Mechanism of Fe-Mn-Al alloy steel ingot failure from MgO-C refractory corrosion

Fe-Mn-Al alloy steel was melted in a mass production tonnage arc furnace equipped with ladle refining facilities. The ingots were cracked and torn apart on hot rolling. Blue flames erupted from the cracks and became red. A white powder was observed adjacent to the cracks in ingots. The white powder was identified as magnesia. Concentrations of Mg and Ca were high in the centre of the ingot, implying the segregation of impurities. Quantitative elemental analysis and microstructural investigation revealed Mg, Si, Ca and S containing impurities and Cr, Mo and Si carbides were segregated within grain boundaries. The segregation was the main cause of ingot cracking. The 1600°C static cup test for carbon containing MgO-C refractories exhibited the reduction reaction, which raised the Mg concentration up to 0.017 wt% in Fe-Mn-Al alloy steel, whereas the pure MgO refractory cup test showed inertness to Fe-Mn-Al alloy.     

Refining of metallurgical silicon by directional solidification

The directional solidification of a typical and a previously refined metallurgical silicon was carried out in a vertical Bridgman furnace. The mold velocity out of the hot zone of the furnace changed from one experiment to another in the range between 5 and 110 μm s⁻¹. Samples were extracted from the cylindrical ingots obtained in the experiments to investigate the effects of the mold velocity on the micro and macrostructures and on the concentration profiles of impurities along the ingots. At the lowest mold velocity, the macrostructures consist of columnar grains oriented approximately parallel to the ingot axis. As velocity increases, grains become thinner and more inclined in the radial direction. Precipitated particles containing Si, Fe, Al, and Ti are observed at the top of all ingots and, as the mold velocity increases, they are also seen at the ingot bottom and middle. The concentration profiles of several impurities have been measured along the ingots by inductively coupled plasma atomic emission spectrometry (ICP), indicating an accumulation of impurities at the ingot top. Consequently, the bottom and middle of the ingots are purer than the corresponding metallurgical silicon from which they solidified. Slices from the ingot bottom have also been analyzed by the glow discharge mass spectrometry technique (GDMS), allowing measurement of impurity concentrations that were below the quantification limit of the ICP. The purification effect and the accumulation of impurities at the ingot top are more pronounced as the mold velocity decreases. In the ingots obtained from the typical metallurgical silicon at the lowest mold velocities (5 and 10 μm s⁻¹), except for Al, all impurities are in concentrations below an important maximum limit for the feedstock of solar grade silicon. At the same mold velocities, the concentrations of Fe, Ti, Cu, Mn, and Ni measured at the bottom of the ingots obtained from both types of metallurgical silicon (typical and previously refined) are even below some limits suggested directly for solar grade silicon.     

富氧顶吹熔融还原技术冶炼高磷铁矿的研究

云南省的高磷铁矿具有其独特性,不但难选,而且难以大量应用于高炉炼铁,因此高磷铁矿的处理是云南省钢铁企业亟待解决的一个问题。主要介绍了一种富氧顶吹熔融还原冶炼技术,并采用该技术处理一种高磷铁矿,研究了富氧率和风量对炉温的影响,重点分析了炉渣的性质,并通过测定冶炼后生铁和炉渣中各元素的百分含量讨论了此工艺的冶炼效果。     

Liquid metal induced embrittlement in fuel line braze joints

This paper documents an instance of liquid metal induced embrittlement (LMIE) that occurred during the manufacture of automobile engine fuel lines. The fuel lines were constructed of low-carbon steel fittings press fit onto 304 stainless steel tube. The joints were subsequently furnace brazed with copper braze paste. The carbon steel fittings were occasionally observed lying separately as the part exited the brazing furnace. Failure analysis using metallographic techniques, scanning electron microscopy, and energy-dispersive spectroscopy indicated that the fractures were intergranular with no observable ductility. Copper from the braze filler metal was observed on the stainless steel tube fracture surfaces and within the tube wall grain boundaries. Copper and austenitic stainless steel are a well-known LMIE couple. Subsequent investigation indicated the stress required for rapid fracture was due to impacts of the parts against one another and the brazing furnace stationary components while the braze metal was liquid.     

Silicon nanoparticles synthesised through reactive high-energy ball milling: enhancement of optical properties from the removal of iron impurities

Alkyl-terminated silicon nanoparticles (SiNPs) were prepared through reactive high-energy ball milling with contamination of iron from the steel milling materials. Iron impurities in the form of iron nanoparticles cause a decrease of photoluminescence intensity and an increase of the UV absorption. The iron impurities were removed either by gel permeation chromatography separation or by treatment with an aqueous HCl solution. The photoluminescence properties of the SiNPs were enhanced after the removal of iron. Transmission electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy and proton nuclear magnetic resonance were used to determine morphology, elemental composition and surface passivation of SiNPs.     

Digital Mine Research and Practice Based on Mining and Metallurgy System Engineering

Iron ore is necessarily basic raw material for industrialization and urbanization and related to the industrial distribution and development programming. The lean iron ore resource in our country is high-cost and low-efficiency, which cannot meet the demand of iron and steel industry, and even endanger the safety of industrial economy. Ansteel mining has created “grade decision-based multi-system integration” mode of mining and metallurgy system engineering and realized scale and efficient development of lean iron ore on basis of the construction of digital mines. Moreover, the “wisdom mines” was proposed and had led to the transformation and upgrading of iron developments.     

Analysis of solidification in Bridgman furnace as simulation of DC casting of aluminium alloy slabs

Abstract

The vertical Bridgman directional solidification equipment has been used in several investigations to simulate direct chill casting of wrought aluminium alloys. As a basis for such investigations and alloy developments, it is important to have an understanding of the performance of the furnace used during simulation of the casting conditions. In this investigation the thermal conditions in the furnace have been analysed in detail, both by measurements and by mathematical modelling. The growth characteristics of the furnace, such as gradient, growth rates and cooling rates have been compared to conditions in large ingots. The direct chill casting conditions, which the simulations have been compared to, are casting of slabs of 330 and 600 mm thickness of an aluminium AA3003 type alloy. The results show that the experiments are able to simulate the cooling conditions in the ingots except from the surface zone. Comparisons of the microstructures have been made and a good agreement has been obtained for structure parameters such as grain size and DAS.     

Analysis of thermal stresses in massive steel ingots in the process of their heating in a furnace prior to their treatment by pressure

A method of estimating the thermally stressed state of massive steel ingots heated in a combustion furnace has been developed. The rate of heating of these ingots as permitted by their technological parameters (admissible thermal stresses), was estimated. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 81, No. 1, pp. 117–122, January–February, 2008.     

A novel material for grounding resistance reduction agent

Abstract

Basic oxygen furnace slag is a major by‐product of steel‐manufacturing plants, yet is often wasted. Over one million metric tons of basic oxygen furnace slag are generated annually. Recycled basic oxygen furnace slag is generally used in the cement industry, road construction and agriculture. The disposal of large amounts of basic oxygen furnace slag is a big problem for both steel‐makers and government agencies. However, the recycling of basic oxygen furnace slag still faces numerous technical, economical and environmental challenges. This study proposes using basic oxygen furnace slag as the main material in a grounding resistance reduction agent. Adding different proportions of water, cement and salt, the resistivity and clot strength of grounding resistance reduction agent was considered to find an effective combination. The effectiveness of the identified combination in reducing grounding resistance was experimentally verified.     

Homogenisation of 20SiMn2MoV steel ingots: thermodynamic/kinetic simulation and experimental validation

Abstract

The phase transformation behaviour and microsegregation profiles during solidification process of the 20SiMn2MoV as cast ingot was analysed using a computational simulation. Then a conservative estimate of the solidification temperature range was provided by the modified Scheil module of Thermo-Calc software, the result was combined with the previous results of the computational simulation and that were used to simulate the homogenization kinetics of this steel. The simulated maximum/minimum dendrite compositions were confirmed by the experimental composition analysis, the good agreement between the simulation and experiments shows that the thermodynamic calculation and kinetic simulation approaches can be used to design an optimised homogenisation heat treatment for as cast ingots, this method does not need a large amount of experimental data of test ingots.     

热锭加热的理论计算法

钢锭在高温炉中加热主要靠辐射热交换。本文给出辐射热交换条件下圆柱体导热微分方程的解,并论证了解的收敛性。锻造钢锭多采用热锭加热,其特点是在恒温炉中一段加热和在变温炉中两段或三段加热方式。本文给出适应各种加热情况的公式和曲线图。利用根据上述公式编制的计算机辅助设计程序计算了10. 3吨和88吨锭的加热过程,结果与实测结果接近。     

Oxide/metal interface on 20Cr–25Ni–Nb stainless steel

Abstract

20Cr–25Ni–Nb stabilised stainless steel is used to contain the fuel in the advanced gas cooled reactor. During operation, this steel must withstand temperatures from 600 to 1073 K in CO₂ gas at 40 atm pressure. It is important that the oxide which forms on this steel is thoroughly characterised and the adherence of the oxide to the metal is understood. A technique of sputter ion plating has been used to remove the oxide from the metal without destroying either metal or oxide. This involves plating the oxide with nickel or molybdenum at a temperature of 600 K, while sputtering the surface with argon ions. On cooling, stresses set up between the oxide and the metal cause the oxide plus sputtered layer to peel off allowing both the metal and oxide sides of the interface to be examined. Results are presented from studies of the metal/oxide interface using scanning Auger microscopy. Analysis of grain centres and grain boundaries indicates that silicon and chromium play an important role in oxide/metal adhesion and, together with conventional analysis of the bulk oxide, assist in determining the oxidation mechanism.

MST/862     

Microstructure and strength of TiAl/steel joint induction brazed with Ag-Cu-Ti filler metal

Abstract

Intermetallic TiAl was induction brazed to steel in an induction furnace with Ag-Cu-Ti filler metal at 1143 K for 0·2–2·4 ks. Microstructural analysis indicates that Ti, Al, and C atoms in base metal diffuse to the interface and react strongly with the filler metal during brazing. The interface structure of the joint can be divided into three distinct zones: the reaction layer near TiAl, composed of Cu-Al-Ti compounds and Ag based solid solutions; the central zone of the interface, consisting of Ag based solid solutions in which Ag-Cu eutectic phases are dispersed; a TiC reaction layer adjacent to the steel. The relationship between brazing parameters and tensile strength of the joints is discussed, and the optimum induction brazing parameters obtained. When brazed at 1143 K for 0.9 ks, the tensile strength of the joint is 298 MPa.     

Thin-slab casting: New possibilities

Changes in the IT industry are known to proceed at a scorching pace. In sharp contrast, the rate of development in the steel industry is generally slow. Nonetheless, the unpact of recenr technical development on the steel industry has been quite significant. The production chain from iron ore to final rolled steel is a long one and the shortening of this length has long been the endeanvour of scientists and engineers. The initial development came in the form of speeding up the process of steelmaking by reducing the slow open-hearth process (8 h tap-to-tap time with the 45 min tap-to-tap time of the Basic Oxygen Furnace (BOF) process. Significant development thereafter have been in the process of continuous production of billets and blooms from liquid steel thereby doing away with the large blooming mills needed for rolling ingots. For a fairly long time after the stabilition of continuous casting, hot rolling involved reheating thick (200–250mm) slabs and reducing them in a hot strip mill. The advent of thin-slab casters has made even these large hot strip mills redundant. The new installations produce thin slabs (50–70mm) that are directly nil led hin strip without the need of an intermediate furnace for raising the stock temperature; the so-called cunnel furnace prior to the rolling stands serving only to equalise stock temperatures. Additionally, what started as a step for reducing investment in hot rolling has in fact, given new opportunity for direct hot rolling of thickneses that were, for long, considered to be feasible only through the cold-rolling route. This article discusses the slow but steady encroachment of hot-rolled sheets into the domain of strip thicknesses hitherto produced by cold rolling and tries to show how the development of thin-slab casters has allowed this process to be accelerated. A techno-economic analysis of thin-slab casting: been presented along with the benefits that arise when a thin-slab caster is linked to the blast furnace and basic oxygen route of steel making.     

High resistance boron treated steels for railway applications

Abstract

The present study analyses different boron contents (between 10 and 160 ppm) on the structure of a 0·2C–2Mn–1Si (wt-%) steel deformed at a starting temperature of 1050°C in a T. J. Pigott laboratory rolling mill. The steel was made in a laboratory open induction furnace using high purity raw materials and cast into metallic moulds. This experimental steel has proved to have tribological properties, under dry rolling/sliding contact, as good as those for the 0·8% pearlitic steels used in railway applications. Before thermomechanical processing, the steel ingots (70 × 70 × 70 mm) were homogenised at 1100°C for 1·5 h. The thermomechanical treatment was carried out by a reversed multipass process to reach a level of deformation of 60%. Plastic deformation was finished at ～920°C for all the rolled steels and the plates (70 × 150 × 20 mm) were then water quenched and/or air cooled to room temperature. Results show more bainitic structures as boron content increases in the air cooled steel after hot rolling. For the quenched steels, the structure becomes more martensitic as boron content increases. The best combination of mechanical properties was obtained for the air cooled 76 ppm boron containing steel, which had a lower bainitic structure. This steel had the yield strength of 750 MPa, 15% elongation and the hardness of 40 HRC. Materials characterisation was carried out by optical and transmission electron microscopy (TEM). Results are discussed in terms of the boron segregation towards grain boundaries, the effect of boron on the steel hardenability, as well as on the boron carbonitrides (CNB) precipitation.     

Construction of Gallium Point at NMIJ

Two open-type gallium point cells were fabricated using ingots whose nominal purities are 7N. Measurement systems for the realization of the melting point of gallium using these cells were built. The melting point of gallium is repeatedly realized by means of the measurement systems for evaluating the repeatability. Measurements for evaluating the effect of hydrostatic pressure coming from the molten gallium existing during the melting process and the effect of gas pressure that fills the cell were also performed. Direct cell comparisons between those cells were conducted. This comparison was aimed to evaluate the consistency of each cell, especially related to the nominal purity. Direct cell comparison between the open-type and the sealed-type gallium point cell was also conducted. Chemical analysis was conducted using samples extracted from ingots used in both the newly built open-type gallium point cells, from which the effect of impurities in the ingot was evaluated.     

Waterwall corrosion in pulverized coal burning boilers: root causes and wastage predictions

Abstract

Waterwall corrosion has become a serious problem in the USA since the introduction of combustion systems, designed to lower NO_x emissions. Previous papers have shown that the main cause of the increased corrosion is the deposition of corrodants, iron sulfides and alkali chlorides, which occurs under reducing conditions. In this paper, the contribution of various variables such as the amount of corrodant in the deposit, the flue gas composition and the metal temperature, is further quantified in laboratory tests, using a test furnace allowing thermal gradients across the deposit and the metal tube samples. Approximate deposit compositions were calculated from the coal composition, its associated ash constituents and corrosive impurities. A commercially available thermochemical equilibrium package was used, after modifications to reflect empirical alkali availability data. Predictions from these calculations agreed reasonably well with the alkali chloride and FeS content found in actual boiler deposits. Thus approximate corrosion rates can be predicted from the chemical composition of the coal using corrosion rates from laboratory tests, adjusted to account for the short duration (100 hours) of the laboratory tests. Reasonable agreement was again obtained between actual and predicted results.     

Experimental investigation on induction brazing of diamond with Ni-Cr hardfacing alloy under argon atmosphere

In recent years direct brazing of a monolayer of diamond crystals on a steel substrate with active filler metals has gained tremendous importance in the industry, with a view to developing tools which can out-perform the conventional galvanically bonded diamond tools. An existing proprietary process uses a specially prepared Ni-Cr filler metal to facilitate its application on a steel substrate. The brazing is done either in a vacuum or a dry hydrogen furnace. The present study has shown that a commercially available Ni-Cr hardfacing alloy, flame-sprayed on a steel substrate with an oxyacetylene gun, could be used for direct brazing of diamond particles. Induction brazing was carried out in an argon atmosphere only for short durations. During brazing under such conditions, the chromium present in the alloy segregated preferentially to the interface with diamond to form a chromium-rich reaction product promoting the wettability of the alloy. It has been further revealed that under a given set of brazing conditions, the wettability of the Ni-Cr hardfacing alloy towards diamond grits primarily depended on its layer thickness. Such dependence resulted in significant variation of topographical features of the tool and its wear mode in simulated grinding tests.     

Effects of surface alloying on microstructure and wear behavior of ductile iron

In this research, the effect of austenitic stainless steel cladding on improving the wear behavior of ductile iron was studied. Samples made of ductile iron were coated with steel electrodes (E309L) by manual shielded metal arc welding. The effect of coated layer thickness on microstructure, hardness, and wear resistance of the surface were investigated. Wear resistance of the samples was measured using the pin-on-plate technique. Optical microscopy and scanning electron microscopy were used to investigate microstructure and wear mechanisms. The phases in the interface of both the coating and the substrate were studied by X-ray diffraction. The results showed that a film of white chromium-enriched iron formed at the interface between the substrate and coating which contained iron–chromium complex carbides. It was, therefore, concluded that enhanced properties would be obtained if the coating thickness and the carbides deposited on the surface were reduced. In samples with a thin coating, surface hardness rose to above 1150 HV (five times higher than that of the substrate) and wear resistance increased significantly.     

Corrosion behaviour of four high temperature engineering materials exposed to landfill gas engine flue gas

Abstract

Four engineering materials commonly used in high-temperature applications were exposed to landfill gas engine flue gas for 25 days at 411°C. The flue gas was composed mostly of nitrogen and oxygen, although some carbon monoxide and hydrocarbons were detected together with low concentrations of hydrogen chloride gas and sulphur oxides. Hydrogen fluoride was not analysed, but, due to the nature of the fuel, may have been present at low levels. The materials tested were a carbon steel, a low alloy steel, a 12%Cr stainless steel and an austenitic stainless steel. Thin, protective films were formed on the 12%Cr and austenitic stainless steels, however, the corrosion products formed on carbon and low-alloy steels consisted of several layers, the outermost of which were extremely friable and non-adherent, especially on cooling from the flue gas working temperature. The maximum corrosion rate obtained was 70 μm yr^–1 for carbon steel, which may be acceptable for a number of flue gas applications. The exposed coupons were examined using conventional techniques such as X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM). The predominant scale formation mechanism controlling the corrosion appeared to be simple oxidation type reactions, however, the influence of HCl, as evidenced by the presence of akaganeite (β-FeO(OH)), was hypothesised. A model describing the corrosion mechanism is proposed in order to provide an improved life prediction capability for such flue gas environments.