Steels scale structure

Table 1.3 shows a rough breakdown of material prices. Materials for large-scale structural use - wood, cement and concrete, and structural steel - cost between UK 50 and UK 500 (US 75 and US 750) per tonne. There are many materials which have all the other properties required of a structural material - nickel or titanium, for example - but their use in this application is eliminated by their price. 

Hydrogen permeation tests on ferritic stainless steels indicated that hydrogen can diffuse through the alloys, though the permeation was drastically decreased by formation of chromia scale on the alloys. - The mechanisms by which the presence of hydrogen or protons at the air side affects the oxide scale structure and growth are not clearly understood at this time. Several mechanisms have been proposed to tentatively explain the observed anomalous oxidation behavior. ... 

Steel used in the shipbuilding industry and for other large-scale structures such as oil production platforms is often exposed to the weather during storage before construction and during construction, and it is generally protected... 

The full scale structure is a big steel bquid storage tank typically installed in petrochemical plants. The dimensional characteristics of the tank are the following radius R=27.5m, height Hs=15.60 m, liquid level H=13.7 m, liquid density p=900 kg/m3, and wall thickness variable from 17 to 33 mm. The scale model has radius R=2 m, height Hs=1.45 m and wall thickness s=l mm (Figure 12a). Thus, the scale ratio is about 13.7. 

The gas-phase microreactor can be used on the laboratory scale under maximum conditions of 3 bar and 500 °C. It is made up of a stack of stainless-steel micro-structured plates that are arranged for counter-flow or co-current flow practice. Already tested applications of this reactor include the dehydration of 2-propanol [109]. 

In terms of constituents, describe the microscopic (on an atomic scale) structures of tempered eutectoid, hypo-eutectoid, and hyper-eutectoid steel. 

It has been reported that thermal stresses may cause failures or even a collapse of grain silo structures [2]. Some results of earlier investigations on a steel storage silo in the USA have showed that a drop of ambient temperature over 4 °C per day accompanied by low external temperature (tg < -9°C) may cause a serious catastrophe [3]. Effects of temperature on a steel silo model [4] and also full-scale structures [5], [6] have been studied both in theory and experimentally. It has been stated that thermal actions on cylindrical reinforced concrete silo bin produce the following forces [7] ... 

As the samples were heated in air, a thin magnetite layer would have formed first when the sample was heated from room temperature to 570°C. Once the temperature was raised to above 570°C, wustite would start to nucleate at the magnetite-steel interface. However, a stable wustite phase could not form during heating before the sample temperature reached a temperature above 650°C. After the formation of stable wustite nuclei, they would grow into a continuous layer, and after a sufficiently long period of time, become the predominate layer in the scale. The initial scale structure... 

In more recent studies, in order to interpret the observed accelerated steel oxidation in steam in the range of 400-900°C for power plant applications, various possibilities of water , oxygen and hydrogen transports through the oxide scales were again considered [47], but no conclusion was made as to which species was the predominant one and responsible for the observed oxidation rates and scale structures developed. 

Oxide scale structures observed were found to be different on the two sides of the samples used (steel samples 25 x 9 x 8 mm iron samples 25 x 9x2 mm) when the reaction gas was supplied at a rate of 1 litre/min. In all cases, it was found that the typical three-layered (hematite/magnetite/wustite) oxide scale structures with a smooth scale surface were observed on the sample surfaces facing the gas flow, whereas hematite and magnetite were absent at the back surfaces and the scale had the idiomorphic [56,64], multi-faceted appearance associated with scaling in pure carbon dioxide and steam, as will be further discussed later. The relatively slow gas flow rate... 

It is not clear why the author used 10% CO2, instead of water vapour, in the reaction gas to simulate steel reheating. The scales presented were all detached from the steel substrates with some minor preparation damage inside the scale layers, but the typical porous scale sttuctures normally observed in mill scales were not generated in their simulations. This formed a clear contrast to the situation presented by Sheasby etal. [56] who demonstrated that the typical porous scale structure could be reproduced in an atmosphere containing N2-2OH2O-4O2. This poses a question whether porosities can actually form without the presence of water vapour in the atmosphere. Further studies will be required to clarify this. 

In other recent studies of Chen and Yuen [45,46], the oxidation behaviour of a Ti-IF steel (Al-killed) was found to be completely different from those of iron and Al-killed low carbon steel. Because of its very low carbon content and low levels of other alloying elements, this steel was expected to behave similarly to pure iron. However, much slower oxidation rates were observed at temperatures below 850°C. The scale structures developed were also different. At 700-800°C, the steel was very resistant to oxidation with very thin scale formed even after several hours of exposure to dry air (<0.1% H2O) and moist air (2.8% H2O), although the rates were higher in the latter case. The oxidation resistance was reduced partially at 850°C and lost nearly... 

There were several types of genuine porosities observed by various researchers. The first type was a void observed in the magnetite layers formed on iron and steel at temperatures below 570°C [91,93], where the gas used could be moist air, dry air or CO2. The voids were very fine in size and were observed at grain boundaries as well as inside magnetite grains. This type of porosity is also known as Kirkendall voids [100]. The formation of voids appeared to be associated with the formation of a duplex scale structure [101]. Recently, some theoretical treatments using conventional diffusion theories were made by Maruyama etal. [102] and Ueda etal. [92] to provide a semi-quantitative and quantitative explanation of their formation mechanism and their location in the scale. 

R. Y. Chen and W. Y. D. Yuen, Oxide-scale structures formed on commercial hot-rolled steel strip and their formation mechanisms , Oxid. Met. 56, 89-118 (2001). 

The most notable part of the traction engine made from copper is the boiler and its firetubes (see Fig. 1.1). In full size this would have been made from mild steel, and the use of copper in the model is a nice example of how the choice of material can depend on the scale of the structure. The boiler plates of the full-size engine are about 10 mm thick, of which perhaps only 6 mm is needed to stand the load from the pressurised... 

Cement and concrete are used in construction on an enormous scale, equalled only by structural steel, brick and wood. Cement is a mixture of a combination of lime (CaO), silica (SiOj) and alumina (AI2O3), which sets when mixed with water. Concrete is sand and stones (aggregate) held together by a cement. Table 15.4 summarises the most important facts. 

This method is generally not capable of achieving a uniform standard of cleanliness on structural steel. It is not effective in removing intact mill scale or corrosion products from pitted surfaces. The durability of subsequent coats is therefore variable and unpredictable, and depends on the thoroughness of the operation and the exact nature of the contaminants left on the surface. The method should be confined to non-aggressive environments or where short-term durability is economically acceptable. 

Since the paper by Pilling and Bedworth in 1923 much has been written about the mechanism and laws of growth of oxides on metals. These studies have greatly assisted the understanding of high-temperature oxidation, and the mathematical rate laws deduced in some cases make possible useful quantitative predictions. With alloy steels the oxide scales have a complex structure chromium steels owe much of their oxidation resistance to the presence of chromium oxide in the inner scale layer. Other elements can act in the same way, but it is their chromium content which in the main establishes the oxidation resistance of most heat-resisting steels. 

It has been noted that the total current required to protect large structures can be substantial even in mildly corrosive environments. In seawater, for example, an initial current in the region of 200mA/m for bare steel might well be required in the North Sea. This is because the relatively high oxygen concentration and the tide and wave action all contribute to a facile cathodic reaction. Fortunately this current diminishes with time. The reason for this is the protective scale on the steel surface which forms during cathodic protection by decomposition of the seawater. 

Cathodic Current Densities for Protecting Steel Examples of current density requirements for the protection of steel (to achieve a steel potential of —0-8 V vs. Ag/AgCl/seawater) are given in Tables 10.13 and 10.14. It should be realised that the current demand of a structure will be influenced by, inter alia, temperature, degree of aeration, flow rate, protective scales, burial status, presence of bacteria and salinity. 

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