Metallographic Observations Research Articles

Leakage failure analysis on water wall pipes of an ultra-supercritical boiler

Water wall pipe is one of the key components of thermal power plant, and their premature failure is a common appearance. The occurrence of a leakage accident involving water wall pipes in a thermal power plant boiler prompted an analysis of the causes of cracking and leakage. This analysis was conducted using various methods including optical light microscope (OLM) observation, chemical composition testing, hardness testing, metallographic organization observation, and energy dispersive spectroscopy (EDS) analysis. Based on the findings, relevant suggestions were provided to prevent the recurrence of similar accidents. The test results indicate that the proximity of the leaking pipes to the soot blower and manhole door has created a complex stress environment. Also, the inner wall of the pipes displays passivated quenching cracks, while the outer wall exhibits black dense materials in the high temperature oxidation cracks. The fracture surface clearly exhibits a fatigue fracture pattern, with all cracks being transgranular and oriented perpendicular to the pipe wall. Additionally, coarse grains are observed near the outer surface of the cracks and in some areas in the middle of the pipe wall. The root cause of the failure is inferred to be that the passivation quenching cracks were generated due to improper process control during the manufacturing of the pipe, and the quenching cracks were further expanded by the thermal alternating stress, finally penetrated and leaked under the promotion of large stress environment, uneven grain structure and other factors.

Behaviour of high strength steel butt joints exposed to arctic low temperatures

The present study investigated the tensile behaviour of high strength steel (HSS) butt joints exposed to arctic low temperatures. Two HSS grades, namely Q890 and Q960, with thicknesses of 6 mm and 8 mm, were employed. Single-V-groove butt joint specimens were fabricated by means of robotic gas metal arc welding using 1.2 mm-diameter ER100S-G and ER120S-G feedstock wires. The single-V-groove and butt joint tensile specimens were designed according to the Structural Welding Code-Steel AWS D1.1. Metallographic observations and Vickers hardness tests were performed on five series of HSS butt joint. Moreover, a total of 25 butt joint specimens were tested under uniaxial tension in a liquid nitrogen cooling chamber, with temperatures ranging from -75 °C to 25 °C. The stress-strain histories, tensile strengths and failure modes of the HSS butt joints exposed to arctic low temperatures are fully reported. The influence of low temperature exposure, parent steel grade and weld material selection on the tensile behaviour of the HSS butt joints is discussed. Based on the test results, recommendations for weld material matching for Q890 and Q960 HSS butt joints are provided. Additionally, prediction equations for HSS butt joint strengths exposed to arctic low temperatures are proposed using the best subset regression analysis. The findings from this study contribute to a better understanding of the behaviour of HSS butt joints under arctic low temperatures, and can be used to inform design and fabrication practices of HSS structures in cold environments.

Multi-Objective Optimization of The Low-Pressure Casting of Large-Size Aluminum Alloy Wheels through a Systematic Optimization Idea.

The process parameters in the low-pressure casting of large-size aluminum alloy wheels are systematically optimized in this work using numerical casting simulation, response surface methodology (RSM), and genetic algorithm (NSGA-II). A nonlinear input-output relationship was established based on the Box-Behnken experimental design (BBD) for the crucial casting parameters (pouring temperature, mold temperature, holding pressure, holding time), and response indicators (defect volume fraction, spokes large plane mean secondary dendrite spacing (SDAS)), and a mathematical model was developed by regression analysis. The Isight 2017 Design Gateway and NSGA-II algorithm were used to increase the population and look for the best overall solution for the casting parameters. The significance and predictive power of the model were assessed using ANOVA. Casting numerical simulation was used to confirm the best option. To accomplish systematic optimization in its low-pressure casting process, the mold cooling process parameters were adjusted following the local solidification rate. The results showed that the mathematical model was reliable. The optimal solutions were a pouring temperature of 703 °C, mold temperature of 409 °C, holding pressure of 1086 mb, and holding time of 249 s. The mold cooling process was further optimized, and the sequence solidification of the optimal solution was realized under the optimized cooling process. Finally, the wheel hub was manufactured on a trial basis. The X-ray detection, mechanical property analysis, and metallographic observation showed that the wheel hub had no X-ray defects and its mechanical properties were well strengthened. The effectiveness of the system optimization process scheme was verified.

Mechanical properties and microstructure of aluminum alloy 7075-T6 spot-welding joint

In this study, an orthogonal experiment was conducted to investigate the effects of various process parameters on the mechanical properties of resistance spot welding joints in AA7075-T6 aluminum alloy. A numerical simulation was also performed to determine the optimal parameter combination. The mechanical properties, macro-morphology, and microstructure of the joints were characterized and analyzed through a series of tests including tensile testing, super depth-of-field testing, metallographic observation, and microhardness testing. The results show that the welding parameters have a significant effect on the mechanical properties of the joints with the following order of influence: welding time > welding current > electrode pressure. The combination of 60 ms welding time, 17 kA welding current, and 0.22 MPa electrode pressure yielded the best comprehensive mechanical properties with an average tensile load of 2364N and an energy absorption value of 6.8 J. The numerical simulation results are consistent with the nugget size of this parameter combination. During spot welding, spatter defect occurs, shrinkage cavity exists at nugget interface, strengthened T-phase and finer equiaxed grain structure precipitate in nugget center. After welding process and microstructure change, the hardness of the nugget zone is lower than that of the base material while the hardness of the nugget center is higher than that of the edge. The failure mode of the joint is interface fracture.

Effect of Cold and Hot Compounds Forming on Microstructures and Mechanical Properties in the Deformation Zone of Sharp-Edged High-Strength Steel Sections.

We researched the cold and hot composite-forming process, setting the forming speed at 2 m/min, the induction heating frequency at 40 KHz, and the induction current at 3000 A, and manufactured curtain wall steel with sharp corners. We analyzed the microstructure and mechanical properties of the deformation zone of the sharp-edged section using a tensile test, impact test analysis, metallographic observation (OM), fracture morphology observation, and electron backscatter diffraction (EBSD) analysis. The results show that the formed profile had a 96% reduction in the radius of the outer fillet and a 76% increase in the thickness of the corner compared to the pre-formed shape. The tensile strength increased by 3.6%, and the elongation after break increased by 13%. A forming temperature of 850 °C and forming deformation of 70% were determined as the optimum process parameters.

Classification of 3D Casting Models for Product Lifecycle Management and Corporate Sustainability

The purpose of this study was to combine simulations and experiments in order to present the first stage of construction in product lifecycle management. Based on the simplification of casting models, the relationship between the filling and solidification characteristics, casting methods, and geometrical classifications of aluminum alloy precision casting products was investigated. By rearranging and summarizing the data, the casting models could be digitally managed; moreover, the digitized data could be used as the basis for intelligent processes in further developments. The simulations calculated and analyzed the casting speeds, defect locations, material densities, and critical fraction of a solid A356 aluminum–silicon alloy; the actual casting was carried out and samples were taken for metallographic observation to confirm the simulation results. The part model was simplified with four basic geometric shapes: solid cylinder, tubular, block rectangle, and thin-shell rectangle. The 150 casting models were summarized using 37 combinations, which were further classified into five main categories to match the casting method: solid cylindrical, tubular, and thin-shell rectangular for side casting, and discoidal and plate rectangular for bottom casting. File-compression rates of up to 75% were achieved after classification and archiving, and data integrity was maintained. Finally, model training using random forest classification resulted in an 88.8% accuracy when predicting the casting method. This research is based on the practical issues raised by business owners and R&D engineers, and a solution was obtained. From the perspective of product lifecycle management, the results of this study show the consistency and uniformity of product design rules, as well as the reusability of product process planning, which can be integrated with carbon emissions trading and carbon taxes to save energy and achieve corporate sustainability.

Improving the Properties of Gray Cast Iron by Laser Surface Modification.

Laser surface modification is a widely used technology to improve the properties of functional surfaces. In this study, the properties of gray cast iron are modified by laser surface modification, and the influence of laser quenching on the properties of cast iron in terms of frictional vibration and noise, friction and wear, internal structure, residual stress, hardness, and corrosion resistance is investigated. The experimental results show that, after high-power laser quenching, the frictional vibrations and noise of most gray cast iron specimens are decreased, but the coefficients of friction against a bearing steel counterface are increased and more stable. The surface and sub-surface hardness of all laser-quenched cast iron specimens is significantly increased. The residual stresses on the surface of the cast iron specimens are significantly increased and changed from tensile to compressive residual stresses. Experimental modal testing results show that the modal damping ratios of the laser-treated specimens are increased significantly, although their modal frequencies are not significantly changed. In addition, through the metallographic observation, XRD (X-ray diffraction) analysis, and TEM (transmission electron microscopy) observation, it is found that the microstructures of the cast iron specimen after high-power laser modification become fine-grained, and the pearlite and ferrite in the matrix become fine martensite, which leads to the improvement of the dynamical, tribological, and chemical properties of cast iron after laser modification.

Study on the Microstructure and Mechanical Properties of Martensitic Wear-Resistant Steel

In order to improve the overall performance of edge plates such as bulldozer blades, composition and heat treatment processes were optimized on the martensitic wear-resistant steel grade 400 HB. Steel billets were first obtained through smelting in a state of hot rolling, followed by quenching and tempering to obtained wear-resistant steel (HB400). Then, HB400 was subjected to metallographic observation, electron backscatter diffraction (EBSD) testing, and transmission electron microscope (TEM) characterization and property testing. The results showed that HB400 exhibited microstructural refinement, characterized by narrower martensite laths and finer grains. The EBSD results indicated a uniform microstructure with a low content of the residual austenite (0.5%), indicating good hardenability. TEM observation of the martensite matrix revealed the presence of substructures, i.e., numerous dislocations in martensite laths. The average Rockwell hardness (HRC) of HB400 was 46.3, and the average Brinell hardness (HB) was 402. A mechanical properties test demonstrated comprehensive properties, which showed that the ultimate tensile strength and yield strength of HB400 were 1495 MPa and 1345 MPa, respectively, with a relative elongation of 12%. Friction and wear experiments showed that the friction coefficient and wear rate in reciprocating mode decreased by 16.1% and 45.4%, respectively, while in rotating mode, they decreased by 27.6% and 2.1%, respectively, as the load increased from 100N to 300N. According to the wear morphology, the main wear mechanisms were identified as adhesive wear, abrasive wear, and oxidative wear. The lubricating effect of the oxide layer generated by wear was identified as the primary reason for the reduction in the friction coefficient. The relationship between microstructures and properties was discussed based on grain refinement strengthening and dislocation strengthening.

Study on the effect of annealing and solution treatment on the peripheral coarse grain of 2195 alloy

In the actual production process, some aluminum alloys need to go through annealing heat treatment followed by solution treatment; this paper explores the evolution of peripheral coarse grain (PCG) structure during annealing and solution treatment by metallographic observation and electron back-scattered diffraction (EBSD) of materials in the extruded state, after different annealing heat treatments and different solution treatments. It was found that recrystallization occurred in the surface layer of the material during the extrusion process, and the recrystallized grains developed into PCG during the solution process. The PCG was smaller when the annealing temperature was too low, or the solution temperature was too high, but the PCG structure was thicker. The annealing time had no significant effect on the PCG structure. The coarse grains appeared first at the edge of the material and then engulfed each other to develop into several huge grains.

Failure analysis of pulverizer pipe elbow in PLTU boiler

Erosion occurs due to several different mechanisms, depending on the composition, size, shape of the eroding particles, speed, angle of impact, and surface composition of the eroded components. The pulverizer pipe elbow has become worn out due to the pulverized coal fluid abrasion flowing on the pipe, which the type is AISI Grade 1026. This study was carried out on the causes of this damage case. Damage to the elbow in the boiler needs to be analyzed for the failure of the elbow so that the damage's cause is known and it becomes a lesson so that the same damage does not occur again. The research aims to: 1. Find out the cause of damage to the pulverizer elbow on the boiler; 2. Know the correct maintenance strategy to increase the reliability of pulverizer pipes in boilers; 3. Simulate erosion due to coal particles in the pulverizer pipe using the Autodesk Simulation Computational Fluid Dynamics software program; 4. Analytical calculations of the erosion rate that occurs at the bend of the pulverizer pipe (elbow) in the boiler. The analysis was done by visual observation, hardness testing, metallographic observation, simulation of the ANSYS CFD program, and analytical calculation. The result of the ANSYS simulation showed that the main factor causing the leakage was erosion-corrosion. In the leaking area, the corrosion concentration was higher than in other areas, indicated by the red color in that area. From the calculation results, it was concluded that the largest erosion rate occurs at the angle of 200 with the value is 4.9548 x 10- 11 m3 / s, the smaller the pulverized coal’s angle of impact crashed the pulverizer pipe elbow, the greater the erosion.

The Effect of Temperature on Electrochemical Corrosion Behavior of HDR Duplex Stainless Steel

To study the effect of temperature on the electrochemical corrosion behavior of HDR duplex stainless steel, the corrosion resistance of HDR at different temperatures was investigated using the electrochemical test method and combined with metallographic microstructure observation and the EDS analysis based on the characteristic that the metal structure determines. The results show that the HDR contains inclusions such as silicide and nitride, mainly distributed in the ferrite phase and its boundary. The pitting of HDR occurs when the polarization potential of -0.5–1.5 V is applied at different temperatures ranging from 10 °C to 80 °C. The pitting occurs mainly in the ferrite and its boundary below 50 °C and in the austenite and ferrite tissues above 60 °C. Cl−, inclusions, and Cr oxides are the important factors affecting corrosion. The corrosion rate of HDR increases with the increase in temperature; the higher the temperature is, the more serious the corrosion is. The critical pitting temperature of HDR is between 50 °C and 60 °C. The pitting potential below 50 °C is above 0.65 V, while that above 60 °C is below 0.60 V.

Effect of Ni-based Fillers on Corrosion Resistance of Heat Affected Zone of Super Austenitic Stainless Steel (UNS S31254) Welded by GTAW

The effect of Ni-based fillers on corrosion resistance of welded super austenitic stainless steel (SASS) was examined. The use of Ni-based fillers for gas tungsten arc welded SASS leads to the formation of fine Cr-Mo enriched phase in its weld unmixed zone, which makes it more susceptible to the localized corrosion. The metallographic observations showed that the pits were initiated around the fine Cr-Mo enriched phase precipitated in unmixed zone, and they were propagated along the base metal. The degree of corrosion damage among the welded samples using the three types of Ni-based fillers (Inconel 625, Inconel 622, Hastelloy 276) increased with the Mo contents in the fillers applied. The higher the Mo contents in the fillers, the higher the size/number ratio of Cr-Mo enriched phase with higher concentration of Mo, precipitated in unmixed zone. Based on the results, it is proposed that the Mo contents in Ni-based high alloyed fillers can have a great effect on the precipitation behavior of Cr-Mo enriched phase in unmixed zone, causing deterioration of corrosion resistance of SASS welds. Therefore, the Mo content in the filler should be optimized with the content similar to that of the base metal so as not to susceptible to localized corrosion.

Study on thermal deformation behavior and microstructure evolution of P550 high nitrogen austenitic stainless steel

The high nitrogen austenitic stainless steel is widely used as power generation and geological exploration equipment materials because of its excellent strength, corrosion resistance and non-magnetic. In this paper, the mechanical behavior and microstructure evolution of P550 steel in the range of 900 °C–1200 °C and 0.001–10 s−1 deformation conditions were studied by physical and heat treatment simulations, metallographic observations and thermal processing maps. The results showed that the flow curves quickly reach the peak and then soften to a steady state, which indicates dynamic recrystallization (DRX) behavior. DRX is easy to occur when the deformation temperature is above 1080 °C. The activation energy of the forged P550 stainless steel was calculated as 519 kJ mol−1. There is a positive correlation between the peak stress, DRX critical stress, strain and Z value of the tested steel. The instability of the tested steel is easy to produce in the high strain rate region and low temperature region during hot working. Crack germinates and expands preferentially at the ‘necklace structure’ of inadequate dynamic recrystallization. Under the deformation state of 0.001 s−1, coarse crystals and mixed crystals are easily emerged during subsequent heat treatment. Combining the hot working map, the maximum deformation resistance and the grain evolution behavior during hot working and heat treatment, the suggested working window is T = 1020 °C–1200 °C and έ = 0.01–1 s−1.

The Failure Analysis of Ф 88.9mm × 6.45mm P110 Tubing Fracture Used in A Well in Western Chinca

In this paper, it aims at the fracture tubing of Ф88.9mm×6.45mm P110 used in a well, magnetic particle flaw detection, material physical and chemical property test and scanning electron microanalysis system are used to carry out macro dimension measurement, defect flaw detection, chemical analysis, mechanical property test, metallographic structure observation, macro and micro morphology observation and micro area composition analysis of the fractured tubing. The results show that the fracture of the tubing belongs to sulfide stress corrosion cracking, which is mainly due to the use of non sulfur resistant tubing in sulfur-containing, resulting in a large number of sulfide stress corrosion cracks on the outer wall of tubing until fracture.

Hot Deformation Behavior and Processing Maps of Pure Copper during Isothermal Compression.

In this study, pure copper's hot deformation behavior was studied through isothermal compression tests at deformation temperatures of 350~750 °C with strain rates of 0.01~5 s-1 on a Gleeble-3500 isothermal simulator. Metallographic observation and microhardness measurement were carried out of the hot compressed specimens. By analyzing the true stress-strain curves of pure copper under various deformation conditions during the hot deformation process, the constitutive equation was established based on the strain-compensated Arrhenius model. On the basis of the dynamic material model proposed by Prasad, the hot-processing maps were acquired under different strains. Meanwhile, the effect of deformation temperature and strain rate on the microstructure characteristics was studied by observing the hot-compressed microstructure. The results demonstrate that the flow stress of pure copper has positive strain rate sensitivity and negative temperature correlation. The average hardness value of pure copper has no obvious change trend with the strain rate. The flow stress can be predicted with excellent accuracy via the Arrhenius model based on strain compensation. The suitable deforming process parameters for pure copper were determined to be at a deformation temperature range of 700~750 °C and strain rate range of 0.1~1 s-1.

The Influence of MSR-B Mg Alloy Surface Preparation on Bonding Properties.

Nowadays, industrial adhesives are replacing conventional bonding methods in many industries, including the automotive, aviation, and power industries, among others. The continuous development of joining technology has promoted adhesive bonding as one of the basic methods of joining metal materials. This article presents the influence of surface preparation of magnesium alloys on the strength properties of a single-lap adhesive joint using a one-component epoxy adhesive. The samples were subjected to shear strength tests and metallographic observations. The lowest properties of the adhesive joint were obtained on samples degreased with isopropyl alcohol. The lack of surface treatment before joining led to destruction by adhesive and mixed mechanisms. Higher properties were obtained for samples ground with sandpaper. The depressions created as a result of grinding increased the contact area of the adhesive with the magnesium alloys. The highest properties were noticed for samples after sandblasting. This proved that the development of the surface layer and the formation of larger grooves increased both the shear strength and the resistance of the adhesive bonding to fracture toughness. It was found that the method of surface preparation had a significant influence on the resulting failure mechanism, and the adhesive bonding of the casting of magnesium alloy QE22 can be used successfully.

Mechanical properties and biocompatibility of various cobalt chromium dental alloys

Cobalt-based metal alloys are often used in dentistry for prosthetic restorations because of their considerable wear and corrosion resistance. For this reason, the effect on the human body of three Co–Cr based alloys, named "Co–Cr", "C" and "L", differing in their concentrations, for subsequent use as dental materials, has been studied and compared. Metallographic observations of the microstructure, electrochemical and three-point bending tests were performed. The metallographic investigations showed similar dendritic microstructure of the Co–Cr and L dental alloys, as well similar porosities and defects. From the corrosion and pitting potential tests, it was found that the samples show surface passivation, reaching closed corrosion potential values from −0.280 V vs SCE to −0.250 V vs SCE. The Pitting Resistance Equivalent Number (PREN) values are above 38 for all the samples, which means that the studied alloys have a high resistance to pitting corrosion. Likewise, when applying the Electrochemical Impedance Spectroscopy technique, a slightly higher corrosion resistance was observed for sample L, since corrosion resistance tends to increase as the applied potential is more positive and as the impedance and phase angle values are higher. Moreover, in the three-point bending test, the alloy C presents the lowest values of the elastic modulus of 134 ± 13 GPa. The analyzed Co–Cr alloys are recommended for the efficient treatment of patients with dental prostheses that have metal frameworks, as shown by all of the obtained results.

Effect of gap and overlap on mode II interlaminar fracture toughness of automated fiber placement prepreg laminates

Automated Fiber Placement (AFP) technology facilitates the manufacturing process of composite structures with complex geometry owing to its high efficiency and accuracy. However, the unavoidable imperfections induced by the automated layup method bring challenges to the stability of the final mechanical properties of composites. The influence of AFP-induced gaps and overlaps on the mode II interlaminar fracture process of oven-cured laminates is experimentally investigated and explicitly revealed. End-Notched Flexure (ENF) tests were performed to measure the interlaminar fracture toughness of unidirectional laminates under three different defect configurations, namely “Gap” “Overlap” and “Gap & Overlap”. A marginal decline (about –1.2% in non-precracked tests, –5.1% in precracked tests) and an obvious increase (about 15.2% in non-precracked tests, 5.2% in precracked tests) in fracture toughness were observed for the “Gap” group and the “Overlap” group, respectively. Fractographic studies which involve the metallographic observation and Scanning Electron Microscopy (SEM) observation revealed that the increases in fracture area and crack length helped improve the fracture toughness of the “Overlap” group. However, the resin-rich area with high porosity induced by gaps was detrimental to the delamination resistance. To capture the interlaminar stress distribution local to imperfections, a three-dimensional numerical model was established. Gap defects facilitate the local delamination process, and the overlap defect postpones the damage onset and the propagation.

Dwell-fatigue crack growth behaviour of Alloy 709

Dwell-fatigue crack growth behaviour of an advanced austenitic stainless steel Alloy 709 has been investigated and compared with that of a conventional Type 316H stainless steel. The test procedure employed alternation of 1 h dwell-fatigue loading and 0.25 Hz fast cycling so that crack growth rates (da/dN) obtained from dwell-fatigue loading can be compared to those purely result from fatigue mechanism on the same test-piece. Tests were conducted at 550, 650 and 750 °C in air using 0.5 T compact tension test-pieces under a fixed maximum load of 8 kN and a stress ratio, R, of 0.1. For the investigated temperature and ΔK ranges, crack growth mechanisms of fatigue alone, creep alone and mixed fatigue-creep have all been observed. Detailed fractographic and metallographic observations were conducted to interpret failure mechanisms and regimes of different failure modes. Compared to crack growth rates obtained under 0.25 Hz fatigue loading, dwell-fatigue produces: no obvious increases in crack growth rates at a test temperature of 550 °C; a moderate increase (∼ 2–5 times) at a test temperature of 650 °C; and, over a tenfold increase at a test temperature of 750 °C. Compared to 316H, Alloy 709 has much improved resistance against creep-fatigue crack growth, here confirmed at the single test temperature of 650 °C.

Effect of on-site service for 16,000 and 38,000 h on microstructure and mechanical properties of austenitic steel HR3C reheater tubes

The microstructure and mechanical properties of austenitic heat-resistant steel HR3C reheater steam tubes were extensively investigated after 16,000 and 38,000 h exposure at 623 °C/7.2 MPa. The surfaces of the tubes are mildly oxidized, and the oxidation layers grew slowly with service time. Metallographic observation shows that only M23C6 carbides formed at the grain boundaries (GBs) and fine NbCrN nitrides that precipitated dispersedly in the matrix after 16,000 h in service. When service time up to 38,000 h, σ phases, (Cr3Ni2Si)C carbides precipitated at GBs. The large σ phase precipitated preferentially at the triangular GBs. The (Cr3Ni2Si)C carbides precipitated by the M23C6 particles or formed inside the σ phases at the GBs. In addition, granular and needle-like M23C6 carbides formed in the matrix, and more of them precipitated close to GBs. With service time prolonged, the precipitates close to the tube walls became plainly denser and larger. The mechanical test results indicate that the strength and hardness of the tubes increased with service time, but plasticity and impact toughness decreased. The large σ phases precipitated at the triangular GBs and M23C6 carbides obvious coarsened at GBs led to the degradation trend, specially deteriorated the tube surface performance.