Limiting carbon content

The evaluation of heat treatments or the effectiveness of stabilisation by limiting carbon content of these stainless steels can be determined by subjecting specimens to the ASTM standardised acid copper sulphate test or boiling nitric acid test (ASTM A262 1986 see also Sections 9.5 and 1.3). 

The carbon content of DRI depends primarily on the direct reduction process used and the way the process is operated. Carbon content can be adjusted within limits by operating changes within the DR process. Most steelmakers prefer slightly more carbon than is required to balance the remaining FeO in the DRI. DRI from gas-based processes typically contains 1 to 2.5% carbon, mostly in the form of cementite [12169-32-3] Fe C. DRI containing approximately 6 to 7% carbon in the form of cementite is called iron carbide. DRI from coal-based, rotary-kiln processes contains very low (ca 0.5%) levels of carbon. 

If the gas has the correct composition, the carbon content at the surface increases to the saturation value, ie, the solubiUty limit of carbon in austenite (Fig. 2), which is a function of temperature. Continued addition of carbon to the surface increases the carbon content curve. The surface content is maintained at this saturation value (9) (Fig. 5). The gas carburizing process is controlled by three factors (/) the thermodynamics of the gas reactions which determine the equiUbrium carbon content at the surface (2) the kinetics of the chemical reactions which deposit the carbon and (J) the diffusion of carbon into the austenite. 

Electron spin resonance (esr) (6,44) has had more limited use in coal studies. A rough estimate of the free-radical concentration or unsatisfied chemical bonds in the coal stmcture has been obtained as a function of coal rank and heat treatment. For example, the concentration increases from 2 X 10 radicals/g at 80 wt % carbon to a sharp peak of about 50 x 10 radicals/g at 95 wt % carbon content and drops almost to zero at 97 wt % carbon. The concentration of these radicals is less than that of the common functional groups such as hydroxyl. However, radical existence seems to be intrinsic to the coal molecule and may affect the reactivity of the coal as well as its absorption of ultraviolet radiation. Measurements from room... 

Figures 11.2-11.6 show how the room temperature microstructure of carbon steels depends on the carbon content. The limiting case of pure iron (Fig. 11.2) is straightforward when yiron cools below 914°C a grains nucleate at y grain boundaries and the microstructure transforms to a. If we cool a steel of eutectoid composition (0.80 wt% C) below 723°C pearlite nodules nucleate at grain boundaries (Fig. 11.3) and the microstructure transforms to pearlite. If the steel contains less than 0.80% C (a hypoeutectoid steel) then the ystarts to transform as soon as the alloy enters the a+ yfield (Fig. 11.4). "Primary" a nucleates at y grain boundaries and grows as the steel is cooled from A3...

C) 370/656X brittleness after exposure to temperatures between about 700 to 1. OSO-F. stainless steels. chromium stainless steels, over 13% Cr and any 400 Series martensitic chromium stainless steels low in carbon content (high Cr/C ratio). complex chromium compound, possibly a chromium-phosphorus compound. chromium steels at temperatures above about 700 F (370 C) keep carbon up in martensitic chromium steels and limit Cr to 13% max. 

The grades with the 410 or 420 numerals are the basic 13% chromium type with varied carbon content. The additions of sulphur or selenium (possibly with phosphorus) to some grades (416 group) is to improve machinability. 431S29 has increased chromium content to improve corrosion resistance, but reference to Fig. 3.11 shows that such addition alone would lead to a mixed martensite-5-ferrite structure with certain disadvantages to mechanical properties. The nickel addition is to limit ferrite content. 

The limitations imposed by this formula, together with the fact that the alloys in practice rarely contain more than 35% chromium, suggest that the maximum carbon content of the irons should be 2-3%, and in fact the irons normally contain between 1-0% and 2-0% carbon, unless some property other than corrosion resistance is the most important. 

Under certain conditions it is possible for a weldment to suffer corrosive attack which has the form of a fusion line crack emanating from the toe of the weld this is termed knifeline attack. It is occasionally experienced in welded stabilised steels after exposure to hot strong nitric acids. The niobium-stabilised steels are more resistant than the titanium-stabilised types by virtue of the higher solution temperature of NbC, but the risk may be minimised by limiting the carbon content of a steel to 0-06 Vo maximum (ELC steel). 

Calcium can be used as an indicator for selective carbonate dissolution. Its concentration in the extracts is limited either by its content in the carbonate phase (at low pHs and/or low carbonate content of the soil) or by the acid capacity of the extracting solution (at higher pHs and/or higher carbonate content in the soil). The interplay of these factors is clearly shown in Fig. 4.2. 

The dry combustion-direct injection technique provides many advantages over other methods, such as quick response and complete oxidation for determining the carbon content of water. Its primary shortcoming is the need for rapid discrete sample injection into a high-temperature combustion tube. When an aqueous sample is injected into the furnace, it is instantaneously vapourised at 900 °C and a 5000-fold volume increase can be expected. Such a sudden change in volume causes so-called system blank and limits the maximum volume of injectable water sample, which in turn limits the sensitivity [106,107]. 

The operation of the smoking rooms needs to be addressed further. It is not clear that the misting process employed in the rooms and the associated carbon filters will remove the smoke generated from the contents removed from the TRBPs. If the smoke is not adequately managed, a visible plume will be emitted from this part of the facility. Because the oxygen supply is limited, carbon monoxide formation can be expected, but this appears to not have been addressed in the design. 

The CleanOX process is limited in saturated matrices possessing very low permeabilities and high calcium carbonate levels, as well as in groundwater with high total organic carbon content. In addition, the presence of iron-metabolizing bacteria can reduce the effectiveness of the iron catalyst, resulting in operations and maintenance problems for the system. 

---