Light Optical Metallography Research Articles

The differences of damage initiation and accumulation of DP steels: a numerical and experimental analysis

Many studies have examined the damage behaviour of dual-phase steels already. It is a topic of high interest, since understanding the mechanisms of damage during forming processes enables the production of steels with improved properties and damage tolerance. However, the focus was rarely on the comparison between representatives of this steel class, and the numerical simulation for the quantification of damage states was not thoroughly used. Therefore, this study compares the damage initiation and accumulation of two dual-phase steels (DP800 and DP1000), which are used in the automotive industry. Additionally, parameter sets of a phenomenological damage mechanics model with coupled damage evolution are calibrated for each material. The combined analysis reveals an earlier initiation of damage for the DP800, where the damage accumulation phase is prolonged. For DP1000 the damage nucleates only shortly before material failure. The material model is able to correctly predict the behaviour, while experimental analysis confirms the prediction via light optical and SEM metallography.

Microstructural and Mechanical Characterization of Ti‐Containing Twinning‐Induced Plasticity Steel Welded Joint Produced by Gas Tungsten Arc Welding Process

The aim herein is to study the microstructural and mechanical changes generated in a welded joint of Ti‐containing twinning‐induced plasticity (TWIP) steel by the gas tungsten arc welding (GTAW) process. For this purpose, the welding parameters that allow the union of 5.6 mm‐thick butt plates with full penetration and without filler material are investigated. Microstructural changes are examined by light optical metallography. Segregation and second‐phase precipitated particles are investigated by scanning electron microscopy and electron‐dispersive spectroscopy. Phase transformations are evaluated using X‐ray diffraction. Finally, the mechanical behavior is assessed by Vickers microhardness and microtensile tests. In general, the welded joint shows dendritic structure formations in the fusion zone (FZ) and equiaxed grain growth in the heat‐affected zone (HAZ). The FZ shows a high degree of segregation, where Mn, Si, and C segregate in the interdendritic regions, whereas Al preferentially segregates in dendritic areas. In contrast, no effect on austenite stability is noticed. The welded joint shows an increase in microhardness, which is associated with the formation of precipitated particles. Finally, the microtensile results show a decrease in ultimate tensile strength (UTS) in the FZ, whereas the welding interface and HAZ show an increase in UTS compared with the base material.

Methods for the measurement of ferrite content in multipass duplex stainless steel welds

The phase balance is commonly determined as a part of the duplex stainless steel (DSS) welding procedure qualification. In this work, the aim was to find a reliable, but still fast method to measure the ferrite content of DSS welds. Four methods were compared: image analysis with light optical metallography, magnetic measurements with Feritscope and Magne-Gage, and X-ray diffractometry (XRD). Image analysis with a magnification of ×500 was concluded to be most accurate on condition that the image quality was sufficiently high. The best contrast for image analysis was achieved by covering all surfaces apart from the part of interest with adhesive tape and etching the sample in a modified Beraha II solution. The Feritscope systematically underestimated the average ferrite volume fraction compared with image analysis, and a correction factor of 1.1 is suggested. Magne-Gage in turn resulted in considerably higher ferrite numbers as compared with the Feritscope and would require a correction factor of 1.18. Besides the long duration of XRD measurements, the method proved unsuitable for ferrite measurement due to the coarse texture of the microstructure.

Heat Input Effect on the Microstructure of Twinning-Induced Plasticity (TWIP) Steel Welded Joints Through the GTAW Process

ABSTRACTHigh-Mn Twinning Induced Plasticity (TWIP) steels are an excellent alternative in the design of structural components for the automotive industry. The TWIP steels application allows weight reduction, maintaining the performance of vehicles. Nowadays the research works focused on TWIP steel weldability are relative scarce. It is well-known that weldability is one of the main limitations for industrial application of TWIP steel. The main goal of this research work was studied the effect of heat input on the microstructural changes generated in a TWIP steel microalloyed with Ti. A pair of welds were performed through Gas Tungsten Arc Welding (GTAW) process. The GTAW process was carried out without filler material, using Direc Current Electrode Negative (DCEN), tungsten electrode EWTh-2 and Ar as shielding gas. The microstructure and average grain size in the fusion (FZ) and heat affected zone (HAZ) were determined by light optical metallography (LOM). Elements segregation in the FZ was evaluated using point and elemental mapping chemical analysis (EPMA) by Scanning Electron Microscopy and Electron Dispersive Spectroscopy (SEM-EDS). Phase transformations were evaluated using X-ray diffraction (XRD). Finally, the hardness were measured by means of Vickers microhardness testing (HV500). The results show that the FZ is characterized by a dendritic solidification pattern. Meanwhile, the HAZ presented equiaxed grains in both weld joints. On the other hand, the TWIP-Ti steel weldments did not present austenite phase transformations. Nevertheless, the FZ exhibited variations in the chemical elements distribution (Mn, Al, Si and C), which were higher as the heat input increases. Finally, the heat input reduced the microhardness of TWIP-Ti steel weld joints. Although post-welding hardness recovery was detected, which is associated with precipitation of Ti second-phase particles.

Weldability of High-Mn Austenitic Twinning-Induced Plasticity (TWIP) Steel Microalloyed with Nb

ABSTRACTThe objective of this research work is to study the weldability of a Nb microalloyed TWIP steel through welding nuggets generated by Gas Tungsten Arc Welding process. Weldability was examined by microstructural changes in the fusion zone (FZ) and heat affected zone (HAZ) using light optical metallography (LOM), segregation in the nuggets was evaluated using elemental mappings of chemical analysis by Scanning Electron Microscopy and Electron Dispersive Spectroscopy (SEM-EDS), phase transformations were evaluated using X-ray diffraction (XRD) and the hardness properties were examined using Vickers microhardness testing (HV25). Experimental results show that microstructure of welding nuggets consists of austenitic dendritic grains in the FZ and equiaxed grains in the HAZ. FZ width and HAZ grain growth tend to increase as the heat input increases. Additionally, the studied Nb-containing TWIP steel showed segregation in the FZ, where Mn and Si segregated in the interdendritic regions, while Al and C preferentially segregated in dendritic areas. In general, the data obtained by XRD indicated that GTAW process did not affect austenite stability. Finally, the welding nuggets of studied TWIP steel showed lower microhardness values than the as-solution condition (starting condition). However, the heat affected zone showed hardened areas, which are associated with NbC precipitation hardening.

Typical defects in plate and long steel products

Purpose During steel plate and long-product production, numerous imperfections and defects appear that deteriorate product quality and consequently reduce revenue. The purpose of this paper is to provide a practical overview of typical defects (surface and internal) that occur and their root causes. Design/methodology/approach The data presented here derive from the quality department and from more than 50 technical reports of ELKEME S.A. on the last decade’s production of steel making companies STOMANA S.A. and SIDENOR S.A., with emphasis on the defects occurred in some of the products of the Bulgarian plant. Stereoscopic observations of surface defects, light optical metallography, and scanning electron microscopy with EDS represent the most used techniques to characterize defected macro-/micro-areas and microstructures. Findings In general, the most commonly encountered defects from semi-finished (billets, blooms, and slabs) and final (round bar and plate) steel products are as follows: network cracks, porosity, gas holes, shrinkage, shell, slivers, casting powder entrapment, ladle slag entrapment, other non-metallic inclusions, low hot ductility, centerline segregation cracking, macro- and micro-segregation, and mechanical defects (scratches, transverse cracks, and seams). Practical implications External and internal quality improvement can reduce the production cost (Euro/ton). Social implications Improvement of the quality of industrial plates and long products increases the safety of the further-produced constructions and systems such as bridges, cranes, heavy equipment, automobile parts, etc. Originality/value Root cause analysis and categorization of the most commonly encountered defects can pave the way to production process improvements that directly affect final product quality and the overall per ton production cost. The benefits of this work obviously affect all steel producers/processers, and also society through the safety increase achieved by the quality improvement in the steel products used in constructions and automobile parts.

Ab Initio Study of Weldability of a High-Manganese Austenitic Twinning-Induced Plasticity (TWIP) Steel Microalloyed with Boron

ABSTRACTHigh-Mn Twinning-Induced Plasticity (TWIP) steels are advanced high-strength steels (AHSS) currently under development; they are fully austenitic and characterized by twinning as the predominant strengthening mechanism. TWIP steels have high strength and formability with an elongation up to 80%, which allows reduction in automotive components weight and fuel consumption. Since the targeted application field of TWIP steels is the automotive industry, steels need high mechanical performance with good weldability and excellent corrosion resistance. However, there is lack of information about the weldability behavior of these advanced steels. This research work aims to study the weldability of a new generation of high-Mn austenitic TWIP steels microalloyed with B. Weldability was examined using spot welds produced by Gas Tungsten Arc Welding. Microstructural changes were examined using light optical metallography. Segregation of elements in the weld joint was evaluated using point and elemental mapping chemical analysis by Scanning Electron Microscopy and Electron-Dispersive Spectroscopy; while the hardness properties were examined with Vickers microhardness testing (HV25). Experimental results show that the welded joint microstructure consists of austenitic dendritic grains in the fusion zone, and equiaxed grains in the heat affected zone. Notably, the boron microalloyed TWIP steel exhibited poor weldability, showing hot cracking. Additionally, the studied TWIP steels showed a high degree of segregation in the fusion zone; Mn and Si segregated into the interdendritic regions, while Al and C preferentially segregated in dendritic areas. Finally, the welded joints of the TWIP steels showed microhardness values lower than the base material. In general, the present TWIP steels have problems of weldability, which are corroborated with microstructural changes, elements segregation and microhardness loss.

Determination of defects on a processed disc from S355J2 steel

Purpose – A failed disc that was forged from S355J2 round bar was investigated in order to determine the failure route cause. The purpose of this paper is to determine the defects and route cause analysis regarding their origin. Design/methodology/approach – Macroscopic evaluation, microstructure observation using light optical metallography and scanning electron microscopy with EDX analysis were the techniques used to analyse and characterize the defected areas. Findings – Macro-inclusions (up to 850 µm) that correspond to high melting aluminium rich calcium-aluminate particles were detected. Their formation, possibly due to improper calcium treatment during ladle furnace steel refining process might be associated with clogging problems at casting. SEM-EDX analysis revealed whitish spots containing Zr that could be related to submerged entry nozzle (SEN) erosion/breakage. Characteristic is the large size and unusual shape of the traced particles, as well as the presence of low Si, Na, K. The findings indicated that nozzle clogging and/or breakage at casting was most possibly the root cause of the product’s quality degradation. Originality/value – After extended root cause analysis, specific countermeasures are proposed to avoid clogging phenomena. The suggestions are based on the findings taking into account restrains of the steel-making process. Emphasis was laid in detecting the weaknesses that lead to product quality degradation and consequently in optimizing the steel-making process. Such incidents are often found during steelmaking a useful suggestion to steelmakers is to mark and remove cast parts after SEN problems are encountered. In this way quality issues in intermediate and/or final products will be avoided.

Nano-Scale Fission Product Phases in an Irradiated U-7Mo Alloy Nuclear Fuel

Irradiated nuclear fuel is a very difficult material to characterize. Due to the large radiation fields associated with these materials, they are hard to handle and typically have to be contained in large hot cells. Even the equipment used for performing characterization is housed in hot cells or shielded glove boxes. The result is not only a limitation in the techniques that can be employed for characterization, but also a limitation in the size of features that can be resolved The most standard characterization techniques include light optical metallography (WM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). These techniques are applied to samples that are typically prepared using grinding and polishing approaches that will always generate some mechanical damage on the sample surface. As a result, when performing SEM analysis, for example, the analysis is limited by the quality of the sample surface that can be prepared. However, a new approach for characterizing irradiated nuclear fuel has recently been developed at the Idaho National Laboratory (INL) in Idaho Falls, Idaho. It allows for a dramatic improvement in the quality of characterization that can be performed when using an instrument like an SEM. This new approach uses a dual-beammore » scanning microscope, where one of the beams isa focused ion beam (FIB), which can be used to generate specimens of irradiated fuel (-10µm x 10µm) for microstructural characterization, and the other beam is the electron beam of an SEM. One significant benefit of this approach is that the specimen surface being characterized has received much less damage (and smearing) than is caused by the more traditional approaches, which enables the imaging of nanometer­ sized microstructural features in the SEM. The process details are for an irradiated low-enriched uranium (LEU) U-Mo alloy fuel Another type of irradiated fuel that has been characterized using this technique is a mixed oxide fuel.« less

Microstructures and properties of solid and reticulated mesh components of pure iron fabricated by electron beam melting

This research examines rapidly solidified, atomized pure iron powder and solid and reticulated mesh components fabricated by electron beam melting (EBM) from this powder precursor. Especially significant was the characterization of associated microstructures and corresponding mechanical properties. Atomized Fe powder was used to fabricate solid and reticulated mesh components by EBM. Powder and component microstructures and phase structures were examined by light (optical) metallography, scanning electron microscopy, X-ray diffractometry, and transmission electron microscopy. Corresponding Vickers microindentation hardness measurements were also made and compared to tensile data along with measurements of dynamic stiffness for mesh components having varying densities. The atomized Fe powder was observed to contain δ-Fe which was retained in the solid, EBM-fabricated components where it was observed to be homogeneously distributed in equiaxed α-Fe grains as δ-phase platelets measuring ∼0.5μm to 2μm in length and ∼40nm thick; coincident with the α-Fe matrix {100} or {110} planes. A log–log plot of E/Es versus ρ/ρs resulted in (E/Es)=(ρ/ρs)2.8. Novel, δ-Fe phase platelets have been observed in α-Fe components fabricated by EMB.

Failure Investigation of Cold-Rolled ZnTiCu Alloy Fracture During Bending Under Ambient Temperature Conditions

Bending ductility is an important quality parameter of the ZnTiCu rolled sheet for the fabrication of specially formed members in construction industry. Fabrication of such components is usually realized under severe loading conditions (i.e., high strain rate forming using manual tools or press brake machinery) and even sometimes under low temperature environment in the construction field, which is very often close to or less than 0 °C. Bending orientation is also an important parameter, since texture-sensitive Zn alloy cold-worked strip affects ductility performance in direction transverse to the rolling direction. A failure analysis process was implemented on a ZnTiCu bent sheet possessing cracks on outer bent areas. Light optical metallography, and scanning electron microscopy coupled with energy dispersive spectrometry, along with mechanical testing were the principal investigation techniques employed to evaluate the failure. The presence of pronounced elongated grain structure combined with coarse and continuous Ti-rich intermetallic phase, likely reduces the fracture resistance against the transverse stress field imposed by bending, stimulating, thus crack initiation and propagation.

Failure of a Rear Axle Shaft of an Automobile Due to Improper Heat Treatment

A section of fractured rear axle shaft made of induction-hardened steel and removed from the scene of overturned automobile was analyzed to determine the most probable cause of failure. Light optical metallography and scanning electron microscopy combined with energy dispersive spectroscopy were used to characterize the microstructure and the mechanical strength was evaluated by microhardness measurements. Chemical analysis verified that the shaft was made of AISI 4140 steel as per specifications. However, microstructural characterization and microhardness measurements revealed that the shaft was improperly heat treated resulting in a brittle case, where crack propagation was found to occur by an intergranular mode in contrast with cleavage within the core. This behavior was related to differences in microstructure, which was observed to be martensitic-type within the case with microhardness equivalent to R c 58, and a mixture of pearlite and ferrite within the core with R c 25. Although it was not possible to reconstruct the exact sequence of events leading to fracture, it is possible that it was initiated by large overload within the extremely hard brittle case, which could lead to overturning of the vehicle and final fracture could have occurred by the impact of overturning. However, crack initiation due to hydrogen generated by rust and water pickup as well as the possibility that overturning of the vehicle was the cause of the fracture could not be ruled out.

Root cause analysis of surface and internal defects of micro‐alloyed S355 heavy plates

Purpose – Steel heavy plates, grade S355, micro‐alloyed with Vanadium‐V and/or Niobium‐Nb plus Titanium‐Ti in thicknesses from 5 to 60 mm, 200.000‐350.000 t/y, are produced according to EN 10025 at STOMANA S.A., a company of the SIDENOR Group in Pernik Bulgaria, and are exported to the European Market. These plates fulfil high quality standards as they are used for constructions and engineering applications (e.g. high‐building constructions, bridges, shipping applications, cranes, etc.). Often intermediate and/or final products (slabs and plates, respectively) suffer from surface and/or internal defects, which deteriorate the final product's quality. The purpose of this paper is to look at the challenging task of eliminating the external and especially the internal defects.Design/methodology/approach – ELKEME performs root‐cause analysis and proposes improvement actions. For these purposes light optical metallography (LOM) and scanning electron microscopy (SEM) with EDS were applied. For the analysis a NI...

Comparison of Microstructures and Properties for a Ni-Base Superalloy (Alloy 625) Fabricated by Electron Beam Melting

Nickel-based superalloys processed by additive manufacturing have demonstrated directional solidification, which has been shown to equal or improve mechanical properties compared to cast and wrought alloys. Inconel 625 cylinders have been manufactured by electron beam melting (EBM) and selective laser melting (SLM) and compared. EBM cylinders were built in the Z-axis direction (parallel to the build direction), and SLM cylinders were built in XY-axis (perpendicular to build direction) and Z-axis directions. The microstructures of as-fabricated as well as fabricated and HIPed cylinders were characterized by light optical metallography (LOM), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive (X-ray) spectroscopy (EDS). EBM fabricated components contained columnar plates of ?¢¢ (Ni3Nb) precipitates while SLM components contained columnar arrays of fine ?¢¢ nanoparticles. The EBM fabricated and HIPed samples and SLM fabricated and HIPed samples exhibited equiaxed grains, but both components contained complex arrays of dissimilar precipitates.

Microstructures for Two-Phase Gamma Titanium Aluminide Fabricated by Electron Beam Melting

Pre-alloyed (Ti-48Al-2Cr-2Nb) powder was used to fabricate solid, fully dense (3.85 g/cm3) components by electron beam melting (EBM). Foam components with densities from 0.33 to 0.46 g/cm3 were also fabricated by EBM. Both the solid and the foam component microstructures were characterized by an equiaxed, duplex grain structure (15 μm diameter) containing lamellar α2 (Ti3Al)/γ (TiAl) plates ~20 nm. Light optical metallography and transmission electron microscopy illustrated that these microstructures confirmed the (111)γ/(001)α2 duplex orientation relationship. Solid components annealed at 1150 °C at 5 h and 1380 °C at 1 h exhibited a fine-grain (3 μm diameter) duplex structure and a large, equiaxed, fully lamellar grain structure (550 μm diameter), respectively. The lamellar structure consisted mostly of coherent {111} γ (TiAl) twins with a width of ~0.9 μm. These diverse annealed microstructures exhibited residual hardness values similar to the as-fabricated EBM components.

Metallography, Microanalysis & Corrosion of the Athlit Ram

ABSTRACTThe Athlit ram, a bronze warship ram from a 2nd Century BCE Roman-era galley, was found in 1980 off the coast of Israel at Athlit, and is now displayed at the National Maritime Museum, Haifa, Israel. It meant to fit on the prow of a medium-sized oared warship. This ram is the only known surviving example of this ancient naval weapon. Inside the bronze ram some of the ship’s wood is still preserved. We have recently studied a piece of the ram removed during early conservation. Remnant metal, corrosion products, and mineralized and pseudomorphed wood have all been found and examined by light optical metallography, x-ray diffraction, scanning electron microscopy, and microanalysis using energy dispersive x-ray mapping. The main corrosion product on the Athlit Ram is identified as covellite (CuS), and the entrained material is pseudomorphed cedar wood. Analysis indicates the lumen to be replaced by calcium carbonate and the cell walls to be replaced by covellite, consistent with the matrix.

Imaging systems and materials characterization

This paper provides a broad background for the historical development and modern applications of light optical metallography, scanning and transmission electron microscopy, field-ion microscopy and several forms of scanning probe microscopes. Numerous case examples illustrating especially synergistic applications of these imaging systems are provided to demonstrate materials characterization especially in the context of structure-property-performance issues which define materials science and engineering.

Influence of Processing Route on the Properties of Magnesium Alloys

Magnesium alloys had gained an increasing interest in recent years due to their promising property profile for light weight constructions. They offer drastic advantages in weight reductions in automotive industries compared to steel or even aluminium. Therefore they can be used to decrease the emission of green house gases as requested by the EU directive for the reduction of CO2 emissions and moreover due to their recyclability they also help to fulfill the requirements from the EU directive regarding the end of life of vehicles. But still there are some limitations with regard to strength, mostly at elevated temperatures above 130 °C. To overcome these limitations alloy development as well as process optimization has to be done for further enhancement of the range of magnesium applications. This paper will show and discuss the property profiles of the standard magnesium alloy AZ91D compared to the recently developed, heat resistant magnesium alloy MRI153. The alloys have been processed using normal high pressure die casting (HPDC), New Rheocasting (NRC) and Thixomolding® (TM). As methods of investigation tensile and creep tests have been applied. The creep properties have been determined in the temperature range of 135-150 °C and loads of 50-85 MPa. All these trials have been accompanied by metallographic observations (light optical metallography, SEM) and density measurements to investigate the influence of the processing routes on microstructure and the porosity of the materials. It will be shown that the differences in the property profile of the chosen alloys are dependent on their different chemical compositions as well as on different microstructures that are obtained by the different processing routes. While in the case of AZ91D, TM is showing advantages compared to HPDC for room temperature applications, the NRC in combination with the heat resistant alloy leads to an improvement of creep rates by two orders of magnitudes.

Metallurgical and acoustical characterization of a hydroformed, 304 stainless steel, Caribbean-style musical pan

We report herein the metallurgical and acoustical characterization of hydroformed 304 stainless steel, Caribbean pans. These pans were fully tuned to chromatic tones and compared to a manufactured, low-carbon, Caribbean steel pan standard. Hydroformed platforms had a Vickers microindentation hardness of HV 345, which was reduced by annealing during pan fabrication to HV 270. Skirts welded to the hydroformed head had a microindentation hardness of HV 440. Microstructural characterization by light optical metallography and transmission electron microscopy illustrated microstructures (including grain structures) characteristic of these pan microindentation hardnesses.

On the intergranular embrittlement of a solid-solution strengthened Fe-base alloy by internal oxidation

diameter = 0.11 m, wall thickness = 0.014 m) in the same condition to be used in practice were installed in a benzene plant and exposed to an environment consisting of 64.3% H2-18.92%C1-3.48C2-0.96C30.03C4-0.24C5-1.5%C6-C8 NON-ARO, 5.91% benzene-1.45% C8 aromatics-0.04%C9-0.15% diphenyl at 600 � C. Light optical metallography and scanning electron microscopy were used to characterize the microstructure.