Metallographic Observations Research Articles

Damage behaviors in microstructures and mechanical properties of pure tungsten induced by repetitive thermal loads

Damage behaviors of ITER grade tungsten (W) induced by repetitive thermal loads have been investigated using an electron beam device EBMP-30. The metallographic observation, backscattered electron scanning electron microscopy, electron backscattered diffraction, atomic force microscopy analysis and tensile tests were carried out after the repetitive heat loads. Results indicate that there are negligible effects on the microstructures, surface morphologies and mechanical properties with the absorbed power densities (APD) lower than 10 MW/m 2 . While serious damages including the severe surface roughness with protruding structures along grain boundaries, and significant grains coarsening with the evolution of the grain shape occur undesirably due to the full recrystallization during the repetitive 30 MW/m 2 heat loads. Furthermore, after exposed to an APD of 30 MW/m 2 with 50 heat loads, the tensile properties deteriorate dramatically, resulting in a limited ultimate tensile strength of only 328 MPa and even a zero total elongation at 300 °C. The relationships between the thermal loads, microstructures evolution and degradation of mechanical properties have been analyzed.

Results of metallographic observations of cultivator shares after spot electromechanical processing

Presented are the results of metallographic studies of the structure and properties of a cultivator share, hardened by spot electromechanical processing in its various high-wear areas. In the process of research, the manufacturer was offered recommendations on the technology of forming parts in the process of their manufacture. It has been established that spot electromechanical processing allows to obtain hemispherical hardened areas with a hardness of HV 7000 ... 7600 MPa, which will provide the effect of self-sharpening of their cutting parts during the operation of the cultivator shares. Spot electromechanical processing with two tools allows to provide through hardening of the cultivator share along the ends of the share wings. The proposed technology for hardening cultivator shares during their manufacture and repair will increase the durability of these parts during their operation.

Effect of high-frequency PCB laminate on thermal cycling behavior of electronic packaging structures

PurposeAs the solution to improve fatigue life and mechanical reliability of packaging structure, the material selection in PCB stack-up and partitioning design on PCB to eliminate the electromagnetic interference by keeping all circuit functions separate are suggested to be optimized from the mechanical stress point of view.Design/methodology/approachThe present paper investigated the effect of RO4350B and RT5880 printed circuit board (PCB) laminates on fatigue life of the QFN (quad flat no-lead) packaging structure for high-frequency applications. During accelerated thermal cycling between −50 °C and 100 °C, the mismatched coefficients of thermal expansion (CTE) between packaging and PCB materials, initial PCB warping deformation and locally concentrated stress states significantly affected the fatigue life of the packaging structure. The intermetallics layer and mechanical strength of solder joints were examined to ensure the satisfactorily soldering quality prior to the thermal cycling process. The failure mechanism was investigated by the metallographic observations using a scanning electron microscope.FindingsTypical fatigue behavior was revealed by grain coarsening due to cyclic stress, while at critical locations of packaging structures, the crack propagations were confirmed to be accompanied with coarsened grains by dye penetration tests. It is confirmed that the cyclic stress induced fatigue deformation is dominant in the deformation history of both PCB laminates. Due to the greater CTE differences in the RT5880 PCB laminate with those of the packaging materials, the thermally induced strains among different layered materials were more mismatched and led to the initiation and propagation of fatigue cracks in solder joints subjected to more severe stress states.Originality/valueIn addition to the electrical insulation and thermal dissipation, electronic packaging structures play a key role in mechanical connections between IC chips and PCB.

Failure Analysis Report on Fractured Torsion Bar in Suspension of a Vehicle

Accidents caused by fractured torsion bars in off-road vehicles (ORVs) have occurred during road tests. Through visual inspection of the fracture in the failed samples, the preliminary conclusion is that the typical torsional fracture occurs according to its ratchet shape and 45-degree cracking direction to the bar axis. The crack initiated at the side surface of the fillet on the spline end. All failed parts during road tests have identical fracture modes and crack initiation sites, which implies that they have identical failure reasons. Techniques such as scanning electron microscopy (SEM), metallographic observation, hardness test and spectral component analysis have been performed to obtain detailed information to diagnose the cause of failure. The result reveals that the fracture was caused by fatigue cracking initiated from the spline fillet. In addition, SEM images of the intergranular feature of the fracture initiation zone demonstrate that the untimely formation of a fatigue core and very quick expansion of the crack directly result from over-brittleness. This behavior can be ascribed to insufficient tempering after the quenching process, which was confirmed by a tempering simulation experiment: after the tempering experiment, the artificial struck fracture of the sample became a 100% dimple fracture, and the hardness distribution became more uniform and reasonable. Thus, the brittleness was eliminated, and the strength was ensured after an expanding tempering time. This investigation can provide reference value for solving and preventing such types of failures.

Solidification structure simulation and casting process optimization of GCr15 bloom alloy

Based on the solidification heat transfer model and CAFÉ model, the solidification structure of GCr15 bloom alloy was studied. Using nail shooting and acid etching experiments, the solidification models were verified. The secondary dendrite arm spacing (SDAS) model of GCr15 was obtained by simulation calculation and metallographic observation. With the increase of casting speed, the SDAS, equiaxed crystal ratio (ECR) and average grain size increase. With the rise of superheat, the SDAS increases in the 20–70 mm of thickness and decreases in the 80–160 mm of thickness. The ECR decreases and the average grain size increases with the increase of superheat. With the increase of specific water flow, both the SDAS and ECR decrease. The minimum average grain size is obtained when the specific water flow is 0.20 L·kg-1. The central carbon segregation index is reduced from 1.11 to 1.075.

First-principles study on yttrium inclusions in micro alloyed steels

The removal and control of inclusions has always been a hot issue in the field of metallurgy, and rare earth modifying is a proven method. However, some rare earth inclusions cannot be accurately judged due to the necessary assumptions for thermodynamic calculations and the lack of certain thermodynamic data. On the other hand, most of the currently reported researches focus on the changes in the morphology of the inclusions, and rarely involve the inherent properties of the inclusions. To address these two deficiencies, this work investigated elaborately the stability, elastic modulus, elastic anisotropy and thermal expansion coefficient of yttrium (Y) inclusions in micro alloyed steels by first-principles calculation. Then, the role of yttrium inclusion in the initial stage of crack initiation was discussed. The results showed that it was practical to judge the possibility of unknown inclusions formation in steel by formation enthalpy. The inclusions were changed from Al2O3 to Y2O3, Y2O2S, YAlO3-Y2Si2O7 composite inclusions with yttrium treatment, which was consistent with the metallographic observations. The bulk modulus (B), shear modulus (G), Young’s modulus (E), and Poisson’s ratio (σ) were calculated in a framework of the Voigte-Reusse-Hill approximation. In light of B/G and σ values, Al2O3 showed apparent brittleness, and the toughness of yttrium inclusions were improved to varying degrees compared to Al2O3 inclusions. Moreover, the order of the degree of elastic anisotropy for these inclusions was Y2Si2O7 > Al2O3 > Y2O3 > Y2O2S > YAlO3. Furthermore, the thermal expansion coefficient of Y2O3, Y2O2S, YAlO3 inclusions at any temperature were greater than that of Al2O3 and were relatively close to that of Fe, whereas, Y2Si2O7 inclusion was much larger than that of Fe. Finally, the difference between yttrium inclusions and iron matrix in the aspect of incompressibility, brittleness, toughness, mechanical anisotropy, and thermal expansion was reduced significantly, thereby improving the consistency of the matrix, this could be used to explain the phenomenon of alleviating stresses concentration and delaying the formation of micro voids.

Study on MnS Inclusion Aggregation along Continuous Casting Slab Thickness of Medium Carbon Structural Steel

Precipitation of MnS inclusions in steel affects the mechanical properties of the material significantly. The evolution of MnS inclusions along the continuous casting slab thickness and its influencing factors has not been clearly established and comprehensively studied. In this paper, solidification macrostructure, sulfur segregation and MnS inclusions in the continuous casting slab of medium carbon structural steel 45# were studied by various methods, including the metallographic observations, elemental analysis, scanning electron microscope (SEM) with Energy Dispersive Spectrometer (EDS) observation, automatic particle analysis, and thermodynamic calculations. The 2D/3D morphologies of MnS inclusions suggest that the sulfides turn from globular to rodlike, and further to dendritic shape along the slab thickness progressively. Furthermore, it was found that MnS inclusions are remarkably aggregated in the columnar crystals and the equiaxed crystals mixed zone, where the sulfides have the largest average diameter of 6.35 μm and the second maximum area fraction of 0.025% along the slab thickness. In order to reveal the mechanism of this phenomenon, the precipitation temperature of MnS inclusion in the 45# steel was clarified by thermodynamic calculation and experimental observation, and the quantitative relationships among the distribution of sulfur content, secondary dendrite arm spacing (SDAS), and precipitation area fraction of MnS inclusions were discussed. Moreover, the inclusion size was numerically predicted to compare with the measured value. The results indicate that the large SDAS, high sulfur content and low cooling rate accounting for the large-size aggregated MnS inclusions in the mixed zone. Unfortunately, the dendritic MnS inclusions, even if the average diameter exceeds 52 μm, can act as the nucleation sites for ferrites, and the distribution of the sulfides promotes uneven microstructure in the steel.

Microstructural Evaluation of the High-Frequency Induction Welded Joints of Low Carbon Steel Pipes

Abstract The work presents the results of research on the structure of welded joints in the area of heat affected zone (HAZ). Based on precisely performed metallographic tests, the contribution of individual structural components in the area of welds of pipes welded with the induction method was assessed. The volume fraction of individual structural components in various areas of the heat affected zone, the size of the grain formed in the welding process, as well as its shape coefficients were determined. On the basis of metallographic observations, an attempt was made to describe the course of the pressure induction welding process, taking into account the structural changes, phase changes and the recovering and recrystallization processes taking place in this process.

Usage of Barkhausen Noise for Assessment of Corrosion Damage on Different Low Alloyed Steels

This study deals with corrosion damage of low alloyed feritic steels of variable strength. Three different steels of nominal yield strength 235, 700 and 1100 MPa were subjected to the variable degree of corrosion attack developed in the corrosion chamber under a neutral salt spray (NSS) atmosphere. The corrosion damage was investigated by the use of conventional metallographic observations when the thickness of corroded layer was quantified. Moreover, non-destructive magnetic technique, based on Barkhausen noise, was also employed. It was found that the rate of corrosion damage decreases along with the increasing number of days in the chamber. The similar evolution can be also found for Barkhausen noise emission and the extracted parameters from the emission. It can be reported that conventional rms value of Barkhausen noise signal as well as FWHM (full width at half maximum of Barkhausen noise envelope) can be linked with the corrosion extent, especially in the early phases of corrosion attack. The PP (peak position of Barkhausen noise envelope) values exhibit poor sensitivity.

Effects of rolling reduction on microstructural evolution and mechanical properties of W-0.5wt%ZrC alloys

The effects of hot rolling reduction on microstructures and mechanical properties of W-0.5 wt%ZrC (WZC) materials were investigated. Four WZC specimens with rolling reductions of 46%, 69%, 77%, and 96% were prepared by multi-step rolling processes. Metallographic observation, SEM, EBSD, and TEM characterization were conducted to obtain information on the microstructures of the materials. The metallography and EBSD results indicate that W grains were refined to 0.4–0.5 μm in the 96%-rolled WZC01 specimen. In addition, Vickers microhardness measurements at room temperature (RT) and tensile tests at various temperatures from RT to 500 °C were conducted. The results indicate that hardness and strength increase with the increase in rolling reduction. The 96%-rolled 1-mm thick WZC plate exhibits an ultra-high tensile strength of 1450 MPa at 100 °C. The 77%-rolled 6-mm thick WZC bulk material demonstrates both high tensile strength with an ultimate tensile strength of 620 MPa and good ductility with a tensile elongation of 21% at 400 °C, which are superior to those of most reported bulk W materials. The relationships between microstructure, mechanical properties, and rolling reduction were investigated and discussed systematically.

Evaluation of bulk defects area in friction stir welded 1080 aluminum alloy utilizing electrical resistivity measurements

In this study, the electrical resistivity measurements as an accessible technique for NDT inspection of FSWed sections were investigated. For this purpose, 1080 Aluminum plates were welded by the FSW technique. The welding parameters were optimized using the Taguchi L9 orthogonal design of experiments. The rotational speed, tool shape, tilt angle, and welding speed were taken into consideration as welding parameters. The soundness of welds was inspected by visual tests, radiography, and metallography observations. The electrical resistivity of welded sections was compared with the area of bulk defects. It revealed that there is a linear relationship between the electrical resistivity and the area of bulk defects. The acquired intercept of the linear equation was introduced as a reference point. The results showed that the area of bulk defects could be predicted by an equation with good accuracy, and the joints with lower electrical resistivity than the reference point could be considered as a sound weld.

Effects of Specimen Size and Welded Joints on the Very High Cycle Fatigue Properties of Compressor Blade Steel KMN-I

The effects of specimen size and welded joints on the very high cycle fatigue properties of compressor blade steel KMN-I were studied by ultrasonic fatigue testing. It was found that the S-N curve of large specimens had a slow decline above 107 cycles, and fatigue failure still occurred in the very high cycle regime (>107 cycles), while the very high cycle fatigue characteristics of welded specimens was less obvious, and the fatigue limit was observed. Metallographic observation and SEM analysis were carried out on the fracture of the specimens. The results showed that surface fractures were mostly observed in the large specimens, and only a small number of cracks initiated from non-metallic inclusions above 107 cycles. The cracks of welded specimens initiated from the surface below 107 cycles and initiated from the internal matrix above 107 cycles. In addition, the formation mechanism of GBF (granular bright facet) was analyzed by the “dispersive decohesion of spherical carbide” theory, and the fatigue strength and fatigue life were predicted, which was consistent with the experimental results.

EFFECT OF COOLING RATE ON MICROSTRUCTURE AND HARDNESS IN GRAY CAST IRON CASTING PROCESS

This study aims to investigate the thermal conditions of the molds, changes in microstructure and hardness of casting products using sand mold and permanent mold. The use of sand mold and permanent mold results in different cooling rates. Thermal analysis was performed using a thermocouple to obtain a temperature versus time curve. Metallographic observations were carried out using a Scanning Electron Microscope equipped with Energy-Dispersive X-ray Spectroscopy. The Vickers hardness test was carried out in three areas with different thicknesses. The results showed a constant temperature at 691 oC where the eutectoid phase reaction occurred. Testing with sand mold showed that cast iron with flake graphite was finer and spreader than graphite in cast iron produced by permanent mold. Meanwhile, gray cast iron from a casting process with a permanent mold has a higher hardness than gray cast iron from a casting process using a sand mold.

Cracking Failure Analysis of Pipe to Flange Weld Joint

Cracking failures occurred frequently in blowdown pipe to flange weld joint, and the root crack appeared on the blowdown pipe side of the weld joint and extend into the base metal of pipe. To explore the reason why cracking failures occurred on the blowdown pipe side of the weld joint, the weld joint was analyzed via macroscopic and metallographic structure observation, and blowdown pipe was tested by physical and chemical property testing, and corrosion scale was analyzed by SEM, EDS, and XRD. The dent and thinner wall thickness of pipe are found in the internal pipe to flange weld joint. Widmanstatten metallographic structure is found in heat affected zone (HAZ), fusion zone (FZ) and weld metal zone (WZ) of weld joint. Stress concentration form in the weld joint at blowdown pipe side during welding due to the dent and different wall thickness. Under the action of gas vortex in the dent of weld joint, corrosive droplets aggravate the localized corrosion of the weld joint. Stress corrosion cracking (SCC) were initiated in FZ and WZ from the surface of weld joint and propagated into the base metal of pipe under the combined actions of stress resulting from weld process and during operation, and corrosion.

Analysis on the interference assembly of camshaft with knurled tube and cam

Interference joint is one of the most advanced assembly methods for camshaft. The joining force in the assembly process and the connection strength after assembly are the key aspects of the camshaft assembly system. In this paper, the mechanism of joining force and connection strength are analyzed from two aspects of elasticity and plasticity using thick-wall cylinder model. Joining and torsion experiments are done to determine the joining force and connection strength. Joining forces fluctuating increase in the joining process, which is composed by friction force Fs and shearing force Ff. Fs is 0.87 KN, and Ff is 6.23 KN when the assembly interference is 0.15mm. Joining force linear increases with the interference, and torque exponential increases with it. The empirical formulas between the torque capacity, joining force, and interference of the camshaft are provided by the experiment results. The assembly interference limit of 0.3mm is estimated for the case of this paper. Metallographic observation reveals the changes of metal flow line of the knurled tube. The top of knurled tooth turns over and an inverted triangle formed after joining process.

Effect of powder particle shape and size distributions on the properties of low-viscosity iron-based feedstocks used in low-pressure powder injection moulding

ABSTRACT Low-pressure powder injection moulding (LPIM) is a cost-effective manufacturing technology used to fabricate complex-shaped parts with high mechanical properties at low- or high-volume production. This research work presents an experimental approach to investigate the debound/sintered properties of iron components produced by the LPIM process using iron-based powders exhibiting different particle shapes and size distributions. Four low-viscosity feedstocks were mixed and injected into a rectangular mould cavity before being thermally wick-debound and finally sintered using identical debinding and sintering cycles. This study confirms that both irregular and spherical iron powders can be shaped via the LPIM process. The solid loadings obtained with these two powder morphologies, varying from 58 to 62 vol.-%, represent expected values. The trend in the density values obtained with irregular (∼6.6 g/cm3) and spherical (∼7.5 g/cm3) powders was validated by metallographic observations.

Failure analysis of a commercially pure titanium tube in an air conditioner condenser

Purpose The purpose of this paper is to determine the failure reasons and failure mechanism of the commercially pure titanium air conditioning condenser. Design/methodology/approach In this paper, chemical analysis, metallographic observation, visual examination and scanning electron microscope examination, corrosion products analysis and working conditions analysis were adopted for determining the reasons for the failure of the condenser. Findings The results indicated that TA2 titanium pipe perforation failure is caused by the synergistic effect of crevice corrosion and deposit corrosion. The corrosion processes can be divided into three steps. Originality/value This research is an originality study on the failure case of a commercially pure titanium air conditioning condenser. This study makes up for the shortage of titanium alloy failure cases and also gives the result of the failure reason and failure mechanism for titanium, which has an engineering significance.

ANALYSIS AND THERMODYNAMIC STABILITY OF NUCLEI IN SPHEROIDAL GRAPHITE IN Fe–C–Si ALLOYS

The paper describes the graphite nuclei constitution for spheroidal graphite cast iron melted in a cupola furnace, which is treated by the addition of magnesium and inoculated with a barium-based inoculant. Two samples of spheroidal cast iron were analysed, differing only in tin content. Field-emission scanning electron microscopy (FE -SEM) with energy-dispersive X-ray spectroscopy (EDS) was used to analyse the nuclei. The thermodynamic calculation of the phase equilibria and the associated free formation energies of the alloys were calculated and compared with metallographic observations. It was found that the nuclei in the spheroidal graphite are different in shape and composition. Spherical and rectangular ones were found, and in many cases the porosity was present at the nuclei. The nuclei consisted of different compounds such as (Mg,Ca)S, MgO, (Mg,Al,Si)N. The amount of Sn only affected the pearlite content, and there were no Ba and rare earths present in the nuclei.

Effects of Zr element on the microstructure and mechanical properties of B4C/Al composites fabricated by melt stirring method

In this study, the addition of Zr element (K2ZrF6) was employed to improve the wettability, inhibit the serious chemical reaction of B4C to Al, and uniformly disperse the B4C particles in the Al melt, during the fabrication of B4C/Al composites via melt stirring method. And the particle distribution, interface microstructure and mechanical properties of the fabricated composites were further investigated. The metallography observation shows that the B4C particles could be effectively introduced to the Al melt with the addition of Zr element, while there were few B4C particles observed in the composites without Zr addition. The XRD analysis indicates that the ZrB2 and ZrC peaks arise with the addition of Zr, where the ZrB2 and ZrC have better chemical stabilization and wettability than the B4C ceramic particles. The SEM, EDS, TEM and SAED analysis shows that the ZrB2 and ZrC interface layer with a thickness of about 200–350 nm on the B4C surface was formed, which could avoid the serious chemical reaction and improve the wettability of B4C to Al, simultaneously. In addition, the easy introduction of B4C particles to the Al melt with K2ZrF6, the formation mechanism of ZrB2 and ZrC layer, and mechanical properties of the composites were also discussed.

An innovation in finite element simulation via crystal plasticity assessment of grain morphology effect on sheet metal formability

Experimental and numerical study regarding the uniaxial tensile test and the forming limit diagram are addressed in this paper for AL2024 with the face-centered cube structure. First, representation of a grain structure can be obtained directly by mapping metallographic observations via scanning electron microscopy approach. Artificial grain microstructures produced by Voronoi Tessellation method are employed in the model using VGRAIN software. By resorting to the finite element software (ABAQUS) capabilities, the constitutive equations of the crystal plasticity were utilized and implemented as a user subroutine material UMAT code. The hardening parameters were calibrated by a trial and error approach in order to fit experimental tensile results with the simulation. Then the effect of the changing grain size, the heterogeneity factor, and the grain aspect ratio were studied for a uniaxial tensile test to emphasize the importance of the microstudy behavior of grains in material behavior. Furthermore, the polycrystal plasticity grain distribution was employed in the Nakazima test in order to obtain the forming limit diagram. The crystal plasticity-driven forming limit diagram reveals more accurate strains, taking into account the involving the micromechanical features of the grains. An innovative approach is pursued in this study to discover the necking angle, both in tensile test or Nakazima samples, showing a good agreement with the experiment results.