Carbon content, steels

FIGURE 10.18 Variation of coating hardness with steel carbon content following chromizing at 1050°C for 95 min. (From LA. Menzies and D. Mortimer, Corros. Sci. 5,1965,539. With permission.)... 

Carburization of steel is a high-temperature process. At a temperature of 1273°K, how much time would be required to raise the steel carbon content at a depth of 1 mm from 0.1 to 1.0 percent (carbon mole fraction on steel surface is 0.02) ... 

Carbon content is usually about 0.15% but may be higher in bolting steels and hot-work die steels. Molybdenum content is usually between 0.5 and 1.5% it increases creep—mpture strength and prevents temper embrittlement at the higher chromium contents. In the modified steels, siUcon is added to improve oxidation resistance, titanium and vanadium to stabilize the carbides to higher temperatures, and nickel to reduce notch sensitivity. Most of the chromium—molybdenum steels are used in the aimealed or in the normalized and tempered condition some of the modified grades have better properties in the quench and tempered condition. 

Austenitic steels that are used for nonmagnetic and cryogenic appHcations have lower carbon content than Hadfteld steels and range in composition from 15 to 29% manganese (30). 

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 maximum surface carbon content is usually set by the gas composition via the equiUbtium constant. If the gas reaction kinetics deposit carbon at a rate which carmot be equaled by the diffusion of carbon into the steel, then the surface value may be less than the possible equiUbtium value. 

Fig. 6. (a) The effect of sub2ero cooling on the hardness gradient in a carburized and quenched 3312 steel where (e) is oil quenched from 925 to 20°C and ( ) is cooled to -195°C. The initial quench to 20°C does not convert all of the austenite to martensite because the high carbon content in the surface region lowers the temperature below 20°C. Subsequent cooling to -195°C converts most of the retained austenite to martensite, raising the hardness, (b) The... 

Addition of niobium to austenitic stainless steels inhibits intergranular corrosion by forming niobium carbide with the carbon that is present in the steel. Without the niobium addition, chromium precipitates as a chromium carbide film at the grain boundaries and thus depletes the adjacent areas of chromium and reduces the corrosion resistance. An amount of niobium equal to 10 times the carbon content is necessary to prevent precipitation of the chromium carbide. 

Ferrous foundries consist of two types steel foundries in which electric furnaces (EAF and induction) are used, and iron foundries in which hot-blast cupolas and/or electric furnaces are used. Electric furnaces use virtually 100% scrap charges. Cupolas are shaft furnaces which use preheated air, coke, fluxes, and metallic charges. Scrap is over 90% of the metallic charge. Cupolas accounted for about 64% of total iron foundry scrap consumption in 1994 and electric furnaces accounted for about 34%. The balance was consumed by other furnaces, such as air furnaces. Iron foundry products have a high carbon content and the scrap charge usually contains a high percentage of cast iron or is used in combination with pig iron. 

The durabihty and versatility of steel are shown by its wide range of mechanical and physical properties. By the proper choice of carbon content and alloying elements, and by suitable heat treatment, steel can be made so soft and ductile that it can be cold-drawn into complex shapes such as automobile bodies. Conversely, steel can be made extremely hard for wear resistance, or tough enough to withstand enormous loads and shock without deforming or breaking. In addition, some steels are made to resist heat and corrosion by the atmosphere and by a wide variety of chemicals. 

In the softer grades (average carbon content ca 0.50%), the composition of the bar was uniform throughout. In the harder grades (average carbon content as high as 1.50%), the outside of a bar might have a carbon content of 1.50—2.00%, whereas the center contained 0.85—1.10%. Steels made by this method were called cement steels. 

Changes on Heating and Cooling Hypoeutectoid Steel. Hypoeutectoid steels are those that contain less carbon than the eutectoid steels. If the steel contains more than 0.02% carbon, the constituents present at and below 727°C are usually ferrite and peadite. The relative amounts depend on the carbon content. As the carbon content increases, the amount of ferrite decreases and the amount of peadite increases. 

Ma.rtensite, Martensite is the hardest and most bntde microstmcture obtainable in a given steel. The hardness of martensite increases with increasing carbon content up to the eutectoid composition. The hardness of martensite at a given carbon content vanes only very slightly with the cooling rate. 

In identifying a particular steel, the letters x are replaced by two digits representing average carbon content. Eor example, an AISI 1040 steel would have an average carbon content of 0.40%, with a tolerance of 0.03%, giving a range of 0.37—0.44% carbon. 

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). 

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