Plain-carbon steel

Plain-carbon steels are essentially alloys of iron and carbon together with varying amounts of other elements such as manganese, sulphur, silicon and phosphoms. These additional elements are found in the raw materials used in the steel-making process and are present as impurities. Both sulphur and phosphorus are extremely harmful and cause brittleness in the steel - they are therefore kept to a minimum. The effect of these is offset by the presence of manganese. The carbon content varies up to about 1.4%, and it is this carbon which makes the steel harder and tougher and able to respond to the various heat-treatment processes. 

Medium-carbon steels cover a range of steels with a carbon content above 0.3% up to 0.6%. They can be hardened by direct heating and 

Those steels with 0.6% carbon have a higher tensile strength, 700N/mm in the normalised condition, and can be hardened and tempered up to 850N/mm. They are used where wear properties are of greater importance than toughness, for sprockets, machine-tool parts and springs. 

Plain-carbon steels are essentially alloys of iron and carbon together with varying amounts of other elements such as manganese, sulphur, silicon and phosphorus. These additional elements are found in the raw materials used in the steelmaking process and are present as impurities. 

Both sulphur and phosphorus are extremely harmful and cause brittleness in the steel - they are therefore kept to a minimum. The effect of these is offset by the presence of manganese. 

The carbon content varies up to about 1.4%, and it is this carbon which makes the steel harder and tougher and able to respond to the various heat-treatment processes. 

Low-carbon steels cover a range of steels with carbon content up to 0.3%. These cannot be hardened by direct heating and quenching, but can be case-hardened. Steels containing 0.2% to 0.25% carbon, referred to as mild steels, are used in lightly stressed applications and can be readily machined and welded. They are used for general engineering purposes as bar, plate, sheet and 

In the normalised condition the tensile strength is around 540N/mm, and in the hardened and tempered condition it can increase to around yOON/mml 

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When a component at an austenitizing temperature is placed in a quenchant, eg, water or oil, the surface cools faster than the center. The formation of martensite is more favored for the surface. A main function of alloying elements, eg, Ni, Cr, and Mo, in steels is to retard the rate of decomposition of austenite to the relatively soft products. Whereas use of less expensive plain carbon steels is preferred, alloy steels may be requited for deep hardening. 

The equihbrium carbon content of austenite also depends on the alloy content, eg, Cr or Ni of the steel. A given gas composition equiUbrates with a carbon content of the austenite which is different for a plain carbon steel than for an alloy steel. 

The physical and mechanical properties of steel depend on its microstmcture, that is, the nature, distribution, and amounts of its metaHographic constituents as distinct from its chemical composition. The amount and distribution of iron and iron carbide determine most of the properties, although most plain carbon steels also contain manganese, siUcon, phosphoms, sulfur, oxygen, and traces of nitrogen, hydrogen, and other chemical elements such as aluminum and copper. These elements may modify, to a certain extent, the main effects of iron and iron carbide, but the influence of iron carbide always predominates. This is tme even of medium alloy steels, which may contain considerable amounts of nickel, chromium, and molybdenum. 

Cementite, the term for iron carbide in steel, is the form in which carbon appears in steels. It has the formula Fe C, and thus consists of 6.67 wt % carbon and the balance iron. Cementite is very hard and britde. As the hardest constituent of plain carbon steel, it scratches glass and feldspar, but not quart2. It exhibits about two-thirds the induction of pure iron in a strong magnetic field, but has a much lower Curie temperature. 

If small specimens are prepared in which the austenite can be cooled to 250—500°C sufficiendy rapidly to avoid the above microconstituents, and transformed at temperatures in this range, the formation of a completely different phase, a bcc a-phase supersaturated with carbon and containing small cementite particles (bainite), which is both strong and tough, occurs. Bainite is rarely found in plain carbon steels, but it can be obtained in commercial practice by judicious alloying and is increasing in importance. 

Measurements of stress relaxation on tempering indicate that, in a plain carbon steel, residual stresses are significantly lowered by heating to temperatures as low as 150°C, but that temperatures of 480°C and above are required to reduce these stresses to adequately low values. The times and temperatures required for stress reUef depend on the high temperature yield strength of the steel, because stress reUef results from the localized plastic flow that occurs when the steel is heated to a temperature where its yield strength is less than the internal stress. This phenomenon may be affected markedly by composition, and particularly by alloy additions. 

Properties. The properties of plain carbon steels are governed principally by carbon content and microstmcture. These properties can be controlled by heat treatment as discussed. About half the plain carbon steels are used in the hot-roUed form, although increasingly the property combinations are enhanced by controlled cooling following the last stand of the hot mill for stmctural shapes, sheet, and strip. The other half are cold-roUed to thin sheet or strip and used direcdy or with an annealing treatment such as described. 

Eig. 25. Variations ia average mechanical properties of as-roUed 2.5-cm bars of plain carbon steels, as a function of carbon content (1). 

Fig. 26. Increase of tensile strength of plain carbon steel with increased cold working, where ( ) represents 0.05—0.30 wt % carbon ( ) 0.30—0.60 wt % ...

Carbon Steels and Low—Medium Alloy Steels. Plain carbon steels, the most common cutting tool materials of the nineteenth century, were replaced by low—medium alloy steels at the turn of that century because of the need for increased machining productivity in many appHcations. Low—medium carbon steels have since then been largely superseded by other tool materials, except for some low speed appHcations. 

Fig. 2. Tool wear mechanisms, (a) Crater wear on a cemented carbide tool produced during machining plain carbon steel, (b) Abrasive wear on the flank face of a cemented carbide tool produced during machining gray cast iron, (c) Built-up edge produced during low speed machining of a nickel-based alloy.

We can find a good example of this diffusion-controlled growth in plain carbon steels. As we saw in the "Teaching Yourself Phase Diagrams" course, when steel is cooled below 723°C there is a driving force for the eutectoid reaction of... 

Sketch the time-temperature-transformation (TTT) diagram for a plain carbon steel... 

The figure below shows the isothermal transformation diagram for a coarse-grained, plain-carbon steel of eutectoid composition. Samples of the steel are austenitised at 850°C and then subjected to the quenching treatments shown on the diagram. Describe the microstructure produced by each heat treatment. 

Plain carbon steels rust in wet environments and oxidise if heated in air. But if chromium is added to steel, a hard, compact film of CrjOj will form on the surface and this will help to protect the underlying metal. The minimum amount of chromium needed to protect steel is about 13%, but up to 26% may be needed if the environment is particularly hostile. The iron-chromium system is the basis for a wide range of stainless steels. 

A piece of plain carbon steel containing 0.2 wt% carbon was case-carburised to give a case depth of 0.3 mm. The carburising was done at a temperature of 1000°C. The Fe-C phase diagram shows that, at this temperature, the iron can dissolve carbon to a maximum concentration of 1.4 wt%. Diffusion of carbon into the steel will almost immediately raise the level of carbon in the steel to a constant value of 1.4 wt% just beneath the surface of the steel. However, the concentration of carbon well below the surface will increase more slowly towards the maximum value of 1.4 wt% because of the time needed for the carbon to diffuse into the interior of the steel. 

Fig. A1.41. Pearlite in a eutectoid-composition plain-carbon steel, x500. (After K. J. Pascoe, An Introduction to the Properties of Engineering Materials, Van Nostrand Reinhold, London, 1978.)...

Fig. 2-9 Relation between potential and corrosion rate of a plain carbon steel in slowly circulating water. Definition of symbols ...

Fig. 2-23 S-N curves for plain carbon steel in 0.05 M potassium hydrogen benzoate (pH 4) at 30°C at various potentials U U = rest potential).

In some service stations, stainless steel or aluminum materials are used for all the filters, pipes and fittings to maintain the purity of the fuel. The rest potentials of these materials are different from that of plain carbon steel (see Table 2-4). 

Materials for metal tanks and installations include plain carbon steel, hot-dipped galvanized steel, stainless steel [e.g., steel No. 1.4571 (AISI 316Ti)], copper and its alloys. The corrosion resistance of these materials in water is very variable and can... 

A tank with a fixed cover of plain carbon steel for storing 60°C warm, softened boiler feed water that had a tar-pitch epoxy resin coating showed pits up to 2.5 mm deep after 10 years of service without cathodic protection. Two separate protection systems were built into the tank because the water level varied as a result of service conditions. A ring anode attached to plastic supports was installed near the bottom of the tank and was connected to a potential-controlled protection rectifier. The side walls were protected by three vertical anodes with fixed adjustable protection current equipment. 

The protection current requirement is determined mainly by the uncoated surfaces of the stainless steel whose protection potential is a few tenths of a volt more positive than that of the plain carbon steel, to avoid pitting (sec Section 2.4). The protection current requirement for the turbine section is about 10 A so that the plate anodes are only loaded to about 1 A. 

Material A tar-EPcoated plain carbon steel material B CrNi stainless steel. 

Fig. 21-11 Current density-potential curves for plain carbon steel in hot caustic soda from Refs. 28-31.

Normally concrete is reinforced with plain carbon steel, but under conditions where rapid carbonation can occur or there is a risk of chloride contamination, corrosion-protected or more corrosion-resistant reinforcing steels may be necessary. Currently there are three reinforcing bars which have enhanced corrosion resistance ... 

Galvanised steel provides increased corrosion resistance in carbonated concrete. In concrete with more than 0.4% chloride ion with respect to the cement content, there is an increased risk of corrosion and at high chloride contents the rate of corrosion approaches that of plain carbon steel. In test conditions the rate of corrosion is greater in the presence of sodium chloride than calcium chloride. Fusion-bonded epoxy-coated steel performs well in chloride-contaminated concrete up to about 3.9% chloride ion in content. 

Plain carbon steel containing approximately 2% carbon together with traces of other elements. 

N/mm2 at 25°C falling to 150,000 N/mm2 at 500°C. If equipment is being designed to operate at high temperatures, materials that retain their strength must be selected. The stainless steels are superior in this respect to plain carbon steels. 

The corrosion resistance of the low alloy steels (less than 5 per cent of alloying elements), where the alloying elements are added to improve the mechanical strength and not for corrosion resistance, is not significantly different from that of the plain carbon steels. 

The austenitic stainless steels have greater strength than the plain carbon steels, particularly at elevated temperatures (see Table 7.8). 

As was mentioned in Section 7.3.7, the austenitic stainless steels, unlike the plain carbon steels, do not become brittle at low temperatures. It should be noted that the thermal conductivity of stainless steel is significantly lower than that of mild steel. Typical at 100°C values are, type 304 (18/8) 16 W/m°C... 

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