Chromium
Pure chromium contains only chromium atoms in its crystalline structure. If a piece of pure chromium is heated, the metal expands. On continued heating, chromium reaches its melting point at 3407°F - Fig. 1. The chromium will then melt at a constant temperature.

Metals like chromium have a definite atomic arrangement that conforms to a typical pattern. In chromium's case, the atoms are arranged in cube symmetry, meaning they look like a cube. With chromium, one atom is located at each corner of the cube and one atom is in the cube's center (Fig. 2), an arrangement known as body-centered cubic (BCC). Some other metals that have a BCC atomic arrangement are molybdenum, tungsten, vanadium, and columbium. All of these metals retain the BCC structure from room temperature to their melting temperature.

An exception is iron, which is BCC from room temperature to 1670°F and from 2534°F to its melting temperature at 2790°F. Below room temperature, electroplated chromium is hexagonal. However, the phase is unstable and transforms to BCC slowly.

Copper
When heated, a piece of pure copper goes through the same series of events as does chromium. The only difference is copper has different melting and boiling temperatures - Fig. 3. Copper melts at 1981°F and boils at 4703°F. Although copper also has a cubic arrangement, the atoms are arranged somewhat differently. One atom is located at each corner of the cube and one atom is located at the center of each face of the cube, an arrangement called face-centered cubic (FCC) - Fig. 4. Other metals with a FCC atomic arrangement include aluminum, nickel, silver, gold, and lead, all of which retain their FCC structure to their melting temperature. Again, iron is an exception; iron is FCC from 1670 to 2534°F.

Iron
Pure iron is quite different from chromium or copper. When heated to the correct temperature, it has the ability to transform from one cubic arrangement to another, a phenomenon called allotropy. The temperature transformation stages of pure iron are as follows: It is body-centered cubic at room temperature. Once the metal is heated to 1670°F, it fully transforms to the face-centered cubic form. It stays FCC until it reaches 2534°F. At that temperature, it transforms back to the body-centered cubic form. Heating pure iron above 2534°F to its melting temperature of 2790°F has no effect on the BCC form. Once the metal reaches its melting temperature, the body-centered cubic form begins to break up and the atoms move freely about because the metal is now in the liquid state. Note that these allotropic transformations are only obtained under equilibrium conditions. If the metal is heated or cooled quickly, the allotropic transformation temperatures will change. These transformation points suffer arrests just like a melting point arrest. Another temperature worth mentioning is the Curie point, or magnetic change point. At 1414°F and above, iron is nonmagnetic. This Curie point is not a structural change point.

Pure iron is very ductile and has low tensile strength. There is virtually no use for it in industry; however, when carbon is added to iron its strength increases and good ductility is retained.

Following is a closer examination of the elements that combine to form steel. An element is the smallest unit particle of a substance. There are approximately 102 naturally occurring elements; three-quarters of them are classed as metals.

Carbon. This is the main element in the formulation of steel. Adding up to 0.8% carbon increases hardness and strength. The more carbon added, the more difficult the steel is to weld.

Manganese. At percentages of 0.25 to 1.0, manganese combines with the low sulfur content to form a compound called manganese sulfide. This reduces the chance of cracks occurring at elevated temperatures (termed hot shortness). Manganese is also a hardener and toughener.

Silicon. This element is primarily a deoxidizing and scavenging agent. The amount present in the steel is usually around 0.35%; however, it can be as low as 0.05%.

Sulfur. Sulfur is an impurity that adversely affects impact energy absorption. It is kept to a low of 0.05%. On its own, sulfur combines with iron to form iron sulfide, which has a melting temperature of 1814°F. This is why manganese is added to steel. It combines with the sulfur to form manganese sulfide. Some steels - called free machining steels - have a high sulfur content of around 0.35%. Manganese is added to these high-sulfur steels because it helps the steel to be free cutting. The high manganese combines with the high sulfur to form the manganese sulfide compound. This type of steel is then cold drawn through dies, making the manganese sulfide elongated or needle-like and easy to machine.

Phosphorus. This element is also an impurity. It does have the ability to strengthen steel but at the expense of ductility. Maximum percentage should be approximately 0.05%.
Fig. 5 - The effects of temperature changes on pure iron.

Heat Treatment of Steel
With a basic understanding of atomic arrangements, grain size, and the important elements of steel, we can proceed to the actual heat treatment of steel. It was stated earlier that carbon is the main element in the formulation of steel. Carbon is also an austenite stabilizer; so much so that above 0.5% the delta phase is completely eliminated. Following are definitions of the terms austenite, ferrite, cementite, martensite, and pearlite. Some of these constituents were given names related to their appearance when viewed through a microscope. For instance, cementite is an intermetallic compound of iron and carbon, and pearlite was so named because of its pearl-like appearance. Other structures were named after individuals. E. C. Bain discovered bainite. Martensite was named after Adolf Martens. Austenite was named after James Austen.

Austenite. This is the face-centered cubic structure of steel. It is also known as gamma iron. It is a solid-stage structure stable only at high temperatures. This structure can take on 1.8% C.

Ferrite. Also known as alpha iron, this is the body-centered cubic structure of steel. It is a solid-stage structure stable at room temperature. This structure can only take on 0.008% C at room temperature.

Cementite. This is a crystalline compound of iron and carbon (Fe3C) with an orthorhombic crystal structure. Cementite joins with ferrite to produce pearlite. It contains 6.67% C by weight.

Pearlite. Cementite and ferrite join in a lamellar form to produce pearlite. It results from the transformation of austenite on slow cooling.

Martensite. This structure is obtained only when austenite is suppressed down to a temperature where it has to transform to a body-centered tetragonal. If there is sufficient carbon in the steel, it will not be able to precipitate out of this shear type of transformation and will be trapped in the body-centered tetragonal lattice.

Iron-Iron Carbide Equilibrium Diagram
The iron-iron carbide equilibrium diagram shown in Fig. 6 is helpful when discussing heat treatment. Note that carbon is plotted horizontally and is shown in terms of weight-percent. Temperature is plotted vertically up to the melting temperature of pure iron. While the diagram appears complicated at first glance, the areas of our discussion are quite simple.

Figure 7 shows the cubic lattice structure of ferrite and cementite, and the formation of pearlite. It must be remembered that the stages shown in Fig. 6 are obtained only under equilibrium conditions, meaning that if we heat or cool rapidly, the structures will not form at the temperatures shown or may not form at all.
Fig. 6 - An overall graph of the iron-iron carbide equilibrium diagram.

As stated previously, the most important alloying element in steel is carbon. When carbon is added to iron, the iron is naturally no longer pure, hence the iron-iron carbide diagram shown in Fig. 6 is needed. The top line shows when carbon is added to iron, the melting temperature drops. As you follow the line down, notice it meets another line going up. This point, called the eutectic point, is at a temperature of 2066°F and has a carbon content of 4.3%. It is the lowest melting point of iron carbon alloys. It also solidifies at a constant temperature. At a carbon content above or below 4.3%, the metal will solidify over a range of temperatures. The arrows at the top left-hand side of Fig. 6 show first the amount of carbon that delta can absorb. The maximum amount is 0.1% C.

Next, examine the lower part marked delta and austenite. At 2718°F, a carbon content of 0.1% must reject surplus carbon if cooling is continued. This rejected carbon combines with the iron to produce the interstitial solid solution austenite. This will continue until 2534°F is reached, then it will all be austenite. In discussing heat treatment of steel, however, we are not concerned with this upper part of the graph. Our area of concern is the lower left-hand corner of the overall graph shown in Fig. 6 and on the condensed graph - Fig. 8. In examining Fig. 8, the left area labeled "ferrite with carbon in solid solution" is another area that is not relevant to heat treatment of steel. However, the A1 line - 1333°F - is very important. Steel of any carbon content can be heated to just below this 1333°F line and no structural change will take place; above 1333°F solid structural changes occur.

To understand these structural changes, we will discuss only the steels. Look at the 0.2% carbon steel labeled #1. In examining Fig. 8 just above the A3 line, we can see the steel is all austenite FCC. On cooling the steel very slowly down to the A3 line, the first crystals of ferrite start to form. On continued slow cooling, more ferrite crystals form. At this stage, there is ferrite (BCC) and austenite (FCC). If the steel is cooled to the A1 line (1333°F), there is more ferrite and some untransformed austenite.

We should at this time discuss grain size and its formation relative to carbon steels. Pure iron transforms from BCC to FCC at 1670°F. The grain size is at its smallest at this temperature. Carbon steels, however, start to form austenite at 1333°F (A1 line) and are completely austenitic at the A3 line. This means the grain size starts to become smaller at the A1 line and is at its smallest at the A3 line. If heated way above the A3 line, the grain size increases. We will return to this subject later.

Look at the 0.45% C steel labeled #2. On slow cooling from the A3 line, crystals of ferrite begin forming as they did with the #1 steel. As the steel continues to slowly cool, more ferrite crystals are present. At the A1 1333°F line, ferrite and more untransformed austenite than in the #1 steel are present. Keep in mind this untransformed austenite must transform to the ferrite cementite lamella form called pearlite. Below 1333°F, there is a little less ferrite (BCC) and a little more pearlite than in the #1 steel.

Take a look at the #3 0.8% C steel, which is termed a eutectoid steel. The eutectoid transformation of austenite to the ferrite and cementite lamella called pearlite occurs isothermally. It is also the lowest temperature structural change point for carbon steels. At slightly above 1333°F, the steel is all austenite (FCC). On slow cooling to 1333°F, the ferrite cementite lamella formation takes place. In this case it is 100% pearlite. Pearlite contains 88% ferrite and 12% cementite. On heating the reverse occurs and at 1333°F all the pearlite transforms to austenite. Keep in mind that at this temperature or just above, the grain size is at its smallest.

Thus far, we have discussed what happens when the steel is heated or cooled very slowly. Let's see what happens when we rapidly cool a piece of 0.8% carbon steel from the fully austenitic state. When this steel is rapidly cooled, the FCC state is actually suppressed down to around 200°F. By doing this, the austenite pearlite change has been completely passed and we have a FCC structure that has to change to a BCC structure. The transformation at this temperature is a shear-type transformation. Remember that the FCC structure can take on 1.8% C and the BCC structure can take on only 0.008% C at room temperature. If the 0.8% C steel is cooled in cold water, the FCC state is suppressed down to around 200°F, where it has to immediately transform to the BCC state. Due to this shear type of transformation, the carbon in the FCC state does not have sufficient time to precipitate, resulting in a BCC structure that has more carbon than normal. In fact, the BCC structure is highly distorted. This type of structure is called a highly distorted body-centered tetragonal, more commonly known as martensite. What we have done to this 0.8% C steel is fully harden it. A fully hardened steel has little use as an engineering material; therefore, we usually temper or draw the steel to a desired hardness. Tempering is a heat treating process in which a fully hardened piece of steel is reheated between 300 and 1000°F. Tempering releases some of the trapped carbon atoms. These released carbon atoms combine with iron atoms to produce iron carbide (Fe3C) also known as cementite. The 0.8% C steel was chosen because it is one of the easiest to harden. Theoretically, a 0.1% C steel can be hardened if a fast enough cooling rate can be accomplished. A water quench, however, would not be fast enough; therefore, it is said a 0.1% C steel (mild steel) cannot be hardened. A 0.45% C steel is about the minimum carbon content steel that can be easily hardened using a water quench. It must be remembered that in order to harden steels in the 0.45­0.80% C range, they must be heated to just above the upper critical A3 line, then cooled in the desired cooling medium. Steels above 0.8% C can also be easily hardened. The only difference is they do not have to be heated to the Acm line and then cooled. The steel has to be heated to just above the A1 line then cooled, usually in oil. The reason is that steels above 0.8% C contain cementite as well as martensite on fast cooling. Cementite is a very hard constituent in steels.
Fig. 8 - The cubic lattice structure of ferrite and austenite with carbon in solution.

Much has been said about grain size, and it is very important when hardening steels in the 0.45­0.8% C range. We know now that on heating to the A3 line, a small grain size is obtained and the steel is in the FCC structure. When this steel is cooled in water, martensite is formed and, most importantly, the small grain size is retained. When it is reheated to the desired temper, grain size is not affected but the carbon does precipitate a little and joins with existing iron atoms to form cementite. It produces a tempered, fine-grained steel.

Acknowledgments
Special thanks to the following people for their help in reviewing the article and for their comments: W. R. Irvine, department head, Department of Metallurgy, BCIT; Bruce Hawbolt, professor, UBC Dept. of Metallurgy; and Ernie Gill(deceased), western director, WIC.

Suggested Reading
The following are additional sources of information regarding metallurgy.
1. Metals and their Weldability. Miami, Fla.: American Welding Society.
2. Metals And How To Weld Them. Cleveland, Ohio: James F. Lincoln Arc Welding Foundation.
3. Basic Metallurgy Programmed Instruction. Materials Park, Ohio: The Metals Park Engineering Institute of the American Society for Materials.
4. Linnert, George. Welding Metallurgy, Vols. 1 and 2. Miami, Fla.: American Welding Society.
5. Krauss, George. Principles of Heat Treatment of Steel. Materials Park, Ohio: American Society for Materials.