Hot Tensile Research Articles

Health assessment of 22 years service-exposed radiant tube from an oil refinery

Radiant tube conforming to ASTM A-200 T5 grade of steel and service exposed for 20 years from an oil refinery was investigated for health check and safety aspect. Microstructural study of the service exposed tube revealed a ferritic–martensitic structure with significant spheroidization of second phase particles. It appears that there is total disintegration of martensitic lath boundaries in some areas. The martensitic lath grains seemed to have broadened and lath boundaries are decorated with coarse and rounded carbides. As a result, with carbides the material has softened. Analysis of hot tensile and accelerated stress rupture data revealed that the residual life at 575 °C and an operating hoop stress level of (28.11 MPa) gave a minimum balance of life of approximately four and a half years for the service-exposed radiant tube. This is provided there is no evidence of localised damage in the form of surface cracks, cavitations or dents. A further life assessment of the service-exposed radiant tube is recommended after an additional 2–3 years of service life an economical and safety reasons viewpoint. During the next shut down of the plant, NDT (non-destructive) tests viz. dimensional (thickness and diameter)measurement, hardness measurement and in situ metallography may be carried out to assess the condition of the materials for their future serviceability.

Analysis of Hot Tensile and Compression Curves to Assess the Hot Ductility of C-Mn Steels

The unbending operation is a critical stage of steel continuous casting because it is carried out at thermomechanical conditions for which embrittlement mechanisms can appear leading to transverse cracking. The hot tensile test is commonly used to simulate such thermomechanical conditions, at the surface of the slab, and, the reduction in area of the samples tested to fracture is taken as a measure of the susceptibility to cracking of the steel. However, a further metallographic and fractographic evaluation of the samples is required in order to identify the embrittlement mechanisms. These mechanisms are usually related to transformations in the microstructure, such as precipitation or the appearance of deformation induced ferrite, which imply changes in the strength of the material and should therefore be detectable in the flow curves. However, the features of tensile curves are not usually analyzed when evaluating the hot ductility because necking makes the interpretation of the curves complicated. In this work the hot ductility of a C-Mn steel will be discussed by means of hot tensile and compression tests. The embrittlement mechanism identified for this steel is the appearance of a ferrite layer at austenite grain boundaries. The effect of this mechanism on the features of the tensile curves will be discussed. Moreover, these curves will be compared to compression curves obtained under the same testing conditions to see whether transformation induced ferrite can be detected by means of hot compression testing. The possibility of assessing the ductile behavior of different steel grades through hot compression, which requires less material and is easier to control, will be discussed.

Ductility of Microalloyed Steels during Hot Deformation

The ductility behaviour experienced by steels for linepipe (LP) applications in a temperature range generally from 700°C to 1200°C is a widely studied subject in steel research, especially for its implication on cracking during continuous casting and rolling. Hot tensile / torsion tests on as-cast products, conducted until fracture, are normally used to characterise the hot deformability behaviour. Depending on the industrial hot deformation process within which the steel aptitude is being investigated, other types of tests can be more adequate. With the aim to characterize the hot deformability behaviour in terms of the damages each steel presents at moderate strain levels (i.e. far from the onset of necking), a special device for interrupted hot tensile tests, followed by immediate quenching (i.e. to “freeze” the microstructure) was developed. Various industrial steels with different starting microstructures (ad hoc in-lab heat treatments performed before testing) were tested by this method, and subsequent metallurgical investigations of the strained samples were carried out to identify, for each case, the damage mechanism and the microstructure features having the major influence on ductility loss. As a result, it was found that (i) microstructural damages at moderate strain levels can be much better described throughout interrupted hot tensile tests, (ii) different compositions and starting microstructures within the industrial LP scenario lead always to microstructural damages at relatively high deformation temperatures (e.g. 950°C) and moderate strains (e.g. 0.1 to 0.2, very far from the onset of necking), (iii) the common mechanism by which LP steels start voiding is the grain boundary sliding and (iv) the intergranular voids, once formed, grow longer in coarser microstructures.

Investigation on Uniaxial Tensile Instability of USIBOR1500 Steel Sheets at Elevated Temperature

In the sheet forming process with stretch, diffuse instability and localized instability usually occur in the process one after another. The necking rate has a great impact on the instability process and the forming limits of the rate sensitive material, such as USIBOR1500 steel sheets at elevated temperature. The available reports about this steel mainly focus on hot uniaxial tensile, martensitic transformation and forming process, but there are few investigations on instability behavior and forming limits. Based on Inoue Kachiro's flow stress constitutive model at elevated temperature, combined with Swift's diffuse instability rule and Hill's localized instability rule, the relation is theoretically deduced between the diffuse necking rate along with the localized necking rate and the exponent of strain rate sensitivity together with the strain rate in the case of the USIBOR1500 steel sheets at elevated temperature. According to the time-temperature characteristics of the hot stamping process, tensile tests of the steel sheets were carried out on Gleeble3800, and the stress-strain curves were obtained at different temperatures and different strain rates. Then the values of the exponent of strain rate sensitivity and the hardening exponent were obtained through fitting the curves by least squares. The tests also helped to provide the distribution laws of the major strain of the specimens and the uniform strains as well as the necking width. The uniform strains obtained from the tests are matched well with the theoretical calculations.

Effect of calcium content on the microstructural evolution and mechanical properties of wrought Mg–3Al–1Zn alloy

The effect of Ca element on the hot workability and microstructure evolution of AZ31 magnesium alloy was investigated. Conventional AZ31, AZ31 + 0.7 wt.%Ca, and AZ31 + 2.0 wt.%Ca alloys were tested. Electron back scattered diffraction microscopy revealed that the alloys containing Ca exhibited much finer and more homogeneous microstructure than the conventional AZ31 alloy. Hot compression and tensile tests showed that the Ca element generally increased flow strength and decreased ductility at low temperature. High temperature elongation was considerably improved by the operation of the thermally activated process. TEM work suggested that the large volume fraction of fine (Mg, Al) 2Ca particles played an important role in preventing significant grain growth of the AZ31 + 2.0Ca alloy.

Hot ductility and microstructure in casted 2205 duplex stainless steels

The effects of the warm parameters (the strain rate and forming temperature) on the deformation behavior and microstructure in 2205 duplex stainless steels (DSS) was studied. The hot deformation behavior of 2205 DSS was investigated through hot tensile and hot compression testing on a Gleeble-1500 thermal-mechanical simulator within the temperature range of 1023–1623 K, in order to determine their reduction of area, resistance to deformation, deformability, and microstructure. The experimental results show that the hot deformation curves of the test steel has a high temperature brittlement region at temperatures higher than 1523 K, and the optimum hot ductility region is in the temperature range of 1423–1523 K. The microstructure of the steel obtained after hot deformation is much finer compared with the as-cast microstructure of the steel. It was found that dynamic recovery only occurs within the γ-phase during hot deformation, but the δ phase undergoes dynamic recrystallization. Small secondary γ′ phase islands are precipitated on the δ matrix, as demonstrated by TEM analysis, which can markedly promote hot ductility behavior.

Research into hot ductility of high aluminium dual phase steel

Hot tensile tests of two dual phase steels with different aluminium contents were performed using a Gleeble 2000 thermal simulator. The effect of Al on hot ductility is discussed. The experiment results indicated that the filmy and meshy ferrite which generated along the austenite grain boundary during the transformation from austenite to ferrite were the main cause of the decrease in hot ductility. The start transformation temperature from austenite to ferrite occurred at a higher temperature with the addition of Al increased from 0⋅03 to 0⋅9%. As a result the ductility trough shifted to a higher temperature range 800–1020°C, compared to 710–920°C for the 0⋅03%Al steel.

Hot tensile characterization of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy with an equiaxed microstructure

Hot tensile tests were performed on Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy with an equiaxed microstructure in the temperature range of 900–980 °C with a constant strain rate of 0.001 s −1. The tests results show that the maximum elongation reaches 400% at 960 °C, for Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy with equiaxed grains of about 9 μm. The true stress–true strain curves supplemented with microstructural investigations indicated the identification of mechanisms such as grain boundary sliding, dynamic grain growth and dynamic phase transformation. Dynamic grain growth and dynamic phase transformation led to the increases of grain size and β-phase volume fraction.

Effect of Al and C on structure and mechanical properties of Fe–Al–C alloys

Effect of aluminium and carbon content on the microstructure and mechanical properties of Fe–Al–C alloys has been investigated. Alloys were prepared by combination of air induction melting with flux cover (AIMFC) and electroslag remelting (ESR). The ESR ingots were hot forged and hot rolled at 1373 K. As rolled alloys were examined using optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to understand the microstructure of these alloys. The ternary Fe–Al–C alloys containing 10·5 and 13 wt-%Al showed the presence of three phases: FeAl with disordered bcc structure, Fe3Al with ordered DO3 structure and Fe3AlC0·5 precipitates with L′12 structure. Addition of high concentration of carbon to these alloys resulted in excellent hot workability and superior tensile at room temperature as well as tensile and creep properties at 873 K. An increase in Al content from 9 to 13 wt-% in Fe–Al–C alloys containing the same levels of carbon has no significant influence on strength and creep properties at 873 K, however resulted in significant improvement in room temperature strength accompanied by a reduction in room temperature ductility.

High Temperature Mechanical Properties of Thermal Barrier Coated Superalloy Applied to Combustor Liner of Aero Engines

High temperature load controlled fatigue, hot tensile and accelerated creep properties of thermal barrier coated (TBC) Superni C263 alloy used as a candidate material in combustor liner of aero engines are highlighted in this paper. Acoustic emission technique has been utilised to characterise the ductile-brittle transition temperature of the bond coat. Results revealed that the DBTT (ductile to brittle transition temperature) of this bond coat is around 923 K, which is in close proximity to the value reported for CoCrAlY type of bond coat. Finite element technique, used for analysing the equivalent stresses in the bond coat well within the elastic limit, revealed the highest order of equivalent stress at 1073 K as the bond coat is ductile above 923 K. The endurance limit in fatigue and the life of TBC coated composite under accelerated creep conditions are substantially higher than those of the substrate material. Fractographic features at high stresses under fatigue showed intergranular cleavage whereas those at low stresses were transgranular and ductile in nature. Delamination of the bond coat and spallation of the TBC at high stresses during fatigue was evident. Unlike in the case of fatigue, the mode of fracture in the substrate at very high stresses was transgranular whereas that at low stresses was intergranular in creep.

Investigation on the thermoformability of polyolefin blends by hot tensile and rheological tests

Due to desirable performance/cost properties, polypropylene (PP) is a suitable alternative for acrylonitrile butadiene styrene, polyvinyl chloride and polystyrene in packaging and automotive thermoformed articles. Because of low elasticity and poor melt strength, thermoforming of PP is difficult. In this study, the effect of blending of PP with high-density polyethylene (HDPE) and ethylene propylene diene monomer (EPDM) is presented. Ternary blends of PP/HDPE/EPDM were prepared using a laboratory co-rotating twin-screw extruder. Three levels of HDPE (10, 20, 30 phr) and EPDM (5, 10, 15 phr) and 10 phr of calcium carbonate were incorporated into the PP matrix. Thermoformability of the samples was investigated by hot tensile and rheological tests. The effects of blending and processing conditions, such as sheet temperature and strain rate, were studied. The stress–strain behavior showed that the ternary blending improved thermoformability of PP. In all blends, elastic modulus, load at yield and the post yield modulus decreased in comparison with original PP. Increasing temperature and crosshead speed (strain rate) had significant effects on the hot tensile properties. Rheological properties of the blends were studied using a rheometer. It was found that blends had higher elastic share and lower tan δ, hence more elasticity relative to conventional PP. All of the results confirmed that blending with HDPE and EPDM improved the thermoformability of PP.

The relationship between hot ductility and intergranular fracture in a CuSn6P alloy at elevated temperatures

The hot ductility of tin α-bronzes has been studied by hot tensile and hot torsion tests, microscopic observations of grain boundary sliding, fracture surface observations and Auger's spectrometric technique. It has been found that the minimum plasticity of the tested alloy observed as a function of deformation temperature is directly connected with grain boundary sliding and the segregation of tin atoms and impurities, mainly sulphur, at the grain boundary. The complex effect of these mechanisms determines the intergranular fracture of CuSn6P grade bronze within the temperature range of minimum plasticity (DMT).

Health Assessment of Radiant Heater Tube

A radiant heater tube in an oil refinery made of a particular grade of austenitic steel and service exposed for 61,000 hours was investigated for health assessment and safety. Experimentally determined yield strength and ultimate tensile strength as well as estimated 10,000 hours - 100,000 hours rupture strength as obtained from experimental data in the temperature range of 773 to 873K exhibit a decreasing trend with increasing temperature. Microstructural study did not reveal any significant degradation in terms of creep cavities, cracks, graphitization etc. Analysis of hot tensile and accelerated stress rupture data revealed that the service exposed radiant heater tube could remain in service for a length of more than ten years at the operating hoop stress level and maximum allowable skin temperature (158.78 MPa / 865 K), provided no localised damage in the form of cracks or dents had developed. It is recommended that a similar health check of these tubes should be carried out after 50,000 hours of service exposure at the desired operating conditions.

Hot tensile and fatigue behaviour of zinc–aluminum alloys produced by gravity and squeeze casting

The tensile and fatigue properties of zinc–aluminum alloys (ZA-8, ZA-12 and ZA-27) in squeeze and gravity cast forms have been investigated. Tensile tests were conducted at ambient and elevated temperatures up to 150 °C. At low temperatures, the ultimate tensile strength and yielding strength of the squeeze cast alloys have been found to be superior those of the gravity-cast alloys, as the temperature increased they decreased. In the same way, Brinell hardness of the squeeze cast alloys were obtained at higher values than gravity castings. The fatigue tests were performed at a constant speed of 400 rev/min and under a number of stress levels ranging from 100 to 150 MPa. The fatigue behaviour results of the ZA alloys were similar to obtained from the tensile testing. The squeeze cast alloys exhibited good fatigue resistance in proportion to the gravity castings. Metallography examinations showed that the microstructure of the castings differed according to the method of casting used. It was considered that the mechanical properties of the alloys were affected from these micro-structural changes.

Microstructure and modeling of the high temperature deformation behavior of TBC-coated superalloys

The present study describes the application of hot deformation experiments and microstructure investigations in order to predict the deformation behaviour of uncoated and TBC-coated superalloys NiCr22Co12Mo9 and CoCr22Ni22W14. Estimating parameters like dislocation density and carbide spacing by transmission electron microscopy (TEM), the hot tensile, creep and relaxation behaviour has been connected with the evolution of microstructure. The results of mechanical tests and the TEM-investigations have been used as input data into two models (an effective stress model and a modified constitutive model by Kocks–Mecking), which describe the deformation behavior. The deterioration in the creep properties of the alloy NiCr22Co12Mo9 as a result of coating was a result both of the degraded state of the M 23C 6-precipitate in the substrate metal, and of the weakening of the solid solution- and precipitation hardening mechanisms responsible for the creep strength of the material, in the region of the boundary surface.

Numerical and Experimental Studies of 3-Dimensional Thermoforming Process

Two Acrylonitrile-Butadiene-Styrene (ABS) polymers were thermoformed, and their behavior was compared with numerical simulation. Hot tensile and dynamic oscillatory shear tests were performed at various temperatures to characterize the polymers. Hot tensile test results were used to obtain the material parameters for simulation and to check the relative usefulness of these test parameters in the actual thermoforming process. The thickness distribution obtained from experiments was compared with simulation results. It shows that simulation results based on hyperelastic rubber like model can predict the deformation behavior of a sheet reasonably well. We could also find that the temperature sensitive polymer in hot tensile test shows more temperature sensitive thickness distribution in actual thermoforming process.

Extension of a constitutive model describing the inelastic behaviour of a precipitation hardened alloy

The one intemal variable model by Kocks, Mecking and Estrin has been applied for the description of the hot tensile and creep behaviour under constant loading conditions of Alloy 800H, a carbide precipitation strengthened high temperature alloy. The model was extended by incorporating an expression for a mean free path of mobile dislocations which depends on the kinetics of the precipitation and ripening process of the secondary carbides. Microscopy and mechanical data for three different heats of Alloy 800H at 900°C were evaluated to enable comparison between calculations using the extended model and experimental data. Difficulties were caused by the complex precipitation behaviour of the alloy in which both larger carbides of type M23C6 and smaller ones of type M(C,N) are formed in the investigated range of temperature. Although the microstructural data available is not sufficient for a reliable quantitative description of the evolution with time of the average carbide spacing a qualitative agreement between calculations and mechanical data gives a preliminary verification of the developed extended model.

Effects of various homogenisation treatments on the hot workability of ingot aluminium alloy AA2014

Abstract AA2014 aluminium alloy has high strength but very low ductility, hence low workability. In order to improve the hot workability of this ingot aluminium alloy, different homogenisation conditions have been tested, such as 400, 440, 480, 500 and 510°C for 4, 8, 16 and 24 h, followed by various cooling and strain rates. The effect of homogenisation temperature, time and cooling rate on the workability has been investigated. Hot tensile and torsion tests have been carried out to show the harmony between each other and determine hot workability behaviour. From the experimental work, the data have been converted into equivalent true stress–strain curves. It was observed that the optimum hot ductility for the alloy occurs at the strain rate of 1×10−3 s−1 for the specimens that were homogenised at 500°C for 24 h, then cooled in the furnace at a rate of 12 K h−1.

Crack susceptibility of medium and high alloyed tool steels under continuous casting conditions

Hot tensile tests after partial melting of the specimens were carried out on medium and high alloyed tool steels and their mechanical properties (maximum force and reduction of area) were determined in the temperature range between liquidus and 900°C. Cooling conditions and strain rates were varied in this range. The temperature ranges of reduced ductility were determined, the metallurgical reasons for embrittlement were investigated by means of metallographic and microprobe analysis. The crack susceptibility is related to an enrich‐ment of carbon and carbide forming elements in cold and hot working steels, and to the high S content in the segregated zone of the laminate forming steels. The crack susceptibility can be minimized by low strain rates combined with a strong secondary cooling.

An Integrated Model for Microstructural Evolution in the Hot Strip Mill and Tensile Properties Prediction of Plain and Microalloyed C-Mn Hot Strip

A model has been developed, which computes the microstructural evolution of austenite, phase transformation, final microstructure and tensile properties (YS, TS, El) of plain carbon and microalloyed hot strip steels. This paper describes the principal features of the model and compares the computed and observed tensile properties on a database of hot strip steels extracted from the normal production of the hot strip mill of Sollac Fos.