The Iron-Carbon Diagram
A slightly modified form of the iron-carbon diagram is shown in Fig. 11 -12. The modification is in the naming of the constituents where, in this case, the names used in this text are inserted in place of the usual Greek names or symbols.
The iron-carbon diagram illustrates the temperatures at which the different constituents in an iron-carbon alloy exist under equilibrium (very slow heating and cooling) conditions. It is a most useful tool in predicting the final structure of these alloys.
The principal iron-carbon alloys are steel and cast iron. The region to the right of 2 percent carbon represents cast iron and that to the left represents steel (although the limit of carbon content of commercial steels is actually about 1.5 percent). Furthermore, this diagram represents only plain carbon steels. When large amounts of alloy additions are made to steel, the iron-carbon diagram changes considerably.
As an example, take a.2 percent carbon steel (AISl 1020); as it is cooled slowly from the liquid, the following changes occur on the
iron-carbon diagram. First, some nuclei of delta iron form in the melt. Delta iron is a body-centered cubic form of iron that occurs only at very high temperatures and is shown in the small areas at the top left-hand corner of the diagram. As the liquid cools further, the nuclei form small crystals which continually grow larger.
When the temperature of 2720F has been reached, the melt becomes mushy and consists of many solid crystals and some liquid. At this temperature a change occurs in the solid crystals. The melt briefly enters the austenite 4- liquid region at this temperature and the solid crystals in the melt change from delta iron into austenite, which, it will be recalled, is the face-centered cubic form of iron.
At a slightly lower temperature (approximately 2700F) all of the liquid has solidified. The solidified metal now consists of a large number of individual grains of austenite. All of the carbon atoms are dissolved in the solid austenite crystals or grains.
No further change occurs until the austenite is cooled to approximately 1580F, at which time some of the austenite (particularly in the region of the grain boundaries) changes into ferrite, forming a number of small grains of ferrite. As the metal cools further, these ferrite grains continue to grow and additional grains of ferrite form.
At about 1333F the microstructure consists of almost 75 percent ferrite and 25 percent austenite. Then, at this temperature, the remaining austenite transforms to pearlite. No further changes occur as the metal cools to room temperature,
The final microstructure consists of grains of ferrite and pearlite. It should be remembered that pearlite consists of plates of ferrite and cementite (iron carbide).
Similarly, it is possible to predict the structure of other compositions of iron and carbon from this diagram when they are cooled slowly.
A fine grain size promotes both increased strength and increased ductility in a metal. The grain size in steel may be altered by heat treatment. When it is heated above certain critical temperatures, a phase change occurs and new grains will nucleate and grow within the old grains.
A part of the iron-carbon diagram is shown at the right in Fig. 11-13. The critical temperatures are labeled Ась Асз, and Асз^, and Acm. It will be seen that these temperatures are at the boundaries of the different phases in steel.
The microstructure of an AISI 1020 steel (.2 percent carbon) will consist of grains of pearlite and ferrite. When it is heated no change
t ig. H-13. Schematic drawing of grain-coarsening effect when heating and cooling a 2 percent carbon steel.
will occur until it reaches the lower critical temperature (Aci). At this temperature the pearlite will be transformed into austenite and in this process many new grains of austenite will form in the old grains of pearlite. As the temperature is increased further, more austenite grains are formed, this time within the remaining ferrite grains. This continues until the upper critical temperature for the AISI 1020 steel (Ac3) is reached, at which the microstructure will consist entirely of austenite grains.
Because many new grains of austenite were formed within each of the former grains, the steel now has more grains which are finer than the previous grains. The fact that the average grain size has been decreased upon heating it above the upper critical temperature is shown schematically by the diagram at the left in Fig. П -13,
If this steel is now heated to a temperature above the upper critical (Ac3) temperature, little change in the average grain size will occur until the grain-coarsening temperature is reached. When this temperature is exceeded, the smaller grains will coalesce, or grow together, forming larger grains. Above this temperature, the average grain size will increase rapidly as shown in Fig. 11-13.
It is not possible to define a specific temperature that will be the grain-coarsening temperature for a specific class of steel. Much depends on how it is made; however, it is always above the upper critical (Ac3) temperature.
If this piece of steel is now cooled from above the grain - coarsening temperature, there is apt to be a slight increase in the average grain size until the grain-coarsening temperature is reached,
as shown in Case 1, Fig. 11-13. When the steel reaches the upper critical temperature, new grains of ferrite form in the austenite grains; this process continues until the lower critical temperature is reached. At this temperature, the remaining austenite transforms to pearlite. As in the case of heating the steel, the grain size was refined when the steel passed through the temperature region between the two critical temperatures. Cooling the steei below the lower critical temperature, however, does not change the grain size.
In Case 1, Fig. 11*13, the AISI 1020 steel was heated above the grain-coarsening temperature resulting in a very coarse austenitic grain size. When this steel is cooled to room temperature, the grains will be larger than they were prior to heating.
If this steel had been heated only slightly above the upper critical temperature and then cooled, a significant grain refinement would have occurred, as can be seen from Case 2, Fig. 11-13. In this case, the steel is cooled from a fine austenitic grain size. Sometimes steels are deliberately heat-treated in this manner to refine the grain.
When depositing a weld, the metal adjacent to the weld is heated and cooled in the manner just described. The result is that there will be regions of coarse-grain size and regions of fine-grain size adjacent to the weld. In welding, the metal is heated and cooled more rapidly than described, and for this reason, the grain coarsening and the critical temperatures will be different than shown in Fig. 11-13. However, the effects will be the same, with the changes described occurring at slightly different temperatures.