Molten Zn Research Articles

Study of the zinc-rich corner of the Zn-Fe-Cr system at galvanizing temperatures

The Zn-rich corner of the Zn-Fe-Cr system was experimentally determined at temperatures relevant to galvanizing. The liquid phase was found to be in equilibrium with the ternary extensions of the ζ-FeZn13 phase and the θ-CrZn17 phase. The solid solubility limits of Cr in the ζ phase and of Fe in the θ phase are 3.1 and 4.2 at.%, respectively, at 450 °C. The invariant liquid composition of the liquid-ζ-θ three-phase equilibrium changes noticeably with temperature, shifting from 0.18% Cr and 0.019% Fe at 450 °C to 0.161% Cr and 0.011% Fe at 440 °C. The θ-CrZn17 phase is the only intermetallic phase in the binary Zn-Cr system, and the Cr solubility in molten Zn was grossly overestimated by previous investigators. A schematic Cr-Zn phase diagram is proposed.

Reactions of Co based and Fe based superalloys with a molten Zn – Al alloy

Static corrosion tests of Co based and Fe based superalloys were carried out at 470°C in a molten Zn galvanising alloy containing about 0.22 wt.-%Al and saturated with Fe for various lengths of time up to 168 h (one week). The superalloys readily reacted with the galvanising alloy and CoAl or Fe2Al5 formed initially on the sample surfaces. The reaction front moved towards the depth of the test pieces with increasing dipping time, leaving behind a Zn rich reaction zone. At the same time, other intermetallic phases formed in the Zn rich zone and on the surfaces of the samples. These intermetallic phases are complex in nature and vary with the alloys. In Stellite 6, Fe aluminide was prominently present in the vicinity of surviving carbide particles in the Zn rich zone of the sample tested for 168 h. Some areas of iron aluminide were intimately connected to Fe2Al5 particles building up on the sample surface. This observation suggests that the iron aluminide formed in the reaction zone provided ideal sites for the attachment of Fe2Al5 particles pre-existing in the melt onto the sample surfaces. It also served as nucleation and growth sites for the Fe2Al5 compound on the sample surface. This is one possible mechanism of dross buildup on submerged hardware made of this type of Co based superalloy in continuous galvanising.

Characterization of Surface Defects in High Strength Galvannealed Steels/ Charakterisierung der Oberflächendefekte in hochfesten, nach dem Verzinken wärmebehandelten Stählen

Hot-dip galvannealed steel sheets, in which Fe of the substrate steel diffused into upper coating layer so as to be alloyed with Zn through a galvannealing above 450°C followed by hot-dip galvanizing process, generally show superior corrosion resistance, weldability and paintability. They have been widely used in automobile, construction, appliance industries and others. In particular, many researches have been carrying out to produce defect-free coating for an exposed automotive body panel. In the present study, high strength interstitial-free steel sheets containing Mn and P were galvannealed in an industrial continuous galvanizing line and defects on the coating surface were investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy. It is clear that the quality of the substrate strongly affects galvannealed coating and it is essential to keep clean surface just before immediately dipping into molten Zn pot.

熱延鋼板の内部酸化層が冷間圧延後の再結晶焼鈍時におけるＳｉ，Ｍｎ表面濃化挙動に及ぼす影響

By the heat treatment of a ultra low carbon hot-rolled steel containing 0.31mass%Si-1.02mass%Mn, not only readily oxidizable elements such as Si and Mn but also Fe were internally oxidized at the steel surface region. The scale of the steel acted as the source of oxygen for internal oxidation. The composition of the grain-boundary oxides differed from those in the grain, presumably due to the difference in oxygen potential ; oxides at the grain-boundary consisted mainly of Fe-Si-Mn-O, while oxides in the grain consisted mainly of Si-Mn-O. The internal oxidation layer remained after scale pickling and cold rolling. Fe containing grain-boundary oxides, arranged in layers by the cold rolling process, were considered to act as the oxidant of Si and Mn, which diffuse toward the steel surface in recrystallization annealing. By not only the low Si and Mn activity due to the depletion layer formed during the heat treatment, but also the redox reaction between the grain-boundary oxides and solute Si and Mn, the amounts of Si and Mn selective surface oxidation were remarkably decreased. The resultant suppression of the selective surface oxidation brought about improvement in the wettability by molten Zn of the annealed sheet steel surface.

Sliding wear performance in molten Zn–Al bath of cobalt-based overlayers produced by plasma-transferred arc weld-surfacing

The purpose of this investigation was to improve the wear resistance of part materials (bush and sleeve in sink roll) by applying plasma transferred arc (PTA) surfacing technique in the continuous hot-dip galvanizing facilities. Candidate alloys from the preliminary investigation were cobalt-based Stellite 6 and Tribaloy 800 with WC, WC–Co, TiC, NbC, and Cr 3C 2. The resultant deposits were subsequently characterized using X-ray diffraction, scanning electron microscopy, and the microhardness tests. Microstructural development during the solidification of each deposit is discussed. Pin-on-disk sliding wear tests were conducted in a molten Zn–0.18%Al (wt.%) bath at 470 °C to evaluate the wear performance of the PTA weld-surfaced cobalt-based alloys. Based on microstructural observation and the microhardness measurement, sliding wear performance of the PTA weld-surfaced cobalt-based alloys in molten Zn–Al bath is reported and compared with pin and disk combination.

溶融Ｚｎ‐６ｍａｓｓ％Ａｌ浴に浸漬した炭素鋼の界面反応挙動に及ぼす炭素濃度の影響

The phases and microstructure of the alloy layer formed during hot dipping of carbon steels in molten Zn-Al bath were investigated. Pure iron, 0.2%C and 0.82%C steels and 2.45%C cast iron were used. A plate shape specimen of 300μm in thickness was firstly hot dipped in a molten Zn bath for 3s and then hot dipped in a molten Zn-6mass%Al alloy bath for certain times, and quenched with water. The microstructure of the quenched specimen was observed by using a SEM, and change in thickness of the specimen was measured by using a laser microscope. The pure iron specimen was entirely changed to an intermetallic compound of Fe2Al5(Zn) phase after 1.2ks hot dipping. The thickness of 0.2%C steel specimen was decreased linearly with the dipping time after an early non-reaction period, and 2/3 of the specimen was changed to the Fe2Al5(Zn) phase after 3.6ks hot dipping. It was observed that primarily precipitated ferrite phase preferentially reacted and pearlite structure scarcely reacted. The reacted thickness of the 0.82%C steel and cast iron specimens were only several μm after 86.4Ks hot dipping. In the cast iron specimen, cementite reacted with Al and an Al3C4 phase was formed. The growth of the alloy layer was restricted by the Al3C4 phase. A heat-treated 0.2%C steel specimen with sorbite structure also exhibited the same low reaction behavior as the 0.82% C steel due to homogenized ferrite + cementite structure.

Flow Field Analysis inside a Molten Zn Pot of the Continuous Hot-dip Galvanizing Process.

The flow field inside a molten zinc pot of the continuous hot-dip galvanizing process of steel strips was investigated experimentally. A 1/5-scale transparent water model with induction heaters, scrapers, and baffles was used in this study. Instantaneous velocity fields were measured using a single-frame PIV velocity field measurement technique with varying the strip velocity V s , flow rate Q of the induction heater, scraper location, and baffle type. The general flow pattern inside the strip region is hardly influenced by the strip speed V s , flow rate Q, and the scraper location around the stabilizing roll. When the induction heater is not operated, a pair of vortices is formed in the inner part of the strip: a clockwise rotating flow at the entrance region and a counter-clockwise rotating flow at the exit region. In the exit region outside of the strip, the flow detached from the stabilizing roll divides into two parts: a counter-clockwise rotating flow in the upper region and a clockwise rotating flow in the lower region. For the cases of no scrapper and scrapper, detached from the stabilizing roll, the flow separates from the moving strip and ascends to the free surface. As the flow rate of the induction heater increases, the ascending flow is weakened and the counter-clockwise rotating flow in the upper area of the outside region becomes tranquil. This indicates that the flow in the upper area of the exit region is greatly influenced by the operation of the induction heater. When a scraper is attached onto the stabilizing roll, the separated flow from the strip is guided downward and the up-rising flow around the stabilizing roll becomes slow and tranquil. By attaching baffles near the moving strip in addition to the stabilizing rolls, the flow entrainment into the corner region between the strip and the stabilizing roll is greatly reduced. These flow control devices should be helpful in reducing top drosses in the zinc plating process.

450 °C Isothermal Section of the Fe-Zn-Si Ternary Phase Diagram

The 450 °C isothermal section of the Fe-Zn-Si ternary phase diagram has been determined experimentally using optical microscopy, scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) and X-ray diffractometry. The focus of the work has been concentrated on the Zn-rich corner which is relevant to general galvanizing. The present study has confirmed the existence of the equilibrium state between the liquid, the ξ phase and the FeSi phase. This three phase equilibrium state prevents the equilibrium between the liquid and the δ phase suggested by some researchers. Experimental results indicate that Si solubility in all four binary Zn-Fe compounds is limited. The Fe solubility in molten Zn was found to decrease with increasing Si content in the melt. The liquid phase boundary was determined using a model based phenomenological approach.On a detérminé expérimentalement la section isothermique à 450 °C du diagramme de phase ternaire Fe-Zn-Si en utilisant la microscopie optique, la microscopie électronique à balayage (SEM) conjointement avec la spectroscopie à rayons x à dispersion d'énergie (EDS) et la diffractométrie à rayons x. On a centré ce travail sur le coin riche en Zn qui est important pour la galvanisation générale. La presente étude a confirmé l'existence de l'état d' équilibre entre le liquide, la phase ξ et la phase FeSi. Cet état d' équilibre à trois phases empêche l' équilibre entre le liquide et la phase δ, suggéré par certains chercheurs. Les résultats expérimentaux indiquent que la solubilité du Si dans les quatre composes binaires Zn-Fe est limitee. On a trouvé que la solubilite du Fe dans le Zn fondu diminuait avec une augmentation du contenu en Si dans le liquide. On a déterminé la limite de la phase liquide en utilisant un modèle basé sur une approche phénoménologique.

Flow Control Inside a Molten Zn Pot for Improving Surface Quality of Zinc Plated Strips

The flow fields inside a molten Zn pot of continuous hot-chip galvanizing process were investigated experimentally. With varying several parameters including the strip speed Vs, flow rate Q of induction heater. scrapper location and baffle configuration, instantaneous velocity fields were measured using a PIV velocity field measurement technique. Inside the strip region, counter-clockwise rotating flow is dominant. The general flow pattern inside the strip region is nearly not influenced by the strip speed Vs, flow rate Q and the scrapper location. In the exit region, the flow separated from the moving strip due to the existence of a stabilizing roll ascends to the free surface, for the cases of no scrapper and scrapper detached form the roll. On the other hand, the ascending flow to the free surface is decreased, as the flow rate Q of induction heater increases. By installing a baffle around the uprising strip, the flow moving up to the stabilizing roll decreases. In addition, B-type baffle is better than A-type baffle in reducing speed of flow around the stabilizing rolls. However, the flow ascended to the free surface is largely influenced by changing the flow rate Q, and the scrapper location, irrespective of the baffle type.

Formation of the ζ phase at an interface between an Fe substrate and a molten 0.2 mass% Al–Zn during galvannealing

An interstitial free (IF) steel was dipped in a bath of molten Zn containing 0.2 mass% Al. The as-dipped (galvanized) steel was then annealed above the melting point of Zn (galvannealed), say at 773 K, for a period ranging from 1 to 30 s. Evolution of the microstructure of the Zn-containing coating was examined by transmission electron microscopy (TEM) and the chemical composition around the Zn(Al)/Fe interface was analyzed by energy dispersion spectroscopy (EDS) in a dedicated scanning transmission electron microscope (STEM). In the as-dipped (galvanized) steel a continuous layer of Fe 2Al 5 existed at the Zn(Al)/Fe interface, but no Fe–Zn intermetallic compounds were observed. Galvannealing at 773 K for 1 s resulted in the formation of the ζ phase at the Zn(Al)/Fe 2Al 5 interface, which grew at the expense of the Zn(Al) coating during the subsequent galvannealing. Possible mechanisms of nucleation of Fe–Zn intermetallic compounds are discussed.

In-situ high-voltage electron microscopy observations of reactions between molten Zn and Fe

Reactions between molten Zn and Fe have been observed directly by means of an in-situ heating experiment in a high-voltage electron microscope. In order to suppress evaporation of Zn during the in-situ heating experiment the specimen was coated with a plasma-polymerized hydrocarbon film. Mechanisms for the formation of Fe-Zn intermetallic compounds are discussed and it is concluded that all the models proposed so far are of doubtful validity.

Segregation in hot-dipped galvanized steel

A commercially prepared, hot-dip galvanized, low-carbon steel sheet sample has been studied by means of XPS, SEM, and energy-dispersive x-ray spectroscopy (EDS). The sample was processed in a molten Zn bath containing Al and other additives at low concentrations. Extensive Al segregation to the top of the galvanized layer was observed by XPS. In addition, a considerable segregation of Al at the galvanized coating/steel interface was revealed by SEM/EDS analysis of the sheet cross-section.

Velocity Field Measurements of Flow inside Snout of Continuous Hot-Dip Galvanizing Process Using a Single-frame PIV Technique.

In the continuous hot-dip galvanizing process of steel strips, the snout has been installed at the entering region of feeding strip into the molten zinc (Zn) pot. However, evaporated Zn particles in the snout cause ash imperfection on the galvanized steel strip surface. In order to resolve this problem, the flow field inside the snout, both on the deoxidization gas flow above the free surface and the molten Zn flow in the Zn pot. has been investigated experientially. For a 1/10 scale water model, flow visualization and PIV (Particle Image Velocimetry) velocity field measurements were carried out at the strip speed V s =1.5 m/s Aluminum flakes (1 μm) and atomized olive oil (3 μm) were used as seeding particles to simulate the molten Zn flow and the deoxidization gas flow, respectively. As a result, the liquid flow in the Zn pot is dominantly influenced by the up-rising flow in diagonal direction caused by the rotating sink roll. For gas flow in front of the strip inside the snout, the large-scale vortex formed by the downward moving strip is dominant. In the rear side of the strip, a counter-clockwise vortex is formed and some of the flow following by the moving strip impinges on the free surface of molten Zn. The liquid flow in front of the strip is governed by the up-rising flow entering the snout, caused by the rotating sink roll. The moving strip affects dominantly the liquid flow behind the strip inside the snout and large amounts of liquid are entrained and followed the moving strip toward the sink roll. A thin boundary layer is formed on the front side due to the up-rising flow, however, a relatively thick boundary layer is formed in the rear side of the strip. Inside the snout, the deoxidization gas flow above the free surface is much faster than the liquid flow in the Zn pot. More ash imperfections are anticipated on the rear surface of the strip where larger influx flow moves toward the strip in the region near the free surface.

Post growth n-type doping of ZnSe-bulk single crystals

In this paper we show that the reproducibility of carrier concentration after annealing treatment of ZnSe strongly depends on the state of the original crystals, i.e. their content of residual impurities. Different charges of the ZnSe crystals were investigated by glow discharge mass spectrometry (GDMS), photoluminescence and Hall-effect measurements before and after an annealing treatment in molten Zn (100 h at up to 950°C). These measurements reveal a purification concerning the highly mobile acceptor impurities Li and Na and a dependence between the measured carrier concentration and the net donor impurity concentration (In, Al, Cl … max. 5×10 17 cm −3). In this consequence, we doped high purity vapour grown ZnSe samples intentionally by adding ZnCl 2, In or Al to the Zn melt. The interdiffusion of Al was found to be the most efficient way of post growth n-type doping. Carrier concentrations of up to 10 18 cm −3 were obtained.

Thermochemical Treatments for Protection of Steels in Chemically Aggressive Atmospheres at High Temperatures

Boronizing is a well-known thermochemical treatment process that has found applications in a variety of industries. Boride layers on Fe-alloys present high hardness values (∼ 1900 Hv), good wear properties and are resistant to various aggressive chemical environments. The mechanical behavior of boride layers on Fe-alloys has been well documented, but there seems to exist limited information on the chemical behavior of these coatings. We present in this paper the results of a systematic study of the chemical properties of boride layers, in aggressive environments that include dilute HCl, H3PO4, H2SO4 and HNO3, acid solutions and vapors of molten Al alloy and Zn metals in various temperature ranges. Results indicated that with the exception of HNO3 acid, the presence of boride layers on Fe-alloys, greatly improved their corrosion resistance. These results are discussed in terms of the potential applications they may have in various industrial sectors, including field tests carried out in fossil-fuel fired boilers.

Porous Al Alloys by Local Melting and Diffusion of Metal Powders

ABSTRACTA novel method exploiting local melt and diffusion induced porosity is described for producing porous Al alloys. Aluminum powder and zinc powder were blended and cold-compacted to a near net shape. A porous AI-Zn alloy component was then formed by annealing the material above the melting point of zinc but below the solidus temperature of the alloy. Pores were generated by the diffusion of molten Zn into the solid Al grains. The pore structure is shown to be determined by the volume fraction, distribution, and size of the Zn powders in the powder mixture, and by time and temperature of the annealing heat treatment. The pores can be further expanded by incorporating small amounts of titanium hydride into the powder blend and conducting the annealing treatment above the decomposition temperature of the titanium hydride (T>447°C). Porosity levels of 45 vol.% have been achieved to date. This processing approach appears amenable to the production of sandwich panels with a gradient porosity core and fully dense face sheets, and is applicable to other solid solution systems.

Preparation of low resistive unintentionally doped n-type ZnSe

This paper reports on the preparation of low resistive n-type ZnSe bulk material required as substrates for homoepitaxial growth of ZnSe-based devices. ZnSe samples were annealed in molten Zn at temperatures between 600 and 950 °C for 100 h and subsequently quenched to room temperature. Temperature-dependent Hall and conductivity measurements reveal a maximum carrier concentration of n = 1 × 10 cm −3 . Mobilities of up to 550 cm 2 V −1 s −1 at room temperature were observed. These values are higher than previously reported for Cl-doped ZnSe molecular beam epitaxy (MBE) layers. Samples heat-treated under equal conditions but belonging to crystals of different origin show differences in the achieved free carrier concentration and distinguished photoluminescence spectra. We tentatively discuss the results in terms of impurity content and quality of used ZnSe crystals.

溶融Zn浴中ドロスの存在状態

溶融Zn浴中ドロスの存在状態

EM of mechanical plated Zn coating on steel

Zn and Fe-Zn alloy coatings on steel have been widely used for various applications due to their excellent corrosion resistance. The coatings are usually applied by electroplating or hot-dipping in molten Zn, but the mechanical plating process has also been used commercially and has been found advantageous over the conventional coating technologies in some applications with special requirements. The recent progress in investigating mechanically alloyed metals revealed interesting structural features including the formation of nanocrystalline and amorphous structures. Since the microstructures of electroplated and hot-dipped Zn coatings have been studied, and are known to be different from those of mechanically milled, the different properties of mechanically plated coating must be due to different microstructure. As no microstructural information is available in literature, this research project has been undertaken, along with a study of mechanical properties.Samples of commercially produced (Johnson Matthey Co.) fine-grained (325 mesh, or 44 (a.m) Zn (99.9%)flakes (1.1 μ m thick) were used as the coating materials.

Recovery and mutual separation of noble metals from the simulated insoluble residue of spent fuel

The recovery of noble metals from the quaternary Mo-Ru-Rh-Pd alloy in the simulated insoluble residue was studied by means of Pb extraction and the recovery yield was found to be more than 90% for Ru, Rh and Pd. The recovery yield of Mo was very small due to the oxidative vaporization in air. The decontamination factor for Ce as a stand-in of Pu was determined to be around 200 for one stage. These results on the recovery yields of noble metals from the quaternary alloy and the decontamination factor for Ce are in good agreement with those for the ternary Mo-Ru-Pd alloy previously reported by the present authors. The mutual separation of noble metal elements recovered in Pb button was examined by means of re-extraction with molten Zn and the dissolution in boiling 3M or 6M nitric acid solution. The dissolution fractions of Pd and Rh recovered in the Pb button in boiling 3M or 6M nitric acid solution for 12 h were about 90 and 30%, respectively, and that of Ru was extremely low (~ 0.02%), which is also in good agreement with the previous results of the ternary alloy. The effects of the addition of Ag, Bi and Pd to Pb extraction on the recovery yield of noble metals in Pb button were examined and the dissolution of the Pb button, thus obtained, in nitric acid solution was also examined. By the addition of Ag, both the recovery yield in Pb button and the dissolution fraction in nitric acid solution of all noble metals Ru, Rh and Pd remained almost unchanged. By the addition of Bi and Pd, the recovery of all those noble metals in Pb button and the dissolution fractions of Pd and Ru remained unchanged, but the dissolution fraction of Rh increased significantly up to 90%.