasing with reducing specimen thickness. Testing of defect ridden stainless steel revealed the sensitivity to failure through strain localization and the influence of defect size was captured in the strength measurements.

The Use of Miniature Specimens to Determine Local Properties and Fracture Behavior of LPBF-Processed Inconel 718 in as-Deposited and Post-Treated States.

This paper summarizes the assessment of directional anisotropy in local mechanical properties for Laser Powder Bed Fusion (LPBF) IN-718 bulk samples via the use of miniature samples excised from the bulk for both as-deposited and post-treated states. The quasi-static tensile properties at room temperature are investigated at several different locations along the build direction and at different orientations for both considered states. A comparison between the excised miniature tensile specimens and standard-sized sample results have also been conducted and exhibit very good agreement. Significant anisotropy is present in mechanical properties at different build heights for the as-deposited state, while the post-treated material exhibited more homogenous properties, both along the height and for different sampling orientations. However, significant reductions (e.g., >30%) in the strength (Yield, UTS) along with a significant increase in the reduction in area at fracture is found for post-processed materials. Metallography and fractography analyses were conducted in order to begin to determine the source(s) of this anisotropy for the as-deposited state.

In situ synchrotron tensile investigations on ultrasonic additive manufactured (UAM) zirconium

The microstructure evolution during room temperature uniaxial tensile straining of ultrasonic additive manufactured (UAM) zirconium was evaluated using the Advanced Photon Source (APS) facility. Miniature dog-bone tensile specimens of two orientations were cut from a UAM-fabricated zirconium bar for in situ synchrotron tensile tests. Wide-angle X-ray scattering (WAXS) scanning was used to unveil the changes in microstructure of the entire gauge regions throughout the straining. A series of WAXS data analysis methods were utilized to quantify both elastic and plastic deformation mechanisms within the strained specimens. Stress concentrations were identified during early stages of plastic deformation, which become candidate necking positions and eventually lead to failure. Fracture surface analysis implied that these stress concentration locations may be correlated to the fabrication defects, providing insightful guidance for future improvements of the UAM zirconium fabrication process.

Effects of microstructural anisotropy and helium implantation on tensile properties of powder-metallurgy processed tungsten plates

Powder metallurgy (PM)-processed tungsten (W) materials subjected to rolling are being developed for the monoblock divertors of fusion reactors. PM-W exhibits high dislocation density, fine grains, and a layered grain structure, which is introduced by the rolling process. It is expected that its microstructure control will be improved by the brittleness of PM-W at low temperatures. However, it is well known that, in rolled metals, the grain structure depends on the rolling ratio, direction, and temperature. This can cause anisotropy in the mechanical properties. The purpose of this study was to elucidate the effects of microstructural anisotropy and helium (He) implantation on the tensile properties of PM-W plates. The PM-W plates subjected to rolling and stress relief were examined. Miniature tensile specimens were prepared along the following three directions: the tensile directions were parallel to the rolling direction (L.D.), perpendicular to the rolling direction (T.D.), and parallel to the thickness direction (S.D.). The He implantation experiments were performed using a 50 MeV He2+ ion beam of a cyclotron accelerator. The implanted He concentration was approximately 20 appm. Tensile tests were conducted at 100–1100 °C and the ruptured surfaces were observed by scanning electron microscopy. From the results of the as-received condition, microstructural anisotropy was observed in the case of total elongation at temperatures lower than 500 °C. Cracks formed owing to intergranular fracture were observed at the edges of the gauge section along S.D., and anisotropy was observed in the tensile fracture behavior. It is assumed that the cracks tend to propagate along the layers of the grain boundaries. Although the ductility remained almost unchanged along the T.D. after He implantation, a decrease in ductility and brittle fracture were observed along the S.D.. Thus, the effect of He on crack propagation was significant along the S.D..

The development of miniature tensile specimens with non-standard aspect and slimness ratios for rapid alloy prototyping processes

This work aims to evaluate the use of miniaturized tensile specimen (MTS) to characterise the mechanical properties of alloys developed through rapid alloy prototyping (RAP), where high throughput tests are required on relatively small amounts of material. Tensile tests were conducted at a variety of strain rates and with increasingly smaller specimen sizes, ranging from larger specimens conforming to ASTM/ISO standards, down to small non-standard specimens. The gauge lengths of the specimens ranged from 50 to 80 mm for the standard specimens down to 5–10 mm for the non-standard specimens. To generalize the non-standard MTS designs, three alloys, DP800, DP600 and 316L stainless steel, were adopted. The results obtained from non-standard designs were compared with those from standard designs. The results show that non-standard designs can give repeatable results for yield strength (YS), ultimate tensile strength (UTS) and uniform elongation (eU). The maximum result differences of YS, UTS and eU are 7.37%, 7.71% and 11.9%, respectively, for DP alloys comparing standard and non-standard dimensions. These values are 13.56%, 14.03% and 19.5%, respectively 316L steel. The total elongation (ef) increases as the specimen dimension decreases. The geometrically dependent constants (n) are 0.2, 0.31 and 0.11 for DP800, DP600 and 316L, respectively. However, the Young's modulus is hard to determine precisely from the miniaturized designs. The conclusion from this work is that miniaturized tensile testing can be used with confidence as a high throughput means of predicting standard mechanical properties across a range of steels.

In-Situ Hollow Sample Setup Design for Mechanical Characterisation of Gaseous Hydrogen Embrittlement of Pipeline Steels and Welds

This work discusses the design and demonstration of an in-situ test setup for testing pipeline steels in a high pressure gaseous hydrogen (H2) environment. A miniature hollow pipe-like tensile specimen was designed that acts as the gas containment volume during the test. Specific areas of the specimen can be forced to fracture by selective notching, as performed on the weldment. The volume of H2 used was minimised so the test can be performed safely without the need of specialised equipment. The setup is shown to be capable of characterising Hydrogen Embrittlement (HE) in steels through testing an X60 pipeline steel and its weldment. The percentage elongation (%El) of the base metal was found to be reduced by 40% when tested in 100 barg H2. Reduction of cross-sectional area (%RA) was found to decrease by 28% and 11% in the base metal and weld metal, respectively, when tested in 100 barg H2. Benchmark test were performed at 100 barg N2 pressure. SEM fractography further indicated a shift from normal ductile fracture mechanisms to a brittle transgranular (TG) quasi-cleavage (QC) type fracture that is characteristic of HE.

Characterization of inhomogeneous microstructure and mechanical property in an ultra-thick duplex stainless steel welding joint

Ultra-thick duplex stainless steel welding plates are widely used in various industries. However, one of the significant concerns is the inhomogeneous microstructure and mechanical properties along the thickness, which reduces welding efficiency and degrade service performance. In this study, a 55 mm thick duplex stainless steel welding joint with an X-type weld groove was prepared to study the variation of microstructure and mechanical properties along the through-thickness direction by both the miniature tensile specimen and cross-weld tensile specimen. The correlation between the microstructure and mechanical property was also studied. The results showed that the weld metal was characterized by abundant columnar grains with decreasing austenite content from surface to mid-thickness. Besides, the total fraction of low angle grain boundaries increased more than one-fold, while the total fraction of high angle grain boundaries decreased from 48.03% to 4.52% with increasing depth to the mid-thickness. Furthermore, substantial substructured grains and recrystallized grains were dominated in the weld metal and base metal. Along the thickness direction, the mechanical strength almost decreased and the elongation of welding joint increased with the increasing distance from the surface. Microhardness had a similar trend to that of strength, which all can be attributed to the variation of austenite content and grain boundary character distribution. Along the transverse direction, weld metal had the highest strength of 597.67 MPa, followed by the heat-affected zone of 549.42 MPa and base metal of 510.20 MPa, while the trend of elongation was opposite to that of strength. The microhardness distribution was not consistent with that of strength and had a maximum at the heat-affected zone.

Determination of local constitutive properties in similar and dissimilar friction stir welded joints from DIC based surface strain measurement in two mutually perpendicular surfaces

In the present work, local constitutive properties of similar and dissimilar friction stir weld (FSW) joints of AA6061-T6 and AA7075-T6 are evaluated using a modified non-uniform stress method (nUSM). The modified nUSM considers the instantaneous cross-sectional area at any section of a macro tensile (MaT) specimen as a two-variable function. These two variables are the strain in width and thickness direction during uniaxial tensile test. The strain in width and thickness direction is measured using two cameras of a stereo digital image correlation (DIC) testing instrument. These two cameras work as two separate 2-dimensional DIC placed perpendicular to width and thickness direction. However, strain data from this two 2D-DIC are arranged in a global euclidian space while maintaining a fixed pixel/mm ratio. The local constitutive properties evaluated from nUSM are then compared with the local properties extracted from longitudinal miniature tensile (MiT) specimens. The local constitutive properties evaluated using modified nUSM for the similar AA6061-AA6061 FSW joint have shown a good agreement with the properties calculated from the MiT specimen. In a similar AA7075-AA7075 FSW joint, the accuracy of the modified nUSM is affected by the relatively less deformation present in some portion of the FSW joints. The nUSM did not evaluate local constitutive properties in the AA7075 side of a dissimilar AA6061-AA7075 FSW joint, as this portion remains elastic during the uniaxial tensile testing of the MaT specimen.

Mechanical behavior of additively manufactured and wrought 316L stainless steels before and after neutron irradiation

Fabrication of nuclear reactor components using additive manufacturing (AM) methods is now a practical option since the AM technologies have advanced to allow for building of complex parts with high quality materials. To assess the mechanical performance of printed components in reactor-relevant conditions and to build a property database for the AM 316L stainless steel (SS), mechanical testing and characterization were performed before and after neutron irradiation. Miniature tensile specimens were irradiated at the High Flux Isotope Reactor (HFIR) to 0.2 and 2 displacements per atom (dpa) at 300 and 600°C. The AM 316L SS was tested in the as-built, stress-relieved, and solution-annealed conditions, and the wrought (WT) 316L SS in solution-annealed condition as a reference alloy. The baseline test result showed that the AM 316L SS, regardless of the post-build heat treatment, had higher strength than the WT 316L SS, but similar ductility. Post-irradiation tensile testing was conducted at RT, 300°C, and 500°C for selected irradiation conditions. Neutron irradiation induced significant changes in the mechanical behavior of the AM stainless steels, including both hardening and softening. Although the as-built 316L steel after 300°C irradiation showed necking just after yielding, the overall property changes of the as-printed alloy became less significant after 600°C irradiation. Irradiation-induced ductilization was also observed after the higher temperature irradiation. In general, the strength change was smaller in the relatively stronger as-built and stress-relieved AM SSs than in the solution-annealed AM and WT SSs. These relatively lower strength 316L SSs overall retained higher ductility in the irradiation conditions tested, but the stronger 316L SSs demonstrated a similar level of ductility after the higher temperature (600°C) irradiation. It is a positive assessment for the AM 316L materials that no embrittlement was observed within the test and irradiation conditions of the experiment.

Stir zone anisotropic work hardening behavior in friction stir processed EN8 medium carbon steel

The SZ anisotropic work hardening behavior in duplex ferrite-martensite phase (DFM) microstructures has not been investigated yet. It is an important consideration for the selection of the stretch formability direction of the friction stir processed blank used for sheet metal forming applications. The anisotropic work hardening behavior is characterized by using miniature tensile specimens prepared along longitudinal (LD) and transverse (TD) directions. The tribological parameters such as coefficient of friction (COF) and wear rate at LE value of 6 are 0.57 and 0.5 × 10 − 10 m 3 / Nm . The effect of surface texture on the work hardening response is observed to be negligible. The plastic anisotropy in terms of three and multi-stage work hardening behaviors is investigated at a linear energy ( LE) value of 6. The global true stress-strain responses and fracture surface morphologies suggested that the deformation occurs in different stages. In the initial stage, the deformation is mainly localized in the soft ferrite phase followed by the distinct work hardening stages in the plastic flow curves. In the elastic and elastoplastic region of the flow curve, the soft ferrite phase plastically deformed, followed by stress transfer to the martensite/retained austenite phases in the fully plastic region. The (110) and (111) pole figure suggested that the active in-habit-plane slip system is (110) and [ 1 1 ‾ 1 ] . The strain localizes more in the V19 + V22 block having the critical resolved shear stress (CRSS) value of 445.5 MPa. The retained austenite (RA) is having ( − 11 − 1 ) A / / ( 0 − 11 ) F and [ 011 ] A / / [ 111 ] F type Kurdjumov-Sachs (K–S) orientation relationship with the ferrite phase. A BCC {112} <111> -type twinned laths is found to be present within the martensite laths. The recommended stretch formability direction (SFD) should coincide with the longitudinal or processing direction of the friction stir processed blank.

Non-contact strain evaluation for miniature tensile specimens of neutron-irradiated F82H by digital image correlation

• Non-contact strain evaluation by DIC using speckle patterns of surface flaws was applied and reliability of the technique was demonstrated. • Room temperature tensile behavior of Japanese RAFM, F82H, irradiated to ∼70 dpa at 573 K was preliminarily evaluated. • No possible irradiation-induced change of the elastic properties in both axial and transverse directions for irradiated F82H was demonstrated. The small specimen tensile test is commonly used as the basic test to evaluate the irradiation effect on the strength of fusion structural materials after irradiation, and a lot of attempts were made to qualify the data obtained by small size specimen, but the specific attention on the evaluation of strain were not satisfactory given. This study therefore aims to propose a novel non-contact strain measurement technique, based on digital image correlation (DIC) method, for reduced-activation ferritic/martensitic (RAFM) steel, e.g., F82H, as the leading candidate structural material of fusion in-vessel components in Japan. A flat rectangular miniature specimen, e.g., SS-J3 type, was adopted to evaluate non-irradiated and neutron-irradiated tensile properties. In the proposed DIC, very tiny surface machining flaws were recognized by the DIC computation program and utilized to set several pairs of the gauge end points with a fixed gauge length. In case of the room-temperature test, it was clearly demonstrated that this procedure could provide more precise and reliable data compared with the previous approach of the single-scan measurement under floated gauge length. Superior linearity was achieved and recognized DIC points of concern were traceable during the entire period of the test. By applying this technique, superior high-dose stability of elastic properties of F82H was first demonstrated.

On the mechanical integrity of AA6082 3D structures deposited by hybrid metal extrusion &amp; bonding additive manufacturing

Hybrid Metal Extrusion and Bonding Additive Manufacturing (HYB-AM) is a new solid-state process for the production of 3D metal structures. In HYB-AM, the wire feedstock is continuously processed through an extruder and deposited in a stringer-by-stringer manner to form layers and eventually a near net-shape component. In this work, the layer bonding of AA6082 samples produced by this process has been investigated by means of tensile testing, hardness measurements and microscope analyses. Furthermore, a novel method for the fabrication of miniature tensile specimens for assessing the bond strength across the layers is presented and applied. The test results reveal that the ultimate tensile strength is approaching that of the substrate material of the same alloy, yet with a somewhat lower elongation prior to fracture. Microscope analyses show that the bonded interfaces are fully dense; however, the fracture surfaces reveal regions of kissing-bonds and lack of bonding. Still, these preliminary investigations indicate that the HYB-AM process, upon further optimization, has the potential of processing high quality aluminum alloy components.

Development and Validation of a Miniature Tensile Specimen for Determination of Mechanical Properties

Abstract In this work, a miniature tensile specimen with nominal dimensions of 3.0-mm gage length (GL), 1.5-mm gage width, and 0.5-mm thickness that was carved out of a 10.0-mm-diameter disk has been developed and standardized using analytical and experimental methods. The geometry of the miniature specimen, called ultra sub-size (USS), was optimized using finite element analysis to determine the fillet radius and appropriate geometrical tolerances for specimen dimensions like gage width and thickness. The methods of specimen preparation, gripping, and tensile testing with the use of digital image correlation for strain measurement were standardized. The 0.2 % offset yield strength, ultimate tensile strength, and uniform strain of different materials obtained using this specimen geometry were analyzed and compared with the results of ASTM sub-size (GL: 25 mm) and further sub-size geometry (GL: 12.5 mm). The results of this study show that USS tensile specimen geometry developed in this work can be reliably employed for mechanical property evaluation in situations where tensile testing using standard-size specimens is practically not possible.

Optimization of the Continuous Galvanizing Heat Treatment Process in Ultra-High Strength Dual Phase Steels Using a Multivariate Model

The main process variables to produce galvanized dual phase (DP) steel sheets in continuous galvanizing lines are time and temperature of intercritical austenitizing (tIA and TIA), cooling rate (CR1) after intercritical austenitizing, holding time at the galvanizing temperature (tG) and finally the cooling rate (CR2) to room temperature. In this research work, the effects of CR1, tG and CR2 on the ultimate tensile strength (UTS), yield strength (YS), and elongation (EL) of cold rolled low carbon steel were investigated by applying an experimental central composite design and a multivariate regression model. A multi-objective optimization and the Pareto Front were used for the optimization of the continuous galvanizing heat treatments. Typical thermal cycles applied for the production of continuous galvanized AHSS-DP strips were simulated in a quenching dilatometer using miniature tensile specimens. The experimental results of UTS, YS and EL were used to fit the multivariate regression model for the prediction of these mechanical properties from the processing parameters (CR1, tG and CR2). In general, the results show that the proposed multivariate model correctly predicts the mechanical properties of UTS, YS and %EL for DP steels processed under continuous galvanizing conditions. Furthermore, it is demonstrated that the phase transformations that take place during the optimized tG (galvanizing time) play a dominant role in determining the values of the mechanical properties of the DP steel. The production of hot-dip galvanized DP steels with a minimum tensile strength of 1100 MPa is possible by applying the proposed methodology. The results provide important scientific and technological knowledge about the annealing/galvanizing thermal cycle effects on the microstructure and mechanical properties of DP steels.