Low Melting Point Eutectics Research Articles

Influence of Post-Bond Heat Treatment on Microstructure and Creep Behavior of the Brazed Single-Crystal Nickel Superalloy.

Post-bond heat treatment (PBHT) is an effective way to improve the bonding quality of a brazed joint. Herein, brazing of a nickel-based single crystal superalloy is carried out using a Ni-Cr-Co-B-Si-Al-Ti-W-Mo filler alloy, and the microstructure and creep property of the brazed joint are systematically investigated using scanning electron microscopy (SEM), Thermo-Calc software, an electron probe micro-analyzer (EPMA), X-ray diffractometer, confocal scanning laser microscope (CSLM), and transmission electron microscopy (TEM). The results reveal that the as-prepared joint only consists of an isothermally solidified zone (ISZ) and an athermally solidified zone (ASZ), where the cubic γ′ phase is observed in the ISZ, and skeleton-like M3B2, γ + γ′ eutectic and reticular G phases are observed in the ASZ. Furthermore, the γ + γ′ eutectic and G phases disappear and the M3B2 alters from a skeleton-like to block-like shape in the ASZ after PBHT. Meanwhile, some lath-like M3B2 phases are precipitated at the edge of the ISZ and several M3B2 phases are precipitated in the base metal, forming a new zone in the brazed joint, namely at the diffusion affected zone (DAZ). Owing to the removal of low melting point eutectics from the as-prepared joint, the creep life also increases from 188 h to 243 h after PBHT. The current work provides a method for the optimization of brazed joints based on the Ni-based single crystal superalloy.

A new insight into rapid oxidation of alloy 925 contaminated by oxide powder

Rapid oxidation takes place both in the laboratory and in industrial conditions when alloy 925 is contaminated by oxide powder, and the oxidation rate is much higher than normal. Herein, the rapid oxidation behavior and mechanism of alloy 925 were investigated by a series of comparison tests at 1160 °C. It is found that the oxide powder produced during the oxidation process is mainly composed of NiCr2O4 spinel, accompanied by NiO and NiMoO4. The oxide powder plays a triggering role in the rapid oxidation of alloy 925, as NiO has a strong affinity with O. A composition adjustment experiment of alloy 925 shows that rapid oxidation is the synergetic effect of Mo and Cu in the alloy. Mo and Cu easily combine to form low melting point eutectics. The formation and volatilization of MoO3 oxide can destroy the completeness of the protective oxide layer. The MoO3 flux can dissolve protective Cr2O3 and prevent the repair of the oxide layer and also promote the formation of nonprotective and easy-spall NiCr2O4 spinel. The synergetic effect of Cu and Mo in accelerating oxidation should be considered in any nickel-based alloy with a high content of Cu and Mo.

Evaluation of the fusion and agglomeration properties of ashes from combustion of biomass, coal and their mixtures and the effects of K2CO3 additives

This study investigated the ash fusion and agglomeration characteristics of a Chinese bituminous, “Neimongol” (NM) coal, soybean straw (SS) material and their blends, as well as the effects of high biomass blending ratio (20–80 wt%) and the addition of K2CO3 (0–20 wt%) on the ash agglomeration propensity of the mixtures in a muffle furnace at 815 °C. The ash fusion and agglomeration characteristics of the fuel blends and their modified behaviours based on the blending ratio and flux were examined using ash fusion temperature (AFT) analysis, X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX). The results indicated that, compared to Neimongol coal ash (NMA), the contents of SiO2 and Al2O3 in soybean straw ash (SSA) were lower, and the contents of basic oxides were higher, which was the main reason for the lower AFT of SSA. The AFT curves for the NM/SS blends showed a “bathtub” shaped trend with increasing SS mass ratios. For a given coal/biomass blend, XRD analysis indicated that for blends without added K2CO3, mineral phases were dominated by arcanite and quartz; with increasing SS mass ratios, the diffraction intensity of quartz decreased obviously. With increasing the K2CO3 additions, the content of potassium feldspars and sylvine in ash increased, and the quartz content decreased. The EDX analysis confirmed that the K2CO3 addition made it possible for kyanite and mullite to react with K-bearing minerals and form low melting point eutectics. The SEM images revealed that the fused particles in SSA were coated with KCl, whereas the NMA particles mostly contained aluminosilicate minerals. As the K2CO3 addition increased, the morphologies of the ashes from blends changed from slightly agglomerated to seriously slagged, and the molten and solidified zone on the slag surface featured high contents of sylvine, quartz, and potassium aluminosilicates.

Comprehensive coal quality index for evaluation of coal agglomeration characteristics

A comprehensive analysis of coal quality factors affecting coal agglomeration characteristics was carried out. The influence weight of seven main coal quality indices on agglomeration was calculated by the entropy method. Furthermore, a reliable and comprehensive coal quality index was obtained to evaluate the characteristics of coal agglomeration by the agglomeration experiment results. The proximate analysis, ultimate analysis, and petrographic analysis results were applied for the analysis of organic constituents. The XRF and ash fusion temperatures were applied for the analysis of inorganic constituents. Results show that there is an absolute positive correlation between the C/H, FC/V and the characteristics of coal agglomeration. Both of the vitrinite content and the average reflectivity of the vitrinite have a positive effect on the coking ability of coal. Alkali oxides not only have a low melting point but also can react with Si and Al to form low melting point eutectics. Both of these characteristics result in coal ash slagging. The entropy method was adopted to analyze the effect weight of C/H, FC/V, vitrinite content Vi, vitrinite average reflectivity Rave, ash deformation temperature (DT), Base/Acid ratio (B/A) and Silicon ratio (G) on the agglomeration characteristics of coal. The weights are 11%, 10%, 26%, 12%, 9%, 14% and 18%, respectively. Among them, vitrinite content had the largest effect, and DT had the least effect. In addition, an empirical index of coal agglomeration characteristics is proposed, and the discriminant limits are provided. The results are helpful to the evaluation of coal agglomeration, and useful to the safe and efficient operation of boilers.

Microstructural evolution and solidification cracking susceptibility of Fe–18Mn–0.6C–xAl steel welds

In this work, the solidification behavior and solidification cracking of Fe–18Mn–0.6C–xAl (x = 1.49, 2.37, 4.79, 6.04 wt%) alloys were investigated. A longitudinal Varestraint test was applied to evaluate the solidification cracking tendency of Al-added high-Mn steel welds. In terms of total crack length and maximum crack length at 4 % applied strain, the solidification cracking susceptibility of high-Mn steel decreased with increasing Al content. Addition of Al suppressed the formation of low melting point eutectics (γ + (Fe,Mn)3C) along the grain boundaries during the final stage of solidification, which resulted in the decrease of solidification cracking tendency. The Al segregated extensively to the dendrite core opposite to Mn and C during solidification, which promoted the formation of δ ferrite. Further, the transition of the solidification sequence from the primary austenitic to primary ferritic mode provided a noticeable improvement in solidification cracking resistance in high-Mn steel welds similar to austenitic stainless steel welds.

Synthesis of β-Si3N4 powder from quartz via carbothermal reduction nitridation

Abstract β-Si 3 N 4 powder was synthesized via carbothermal reduction nitridation (CRN) method using quartz as starting material. The influence of synthesis temperature, C/SiO 2 molar ratio and amount of Fe 2 O 3 additive on the phase transformation and micro-morphology of β-Si 3 N 4 powder was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The results showed that the optimal experimental conditions for the CRN synthesis of β-Si 3 N 4 powder were 3 h sintering at 1600 °C in flowing nitrogen atmosphere with C/SiO 2 molar ratio of 2.0 and amount of Fe 2 O 3 additive of 5.0 wt.%. The prepared β-Si 3 N 4 grains present a short hexagonal, rod-like morphology. Changes in experimental parameters affect the phase compositions and the morphology of the product. Excessive carbon promoted the formation reaction of β-SiC. Fe 2 O 3 additive exerted a significant promotion on the formation of β-Si 3 N 4 in CRN process by forming the low melting point eutectics.

Effect of Ti on hot cracking and mechanical performance in the gas tungsten arc welds of copper thick plates

The new welding material – ERCuTi alloys filler metals were developed for gas tungsten arc welding (GTAW) of copper. The cracking susceptibility of the welds with ERCuTi and ERCu separately in GTAW of 10mm copper thick plates was investigated. The formation causes of hot cracking was researched by using ERCu and the suppression mechanism of hot cracking when using ERCuTi alloy filler was proposed. It has been found that, when element Ti is added into the welding pool, the Ti will combine with O preferentially rather than Cu to generate TiO2, which process can suppress the formation of Cu2O. The hot cracking force and the hot ductility of the welds in brittle temperature range (BTR) could be improved effectively by adding Ti in filler metal compared with that of the welds without Ti. But the degree of addition of Ti (2–4wt%) is critical when the susceptibility of cracking is to be suppressed. If the level is allowed to exceed 4wt%, more low-melting point eutectics (β-TiCu4 and TiCu2) will be formed in the welds, and cracking susceptibility will be increased again. Results of mechanical properties tests show that although adding Ti increases the hardness and strength of the weld compared to the base metal, the impact ductility and the plastic properties are not decreased significantly.

GTA Welding of Copper Thick Plates by Using ERCuTi Welding Materials

The new welding material – ERCuTi alloys filler metals were developed specially for gas tungsten arc welding (GTAW) of copper. The hot cracking in welding copper is inhibited completely as the addition of de-oxidant element Ti in copper welding material. The degree of addition of Ti (2-4wt%) is critical when the susceptibility of cracking is to be suppressed. If the level is allowed to exceed 4wt%, more low-melting point eutectics (β-TiCu4and TiCu2) will be formed in the welds, and cracking susceptibility will be increased again. Results of mechanical properties tests show that although adding Ti increases the hardness and strength of the weld compared to the base metal, the impact ductility and the plastic properties are not decreased significantly.

ANÁLISE DE FALHA DE UM COMPONENTE ESTRUTURAL DE PLANTA DE PELOTIZAÇÃO DE MINÉRIO DE FERRO

Grate bars of pallet cars employed in iron ore pelletizing process presented severe abrasive wear due to the contact with the pellets, and erosive wear as a consequence of the hot air flow containing iron ore particles. Burning fossil fuels and the presence of contaminants in the pellets resulted in low melting point eutectics of metal alkali salts. Liquid salt deposits on the grate bar surface dissolved the protective oxide layer, inducing hot corrosion degradation of the component. Thermal fatigue induced-spalling further aggravated the problem. It was recommended a higher Cr-content stainless steel in order to supply the near surface Cr-depleted zone as well as higher levels of Si and Al aimed at increasing global oxidation resistance.

Hot corrosion mechanism of tundish plaster with steel slags in continuous casting

This study concerns the chemical reactions involved and the phases formed during penetration of slags of variable composition into porous plaster structure of tundish. Tundish plaster is mainly composed of MgO with minor amounts of SiO2 and impurities, with a grain size of less than 1 mm, inorganic or organic fibers in order to decrease density and provide porosity for insulation, plasticizers and stiffening agents and some other additions. Tundish slag analysis for different grades of steel (7176D and 1191D, according to DIN standard) at different sequences, indicated a very variable composition of CaO (11–42%), SiO2 (28–46%), Al2O3 (6–12%), MgO (11–20%), MnO (0–13%) and some minor variations of Fe, FeO and TiO2. Experimental work indicated that slag when penetrated into pores of plaster, develop the phases of Monticellite (CaO · MgO · SiO2) and Merwinite (CaO1.5 · MgO0.5 · SiO2) around MgO particles and decrease the liquidus temperature from 2,800 °C to about 1,500 °C and provide dissolution of MgO grains in steel making process. Calculation based on two kinetic equations developed for diffusion controlled dissolution, indicated that the dissolution of MgO in tundish plaster is not a diffusion controlled process and is affected by turbulent flow parameters. Phase diagram of CaO–SiO2–MgO indicates that decreasing SiO2 to below 20% and increasing CaO content to as high as possible, increases the liquidus temperature to above 2,000 °C. Sources of SiO2 in the process are the rice husk addition, which is used as an insulating material on top of the melt, and the slag flux addition. These sources should be reduced to as low a level as possible. This fact does not affect the fluidity of slag which is required for inclusion removal. Fluidity of slag comes from low melting point eutectics in CaO–Fe2O3 and CaO–FeO (about 1,200 °C) due to iron oxide on top of the steel melt.

Effect of CaF2 on the sintering and thermal expansion of La0.85Sr0.15Cr0.95O3

Lanthanum chromite based perovskite have been widely accepted as ceramic interconnects and as protecting layers for metallic interconnects in solid oxide fuel cells (SOFCs) because of thier high electronic conductivity, chemical stability in both oxidizing and reducing atmospheres as well as their chemical and thermo-mechanical compatibility with other cell components under operating conditions [1, 2]. Strontium-doped LaCrO3 has emerged as a primary candidate for SOFC interconnects over other alkaline earth oxides such as CaO and MgO [3]. From the viewpoint of compatibility of thermal expansion among SOFC components, Sr-doped lanthanum chromite can match the coefficient of thermal expansion (CTE) to the Y2O3-stabilized ZrO2 (YSZ) electrolyte using less than 20 mol% Sr [4]. Lanthanum chromite interconnects must be a dense in order to separate fuel gas from the oxidizing agent. They are usually sintered above 1,600 C to reach a 94% relative density [5]. From cost considerations associated with commercialization, it is desirable for all components of SOFCs to be co-sintered in air to reduce manufacturing costs. However, this may cause severe reactions among the components at high temperature. Therefore, lowering the sintering temperature of the LaCrO3-based interconnect is favored. However, the volatilization of CrO3 in LaCrO3 at high temperature in air makes it difficult to sinter. Some researchers have attempted to improve the sinterability of LaCrO3 powders in different ways. Sakai et al. [6] found that chromium deficient lanthanum calcium chromites (La0.7Ca0.3Cr1)yO3, y = 0.02) could be sintered to 94% theoretical density at 1,200 C in air. Small particle sizes of the starting powders prepared by chemical process are usually beneficial to lower the sintering temperature [7]. Doping with sintering additives is another effective method to densify LaCrO3. A small amount of Sr3(VO4)2 [8] was found to produce high density LaCrO3-based perovskite materials sintered at 1,550 C in air. Alternatively, low melting point eutectics, such as LaF3, YF3 and MgF2, up to 8–10 wt% were added to densify LaCrO3 at 1,400 C in air [9]. The aim of the work described here is to experimentally investigate the sintering and thermal expansion behaviors of La0.85Sr0.15Cr0.95O3 (LSC) ceramics by using calcium fluoride as an additive. Strontium-doped lanthanum chromite powders were synthesized using traditional ceramic processing methods. The starting powders La2O3, SrCO3, Cr2O3 (>99.9 wt%) were mixed with distilled water using ZrO2 milling media in a ball mill. After drying, the powders were calcined at 1,000 C prior to adding the sintering additive. Then various amounts of CaF2 (0–0.9 wt%) were added to the calcined powders. The mixed powders were milled again and dried. After granulitization, the powders were pressed into pellets 20 mm in diameter and 2 mm in thickness under a pressure of 72 MPa. The green bodies were then sintered at selected temperatures with a heating rate of 2 C/min in air and a holding time of 3 h. The particle size distribution of the calcined powders was measured using a particle-size analyzer (NSKC-A, Nanjing University of Technology, Nanjing, China). The bulk densities of the sintered pellets were measured by Archimedes method with de-ionized water. The crystal structure of the sintered samples was analyzed using X-ray diffraction (XRD, D/max-b, Rigaku) with CuKa radiation in the X. Ding AE L. Guo (&) College of Materials Science and Engineering, Nanjing University of Technology, Nanjing, Jiangsu 210009, People’s Republic of China e-mail: lc-guo@163.com J Mater Sci (2006) 41:6185–6188 DOI 10.1007/s10853-006-0161-1

Laser Sintering of Silica Sand ? Mechanism and Application to Sand Casting Mould

Silica sand is commonly used in the foundry industry. With a high melting point of 160°C, the silica sand is normally sintered in a high-temperature furnace. However, silica with contents of calcium, aluminium, magnesium, and chlorine, etc. can form low-melting point eutectics. Therefore, a relatively low-power laser can be used to sinter the silica sand directly. The investigation of the mechanism and process for direct laser sintering of the silica sand, without any binder, is presented in this paper. Combined with rapid prototyping (RP) technology, the laser sintering of the silica sand can be used to directly fabricate a sand casting mould, called a rapid sand casting mould. By avoiding the time-consuming process of fabricating a pattern, the rapid sand casting mould process has the potential of further reducing the lead time for producing a casting product. Some important issues, such as the lead time of producing a sand mould, its accuracy, and surface finish, etc., are discussed.

Prediction of evolution characteristics of alkali metal compounds in coal combustion/gasification from coal properties

A series of experiments has been conducted to study the transformations of the organically bound inorganic elements and inherent sodium chloride under conditions relevant to fluidized bed combustion of low rank coals. Large coal particles (5.5–9.0 mm) were suspended in a convective gas flow in a single particle furnace operated at 700–830 °C. The experiments were performed under pyrolysis as well as combustion conditions. The product particles withdrawn from the furnace at various residence times were analysed using chemical methods, X-ray diffraction and electron microscopy. It was found that the transformations of the inorganic matter in high sodium, high sulphur low rank coals result in the formation of a molten ash layer on the char surface, providing evidence of the formation of low melting point eutectics. The molten ash consisted of a matrix of mixed alkali sulphates. The findings are expected to assist in explaining the role of the inorganic matter in causing operational problems during the fluidized bed combustion of low quality, low rank coals.

Improved Performance Rhenium Containing Single Crystal Alloy Turbine Blades Utilizing PPM Levels of the Highly Reactive Elements Lanthanum and Yttrium

Turbine inlet temperatures have now approached 1650°C (3000°F) at maximum power for the latest large commercial turbofan engines, resulting in high fuel efficiency and thrust levels approaching or exceeding 445 kN (100,000 lbs.). High reliability and durability must be intrinsically designed into these turbine engines to meet operating economic targets and ETOPS certification requirements. This level of performance has been brought about by a combination of advances in air cooling for turbine blades and vanes, computerized design technology for stresses and airflow, and the development and application of rhenium (Re) containing, high γ’ volume fraction nickel-base single crystal superalloys, with advanced coatings, including prime-reliant ceramic thermal barrier coatings (TBCs). Re additions to cast airfoil superalloys not only improve creep and thermomechanical fatigue strength but also environmental properties, including coating performance. Re slows down diffusion in these alloys at high operating temperatures [1]. At high gas temperatures, several issues are critical to turbine engine performance retention, blade life, and integrity. These are tip oxidation in particular for shroudless blades, internal oxidation for lightly cooled turbine blades, and TBC adherence to both the airfoil and tip seal liner. It is now known that sulfur (S) at levels, <10 ppm but >0.2 ppm in these alloys reduces the adherence of α alumina protective scales on these materials or their coatings by weakening the Van der Waal’s bond between the scale and the alloy substrate. A team approach has been used to develop an improvement to CMSX-41 alloy which contains 3 percent Re, by reducing S and phosphorus (P) levels in the alloy to <2 ppm, combined with residual additions of lanthanum (La) + yttrium (Y) in the range 10-30 ppm. Results from cyclic, burner rig dynamic oxidation testing at 1093°C (2000°F) show thirteen times the number of cycles to initial alumina scale spallation for CMSX-4 [La + Y] compared to standard CMSX-4. A key factor for application acceptance is of course manufacturing cost. The development of improved low reactivity prime coats for the blade shell molds along with a viable, tight dimensional control yttrium oxide core body are discussed. The target is to attain grain yields of single crystal CMSX-4 (ULS) (La + Y) turbine blades and casting cleanliness approaching standard CMSX-4. The low residual levels of La + Y along with a sophisticated homogenisation/solutioning heat treatment procedure result in full solutioning with essentially no residual γ/γ’ eutectic phase, Ni (La, Y) low melting point eutectics, and associated incipient melting pores. Thus, full CMSX-4 mechanical properties are attained. The La assists with ppm chemistry control of the Y throughout the single crystal turbine blade castings through the formation of a continuous lanthanum oxide film between the molten and solidifying alloy and the ceramic core and prime coat of the shell mold. Y and La tie up the <2 ppm but >0.2 ppm residual S in the alloy as very stable Y and La sulfides and oxysulfides, thus preventing diffusion of the S atoms to the alumina scale layer under high temperature, cyclic oxidising conditions. La also forms a stable phosphide. CMSX-4 (ULS) (La + Y) HP shroudless turbine blades will commence engine testing in May 1998.

The fate of organically bound inorganic elements and sodium chloride during fluidized bed combustion of high sodium, high sulphur low rank coals

A series of experiments has been conducted to study the transformations of the organically bound inorganic elements and inherent sodium chloride under conditions relevant to fluidized bed combustion of low rank coals. Large coal particles (5.5–9.0 mm) were suspended in a convective gas flow in a single particle furnace operated at 700–830 °C. The experiments were performed under pyrolysis as well as combustion conditions. The product particles withdrawn from the furnace at various residence times were analysed using chemical methods, X-ray diffraction and electron microscopy. It was found that the transformations of the inorganic matter in high sodium, high sulphur low rank coals result in the formation of a molten ash layer on the char surface, providing evidence of the formation of low melting point eutectics. The molten ash consisted of a matrix of mixed alkali sulphates. The findings are expected to assist in explaining the role of the inorganic matter in causing operational problems during the fluidized bed combustion of low quality, low rank coals.

Precipitate formation in austenitic stainless steel welds

Recent research efforts have revealed many of the factors which control the hotcracking susceptibility of austenitic stainless steel weld metals. It is generally agreed that the segregation of impurity elements to solidification grain boundaries is a primary cause of the hot-cracking tendency of these alloys in that elements such as phosphorus and sulfur form low melting point eutectics with the metal matrix. Furthermore, these elements can also degrade cracking resistance by lowering the surface tension of the interdendritic fluids to which they segregate. In the case of phosphide precipitation in austenitic stainless steel weld metal, there is apparently some question in the literature as to the specific phosphide formed. This study was undertaken to determine the microstructural relationships, relative to phosphide formation, pertaining to hot-cracking of austenitic stainless steel weld metals. The results confirm that phosphides of the type (Fe, Cr)/sub 3/P form in austenitic stainless steel welds. These phosphides appear at both eutecticferrite/austenite interfaces and along solidification hot-cracks, where they most likely form via an eutectic reaction. These results are inconsistent with those of Brooks et al. in that no evidence of either the MP or MP/sub 3/ phosphide was found. No reference could be found, either in themore » published literature or in standard compilations of crystallographic information, to verify the existence of a CrP/sub 3/ phase, suggesting that the theory of chromium depletion (CrP/sub 3/ vs. CrS) needs further substantiation before it is accepted.« less