Carbon is the basis for the wide range of properties obtainable in iron and steel. It forms different compositions with iron when combined in different ways and amounts. Thus carbon is
Figure 3-12. Schematic diagrams of the growth of dendritic grains.
the primary means for making iron or steel soft and ductile, tough or hard. Most other alloying elements, in effect, modify or enhance the ben- efits of carbon.
The fundamental effects of carbon on iron are shown by the iron and iron-carbide equi- librium diagram shown in Figure 3-13: This is a type III diagram. The carbon commonly appears as iron carbide, but not always, for example, graphite in cast iron. Iron carbide consists of 6.67% carbon and 93.33% iron by weight and has the average formula, Fe3C. It is the hardest constituent in carbon iron and carbon steel and is quite brittle and white in color. It is called cementite.
Iron and Iron Carbide Solid Solutions
The line for zero carbon in Figure 3-13 shows that pure iron or ferrite solidifies at about 1,538° C (2,800° F). Over a short range of high temperature, it has a body-centered- cubic structure called delta iron. When cooled to 1,400° C (2,552° F), the structure changes to face-centered-cubic gamma iron. This is the main part of austenite, which may dissolve up to 2.11%
carbon as iron carbide. Below 910° C (1,670° F), ferrite transforms to body-centered-cubic alpha iron. Thus, iron is allotropic. It also changes from a nonmagnetic material at high tem- peratures to a magnetic one somewhat below 799° C (1,470° F). Alpha iron dissolves only a small amount of carbide. Ferrite can also hold such elements as nickel, silicon, phosphorus, and sulfur in solution in amounts depending on temperature.
Pearlite
Steel containing 0.77% carbon is an important iron alloy. It starts to solidify when the molten solution is cooled to about 1,482° C (2,700° F) and is completely solid at about 1,249° C (2,280° F).
No change occurs in the austenite until the low temperature of 727° C (1,341° F) is reached. This is at minimum in a solid solution and comparable to a eutectic in a liquid solution, and is called a eutectoid. At this point, the gamma turns to alpha iron, and the iron carbide is forced out of solution if cooling is slow. The eutectoid alloy increases in vol- ume on transformation, and the resulting forma- tion consists of a series of plates of iron carbide
interspersed with plates of ferrite in each grain.
This lamellar structure is known as pearlite and is illustrated under high magnification in Figure 3-14. What is shown is coarse pearlite produced by slow cooling. Faster cooling rates cause closer spacing of the plates in what is known as medium and then fine pearlite. Hardness increases from coarse to fine pearlite.
The interlayers of the two phases in pearl- ite reinforce each other. Ferrite has a tensile strength of about 293 MPa (42,500 psi) and elongation of 40%, and cementite a strength of 35 MPa (5,000 psi) and negligible elongation.
The pearlite combination has a tensile strength of 827–862 MPa (120,000–125,000 psi) and elon- gation of 10–15%.
Hypoeutectoid and Hypereutectoid Steels
Steel containing less than 0.77% carbon is called hypoeutectoid and with more than 0.77%
carbon, hypereutectoid. The first may be exem- plified by a steel containing 0.4% carbon on the diagram of Figure 3-13. This composition starts to solidify at around 1,495° C (2,723° F). First, delta iron is precipitated. Then the delta iron changes to gamma iron, and solidification is complete in austenite at 1,394° C (2,541° F). No further change occurs with falling temperature until a little above 799° C (1,470° F) is reached at the A3 line. Then ferrite precipitates as the temperature is lowered to 727° C (1,341° F), at which point the remaining austenite transforms to pearlite. The resulting structure consists of about 55% grains of ferrite interspersed with 45% grains of pearlite. A hypoeutectoid microstructure is depicted in Figure 3-15. The magnification is not sufficient to reveal the lami- nations in the pearlite grains. The proportion of ferrite to pearlite depends on the carbon content.
A hypereutectoid alloy may be exemplified by a steel containing 1.4% carbon. Solidification commences at about 1,449° C (2,640° F) with austenite separating out until solidification is complete at about 1,160° C (2,120° F), and the structure becomes all austenite. As indicated in Figure 3-13, no change occurs in the austenite until the temperature falls to about 1,038° C (1,900° F). Iron carbide is rejected below that point; at 899° C (1,650° F), for instance, the aus- tenite contains only 1.1% carbon, and at 799° C
(1,470° F), a little over 0.77% carbon. The excess iron carbide is rejected by the gamma iron. At 727° C (1,341° F), the remainder of the austen- ite is transformed to pearlite on slow cooling
Figure 3-13. Equilibrium diagram of iron and iron carbide. Dashed lines represent conditions where carbon is present as graphite. (Metals Handbook Desk Edition 1998) with the rejected iron carbide interspersed in the structure and in the grain boundaries. The lever rule (Equation 3-1 and Equation 3-2) ap- plied to the region between the eutectoid at
0.77% carbon and cementite of 6.69% carbon shows the proportions of the constituents of the 1.4% carbon steel to be:
and
Cementite outside the pearlite exists in hy- pereutectoid steel in proportion to the amount Figure 3-14. Microstructure of pearlite (etched and magnified 1,000×).
Figure 3-15. Microstructure of typical ferrite-pearlite structural steels at two different carbon contents: (A) 0.10% carbon; (B) 0.25% carbon, 2% nital + 4% picral etch, magnified 200× (Metals Handbook Desk Edition 1998).
6 67 1 4
5 92 100 89 4
. .
.− × = . % pearlite
1 4 0 77
5 92 100 10 6 . .
. %
− × = . cementite
of carbon beyond the eutectoid point. Iron con- taining more than 2.11% carbon is usually not considered to be steel.
Martensite
The structures considered up to now have been those formed by cooling at a slow rate. Now, assume the hypoeutectoid steel (Figure 3-13) is heated from room temperature to above 727° C (1,341° F). As the temperature is raised above that point, austenite is formed and dissolves all the carbon. The excess ferrite is dissolved be- tween the A1 and A3 lines. At a temperature above the A3 line, the metal is all austenite and contains all carbon, in an interstitial solution.
If the alloy is quenched in water from above the A3 temperature, it is cooled so quickly that the transformation of gamma to alpha iron does not have time to occur at 727° C (1,341° F). Instead, the change is suppressed to some low tempera- ture, say around 204° C (400° F), and there is not enough energy available to cause diffusion of the carbon atoms. They remain in and distort the lattice. This makes a hardened steel called martensite. Its needle-like or acicular micro- structure is depicted in Figure 3-16. The more highly strained layers in the lattices are at- tacked more readily by the etchant and appear as dark lines. Martensite is formed and harden- ing occurs when any steel (hypoeutectoid or
hypereutectoid) is quenched fast enough from above the critical temperature.
Other Structures of Steel
Other steel structures may be produced by various heat treatments as described in Chapter 6. If instead of being quenched from the austen- ite region, a steel is cooled slowly and held for a period of time at around 704° C (1,300° F), the iron carbide will disperse in small spheroidized particles instead of lamellar plates in the ferrite.
This is called spheroidite. On the other hand, if a steel is not fully quenched, but cooled quickly to and held at a temperature above 232° C (450° F) for a period of time, it is transformed to bainite.
This is an intermediate structure between fine pearlite and martensite and appears to be a me- chanical mixture of ferrite and minute carbide particles.
Practical Aspects of Carbon in Steel
Steels are selected with specific carbon con- tents to suit certain purposes. Easily formed sheet steel for cans and automobile fenders and body panels contains 0.1% carbon or less.
Tensile strength is about 345 MPa (50,000 psi) and elongation is 35% or more. Structural steel, like that in I-beams and channels for buildings, has around 0.2% carbon with tensile strength
Figure 3-16. (A) Microstructure of typical lath martensite, 4% picral + HCl (hydrochloric acid) magnified 200×. (B) Microstructure of typical plate martensite, 4%
picral + HCl, magnified 1,000× (Metals Handbook Desk Edition 1998).
of about 414 MPa (60,000 psi) and elongation of 30% or so. Medium carbon steels, widely used for machine parts like gears and axles, contain 0.30–0.45% carbon, have up to 689 MPa (100,000 psi) tensile strength, and 20–25% elongation.
Steels with carbon contents over 0.30% have the added advantage that they can be hardened, particularly on wearing surfaces (to the extent indicated in Figure 6-4). A carbon content of 0.70% may be chosen for wear resistance without brittleness, as for railroad rails, with a tensile strength of about 827 MPa (120,000 psi) and elongation of 10%. Higher-carbon steels are used largely for tools and dies. Other alloying elements enhance heat treatment and properties of steel as described in Chapter 4 and Chapter 6.
Grain Size of Steel
The grain size of a steel nominally refers to the size of the austenitic grains before the steel is cooled to room temperature. Grain size is impor- tant because it influences many of the physical properties of steel. Occasionally, the size of the ferrite grains is important, as in deep-drawing sheet steel, where “ferrite grain size” is defi- nitely specified. Actually, the austenitic grain size is not altered much by the rate of cooling to room temperature. Coarse austenitic grains raise hardenability, normalize tensile and creep strength, and improve rough machinability.
Fine grains increase impact toughness, improve machining finishes, and mitigate quenching cracks, distortion in quenching, and surface decarburization.
The grain size of a steel depends upon its method of manufacture, heat treatment, the amount of hot and cold working, and alloying elements. A deoxidizer added to a molten steel to eliminate dissolved gases and reduce FeO leaves minute oxide particles in the metal. These act as nuclei and, if numerous, promote a fine- grained steel. Steel recrystallizes when heated above the A1 line (Figure 3-13). The higher the temperature and the longer the time in the aus- tenitic region, the more the grains grow in size, although the actual amount of growth in any case depends upon the composition of the steel.
A highly worked steel starts with many small grains from numerous nuclei when recrystal- lized. As a rule, alloying elements like vanadium that form carbides or fine oxides tend to increase resistance to grain coarsening. This is thought to occur due to the fact that the carbides and oxides resist solution in the austenite and help fix grain boundaries.
Solidification of Cast Iron
The changes that occur when cast iron cools will be described for a composition of 3%
carbon. Solidification from the liquid occurs at about 1,329° C (2,425° F), and austenite, with a lower carbon content, separates from the melt. The remaining liquid is enriched with carbon and the last of it reaches the eutectic composition of 4.3% carbon when the temperature has fallen to the solidus at 1,148° C (2,098° F).
At that point, the austenite contains 2.11%
carbon and the last of the liquid solidifies as a eutectic mixture (of austenite and primary car- bide) known as ledeburite. On further cooling at a rate to prevent breakdown of the carbide, secondary carbide is rejected from the austenite in the mixture. At 727° C (1,341° F), the austen- ite contains 0.77% carbon and is transformed to pearlite. This includes the austenite in the eutectic mixture (ledeburite), and the resulting mixture of primary cementite and pearlite is called transformed ledeburite.
With slow enough cooling or with alloys that reduce the stability of iron carbide, graphite is
formed instead of iron carbide. That is, graphite is deposited when the eutectic solidifies and is rejected by the austenite on further cooling. At transformation temperature, the austenite may be changed to pearlite or partially or wholly to graphite and ferrite. Graphite commonly exists in cast iron in flakes (as shown in Figure 9-34A), but under certain conditions may also appear as nodules. The iron-graphite diagram is slightly different from the iron-carbide diagram.
Differences in cooling rates, compositions, al- loying elements, and subsequent heat treatment produce a variety of cast irons. The main ones are described in Chapter 6.