Plain-carbon steel 213 heat-treatment

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. 

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. 

To stress relieve or postweld heat treat carbon and low-alloy steels, they are typically heated to 1100°F to 1350°F (595°C to 730°C) for extended time, followed by air coohng. The minimum time is specified by the relevant engineering code, and the temperature must be less than the lower transformation temperature of the steel, which is the lowest temperature at which austenite starts to form, for example, 1333°F (720°C) for plain carbon steels. In order to avoid degrading the required mechanical properties of a heat treated alloy, subsequent fabrication heat treatment temperatures, such as those for stress relief and PWHT, must not exceed the tempering temperature (discussed in the next section). 

Carbon steels are the most widely used materials of construction. Unalloyed carbon steels typically contain nominal amounts of manganese, silicon, phosphorus, and sulfur. They are normally supplied with a pearlitic-ferritic microstructure (see Figure 21.3) produced by air cooling a hot-formed product (e.g., hot-rolled plate) or by a normalizing heat treatment. They are available as either killed carbon steel or 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. 

Nickel is known to improve both toughness and strength as measured by impact and room temperature tension stress [1968Jol]. For a particular heat treatment, nickel refines the ferrite grain size as eompared with that of a plain carbon steel of similar eomposition. This grain refinement would produee an improvement of the yield strength and a better resistance to brittle fracture. 

Carbon steels, also called plain carbon steels, are primarily Fe and C, with small amounts of Mn. Specific heat treatments and slight variations in composition will lead to steels with varying mechanical properties. Carbon is the principal hardening and strengthening element in steel. Actually, carbon increases hardness and strength and decreases weldability and ductility. For plain carbon steels, about 0.20 to 0.25 wt.% C provides the best machinability. Above and below this level, machinability is generally lower for hot-rolled steels. Plain carbon steels are usually divided into three groups ... 

Contains up to 3 or 4% of one or more alloying elements and is characterized by possessing similar microstructures, and requiring similar heat treatment to, plain carbon steels, but improved strength and toughness over plain carbon steels with the same carbon content. 

High-alloy steels are those that possess structures and require heat-treatment that differ considerably from those of plain carbon steels. Generally they contain more than 5% of the alloying element. A few examples of some high alloy steels are ... 

Figure 11.11 The iron-iron carbide phase diagram in the vidnity of the entectoid, indicating heat-treating tem-peratnre ranges for plain carbon steels. (Adapted from G. Krauss, Steels Heat Treatment and Processing Principles, ASM International, 1990, page 108.)...

Another possibility is to perform a series of heat treatments in which the steel is austenitized, quenched (to form martensite), and finally tempered. Let us now examine the mechanical properties of various plain carbon steels and low-alloy steels that have been heat-treated in this manner. The surface hardness of the quenched material (which ultimately affects the tempered hardness) depends on both alloy content and shaft diameter, as discussed in the previous two sections. For example, the degree to which surface hardness decreases with diameter is represented in Table 11.12 for a 1060 steel that was oil quenched. Furthermore, the tempered surface hardness also depends on tempering temperature and time. 

As-quenched and tempered hardness and ductility data were collected for one plain carbon steel (AISI/SAE 1040) and several common and readily available low-alloy steels, data for which are presented in Table 11.13. The quenching medium (either oil or water) is indicated, and tempering temperatures were 540°C (lOOO F), 595°C (1100°F), and 650 C (1200"F). As may be noted, the only alloy-heat treatment combinations that meet the stipulated criteria are 4150/oil-540°C temper, 4340/oil-540 C temper, and 6150/oil-540°C temper data for these alloys/heat treatments are boldfaced in the table. The costs of these three materials are probably comparable however, a cost analysis should be conducted. 

Brooks, C.R. Principles of the Heat Treatment of Plain Carbon and Low Allov Steel, ASM International, Materials Park. OH, 1996. 

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